IN ORDER TO PERFORM their duties, 22% of active. Visual and Flight Performance Recovery After PRK or LASIK in Helicopter Pilots RESEARCH ARTICLE

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1 RESEARCH ARTICLE Visual and Flight Performance Recovery After PRK or LASIK in Helicopter Pilots Corina Van De Pol, Joanna L. Greig, Art Estrada, Gina M. Bissette, and Kraig S. Bower VAN DE POL C, GREIG JL, ESTRADA A, BISSETTE GM, BOWER KS. Visual and flight performance recovery after PRK or LASIK in helicopter pilots. Aviat Space Environ Med 2007; 78: Introduction: Refractive surgery, specifically photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK), is becoming more accepted in the military environment. Determination of the impact on visual performance in the more demanding aviation environment was the impetus for this study. Methods: A prospective evaluation of 20 Black Hawk pilots pre-surgically and at 1 wk, 1 mo, and 6 mo postsurgery was conducted to assess both PRK and LASIK visual and flight performance outcomes on the return of aviators to duty. Results: Of 20 pilots, 19 returned to flight status at 1 mo after surgery; 1 PRK subject was delayed due to corneal haze and subjective visual symptoms. Improvements were seen under simulator night and night vision goggle flight after LASIK; no significant changes in flight performance were measured in the aircraft. Results indicated a significantly faster recovery of all visual performance outcomes 1 wk after LASIK vs. PRK, with no difference between procedures at 1 and 6 mo. Low contrast acuity and contrast sensitivity only weakly correlated to flight performance in the early post-operative period. Discussion: Overall flight performance assessed in this study after PRK and LASIK was stable or improved from baseline, indicating a resilience of performance despite measured decrements in visual performance, especially in PRK. More visually demanding flight tasks may be impacted by subtle changes in visual performance. Contrast tests are more sensitive to the effects of refractive surgical intervention and may prove to be a better indicator of visual recovery for return to flight status. Keywords: refractive surgery, photorefractive keratectomy, laser in situ keratomileusis, helicopter flight performance, contrast sensitivity, low contrast acuity, military. IN ORDER TO PERFORM their duties, 22% of active duty and 32% of National Guard U.S. Army aviators require refractive correction, either spectacles or contact lenses (15,19). Often spectacles cause an interface problem with head-mounted displays or protective mask systems. Contact lenses are a solution that not all aviators are able to adapt to and in some cases are impractical for military deployment conditions. Photorefractive keratectomy (PRK) has been under investigation for military personnel since 1994 (13,17,18) and has proven to be a viable option for military aviators, as indicated by military policy shifts in recent years. Not all reports have been positive, however; Air Force researchers have identified contrast sensitivity decreases under glare conditions in some PRK cases (3). Laser in situ keratomileusis (LASIK) is a potential improvement over PRK in terms of faster visual recovery; however, it has certain unconfirmed risks in the military flight environment, specifically with regards to stability of the LASIK flap and variable effects on quality of vision (9,11,14). One of the primary concerns for the helicopter flight environment is the impact of PRK or LASIK on vision and flight performance under low contrast, low luminance conditions. Studies continue to show that after conventional PRK or LASIK, night vision symptoms are reported by up to 46% of patients, with complaints ranging from very mild awareness of halos to disabling visual symptoms (1,8,16). Another concern is how quickly vision recovers after either PRK or LASIK, as returning a pilot to flight status is key to military operational readiness. This study describes a prospective evaluation of the recovery of vision and the impact of PRK or LASIK on flight performance of current and rated U.S. Army Black Hawk (UH-60) pilots. METHODS The study protocol was approved in advance by the Clinical Investigation Division, Walter Reed Army Medical Center, and the U.S. Army Surgeon General s Human Subjects Regulatory Review Board. All participants provided written informed consent for the study at the U.S. Army Aeromedical Research Laboratory (USAARL) and for the surgical procedure at Walter Reed Army Medical Center (WRAMC). There were 21 U.S. Army active duty and National Guard Black Hawk pilots who were enrolled in this prospective study. One presbyopic subject disenrolled from the study prior to surgery, as he did not want to trade the requirement to wear