Corneal refractive surgery for aviators
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《中华医药杂志》英文版 2006年第3期
Correspondence toPin Min LAM,Ophthalmology Service,KK Women’s & Children’s Hospital,100 Bukit Timah Road，229899,Singapore
Tel:+65-6394 1157Fax: +65-6291 0161E-mail: vpmlam@yahoo.com.sg
INTRODUCTION
The importance of the visual system in ordinary life is unquestionable, and the necessity for good vision for the performance of critical tasks when pilots fly the sophisticated aircraft is increasingly evident. The most important sensory input that an aviator requires is, therefore, undoubtedly, visual.
Flying is widely viewed as a mentally and visually demanding task. For many years, aviation authorities in the world have adopted a very conservative approach in disallowing the use of corneal refractive surgeries for the correction of refractive errors in aviators, due to the many uncertainties of the procedures then. However, with the rapid advancement in corneal refractive technology and the improved safety profile and predictability of these procedures, many civil aviation authorities, including the military, are starting to exercise waiver policies for certain types of corneal refractive procedures for their flying crew.
PECULARITIES OF AVIATION ENVIRONMENT
The aviation environment is very different from the sea-level ground environment the human is accustomed to. During flight, the pilot encounters a combination of peculiar environmental factors that includes low humidity, reduced air pressure, changes in cabin pressures, low oxygen partial pressure, acceleration forces (such as Gz forces) and even windblast forces during an uncommon event of ejection in an aircraft emergency. These environmental factors may influence the visual function, and thus have a direct impact on the type of corneal refractive surgeries approved in the flying community.
Low Cabin Atmospheric Pressure / Hypoxia
Subsonic commercial airliners normally cruise at altitudes of 30 000~33 000 feet. The outside atmospheric pressure at these cruising altitudes is extremely low, with a corresponding partial pressure of oxygen of only about 40 mm Hg[1]. Such low level of oxygen tension can only sustain life for a few minutes without supplemental oxygenation. For this reason the cabin of airliners are usually pressurised to an equivalent outside atmosphere of around 6 000~8 000 feet. At this cabin altitude, the partial pressure of oxygen drops from the sea-level value of 148~108 mm Hg, equivalent to a fall of about 27%. Every passenger is essentially mildly hypoxic although certainly not symptomatic[2].
In military aircraft, especially high performance combat aircraft, the cabin altitude can vary from sea-level up to 22 000 feet, depending on the flight altitude. This would mandate the use of oxygen breathing system to prevent hypoxia at such high cabin altitudes.
The low atmospheric pressure and its resultant low partial pressure of oxygen may result in hypoxia of the cornea. This is especially so in long haul flights where the duration of exposure can last up to 18~19 hours. There are concerns that hypoxia can cause the refractive status of the post-operative eye to be unstable and therefore affect the final visual acuity of the aviator.
Changes in Cabin Pressurisation
The cabin atmospheric pressure changes during ascent and descent[1].The maximum rate of increase in cabin pressure adopted for most commercial airliners is 1 kPa (0.15 lb/in2) / minute, that is, about 300 feet / minute. Gas expansion, in closed cavities, expands because of reduction of environmental pressure.
Small air bubble trapped underneath rigid contact lenses may expand during ascent (especially during a cabin rapid decompression) and can result in dislodgement of the contact lenses.
Patients who had just undergone lase-assisted in situ kera-tomileusis(LASIK)surgery may have micro-air bubbles under the corneal flap. These micro-air bubbles may expand and cause dislodgement of the flap during flight ascent or during rapid decompression of the cabin aircraft. Patients are generally not advised to travel by air at least one week after the operation.
Low Humidity
The relative humidity in aircraft cabin is low, usually less than 20%. Low humidity may cause discomfort of the eyes, mouth, and nose but presents little risk to health. Pilots who wear contact lenses may feel the effects of the low humidity more and may require more frequent lubrication with artificial tear supplements.
Many patients who had undergone corneal refractive surgery, especially PRK and LASIK, complain of dry eyes. This would usually improve over 2~3 months after the surgery. The low humidity in aircraft cabins can exacerbate the symptoms and patient should be warned and advised accordingly.
Acceleration Forces (G-forces)
High performance aircraft pilots experience “G” or gravity acceleration forces when they perform or are involved in some flight manoeuvres. High “G” loads upon the human body can cause disorientation, blackout and could have a fatal result. To train as a high performance pilot, a device called a centrifuge has been developed to simulate “G” loads on a human. Many evaluations of the suitability of refractive surgery for flying crew are performed under controlled conditions in the centrifuge.
Positive Gz forcesRapid pullups and steeply banked, level turns create positive Gz - forces acting in the direction of the pilot’s feet. They displace blood and the body’s organs toward the lower extremities. As the blood circulation to the brain decreases under positive Gz, the pilot’s visual field narrows. If positive Gz increase, the pilot loses color vision and eventually loses consciousness, known better as G-induced loss of consciousness (G-LOC).
Negative Gz forcesRapid pushovers and “outside” aerobatic manoeuvres create negative Gz - forces acting toward the pilot’s head. Depending on forces involved and individual tolerance, a pilot may experience discomfort, headache, “redout” caused by excessive blood flow in the eyes, and even unconsciousness. Most people have a much lower tolerance for negative Gz than for positive Gz.
There were concerns that the G-forces experienced in flying a high performance aircraft may result in shifting or dislodgement of the corneal flap of patients who had undergone LASIK procedure.
Ejection and Windblast Forces
The ejection seat is the most commonly used equipment in flight abandonment, although it is only available to military fast jet aviators. The initial mechanism and ejection from the cockpit applies accelerations in excess of 12 G for up to 500 ms with an onset rate up to 300 G/s. As the cockpit canopy is released from the aircraft the pilot is subjected to the full blast of the air whilst still traveling at approximately the same speed of the aircraft. Windblast pressures vary with the density of the air and the square of the velocity. The effect of windblast is due to the sudden application of force to the chest and the abdomen. Significant effects on the face (and eyes) will be experienced if the visor is not drawn down during the ejection process.
There were concerns that the ejection forces and windblast forces may result in shifting or dislodgement of the corneal flap of patients who had undergone LASIK procedure. There were also concerns about the corneal integrity and strength after corneal laser refractive surgery as the cornea thickness is significantly reduced, especially in those patients with moderate to high myopia.
VISION AND AVIATION ENVIRONMENT
The aviation environment is a potentially hazardous environment and aviators are frequently exposed to extreme environmental factors mentioned above. Optimum vision is essential for pilots to ensure safe flight operations. The pilots must see well at distance to detect and identify airborne traffic, as well as hazards that may be on runways and taxiways. Good intermediate and near vision is also important since cockpit instruments and aviation materials such as flight manifests, charts and maps must be clearly visible to properly execute flight procedures in varying cockpit light ambient conditions. Blurred vision from refractive error can interfere with a pilot’s ability to efficiently perform operational tasks and can compromise aviation safety. Common refractive conditions that are correctable with ophthalmic devices (such as spectacles and contact lenses) include myopia, hyperopia, and astigmatism. Good vision is especially important in the military aviation setting where quick precise target acquisition and recognition is crucial in the combat situation. A split second delay in locating the enemy aircraft may result in a catastrophic outcome.
