Photorefractive keratectomy , or simply PRK, is a surgical procedure in which an excimer laser under computer control removes a lenticule of stromal tissue resulting in a permanent refractive change, thus allowing for the correction of myopia, hyperopia, and astigmatism. The excimer laser, a 193-nm ultraviolet energy beam, produces a photochemical disruption of molecular bonds. During PRK, the corneal epithelium is typically removed mechanically with a brush, and the excimer laser treatment is then applied to the exposed cornea. Ablation algorithms for conventional treatments are determined by the Munnerlyn formula. The epithelial defect resolves in most cases by 48 h and is typically completely healed within 4–5 days, though the refractive properties and ocular surface abnormalities continues to heal for some time thereafter. Vision considerably improves within 3–4 days following surgery, with correction stabilizing within 3–6 months.

Closely related to PRK is laser epithelial keratomileusis (LASEK), also known as epithelial-LASIK, E-LASIK, or Epi-LASEK. In this variant, a thin flap consisting of just epithelium is carefully separated from the underlying stroma at a hinge for ablation and then reseated following treatment. This variant was developed to potentially mitigate some of the pain and short-term visual disruption that patients can experience after PRK, although many studies have shown no significant advantage when compared to simply removing the epithelium.

Early community studies of conventional PRK treatments on myopic patients were concerning for corneal haze and visual symptoms of haze, glare, or halo, particularly under mesopic conditions [41]. Crucial to the evolution of PRK for use in aviators were efforts to assess the risk of these unfavorable symptoms, and weigh the potential benefits of surgery with the potential loss of an aviator to poor visual outcomes. With study, it was determined that these symptoms were relatively common in the early postoperative period, but tended to resolve by 6 months to 1 year [11]. The use of larger optical zones was noted to reduce visual symptoms [18]. Postoperative refractive predictability was improved over RK but could still result in over or under correction. Fine-tuning of treatment algorithms, smoother ablation profiles, and increased surgeon experience contributed to improved refractive outcomes and patient satisfaction.

After several years of community study, in 1993 the US Navy began investigating the role of PRK in the military. The first study was sponsored by the Special Warfare Command (Sea, Air, Land team or SEALs), who eventually embraced the procedure as a way to reduce dependence on corrective lenses – an operational necessity for these service members. Thirty myopic active duty Navy and Marine personnel (−2.00 D to −5.50 D, ≤1 D astigmatism) were treated with the Summit OmniMed excimer laser and found no loss of BCVA, with all treated eyes reaching 20/20 UCVA (uncorrected visual acuity) [50]. Glare testing (contrast acuity and intraocular light scatter) was studied noted to return to preoperative levels by 12 months after surgery. Additional PRK studies evaluated mesopic, low-contrast acuity which also showed a return to baseline after surgery. With these early investigations, it was clear that PRK had potential to safely reduce dependence on corrective lenses in military personnel . However, there were several barriers to refractive surgery in aircrew personnel: quality of vision after surgery needed more extensive investigation, and, based on the experience of RK, there were justifiable concerns about refractive stability in hypoxic, hypobaric conditions.

Ongoing with the evolution of PRK were efforts to identify alternative metrics of optical performance in aviators beyond high-contrast Snellen acuity [60]. The limitations of high-contrast acuity testing is well known, especially since vision in flight involves low-light and low-contrast conditions. Two tests in particular were shown to be of value: low-contrast acuity and low-light low-contrast acuity. This is particularly important in aviators; for instance, night carrier operations are widely regarded as the single most challenging and visually demanding task an aviator must complete (Fig. 3.2). As such, visual quality and, in particular, low-contrast acuity were closely followed in early trials with the foresight toward the use of refractive procedures in aviators. Ongoing study of PRK on non-aircrew military personnel reassuringly supported that mean mesopic acuity was as good as or better than preoperative performance after surgery [50].

Refractive stability at altitude was one of the key areas of interest prior to approval. PRK was studied alongside RK and control myopic patients at the US Army Pikes Peak Research Laboratory (elevation 14,100 ft) [27]. In this study, control myopic eyes and PRK-treated eyes exhibited slight corneal thickening but no change in refractive error at altitude.