distance correction for an increased dependence on near correction. All subjects entered in the study were between 22 and 50 yr of age, rated and current in the UH-60 aircraft, meeting all current flight duty medical exam (FDME) requirements, with stable refractive error of at least 0.75 diopter or at From the U.S. Army Aeromedical Research Laboratory, Fort Rucker, AL. This manuscript was received for review in August 2004, and then withdrawn. It was resubmitted in January It was accepted for publication in March Address reprint requests to: Diana Hemphill, U.S. Army Aeromedical Research Laboratory, P.O. Box , Fort Rucker, AL 36362; Diana.Hemphill@us.army.mil. Reprint & Copyright by Aerospace Medical Association, Alexandria, VA. 547

2 least 2.00 diopter in any meridian, with uncorrected vision worse than 20/20, correctable to 20/20. There was no restriction on pre-surgical astigmatism or gender. Patients with ocular or systemic conditions contraindicated for refractive surgery were excluded. Subjects were randomized to LASIK or PRK; however, subjects not suited for LASIK due to pre-surgical corneal shape or thickness received PRK. Of the subjects, 9 (18 eyes) had LASIK and 11 (22 eyes) had PRK. All treatments were conventional (not wavefront-guided) refractive surgeries performed with the LADARVision excimer laser system (Alcon Surgical, Fort Worth, TX). All subjects completed their pre-op, 1-wk, and 1-mo evaluations; 18 completed their 6-mo evaluation (9 LASIK and 9 PRK). Two PRK subjects were unavailable for their 6-mo visual and flight performance assessments at the study site due to deployment. However, both subjects were provided proper follow-up evaluation for their surgical procedure with deployed eye care providers. Ocular and visual data were obtained pre-surgically, 1 wk, 1 mo, and 6 mo post-surgically. Flight performance was assessed in a NUH-60 Black Hawk simulator and UH-60 Black Hawk aircraft under day, night unaided, and night vision goggle (NVG) conditions. Flight assessments were completed on both flight platforms pre-surgically, 1 mo, and 6 mo post-surgically, with an additional post-surgical flight assessment completed at 1 wk in the simulator. Fitness for return to flight duty was assessed at 1 mo post-surgically. Manifest refraction was obtained at the start of each session to determine the optical correction that provided best visual acuity. This refractive result was used as the subject s best optical correction for subsequent testing. For analysis, the refraction was recorded in terms of sphere power, cylinder power, and axis of the cylinder and spherical equivalent power in diopters. Spherical equivalent is a measure of mean refractive power that is determined by combining the sphere power with half of the cylinder power. The expected norm after surgery is an end point within 1.00 diopter spherical equivalent of emmetropia (0.00 diopters). Visual acuities were evaluated using the high contrast and 5% low contrast ETDRS (Precision Vision, LaSalle, IL) retro-illuminated charts at 4 m. Testing was conducted monocularly at a photopic chart luminance level of 200 cd m 2. Threshold acuity was scored on a by-letter basis in logmar units (0.02 log units per letter). The expected outcome for uncorrected high contrast visual acuity (UCVA) is 20/40 (0.30 logmar) or better (consistent with the expected end points for refractive error) for best-corrected high contrast visual acuity (HCVA) is 20/20 (0.00 logmar) or better, and for best-corrected 5% low contrast visual acuity (LCVA) is 20/60 (0.48 logmar) or better. Small letter contrast sensitivity was assessed with a computerized version of the Small Letter Contrast Test (SLCT) at 6 m. This test reveals sensitivity to small targets, comparable to visual acuity, but is comprised of letters of constant size (20/25), which vary in contrast, by row, in 0.1 log unit steps. Testing was conducted monocularly at a photopic luminance chart level (200 cd m 2 ) under best-corrected conditions. Threshold contrast sensitivity was scored on a by-letter basis in logcs units (0.01 log units per letter). The expected outcome for the SLCT is 0.8 logcs or better. Simulator and in-flight testing protocols were used to assess flight performance under day, night unaided, and NVG conditions. In the simulator, lighting levels of the display monitors were set to simulate day and night conditions; for day conditions, the mean luminance was 85 cd m 2, for night and NVG conditions, the mean luminance was 0.14 cd m 2. In the aircraft, day flights were flown between 2 h after sunrise and 2 h before sunset, night flights were flown at least 1 h after sunset and NVG flights were flown at least 2 h after sunset. A visually demanding, 1-h flight profile was flown for each condition and consisted