Civil aviators with refractive surgical procedures have been allowed to obtain Federal Aviation Administration (FAA) medical certificates since the early 1980s. The majority of these aviators had a radial keratotomy (RK) performed to correct their defective distant vision. In 1995, the Food and Drug Administration (FDA) approved the use of the excimer laser to perform refractive surgery. Laser procedures, such as photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis(LASIK), are being performed on a rapidly growing number of people, including civilian pilots. Presently, applicants with refractive surgical procedures may obtain an airman medical certificate without a waiver if they meet the visual acuity standards for the class of medical certificate applied, and an eyecare specialist verifies that their vision is stable, healing is complete, and no glare intolerance is present.
Many armed forces in the world have cautiously begun allowing their aviators to undergo corneal laser refractive surgery, especially PRK, to correct for refractive errors such as myopia, astigmatism and hyperopia. However, the use of LASIK in the military aviation setting is still being evaluated for fear of flap displacement under extreme G forces or ejection forces during emergent regress from a high performance aircraft.
CONVENTIONAL VISION CORRECTION IN AVIATION
Refractive errors and medical ophthalmic conditions are major causes of rejection for flying training selection but less commonly a cause of permanent grounding in established flying crew, due mainly to the stringent entry visual standards requirements. However, these stringent entry visual standards have over the years been relaxed due to the increased prevalence of refractive errors, especially myopia and astigmatism, in many countries in the Far East, such as Singapore, Thailand and Japan.
Corrective Flying Spectacles
Corrective flying spectacles have been the mainstay of refractive error correction amongst flight crew. Spectacles are capable of correcting spherical and astigmatic errors and generally allow good visual quality. Corrective flying spectacles should generally have frames made preferable of metal that are thin, strong and light to reduce visual field interference. The frame should also completely surround the spectacle lens to add strength, minimise peripheral visual field distortion and maximise visual field continuity. The spectacle side pieces should be thin to lessen the risk of breaking the noise seal of the headset and also to reduce hot spots from the pressure of a close fitting helmet. To reduce facial and ocular injury from, for example, bird strike, spectacle lenses should not be made from glass but rather material such as polycarbonate or other synthetic materials with high impact resistance which retain a high level of optical clarity and minimal aberration.
However, lenses of any power can become dirty, scratched or broken. Under emergency conditions, glasses may fog or become covered in water droplets, severely reducing the quality of vision. In military flying where pilots are exposed to high G-forces, pressure spots on the face causing pain and discomfort are often experienced. Lenses for the correction of higher myopic error lead to significant minification of the image with associated restrictions of the field of view. Patients who undergo a successful refractive surgery procedure for the correction of myopia greater than about -5.00 D, tend to demonstrate an increase in the best corrected visual acuity following surgery, as the retinal image is no longer minified. To reduce such optical effects and also for cosmetic reasons, many high-powered spectacle lenses are made from high index materials with aspheric surfaces. The trade-off is a reduction in acuity away from the optical centre of the lens.
Contact Lenses
Contact lenses are often, for the wearer, optically superior to spectacles in many ways. The contact lens moves with the eye and sight through this device will therefore not be subjected to the distortions produced by the peripheral spectacle lens and frame. The size of the retinal image may be more physiological with contact lens rather than the relatively minified images with myopic spectacles and magnified images with hypermetropic spectacles. Contact lenses are bathed in normal tears during each blink and so misting up as occurs in spectacle lenses in humid environment will not occur. Contact lenses, from the 1950s have become an acceptable optical aid in the functional replacement of spectacles although their use in the military is not permitted until the last 1~2 decades. Even then, only certain types of contact lenses have been approved for use in flying.
The problems of contact lenses in aviation are well known from the outcome of numerous military studies. The main difficult for contact lens wearers is the dry atmosphere within the cockpit, reducing tear quality and causing drying of the contact lens and anterior ocular surface. Both soft and rigid lenses are affected by this problem which may lead to symptoms of asthenopia (eye-strain), frontal headache, and reduced visual quality due to poor wetting of the lens surface. Starbursts and reduced contrast sensitivity have been reported, particularly in soft lens wearers. Deposition of the lens surface also degrades the optical performance further.
Rigid lenses can cause temporary disability if small particles, such as dust, become trapped beneath the lens, resulting in excessive lacrimation and discomfort. Many rigid lens wearer also report flare and halos (bright annulus in periphery of vision) when driving at night, due to a mis-match between the pupil and optic zone of the lens. In addition, it has been well demonstrated in many studies that rigid contact lenses can shift or even dislodged when the wearer is exposed to high G-forces. Nowadays, the majority of patients are fitted with soft lenses but such lenses are generally unable to correct the small degrees of astigmatism commonly found among the population. The quality of the vision will depend on the hydration of the lens material, the level of fatigue of the wearer and their sensitivity to blur.
Most contact lens materials are not designed to be worn for more than 12~14 hours a day and are certainly not suitable for wearers to sleep in. Unfortunately, long-haul pilots often wear their lenses for 12 hours or more. With the exception of some rigid lens materials and two new soft lens materials,marketed for extended wear at sea level, the majority of contact lenses do not allow enough oxygen to reach the cornea to prevent swelling at altitude. Four percent corneal swelling is considered an acceptable level since this occurs in the normal closed eye overnight when at sea level and the reduced partial pressure of oxygen at altitude results in the presence of daytime swelling of this magnitude. However the nature of some flight schedules and the tendency for some pilots to sleep in their lenses during long flights, results in a vicious cycle in which the cornea is not given the chance to return to acceptable levels of swelling. Corneal oedema is known to cause an increase in scattered light and hence a reduction in contrast sensitivity.
During the last couple of years, extended wear contact lenses have made a return to the market place. Serious corneal infections occurred in the past when standard soft lenses were worn on an extended wear basis. The new lenses are made from silicone hydrogel materials, allowing significantly more oxygen to reach the cornea. Initial studies suggest that these lenses are a very promising modality although other complications associated with continuous wear have not necessarily been solved.
Complications from contact lens wear,including the silicone hydrogel lenses, are not uncommon. These include corneal neovascularisation, contact lens associated papillary conjunctivitis (soft lenses), or chronic discomfort (particularly with rigid lenses)[3,4].Poor hygiene greatly increases the risk of contact lens-induced bacterial keratitis or acathamoeba keratitis, which are potentially blinding if severe[5].
CORNEAL REFRACTIVE SURGERY IN AVIATORS
The piloting of an aircraft is visually demanding and any significant reduction in visual performance under the specific visual conditions experienced in aviation would be unacceptable. Aviation concerns remain regarding the quality of the resulting vision correction, adverse effects in the aviation environment, and the potential for surgical complications.
The aircraft accident experience of civilian pilots with refractive surgical procedures has been looked into in a retrospective, cross-sectional cohort study[6] conducted by the Office of Aviation Medicine, FAA. It was conducted to determine whether accident rates of airmen with refractive surgery differed significantly from those of airmen without refractive procedures during the study period (1994~1996). The study revealed that the total accident rate was higher for airmen with refractive surgery (3.86/100 000 flight hours) when compared with those without refractive procedures (2.62/100 000 flight hours). However, analysis found that these differences were not statistically significant (P>0.05) for any class of medical certification or the total airman population. In addition, the study found no aviation accident in which refractive surgery was identified as a causal factor.
An ever-increasing range of surgical procedures is available to treat refractive errors. In general, the refractive status of the eye can be modified by changing the corneal shape using a diamond blade (radial keratotomy), an excimer laser (photorefractive keratectomy and laser assisted in situ keratomileusis) or a perspex implant (intra-stromal rings).