Numerous improvements in the PRK procedure during the 1990s resulted in a very low risk for long-term glare disability or reduced contrast sensitivity. In 1999, the US Navy entered into trials permitting PRK to be performed on designated aviators and student aviator applicants, referred to as the “retention” and “accessions” studies. In the “accessions” study, 300 PRK patients entered flight training and were compared to over 4000 controls [53]. The PRK-treated patients had a lower attrition rate and performed as well as or better than their counterparts in academics and flight performance in all study metrics. Following the results of this study, new applicants were allowed to receive waivers for PRK in order to meet visual requirements.

As part of the “Retention of Naval Aviators PRK Study,” 785 aviators (150 pilots and 635 aircrew) received PRK between 2000 and 2005 [53]. In this study, 90% of aviators were eligible to return to flight duty without correction by 6 weeks. No aviator suffered visual complications precluding flight duty. The results of this study supported the role of PRK in designated aviators. With the results of these studies, PRK became approved for waiver in civil airmen and military aviators .

In the wake of the success with the adoption of PRK, attention shifted to laser in situ keratomileusis, or LASIK, a form of corneal refractive procedure which involves the creation of a flap in the corneal stroma, leaving the epithelium intact. After flap creation using a mechanical microkeratome or a femtosecond laser, the corneal flap is folded away from the ablation bed and the excimer laser then applied to the exposed stroma to reshape and treat refractive errors (Fig. 3.3). Following ablation, the flap is reseated and allowed to heal. Most patients experience prompt uncorrected visual improvement, and the visual recovery after LASIK is much faster than PRK. This is an attractive option for aviators as it substantially reduces postoperative downtime. However, similar to PRK, rigorous clinical evaluation was needed before the procedure could be given the green light in aircrew. In the movement toward LASIK, emphasis was placed not only on the quality of visual outcomes but also on the integrity of the corneal flap.

Early retrospective comparisons of conventional PRK and LASIK on non-aircrew personnel suggested that LASIK eyes experienced a mean loss of contrast acuity after surgery, which did not fully resolve over the follow-up time period [9]. In this same study, PRK patients tended to experience fewer symptoms of glare and halo than LASIK. Study of night driving simulator performance on recipients of conventional LASIK revealed a decrease both in target detection and target identification distance after surgery, with and without a glare source. Conventional LASIK, performed with the use of microkeratome for flap creation and standard ablation algorithms, was not ready for aircrew in its early stages.

Several technological advances would occur that would pave the way for the ultimate study and approval of LASIK in aircrew. Notably, the paring of wavefront-guided (WFG) and femtosecond technologies would prove to be the pivotal step in optimizing visual outcomes (Fig. 3.4). Evidence had been accumulating that WFG technology offered the ability to correct optical aberrations, resulting in improved contrast sensitivity [22]. A matched dataset comparison of conventional LASIK and PRK with WFG LASIK provided supportive evidence that WFG treatments produce more predictable refractive outcomes than conventional treatments, with improved best corrected and low-contrast visual acuity [48]. Femtosecond-assisted LASIK had been independently evaluated in numerous studies on non-aviators and demonstrated to be safe and efficacious in the creation of flaps for the treatment of refractive error [12].

In the setting of building evidence supporting the role of these new technologies, direct comparison of mechanical microkeratome and femtosecond-assisted WFG LASIK in military personnel demonstrated faster visual recovery in the femtosecond group, with a higher percentage of eyes reaching uncorrected acuity of 20/16 or greater [57]. Comparison of night driving performance after conventional and WFG femtosecond-assisted LASIK demonstrated significantly improved mean night driving visual performance [51]. The results of this study were instrumental in the decision to allow LASIK to become acceptable for civilian and military aviators, including astronauts.