of 16 flight maneuvers, including in-ground effect and out-of-ground effect hover turns, takeoffs, straight and level flights, descending turns, airfield operations, remote area operations, and an instrument landing system (ILS) landing. Each subject completed a 1-h train-up flight in the simulator, the three flight profiles in the simulator, and the three flight profiles in the aircraft prior to surgery. Pilots were tested in the simulator 1 wk after surgery. All subsequent follow-up sessions at 1 and 6 mo included both simulator and in-flight testing unless the pilot failed to return to FDME flight standards. Objective flight performance assessments were conducted based on measures collected during each simulator and aircraft flight. All simulator flights were performed in the USAARL NUH-60 research flight simulator, a 6-degree-of-freedom motion-based system which includes an operational crew station, computergenerated visual displays, and a multi-channel data collection system. The flight performance variables collected included barometric altitude, radar altitude, airspeed, vertical speed, aircraft heading, roll, slip, turn rate, engine torque, localizer course, glideslope, and aircraft position. All aircraft flights were performed in the USAARL specially equipped JUH-60A helicopter. Flight performance variables (matching those collected in the simulator) were sampled and recorded with a locally manufactured, computerized flight-monitoring package referred to as the Aeromedical Instrumentation System (AIS). Following each flight, data were converted to composite flight scores using specialized software routines developed at the USAARL. The composite flight score was determined based on variability of each of the flight performance parameters applicable to the particular task. For example, in the hover turn the parameters of interest are barometric altitude, radar altitude, turn rate, and slip. In order to score this task, the variability for each of these four parameters was determined and scored and the mean of these four scores formed the composite score. The range of possible composite scores for each flight task was 0 to 100, with 100 indicating absolutely no deviation from all prescribed parameters during the entire maneuver and 0 indicating the entire maneuver was outside of the accepted norms for all prescribed parameters. The limits of each flight parameter were based on the U.S. Army training circular (TC 548

3 Surgery Group TABLE I. VISUAL OUTCOMES ACROSS EYES (MEAN AND SD). Eyes (n) Pre-Surgical Mean (SD) Post-Surgical 1-wk Mean (SD) 1-mo Mean (SD) 6-mo Mean (SD) UCVA (logmar) PRK (0.23) 0.09 (0.15)* 0.03 (0.14)* 0.13 (0.10)* LASIK (0.30) 0.11 (0.11)* 0.12 (0.11)* 0.12 (0.09)* HCVA (logmar) PRK (0.06) 0.07 (0.12)* 0.17 (0.08) 0.20 (0.07) LASIK (0.06) 0.22 (0.07) 0.22 (0.06) 0.22 (0.06) LCVA (logmar) PRK (0.05) 0.33 (0.16)* 0.18 (0.11) 0.15 (0.07) LASIK (0.10) 0.17 (0.11) 0.15 (0.10) 0.11 (0.07) Contrast Sensitivity PRK (0.11) 0.58 (0.36)* 0.95 (0.21) 1.05 (0.12) LASIK (0.16) 0.99 (0.17) 0.99 (0.16) 1.03 (0.14) UCVA uncorrected high contrast visual acuity; HCVA high contrast visual acuity; LCVA low contrast visual acuity; logmar log units by letter; PRK photorefractive keratectomy; LASIK laser in-situ keratomileusis. *p 0.05: significant difference from pre-surgical (baseline); bold p 0.05: significant difference between PRK and LASIK groups. PRK eyes 22 for pre-op, 1 wk, and 1 mo; 18 for 6 mo ) for the UH-60 helicopter. To obtain the overall flight performance score for an entire flight scenario, the mean of all individual flight maneuvers over the 1-h flight was determined. Statistical Analyses Visual outcomes analyzed included refractive error, visual performance, efficacy, and safety of visual and surgical outcomes. Visual performance was defined by visual acuity (uncorrected, high, and low contrast) and contrast sensitivity. The proportion of individuals achieving 20/20 uncorrected visual acuity was used to evaluate efficacy, while the proportion of those losing two or more Snellen lines of spectacle-corrected visual acuity was used to evaluate safety. To evaluate the effect of surgery, pre- and post-surgical visual outcomes were compared at 1 wk, 1 mo, and 6 mo following surgery for all volunteers using the appropriate bivariate analyses. Multivariate linear regression models were constructed to examine the effects of surgery type (PRK or LASIK) on visual performance while controlling for pre-surgical vision. Flight performance scores were analyzed using repeated measures ANOVA and post hoc paired t-test comparisons to determine change in performance from baseline and one-way ANOVA to compare flight performance between PRK and LASIK. Analyses were also completed to determine the correlation of individual