Radial Keratotomy (RK)
Radial keratotomy (RK) uses a micrometer blade to produce radial cuts in the mid-peripheral and peripheral cornea, through approximately 95% of the corneal thickness, leaving a central untreated area of 3~4 mm in diameter[7,8]. The incisions substantially weaken the tissue, causing the peripheral cornea to bulge, resulting in a relative flattening of the central cornea and a reduction in myopia. Permanent radial corneal scars are visible following this procedure. The suitability of RK for aviation is questionable. A hyperopic shift[2,9] in refractive error greater than 1.00 D has been reported in 43% of eyes between 6 months and 10 years post-RK and visually disturbing shifts of -0.50 D or more from morning to evening, are not uncommon. Ongoing glare disability is cited by the French Airforce, as being a significant reason for excluding radial keratotomy patients from flying training. Most surgeons believe that the risks are too high and that the procedure has been superseded by the advances in excimer laser technology.
In a report by Mader et al[10] on the refractive changes in operated eyes after exposure to high altitude for 72 hours, it was found that subject who have had RK induces a significant, progressive, and reversible hyperopic shift in refraction with corresponding video keratographic and keratometric changes. The authors hypothesize that the high-altitude hypoxic environment causes increased corneal hydration in the area of the RK incisions, which may lead to central corneal flattening and a hyperopic shift in refractive error. On the contrary, subjects who have had PRK were not susceptible to this refractive shift.
Intrastromal Corneal Rings (ICR)
Intacs, by KeraVison were approved by the FDA in 1999 for the correction of mild myopia (up to 3 diopters) without significant astigmatism (less than 1 diopter). Intacs are tiny plastic (polymethylmethacrylate, PMMA) ring segments that are implanted in the peripheral cornea. Even though they are very small, their mass is enough to change the shape of the front surface of the eye and correct refractive error. In data from clinical trials[11] presented to the FDA, 98% of patients within the recommended prescribing range could see 20/40 or better one year after the procedure; 78% could see 20/20 or better and 56% could see 20/16 or better. About 7% of patients experienced visual symptoms such as glare or fluctuating vision after the surgery. Complications from surgery included infection (0.2%), too shallow placement of the intracorneal rings (0.2%), anterior chamber perforation (0.4%) and temporary loss of two lines of best-corrected visual acuity (0.2%). The rings are removable and the individual’s eyes will return to their pre-operative near-sighted condition.
The French studied the fluctuation in uncorrected visual acuity[1] of patients who had undergone ICR implantation and found that the uncorrected visual acuity of the subjects varied up to 4 lines without an obvious pattern of progression over time, whilst fifty percent of the eyes had a variation of 2 lines. This is of particular importance during the first 3 postoperative months. However, in all the subjects, the final uncorrected visual acuity was always at least 20/20.
The FAA allows pilots who have had corneal ring implantation to fly after their vision has stabilised following surgery, just as with PRK and LASIK. There is currently no data on the aeromedical disposition of these pilots in military flying. However, because of the inert nature of the implant and the reported efficacy of the procedure in low myopes, the use of ICR should not be contraindicated once the refraction has stabilised.
Holmium Laser Thermokeratoplasty (LTK) and Diode Thermokeratoplasty (DTK)
Both LTK and DTK techniques employ an infrared laser to coagulate the cornea. Spots are arranged in a ring between 6 and 9 mm from the centre of the cornea, and as scar tissue forms, steepening of the central cornea results, causing a reduction in hypermetropia. The central cornea generally remains clear but the recovery time is 6 months or more and the stability of the refractive outcome can be poor[12]. Report by Rocha G et al[13] also showed that results tend to regress by 2 years. Due to its unpredictable outcome, these procedures are not recommended for aviators until more information is available to its safety and predictability in the correction of hypermetropia.
Photorefractive Keratectomy (PRK)
Photorefractive keratectomy (PRK) was first performed on a human cornea in 1988. The procedure involves removing the corneal epithelium to expose Bowman’s membrane. An Argon Fluorine excimer laser (193 nm) is used to remodel the anterior corneal surface by the ablation of stromal tissue. Reprofiling the cornea to treat myopia requires the ablation of more tissue centrally than peripherally, producing a flatter refracting surface. The amount of tissue removed for low myopia is in the region of approximately 50 μm—less than 10% of the overall corneal thickness. Studies have indicated that ocular integrity is not compromised. A subepithelial opacification referred to as haze develops over the first 2~4 weeks and peaks in intensity at 2~3 months, gradually subsiding by approximately 6 months. Once the haze has disappeared, there may be no visible signs that the eye has undergone PRK on slit lamp examination.
As with radial keratotomy, PRK is more successful for lower degrees of myopia.For low and medium degrees of myopia (<-6.00 D), 88%~99% achieve 6/12 or better (uncorrected vision), and 58%~78% achieve 6/6 or better by 12 months post-PRK[14~17]. For higher degrees of myopia (>-6.00 D), 68%~74% achieve 6/12 or better (uncorrected vision), with only around 26% obtaining 6/6 at 12 months. The refractive error tends to be slightly hyperopic initially, drifting towards emmetropia, or mild myopia as the cornea heals. By 12 months, 87%~99% of low and medium myopes (<-6.00 D), and 79%~84% of high myopes are within ± 1 D of emmetropia. Enhancement procedures can be performed to correct residual refractive error but predictability is not as good as for the initial procedure. PRK is now restricted to the treatment of myopia up to about -4.00 D because it is able to produce highly predictable results for this group with rapid stabilisation of the refraction in the vast majority of cases.
As the technology develops, the predictability and accuracy of PRK improves along with a reduction in the risk of complications. Since this procedure was first used on a human eye in 1988, the long-term consequences are unknown but there is no evidence to suggest anything significant to date. Ideally, PRK treatment should be restricted to the lower degrees of myopia because of the increased risk of haze and regression, and the slow stabilisation of the refraction following treatment for myopia greater than -4.00 D.
Many civil aviation authorities, including the FAA, the UK Civil Aviation Authority[18], and the Civil Aviation Authority of Singapore, just to name a few, approve the use of PRK and LASIK in the correction of refractive errors in their pilots and applicants, provided they meet the visual acuity standards for the class of medical certificate applied, and an eyecare specialist verifies that their vision is stable, healing is complete, and no glare intolerance is present.
Many studies have been conducted by the US military to determine the safety and suitability of PRK in aviators. Such studies would normally involve putting the subjects through simulators to reproduce the adverse aviation environment such as temperature, humidity, low atmospheric pressure and G-forces. The results of these studies have reinforced the efficacy and safety of PRK in military flying.
The US Navy[19] also reported a case of a naval aviator, who had PRK performed 6 months ago, ejecting from his Navy S-3B Viking aircraft while performing field carrier landing practice. Post-accident examination of his eyes did not detect any change in his vision of 20/16. His post-PRK status, after thorough investigation, was not listed as a causal factor in the mishap. This case report does provide anecdotal evidence to support the safety of PRK in the aviation community.
As such many military aviation authorities[20~22] such as the USAF, US Army, US Navy[23,24] and the Republic of Singapore Air Force exercise waivers for the use of PRK for the correction of refractive errors. However, the military usually adopts more stringent criteria in the selection of their potential candidates for PRK, such as their pre-operative refractive errors and their aptitude for flying training.
Laser Assisted in situ Keratomileusis (LASIK)
Laser assisted in situ keratomileusis (LASIK) was developed in 1990 after the introduction of the excimer laser to ophthalmology. The LASIK procedure involves placing a suction ring on the peripheral cornea, which momentarily increases the intraocular pressure to >65 mm Hg as the volume of the globe is reduced and the cornea squeezed into the small aperture in order to ensure a regular cut. The microkeratome is inserted into the tracks of the suction ring and activated to pass across the cornea and back, cutting the flap. The vacuum is released and the flap reflected back, exposing the underlying stroma. The ablation is carried out on the dry stromal bed and the flap is repositioned. Bowman’s layer remains intact and therefore there is often a complete absence of postoperative haze and scar formation. Pain is minimal due to the limited disruption to the epithelium and useful vision returns almost immediately after surgery.