Currently, all forms of LASIK and PRK are eligible for waiver for aviators in any role in civilian or military aviation. Formal approval of CRS in civilian aviators occurred in 2004. US Army pilots were eligible for waiver following the introduction of the Army Warfighter Refractive Eye Surgery Program, which was initiated in 2000. Waivers for PRK in US Air Force pilots and aircrew began being granted in 2000. Air Force aircrew received approval for LASIK in 2007. Approval of PRK in US Navy aviators and applicants first occurred in 2004, following the results of the “accessions” and “retentions” studies. Routine waiver recommendations for designated Naval aviators and aircrew for LASIK occurred in 2012 and for student applicants in 2013. LASIK was also approved for use in astronauts in June, 2007. Presently, one current qualified astronaut has received LASIK, and in the most recent applicant cycle, several applicants were noted to have received some form of laser vision correction .

Wavefront-guided technology enables the measurement and quantification of lower- and higher-order optical aberrations through the methods of wavefront mapping. By convention, a clinical refraction, which is used to guide refractive surgery treatments, is composed of sphere, cylinder, and axis: the so called lower-order aberrations. Coma and spherical aberration are common types of higher-order optical aberrations (HOA) which cannot be identified on a routine clinical evaluation, but can contribute to image blur and visual symptoms.

Two advanced ablation profiles have been developed to minimize HOA: (a) wavefront-guided (WFG) ablation , which creates a profile based on all ocular aberrations as measured by an aberrometer device, and (b) wavefront-optimized (WFO) ablation , which creates a profile based on the manifest refraction and is adjusted to reduce the induction of spherical aberration. These advanced ablation techniques have been independently evaluated in both PRK and LASIK. Spherical aberration can be significantly increased after a myopic conventional ablation profile, and both WFG and WFO procedures result in less induction (or reduction) of spherical aberration with resulting improvement in visual quality [52].

Current recommendations for refractive surgery in aviators favor use of an advanced ablation algorithm. Military refractive surgeons routinely perform procedures with these technologies on active duty service members, including aircrew, reverting to conventional ablation algorithms only in rare circumstances.

To date there have been no aviation mishaps directly attributed to complications following refractive surgery. There has been one report of a designated pilot being permanently taken off of flight status after PRK [10]. During the early postoperative period following uneventful PRK, a 46-year-old male C-130 senior pilot was placed on topical steroids for treatment of corneal inflammation and scarring. On postoperative day 24, the patient presented for evaluation with complaint of decreased vision and ocular pain. He was found to have ocular hypertension which necessitated treatment with topical glaucoma medications. A non-arteritic anterior ischemic optic neuropathy was observed by postoperative day 29. On follow-up evaluation approximately 9 months after onset of symptoms, he was found to have severe visual field constriction in the affected eye with a best corrected acuity of 20/50, resulting in removal from flight status.

There have been a number of reports and evaluations of both PRK and LASIK in hypobaric and hypoxic environments. As previously discussed, in study atop the US Army Pikes Peak Research Laboratory at 14,100 ft, PRK-treated eyes exhibited slight corneal thickening but no change in refractive error [27].

In 2001, a published report described a temporary, reversible myopic shift with resulting loss of distance visual acuity in two climbers during an ascent to nearly 23,000 ft. Both climbers had previously received uneventful myopic LASIK [4].

The effect of ambient hypoxia and low humidity was studied on active duty LASIK patients by Larys and Schallhorn (unpublished data) using modified goggles [Reduced Oxygen Delivery Device (RODD), Environics, Hartford, Connecticut, USA]. In this study simulating conditions of a nonpressurized V-22 Osprey aircraft at 25,000 ft, eyes were randomized to dry air or low humidity for 2.5 h. No significant changes in refraction, uncorrected acuity, or contrast sensitivity were observed. Study of exposure to hypoxic conditions simulating an altitude of 35,000 ft similarly revealed no significant changes to corneal curvature, refractive error, or visual performance on post-LASIK subjects [1].