The eye stabilises more quickly than after PRK and there is minimal tissue proliferation resulting in a transparent interface and rapid recovery of corneal sensitivity. In vivo microscopy has revealed corneal flap interface particles in all LASIK patients with microfolds in Bowman’s layer in 96.8% of patients but these problems tend to be clinically insignificant. Postoperatively, a C-shaped ring is visible corresponding to the edge of the flap, which fades with time.
The percentage of eyes achieving 6/12 vision or better has been quoted as 86%~100% at 6 months post-LASIK for corrections of -8.00 D or less, with 88%~100% of eyes achieving a residual refractive error within ± 1.00 D of emmetropia[25,26]. The refraction tends to stabilise within 1~3 months[27,28]. Between zero and 1.2% of eyes treated for myopia <-6.00 D lose two or more lines of best corrected visual acuity[25].
Hyperopic LASIK has proved slightly more successful than hyperopic PRK for the correction of low hypermetropia. In a study by Condon (Condon, 1997), 80% of eyes achieved 6/12 or better. The rate of regression was similar to hyperopic PRK but the stabilisation rate was approximately four times longer than that following myopic treatment [29]. An unacceptably high percentage of patients treated for hypermetropia > +4.00 D lose two or more lines of best-corrected acuity (7.3%)[30].
LASIK has been used to treated a wide range of refractive errors from +8.00 D to -20.00 D but the majority of surgeons now restrict LASIK treatment to those between +4.00 D and -10.00 D due to the reduction in accuracy, increased risk of complications and the small optic zones associated with the correction of high refractive errors [32].
Many civil aviation authorities approve the use of LASIK in the correction of refractive errors in their pilots and applicants, provided they meet the visual acuity standards for the class of medical certificate applied, and an eyecare specialist verifies that their vision is stable, healing is complete, and no glare intolerance is present. However, LASIK is still under evaluation in the military aviation setting. The Israeli air force reported a case of a combat jet aircraft pilot who resumed flying 2 weeks after LASIK, performing several dozens daylight flight uneventfully[33]. However, there is always the theoretical risk of flap movement when the operated eyes are subjected to high G forces or windblast forces during an event of an aircraft ejection.
Goodman et al[34] conducted a animal study to determine the stability of the LASIK flap in a rabbit model when subjected to vertical acceleration at nine times the force of gravity (+9 Gz) in an aircraft cockpit ejection simulator. The results showed that healed LASIK flaps as created in this rabbit model without laser ablation are stable when subjected to a rapid vertical ejection at nine times the force of gravity. More studies are still needed to determine appropriateness of LASIK in military jet aircraft aviation.
OTHER CONSIDERATIONS FOR DETERMI-NING CORNEAL REFRACTIVE SURGERY POLICIES IN AVIATORS
Refractive Restrictions
It is necessary to limit the range of refractive errors accepted for flying crew (both commercial and military aviation) to avoid the increased risk of pathology and optical effects associated with high refractive errors. It is prudent to know that the risk of retinal detachment increases with increasing myopia and that reducing the refractive error does not affect this risk[35]. The choice of procedure is also important as certain procedures, such as PRK are only suitable for refractive error of less than -4.00 D or -5.00 D, above which significant scarring resulting in disabling haze and glare may occur.
Recovery Times
For myopia below -5.00 D, the refraction tends to stabilise within 3~6 months of PRK and 1~3 months following LASIK. However, it can take up to 6~12 months for the visual performance to recover fully, particularly under conditions of low illumination. RK is generally not recommended due to the diurnal variation in visual performance and the reduction in ocular integrity. It is prudent for the attending ophthalmologist to certify the stability of the refractive status of the aviator before returning to unrestricted flying duties.
Screening for Previous Corneal Refractive Surgery
Applicants to professions (especially military flying) that do not allow corneal refractive surgery have been known to attempt to avoid disclosure of their ocular history. Radial keratotomy cases are easily identified since the radial scars are permanent. The detection of those who have undergone photorefractive keratectomy is more problematic because the corneal signs generally disappear once the initial stromal haze has subsided. LASIK tends to result in a faint C-shaped ring but this can fade completely in some patients.
Possible methods for screening out candidates who have undergone a corneal refractive procedure include pachymetry and topography. However, measurements of the central corneal thickness vary significantly within the normal population,with a mean central thickness of 550 nm but a range of 472 nm to 651 nm[36]. Since the cornea also thickens to some extent following both PRK and LASIK, pachymetry is a surprisingly insensitive method of identifying those who have undergone refractive surgery. The use of corneal topography maps to identify cases of refractive surgery has also been considered. Schallhorn et al[37] examined the sensitivity of topography to detect refractive surgery and found that even experienced observers exhibited only 77% sensitivity. Others have demonstrated an artificial neural network for recognition of corneal topography patterns after myopic refractive surgery with 99% accuracy, 99% sensitivity and 100% specificity[38].It is likely that topography in combination with a proven neural network would provide a more suitable test for screening candidates but topography alone would be insufficient.
CONCLUSION
Refractive errors were major causes of rejection for flying training selection in the past. However, with the increasing prevalence of refractive errors, especially in the Far East, over the past decade, many civil aviation authorities and military armed forces are relaxing their vision entry standards to increase the pool of potential candidates. The inherent disadvantages of spectacle correction and contact lenses use has provided an impetus for the authorities, policy makers and researchers to look into assessing the safety and efficacy of corneal refractive surgery in the correction of refractive errors in aviators.
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19. Tanzer DJ, Schallhorn SC, Brown MC. Ejection from an aircraft following photorefractrive keratectomy: a case report. Aviation, Space, And Environmental Medicine,2000,71:1057-1059.
20. United States Air Force (USAF) Aviation and Special Duty Photorefractive Keratectomy (PRK) Waiver and Surveillance Program : SG Policy #005 (02 Aug 2000)
21. United States Army Aeromedical Research Laboratory (USAARL) Corneal Refractive Surgery Surveillance Programme (CRSSP).
22. United Styates Navy and Marine Corps Corneal Refractive Surgery Physical Standards and Waiver Policy (Revised April 10, 2000).
23. Schallhorn S.Photorefractive surgery in the Navy. Navy Medicine,1994,85:27-32.
24. Schallhorn SC, Blanton CL, Kaupp SE,et al.Preliminary results of photorefractive keratectomy in active-duty United States Navy personnel. Ophthalmology,1996,103:5-22.
25. Montes M, Chayet A, Gomez L,et al.Laser in situ keratomileusis for myopia of -1.50 to -6.00 diopters. Journal of Refractive Surgery,1999,15:106-110.
26. Yang CN, Shen EP,Hu FR.Laser in situ keratomileusis for the correction of myopia and myopic astigmatism. Journal of Cataract and Refractive Surgery,2001,27:1952-1960.
27.Balazsi G, Mullie M, Lasswell L,et al.Laser in situ keratomileusis with a scanning excimer laser for the correction of low to moderate myopia with and without astigmatism. Journal of Cataract and Refractive Surgery,2001,27:1942-1951.
28. Magallanes R, Shah S, Zadok D, et al.Stability after laser in situ keratomileusis in moderately and extremely myopic eyes. Journal of Cataract and Refractive Surgery,2001,27:1007-1012.
29. Corones F, Gobbi PG, Vigo L,et al.Photorefractive keratectomy for hyperopia: long-term nonlinear and vector analysis of refractive outcome. Ophthalmology,1999,106:1976-1982.
30. Ditzen K, Huschka H,Pieger S.Laser in situ keratomileusis for hyperopia. Journal of Cataract and Refractive Surgery,2001,24:42-47.
31. Goker S, Er H,Kahvecioglu C.Laser in situ keratomileusis to correct hyperopia from +4.25 to +8.00 diopters. Journal of Refractive Surgery,1998,14:26-30.
32. Puk DE, Probst LE,Holland EJ.Recurrent erosion after photorefractive keratectomy. Cornea,1996,15:277.
33. Levy Y, Zadok D, Barenboim E.Laser in situ keratomileusis in a combat jet aircraft pilot. Journal of Cataract Refractive Surgery,2003,29:1239-1241.
34. Goodman RL, Johnson DA, Dillon H,et al.Laser in situ keratomileusis flap stability during simulated aircraft ejection in a rabbit model. Cornea,2003,22:142-145.
35. Burton TC.Influence of refractive error and lattice degeneration on the incidence of retinal detachment. Transactions of the American Ophthalmology Society，1989，87:143-157.
36. Price FW Jr, Koller DL, Price MO. Central corneal pachymetry in patients undergoing laser in situ keratomileusis. Ophthalmology,2000,107:1967-1968.
37. Schallhorn Sc, Reid JL, Kaupp SE, et al.Topographic detection of photorefractive keratectomy. Ophthalmology,1998,105:1583-1584.
38. Smolek MK, Klyce SD.Screening of prior refractive surgery by a wavelet-based neural network. Journal of Cataract And Refractive Surgery,2001,27:1926-1931.
Ophthalmology Service,KK Women’s & Children’s Hospital,229899,Singapore
(Editor Jaque)(Pin Min LAM)
Tel:+65-6394 1157Fax: +65-6291 0161E-mail: vpmlam@yahoo.com.sg
INTRODUCTION
The importance of the visual system in ordinary life is unquestionable, and the necessity for good vision for the performance of critical tasks when pilots fly the sophisticated aircraft is increasingly evident. The most important sensory input that an aviator requires is, therefore, undoubtedly, visual.
Flying is widely viewed as a mentally and visually demanding task. For many years, aviation authorities in the world have adopted a very conservative approach in disallowing the use of corneal refractive surgeries for the correction of refractive errors in aviators, due to the many uncertainties of the procedures then. However, with the rapid advancement in corneal refractive technology and the improved safety profile and predictability of these procedures, many civil aviation authorities, including the military, are starting to exercise waiver policies for certain types of corneal refractive procedures for their flying crew.
PECULARITIES OF AVIATION ENVIRONMENT
The aviation environment is very different from the sea-level ground environment the human is accustomed to. During flight, the pilot encounters a combination of peculiar environmental factors that includes low humidity, reduced air pressure, changes in cabin pressures, low oxygen partial pressure, acceleration forces (such as Gz forces) and even windblast forces during an uncommon event of ejection in an aircraft emergency. These environmental factors may influence the visual function, and thus have a direct impact on the type of corneal refractive surgeries approved in the flying community.
Low Cabin Atmospheric Pressure / Hypoxia
Subsonic commercial airliners normally cruise at altitudes of 30 000~33 000 feet. The outside atmospheric pressure at these cruising altitudes is extremely low, with a corresponding partial pressure of oxygen of only about 40 mm Hg[1]. Such low level of oxygen tension can only sustain life for a few minutes without supplemental oxygenation. For this reason the cabin of airliners are usually pressurised to an equivalent outside atmosphere of around 6 000~8 000 feet. At this cabin altitude, the partial pressure of oxygen drops from the sea-level value of 148~108 mm Hg, equivalent to a fall of about 27%. Every passenger is essentially mildly hypoxic although certainly not symptomatic[2].
In military aircraft, especially high performance combat aircraft, the cabin altitude can vary from sea-level up to 22 000 feet, depending on the flight altitude. This would mandate the use of oxygen breathing system to prevent hypoxia at such high cabin altitudes.
The low atmospheric pressure and its resultant low partial pressure of oxygen may result in hypoxia of the cornea. This is especially so in long haul flights where the duration of exposure can last up to 18~19 hours. There are concerns that hypoxia can cause the refractive status of the post-operative eye to be unstable and therefore affect the final visual acuity of the aviator.
Changes in Cabin Pressurisation
The cabin atmospheric pressure changes during ascent and descent[1].The maximum rate of increase in cabin pressure adopted for most commercial airliners is 1 kPa (0.15 lb/in2) / minute, that is, about 300 feet / minute. Gas expansion, in closed cavities, expands because of reduction of environmental pressure.
Small air bubble trapped underneath rigid contact lenses may expand during ascent (especially during a cabin rapid decompression) and can result in dislodgement of the contact lenses.
Patients who had just undergone lase-assisted in situ kera-tomileusis(LASIK)surgery may have micro-air bubbles under the corneal flap. These micro-air bubbles may expand and cause dislodgement of the flap during flight ascent or during rapid decompression of the cabin aircraft. Patients are generally not advised to travel by air at least one week after the operation.
Low Humidity
The relative humidity in aircraft cabin is low, usually less than 20%. Low humidity may cause discomfort of the eyes, mouth, and nose but presents little risk to health. Pilots who wear contact lenses may feel the effects of the low humidity more and may require more frequent lubrication with artificial tear supplements.
Many patients who had undergone corneal refractive surgery, especially PRK and LASIK, complain of dry eyes. This would usually improve over 2~3 months after the surgery. The low humidity in aircraft cabins can exacerbate the symptoms and patient should be warned and advised accordingly.
Acceleration Forces (G-forces)
High performance aircraft pilots experience “G” or gravity acceleration forces when they perform or are involved in some flight manoeuvres. High “G” loads upon the human body can cause disorientation, blackout and could have a fatal result. To train as a high performance pilot, a device called a centrifuge has been developed to simulate “G” loads on a human. Many evaluations of the suitability of refractive surgery for flying crew are performed under controlled conditions in the centrifuge.
Positive Gz forcesRapid pullups and steeply banked, level turns create positive Gz - forces acting in the direction of the pilot’s feet. They displace blood and the body’s organs toward the lower extremities. As the blood circulation to the brain decreases under positive Gz, the pilot’s visual field narrows. If positive Gz increase, the pilot loses color vision and eventually loses consciousness, known better as G-induced loss of consciousness (G-LOC).
Negative Gz forcesRapid pushovers and “outside” aerobatic manoeuvres create negative Gz - forces acting toward the pilot’s head. Depending on forces involved and individual tolerance, a pilot may experience discomfort, headache, “redout” caused by excessive blood flow in the eyes, and even unconsciousness. Most people have a much lower tolerance for negative Gz than for positive Gz.
There were concerns that the G-forces experienced in flying a high performance aircraft may result in shifting or dislodgement of the corneal flap of patients who had undergone LASIK procedure.
Ejection and Windblast Forces
The ejection seat is the most commonly used equipment in flight abandonment, although it is only available to military fast jet aviators. The initial mechanism and ejection from the cockpit applies accelerations in excess of 12 G for up to 500 ms with an onset rate up to 300 G/s. As the cockpit canopy is released from the aircraft the pilot is subjected to the full blast of the air whilst still traveling at approximately the same speed of the aircraft. Windblast pressures vary with the density of the air and the square of the velocity. The effect of windblast is due to the sudden application of force to the chest and the abdomen. Significant effects on the face (and eyes) will be experienced if the visor is not drawn down during the ejection process.