Corneal haze is a well-recognized complication of PRK, and is particularly of concern to the aviator given its potential to degrade quality of vision. Haze is developed from subepithelial scarring following ablation and can lead to irregular astigmatism and loss of UCVA. Modern WFG PRK procedures and ablation techniques are the result of a myriad of trials investigating methods to improve postoperative acuity and mitigate undesirable visual symptoms including the development of haze. The risk appears to correlate with the amount of tissue ablation [44], such that highly myopic patients are at relatively increased risk. The prophylactic application of the alkylating agent mitomycin C applied to the ablated corneal stroma following PRK reduces the risk of postoperative haze [61]. This technique, while surgeon dependent, has become relatively widespread in use, including pilots and aircrew.

Following approval, ongoing efforts have been underway to evaluate flap integrity and visual stability under stresses of hypoxic and hypobaric conditions. Complications such as flap displacement or slip, if elicited by forces of acceleration, wind blast, rapid decompression, or other flight-related turbulence would potentially be catastrophic. Study of the LASIK flap in simulated aircraft ejection environments in rabbit models provided the groundwork supporting good flap integrity to extreme windblast forces simulating ejection [16]. Biomechanical studies of the flap revealed a significant wind blast force required to cause flap dislocation [25], and these forces are greater in femtosecond-created flaps when compared to microkeratome flaps [23].

The risk of flap displacement after LASIK has been studied in a large retrospective case series and found to be very low (0.012%) [8]. In this study, all flap displacements occurred within 48 h of surgery and were not preceded by ocular trauma. The risk, while exceedingly low, may be higher following hyperopic treatments or use of microkeratome for flap creation.

There has been a report of a late traumatic flap displacement after LASIK in one activity duty service member, unrelated to aviation duties [14]. This individual experienced flap displacement following blunt trauma to the operative eye 2 months after LASIK, but prompt identification of this vision-threatening injury and transfer to a specialist allowed visual recovery to 20/20 uncorrected by as early as 7 days following the injury.

As a class, the advantages of femtosecond devices over microkeratome to create flaps for refractive procedures are numerous. Femtosecond lasers reliably generate accurate and precise flaps of programmable diameter and thickness, allowing for the tailoring of flap size to ablation zone. The flaps are planar in morphology, as opposed to the meniscus shaped flaps of the microkeratome, which are thicker at the edges. The side-cut angle is often customizable (Fig. 3.5), which can assist with proper flap seating postoperatively and may reduce risk of flap dislocation. Lastly, femtosecond flaps tend to create a robust healing response in the peripheral side cut that has been associated with improved flap adhesion over a microkeratome created flap [23]. Side-cut angle is important to consider in flap adhesion, as inverted side cuts tuck the cut flap edge under adjacent stroma, providing a barrier to dislocation and reducing risk for epithelial ingrowth [24].

Femtosecond-assisted WFG LASIK is considered safe and efficacious in the treatment of refractive error and may facilitate faster visual recovery and a higher proportion of treated eyes reaching uncorrected acuity of 20/16 or better. Large retrospective study published following the approval in civil airmen provided further evidence supporting outcomes of femtosecond-assisted LASIK [55]. However, femtosecond LASIK has its own unique complications, such as rainbow glare and transient light sensitivity syndrome [3, 33], which could pose potential hazards for aviation personnel . The incidence of rainbow glare and transient light sensitivity syndrome are rare, and if it occurs in aviators or aircrew, they should not be flying until it resolves to minimize distractions.

For military aviation, refractive surgery has historically been controversial due to concerns about corneal safety and stability in ejection scenarios . There have been reports supporting the safety of PRK-operated eyes even in these environments which reassuringly supports the role of PRK in military communities [58]. For instance, the case of one aviator, 6 months out from PRK, ejected from a Navy S-3B Viking aircraft while performing field carrier landing practice. There were no visual consequences of the ejection, and follow-up examination demonstrated stable visual acuity. The mishap was unrelated to visual function or surgery. More recently, there has emerged a report of a male F/A-18F Super Hornet naval flight officer who ejected at 13,000 feet at speeds greater than 350 knots. The aviator had received LASIK 7 years prior, and following ejection, no flap related complications or defects were identified, with BCVA of 20/15 in both eyes [66].