There were concerns that the ejection forces and windblast forces may result in shifting or dislodgement of the corneal flap of patients who had undergone LASIK procedure. There were also concerns about the corneal integrity and strength after corneal laser refractive surgery as the cornea thickness is significantly reduced, especially in those patients with moderate to high myopia.
VISION AND AVIATION ENVIRONMENT
The aviation environment is a potentially hazardous environment and aviators are frequently exposed to extreme environmental factors mentioned above. Optimum vision is essential for pilots to ensure safe flight operations. The pilots must see well at distance to detect and identify airborne traffic, as well as hazards that may be on runways and taxiways. Good intermediate and near vision is also important since cockpit instruments and aviation materials such as flight manifests, charts and maps must be clearly visible to properly execute flight procedures in varying cockpit light ambient conditions. Blurred vision from refractive error can interfere with a pilot’s ability to efficiently perform operational tasks and can compromise aviation safety. Common refractive conditions that are correctable with ophthalmic devices (such as spectacles and contact lenses) include myopia, hyperopia, and astigmatism. Good vision is especially important in the military aviation setting where quick precise target acquisition and recognition is crucial in the combat situation. A split second delay in locating the enemy aircraft may result in a catastrophic outcome.
Civil aviators with refractive surgical procedures have been allowed to obtain Federal Aviation Administration (FAA) medical certificates since the early 1980s. The majority of these aviators had a radial keratotomy (RK) performed to correct their defective distant vision. In 1995, the Food and Drug Administration (FDA) approved the use of the excimer laser to perform refractive surgery. Laser procedures, such as photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis(LASIK), are being performed on a rapidly growing number of people, including civilian pilots. Presently, applicants with refractive surgical procedures may obtain an airman medical certificate without a waiver if they meet the visual acuity standards for the class of medical certificate applied, and an eyecare specialist verifies that their vision is stable, healing is complete, and no glare intolerance is present.
Many armed forces in the world have cautiously begun allowing their aviators to undergo corneal laser refractive surgery, especially PRK, to correct for refractive errors such as myopia, astigmatism and hyperopia. However, the use of LASIK in the military aviation setting is still being evaluated for fear of flap displacement under extreme G forces or ejection forces during emergent regress from a high performance aircraft.
CONVENTIONAL VISION CORRECTION IN AVIATION
Refractive errors and medical ophthalmic conditions are major causes of rejection for flying training selection but less commonly a cause of permanent grounding in established flying crew, due mainly to the stringent entry visual standards requirements. However, these stringent entry visual standards have over the years been relaxed due to the increased prevalence of refractive errors, especially myopia and astigmatism, in many countries in the Far East, such as Singapore, Thailand and Japan.
Corrective Flying Spectacles
Corrective flying spectacles have been the mainstay of refractive error correction amongst flight crew. Spectacles are capable of correcting spherical and astigmatic errors and generally allow good visual quality. Corrective flying spectacles should generally have frames made preferable of metal that are thin, strong and light to reduce visual field interference. The frame should also completely surround the spectacle lens to add strength, minimise peripheral visual field distortion and maximise visual field continuity. The spectacle side pieces should be thin to lessen the risk of breaking the noise seal of the headset and also to reduce hot spots from the pressure of a close fitting helmet. To reduce facial and ocular injury from, for example, bird strike, spectacle lenses should not be made from glass but rather material such as polycarbonate or other synthetic materials with high impact resistance which retain a high level of optical clarity and minimal aberration.
However, lenses of any power can become dirty, scratched or broken. Under emergency conditions, glasses may fog or become covered in water droplets, severely reducing the quality of vision. In military flying where pilots are exposed to high G-forces, pressure spots on the face causing pain and discomfort are often experienced. Lenses for the correction of higher myopic error lead to significant minification of the image with associated restrictions of the field of view. Patients who undergo a successful refractive surgery procedure for the correction of myopia greater than about -5.00 D, tend to demonstrate an increase in the best corrected visual acuity following surgery, as the retinal image is no longer minified. To reduce such optical effects and also for cosmetic reasons, many high-powered spectacle lenses are made from high index materials with aspheric surfaces. The trade-off is a reduction in acuity away from the optical centre of the lens.
Contact Lenses
Contact lenses are often, for the wearer, optically superior to spectacles in many ways. The contact lens moves with the eye and sight through this device will therefore not be subjected to the distortions produced by the peripheral spectacle lens and frame. The size of the retinal image may be more physiological with contact lens rather than the relatively minified images with myopic spectacles and magnified images with hypermetropic spectacles. Contact lenses are bathed in normal tears during each blink and so misting up as occurs in spectacle lenses in humid environment will not occur. Contact lenses, from the 1950s have become an acceptable optical aid in the functional replacement of spectacles although their use in the military is not permitted until the last 1~2 decades. Even then, only certain types of contact lenses have been approved for use in flying.
The problems of contact lenses in aviation are well known from the outcome of numerous military studies. The main difficult for contact lens wearers is the dry atmosphere within the cockpit, reducing tear quality and causing drying of the contact lens and anterior ocular surface. Both soft and rigid lenses are affected by this problem which may lead to symptoms of asthenopia (eye-strain), frontal headache, and reduced visual quality due to poor wetting of the lens surface. Starbursts and reduced contrast sensitivity have been reported, particularly in soft lens wearers. Deposition of the lens surface also degrades the optical performance further.
Rigid lenses can cause temporary disability if small particles, such as dust, become trapped beneath the lens, resulting in excessive lacrimation and discomfort. Many rigid lens wearer also report flare and halos (bright annulus in periphery of vision) when driving at night, due to a mis-match between the pupil and optic zone of the lens. In addition, it has been well demonstrated in many studies that rigid contact lenses can shift or even dislodged when the wearer is exposed to high G-forces. Nowadays, the majority of patients are fitted with soft lenses but such lenses are generally unable to correct the small degrees of astigmatism commonly found among the population. The quality of the vision will depend on the hydration of the lens material, the level of fatigue of the wearer and their sensitivity to blur.
Most contact lens materials are not designed to be worn for more than 12~14 hours a day and are certainly not suitable for wearers to sleep in. Unfortunately, long-haul pilots often wear their lenses for 12 hours or more. With the exception of some rigid lens materials and two new soft lens materials,marketed for extended wear at sea level, the majority of contact lenses do not allow enough oxygen to reach the cornea to prevent swelling at altitude. Four percent corneal swelling is considered an acceptable level since this occurs in the normal closed eye overnight when at sea level and the reduced partial pressure of oxygen at altitude results in the presence of daytime swelling of this magnitude. However the nature of some flight schedules and the tendency for some pilots to sleep in their lenses during long flights, results in a vicious cycle in which the cornea is not given the chance to return to acceptable levels of swelling. Corneal oedema is known to cause an increase in scattered light and hence a reduction in contrast sensitivity.
During the last couple of years, extended wear contact lenses have made a return to the market place. Serious corneal infections occurred in the past when standard soft lenses were worn on an extended wear basis. The new lenses are made from silicone hydrogel materials, allowing significantly more oxygen to reach the cornea. Initial studies suggest that these lenses are a very promising modality although other complications associated with continuous wear have not necessarily been solved.
Complications from contact lens wear,including the silicone hydrogel lenses, are not uncommon. These include corneal neovascularisation, contact lens associated papillary conjunctivitis (soft lenses), or chronic discomfort (particularly with rigid lenses)[3,4].Poor hygiene greatly increases the risk of contact lens-induced bacterial keratitis or acathamoeba keratitis, which are potentially blinding if severe[5].
CORNEAL REFRACTIVE SURGERY IN AVIATORS
The piloting of an aircraft is visually demanding and any significant reduction in visual performance under the specific visual conditions experienced in aviation would be unacceptable. Aviation concerns remain regarding the quality of the resulting vision correction, adverse effects in the aviation environment, and the potential for surgical complications.
The aircraft accident experience of civilian pilots with refractive surgical procedures has been looked into in a retrospective, cross-sectional cohort study[6] conducted by the Office of Aviation Medicine, FAA. It was conducted to determine whether accident rates of airmen with refractive surgery differed significantly from those of airmen without refractive procedures during the study period (1994~1996). The study revealed that the total accident rate was higher for airmen with refractive surgery (3.86/100 000 flight hours) when compared with those without refractive procedures (2.62/100 000 flight hours). However, analysis found that these differences were not statistically significant (P>0.05) for any class of medical certification or the total airman population. In addition, the study found no aviation accident in which refractive surgery was identified as a causal factor.
An ever-increasing range of surgical procedures is available to treat refractive errors. In general, the refractive status of the eye can be modified by changing the corneal shape using a diamond blade (radial keratotomy), an excimer laser (photorefractive keratectomy and laser assisted in situ keratomileusis) or a perspex implant (intra-stromal rings).
Radial Keratotomy (RK)
Radial keratotomy (RK) uses a micrometer blade to produce radial cuts in the mid-peripheral and peripheral cornea, through approximately 95% of the corneal thickness, leaving a central untreated area of 3~4 mm in diameter[7,8]. The incisions substantially weaken the tissue, causing the peripheral cornea to bulge, resulting in a relative flattening of the central cornea and a reduction in myopia. Permanent radial corneal scars are visible following this procedure. The suitability of RK for aviation is questionable. A hyperopic shift[2,9] in refractive error greater than 1.00 D has been reported in 43% of eyes between 6 months and 10 years post-RK and visually disturbing shifts of -0.50 D or more from morning to evening, are not uncommon. Ongoing glare disability is cited by the French Airforce, as being a significant reason for excluding radial keratotomy patients from flying training. Most surgeons believe that the risks are too high and that the procedure has been superseded by the advances in excimer laser technology.
In a report by Mader et al[10] on the refractive changes in operated eyes after exposure to high altitude for 72 hours, it was found that subject who have had RK induces a significant, progressive, and reversible hyperopic shift in refraction with corresponding video keratographic and keratometric changes. The authors hypothesize that the high-altitude hypoxic environment causes increased corneal hydration in the area of the RK incisions, which may lead to central corneal flattening and a hyperopic shift in refractive error. On the contrary, subjects who have had PRK were not susceptible to this refractive shift.
Intrastromal Corneal Rings (ICR)
Intacs, by KeraVison were approved by the FDA in 1999 for the correction of mild myopia (up to 3 diopters) without significant astigmatism (less than 1 diopter). Intacs are tiny plastic (polymethylmethacrylate, PMMA) ring segments that are implanted in the peripheral cornea. Even though they are very small, their mass is enough to change the shape of the front surface of the eye and correct refractive error. In data from clinical trials[11] presented to the FDA, 98% of patients within the recommended prescribing range could see 20/40 or better one year after the procedure; 78% could see 20/20 or better and 56% could see 20/16 or better. About 7% of patients experienced visual symptoms such as glare or fluctuating vision after the surgery. Complications from surgery included infection (0.2%), too shallow placement of the intracorneal rings (0.2%), anterior chamber perforation (0.4%) and temporary loss of two lines of best-corrected visual acuity (0.2%). The rings are removable and the individual’s eyes will return to their pre-operative near-sighted condition.
The French studied the fluctuation in uncorrected visual acuity[1] of patients who had undergone ICR implantation and found that the uncorrected visual acuity of the subjects varied up to 4 lines without an obvious pattern of progression over time, whilst fifty percent of the eyes had a variation of 2 lines. This is of particular importance during the first 3 postoperative months. However, in all the subjects, the final uncorrected visual acuity was always at least 20/20.
The FAA allows pilots who have had corneal ring implantation to fly after their vision has stabilised following surgery, just as with PRK and LASIK. There is currently no data on the aeromedical disposition of these pilots in military flying. However, because of the inert nature of the implant and the reported efficacy of the procedure in low myopes, the use of ICR should not be contraindicated once the refraction has stabilised.
Holmium Laser Thermokeratoplasty (LTK) and Diode Thermokeratoplasty (DTK)
Both LTK and DTK techniques employ an infrared laser to coagulate the cornea. Spots are arranged in a ring between 6 and 9 mm from the centre of the cornea, and as scar tissue forms, steepening of the central cornea results, causing a reduction in hypermetropia. The central cornea generally remains clear but the recovery time is 6 months or more and the stability of the refractive outcome can be poor[12]. Report by Rocha G et al[13] also showed that results tend to regress by 2 years. Due to its unpredictable outcome, these procedures are not recommended for aviators until more information is available to its safety and predictability in the correction of hypermetropia.
Photorefractive Keratectomy (PRK)
Photorefractive keratectomy (PRK) was first performed on a human cornea in 1988. The procedure involves removing the corneal epithelium to expose Bowman’s membrane. An Argon Fluorine excimer laser (193 nm) is used to remodel the anterior corneal surface by the ablation of stromal tissue. Reprofiling the cornea to treat myopia requires the ablation of more tissue centrally than peripherally, producing a flatter refracting surface. The amount of tissue removed for low myopia is in the region of approximately 50 μm—less than 10% of the overall corneal thickness. Studies have indicated that ocular integrity is not compromised. A subepithelial opacification referred to as haze develops over the first 2~4 weeks and peaks in intensity at 2~3 months, gradually subsiding by approximately 6 months. Once the haze has disappeared, there may be no visible signs that the eye has undergone PRK on slit lamp examination.
As with radial keratotomy, PRK is more successful for lower degrees of myopia.For low and medium degrees of myopia (<-6.00 D), 88%~99% achieve 6/12 or better (uncorrected vision), and 58%~78% achieve 6/6 or better by 12 months post-PRK[14~17]. For higher degrees of myopia (>-6.00 D), 68%~74% achieve 6/12 or better (uncorrected vision), with only around 26% obtaining 6/6 at 12 months. The refractive error tends to be slightly hyperopic initially, drifting towards emmetropia, or mild myopia as the cornea heals. By 12 months, 87%~99% of low and medium myopes (<-6.00 D), and 79%~84% of high myopes are within ± 1 D of emmetropia. Enhancement procedures can be performed to correct residual refractive error but predictability is not as good as for the initial procedure. PRK is now restricted to the treatment of myopia up to about -4.00 D because it is able to produce highly predictable results for this group with rapid stabilisation of the refraction in the vast majority of cases.
As the technology develops, the predictability and accuracy of PRK improves along with a reduction in the risk of complications. Since this procedure was first used on a human eye in 1988, the long-term consequences are unknown but there is no evidence to suggest anything significant to date. Ideally, PRK treatment should be restricted to the lower degrees of myopia because of the increased risk of haze and regression, and the slow stabilisation of the refraction following treatment for myopia greater than -4.00 D.
Many civil aviation authorities, including the FAA, the UK Civil Aviation Authority[18], and the Civil Aviation Authority of Singapore, just to name a few, approve the use of PRK and LASIK in the correction of refractive errors in their pilots and applicants, provided they meet the visual acuity standards for the class of medical certificate applied, and an eyecare specialist verifies that their vision is stable, healing is complete, and no glare intolerance is present.
Many studies have been conducted by the US military to determine the safety and suitability of PRK in aviators. Such studies would normally involve putting the subjects through simulators to reproduce the adverse aviation environment such as temperature, humidity, low atmospheric pressure and G-forces. The results of these studies have reinforced the efficacy and safety of PRK in military flying.
The US Navy[19] also reported a case of a naval aviator, who had PRK performed 6 months ago, ejecting from his Navy S-3B Viking aircraft while performing field carrier landing practice. Post-accident examination of his eyes did not detect any change in his vision of 20/16. His post-PRK status, after thorough investigation, was not listed as a causal factor in the mishap. This case report does provide anecdotal evidence to support the safety of PRK in the aviation community.
As such many military aviation authorities[20~22] such as the USAF, US Army, US Navy[23,24] and the Republic of Singapore Air Force exercise waivers for the use of PRK for the correction of refractive errors. However, the military usually adopts more stringent criteria in the selection of their potential candidates for PRK, such as their pre-operative refractive errors and their aptitude for flying training.
Laser Assisted in situ Keratomileusis (LASIK)
Laser assisted in situ keratomileusis (LASIK) was developed in 1990 after the introduction of the excimer laser to ophthalmology. The LASIK procedure involves placing a suction ring on the peripheral cornea, which momentarily increases the intraocular pressure to >65 mm Hg as the volume of the globe is reduced and the cornea squeezed into the small aperture in order to ensure a regular cut. The microkeratome is inserted into the tracks of the suction ring and activated to pass across the cornea and back, cutting the flap. The vacuum is released and the flap reflected back, exposing the underlying stroma. The ablation is carried out on the dry stromal bed and the flap is repositioned. Bowman’s layer remains intact and therefore there is often a complete absence of postoperative haze and scar formation. Pain is minimal due to the limited disruption to the epithelium and useful vision returns almost immediately after surgery.
The eye stabilises more quickly than after PRK and there is minimal tissue proliferation resulting in a transparent interface and rapid recovery of corneal sensitivity. In vivo microscopy has revealed corneal flap interface particles in all LASIK patients with microfolds in Bowman’s layer in 96.8% of patients but these problems tend to be clinically insignificant. Postoperatively, a C-shaped ring is visible corresponding to the edge of the flap, which fades with time.
The percentage of eyes achieving 6/12 vision or better has been quoted as 86%~100% at 6 months post-LASIK for corrections of -8.00 D or less, with 88%~100% of eyes achieving a residual refractive error within ± 1.00 D of emmetropia[25,26]. The refraction tends to stabilise within 1~3 months[27,28]. Between zero and 1.2% of eyes treated for myopia <-6.00 D lose two or more lines of best corrected visual acuity[25].
Hyperopic LASIK has proved slightly more successful than hyperopic PRK for the correction of low hypermetropia. In a study by Condon (Condon, 1997), 80% of eyes achieved 6/12 or better. The rate of regression was similar to hyperopic PRK but the stabilisation rate was approximately four times longer than that following myopic treatment [29]. An unacceptably high percentage of patients treated for hypermetropia > +4.00 D lose two or more lines of best-corrected acuity (7.3%)[30].
LASIK has been used to treated a wide range of refractive errors from +8.00 D to -20.00 D but the majority of surgeons now restrict LASIK treatment to those between +4.00 D and -10.00 D due to the reduction in accuracy, increased risk of complications and the small optic zones associated with the correction of high refractive errors [32].
Many civil aviation authorities approve the use of LASIK in the correction of refractive errors in their pilots and applicants, provided they meet the visual acuity standards for the class of medical certificate applied, and an eyecare specialist verifies that their vision is stable, healing is complete, and no glare intolerance is present. However, LASIK is still under evaluation in the military aviation setting. The Israeli air force reported a case of a combat jet aircraft pilot who resumed flying 2 weeks after LASIK, performing several dozens daylight flight uneventfully[33]. However, there is always the theoretical risk of flap movement when the operated eyes are subjected to high G forces or windblast forces during an event of an aircraft ejection.
Goodman et al[34] conducted a animal study to determine the stability of the LASIK flap in a rabbit model when subjected to vertical acceleration at nine times the force of gravity (+9 Gz) in an aircraft cockpit ejection simulator. The results showed that healed LASIK flaps as created in this rabbit model without laser ablation are stable when subjected to a rapid vertical ejection at nine times the force of gravity. More studies are still needed to determine appropriateness of LASIK in military jet aircraft aviation.
OTHER CONSIDERATIONS FOR DETERMI-NING CORNEAL REFRACTIVE SURGERY POLICIES IN AVIATORS
Refractive Restrictions
It is necessary to limit the range of refractive errors accepted for flying crew (both commercial and military aviation) to avoid the increased risk of pathology and optical effects associated with high refractive errors. It is prudent to know that the risk of retinal detachment increases with increasing myopia and that reducing the refractive error does not affect this risk[35]. The choice of procedure is also important as certain procedures, such as PRK are only suitable for refractive error of less than -4.00 D or -5.00 D, above which significant scarring resulting in disabling haze and glare may occur.
Recovery Times
For myopia below -5.00 D, the refraction tends to stabilise within 3~6 months of PRK and 1~3 months following LASIK. However, it can take up to 6~12 months for the visual performance to recover fully, particularly under conditions of low illumination. RK is generally not recommended due to the diurnal variation in visual performance and the reduction in ocular integrity. It is prudent for the attending ophthalmologist to certify the stability of the refractive status of the aviator before returning to unrestricted flying duties.
Screening for Previous Corneal Refractive Surgery
Applicants to professions (especially military flying) that do not allow corneal refractive surgery have been known to attempt to avoid disclosure of their ocular history. Radial keratotomy cases are easily identified since the radial scars are permanent. The detection of those who have undergone photorefractive keratectomy is more problematic because the corneal signs generally disappear once the initial stromal haze has subsided. LASIK tends to result in a faint C-shaped ring but this can fade completely in some patients.
Possible methods for screening out candidates who have undergone a corneal refractive procedure include pachymetry and topography. However, measurements of the central corneal thickness vary significantly within the normal population,with a mean central thickness of 550 nm but a range of 472 nm to 651 nm[36]. Since the cornea also thickens to some extent following both PRK and LASIK, pachymetry is a surprisingly insensitive method of identifying those who have undergone refractive surgery. The use of corneal topography maps to identify cases of refractive surgery has also been considered. Schallhorn et al[37] examined the sensitivity of topography to detect refractive surgery and found that even experienced observers exhibited only 77% sensitivity. Others have demonstrated an artificial neural network for recognition of corneal topography patterns after myopic refractive surgery with 99% accuracy, 99% sensitivity and 100% specificity[38].It is likely that topography in combination with a proven neural network would provide a more suitable test for screening candidates but topography alone would be insufficient.
CONCLUSION
Refractive errors were major causes of rejection for flying training selection in the past. However, with the increasing prevalence of refractive errors, especially in the Far East, over the past decade, many civil aviation authorities and military armed forces are relaxing their vision entry standards to increase the pool of potential candidates. The inherent disadvantages of spectacle correction and contact lenses use has provided an impetus for the authorities, policy makers and researchers to look into assessing the safety and efficacy of corneal refractive surgery in the correction of refractive errors in aviators.
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Ophthalmology Service,KK Women’s & Children’s Hospital,229899,Singapore
(Editor Jaque)(Pin Min LAM)