clinical visual assessments to flight performance under day, night unaided, and NVG conditions for each flight platform. Statistical analyses were performed using STATA version 7.0 (StataCorp LP, College Station, TX) and Statistica 6.0 (StatSoft, Inc., Tulsa, OK). Return to flight status is also reported. Visual assessment by the researchers and flight evaluation by the USAARL test pilot during the testing protocol at 1 mo after surgery was used to determine fitness for return to flight status. RESULTS The 20 subjects were men, mean age 38 (range 28 to 49) and mean pre-surgical prescription was 1.52 diopters spherical equivalent (range 0.75 to 6.00 diopters). The 40 eyes of the 20 subjects enrolled were analyzed. All were experienced Black Hawk pilots with a mean overall flight experience of 1736 h (range 301 to 5000) and mean Black Hawk experience of 814 h (range 100 to 3000 h). No significant differences in demographics were noted between the PRK and LASIK groups. No significant differences in age, spherical equivalent refractive error, visual performance, or flight performance measures were noted between PRK and LASIK groups prior to surgery. There were statistically significant decrements in visual performance from the pre-surgical state 1 wk after refractive surgery in PRK, but not LASIK, eyes in terms of high and LCVA and contrast sensitivity (Table I). At 1 wk post-surgery, LASIK subjects performed significantly better than PRK subjects on UCVA (p 0.01), best-corrected HCVA (p 0.01), LCVA (p 0.01), and contrast sensitivity (p 0.01) tests. LASIK subjects at 1 wk post-surgery were already at the level of, or better than, their pre-surgical state in terms of HCVA; however, LCVA and contrast sensitivity showed a decrement from baseline which was not statistically significant. There were no significant differences between PRK and LASIK subjects in terms of visual acuity (UCVA, HCVA, and LCVA) or contrast sensitivity at 1 mo or 6 mo after surgery (Table I). Nor was there a significant difference for either group from baseline for HCVA, LCVA, or SLCT at 1 mo or 6 mo after surgery. In terms of best-corrected LCVA, preoperatively 100% of both groups were within expected limits, e.g., 20/60 or better LCVA. For contrast sensitivity expected performance on the SLCT (0.8 logcs or better), preoperatively 94.4% of LASIK and 100% of PRK eyes were within expected limits; one LASIK eye scored 0.74 logcs preoperatively. At 1 wk, 100% of LASIK and 86.4% of PRK eyes were 20/60 or better LCVA, a difference which was not statistically significant (p 0.09), and 88.9% of LASIK and 22.7% of PRK eyes were 0.8 logcs or better, which was statistically significantly different (p 0.01). At 1 and 6 mo, 100% of both LASIK and PRK eyes were within expected performance levels on the LCVA test. At 1 mo, 93.8% of LASIK and 72.7% of PRK eyes were within expected limits on the SLCT and by 6 mo 87.5% of LASIK and 94.4% of PRK eyes were within expected limits on the SLCT; neither difference in proportions were statistically significant. Fig. 1 shows the proportion of eyes within expected perfor- 549

4 Fig. 1. Percent of eyes with 5% LCVA (open symbols) and SLCT contrast sensitivity (solid symbols) at or better than expected level of visual performance outcome for each test for each surgical group (LASIK triangles and dotted lines; PRK squares and solid lines). *Significantly different (p 0.05) from baseline. mance levels on the LCVA and SLCT by procedure and time post-surgery. At 1 mo post-op, all LASIK and 10 of 11 PRK subjects had achieved the current FDME visual acuity standards for return to full flight duties (correctable to 20/20 in both eyes and no visual symptoms). The one PRK subject not returned to flight status had persistent corneal haze, which, although correctable to 20/20, was symptomatic. His haze and symptoms resolved by 3 mo post-op and he was returned to flight status. There were no significant differences between PRK and LASIK subjects in terms of baseline performance for any of the flight conditions for either flight platform. The only significant differences in flight performance from baseline were improvements seen in LASIK subjects for night unaided simulator flights for all postsurgical flights (p 0.01) and NVG simulator flights at 1wkand1mo(p 0.05). The only significant difference between groups was a higher level of performance for LASIK than PRK subjects during day flight conditions at 1 mo in the aircraft (p 0.05). Fig. 2 shows flight performance results for both groups over the three flight conditions in both flight platforms. There were no significant correlations between visual and flight performance measures pre-surgically. However, at 1 wk, HCVA, LCVA, and SLCT had significant correlations with simulator flight performance (p 0.05) as follows: HCVA predicted NVG (R ); LCVA predicted Day (R ) and NVG (R ); and SLCT predicted Day (R ) and NVG (R ) performance. UCVA did not correlate with any simulator flight performance outcome at 1 wk. None of the visual measures significantly correlated with 1-wk night unaided simulator performance. There were no significant correlations between visual performance and flight performance at 1 mo and 6 mo for either flight platform. Using multiple regression analysis considering surgical procedure, amount of correction, UCVA, HCVA, LCVA, and SLCT in a forward stepwise regression, at 1 wk performance on the SLCT and the amount of correction predicted Day (p 0.02) and NVG (p 0.01) simulator flight performance. At 1 mo SLCT and HCVA predicted day simulator performance (p 0.01); SLCT predicted NVG simulator performance (p 0.05); and amount of correction, type of surgery, and HCVA predicted day aircraft performance (p 0.04). At 6 mo there were no statistically significant associations of visual performance and level of correction on flight performance. DISCUSSION This report presents results comparing PRK to LASIK for the correction of low refractive error and the impact of these procedures on fitness for flight duty in a sample of U.S. Army aviators. The sample of aviators who participated in this study was demographically typical of the experienced U.S. Army aviator population based on a review of the Army s Aviation Epidemiological Data Registry (AEDR, 2003). The average age of the population for study was within the mean age range of all active-duty aviators (mean age 35.8 yr, range 20 to 58) and of National Guard aviators (mean age 41.0 yr, range 30 to 58). As with the study population, most aviators are men; only 9% of Army aviators are women. In terms of refractive error, Army aviator entry standards are near emmetropia; therefore, this population tends to have a lower refractive error than other career fields. Among the 22 to 32% of Army aviators with refractive error, the average refractive error in terms of spherical equivalent for myopes is 1.34 diopters and for hyperopes is 0.94 (AEDR, 2003). The range of flight experience in this sample was also typical for the target population, ranging from 100 to 3000 h of Black Hawk flight time. LASIK subjects had significantly better visual performance than PRK subjects across all measures at 1 wk and were slightly better than PRK subjects at 1 mo after surgery. This is consistent with previous work indicating a potential for slower refractive recovery due to wound healing after PRK, while LASIK tends to achieve refractive and visual acuity stability earlier (5,21,22). Most notably, LASIK subjects at 1 wk post-surgery 550

5 Fig. 2. Comparison of flight performance from baseline and between groups for LASIK (triangles; n 9) and PRK (squares; n 9) for both flight platforms. Significant differences from baseline are indicated by filled symbols and significant differences between groups are marked by asterisks (p 0.05). were already at the level of, or significantly better than, their pre-surgical state in terms of HCVA. This is to be expected at 1 wk post-surgery, given LASIK s preservation of the epithelial layer and PRK s necessary course of corneal surface healing. Consistent with previous studies comparing conventional PRK and LASIK procedures (4,12), no statistically significant differences in visual performance outcomes were found between PRK and LASIK at months 1 and 6 post-surgically. This study demonstrated that the early post-surgical decrement in PRK-corrected vision affected not only uncorrected visual acuity, but also HCVA, LCVA, and contrast sensitivity. Both flight performance and driving performance have been shown to be related to decreases in contrast sensitivity and LCVA (2,7,10). The recovery and stability of these post-surgical visual parameters, therefore, has implications on the decision as to when to return individuals to full flight duties after either PRK or LASIK. Currently, flight duties may resume 1 mo and 6 wk following PRK surgery for U.S. Naval and U.S. Army aviators, respectively, if specific visual parameters are met. Further investigation of the timeframe necessary for complete recovery of contrast sensitivity and UCVA in larger populations is needed to more thoroughly assess the impact of PRK vs. LASIK 1 mo or sooner after surgery. Based on current FDME standards, which require both eyes have 20/20 correctable visual acuity and no post-surgical complications, all 9 LASIK subjects and 6 of 11 PRK subjects would have been considered for full flight duties at 1 wk post-op. Per study protocol, no subjects were returned to flight duty at this post-op period, however. Of the 20 pilots, 19 were returned to flight status at 1 mo. The one subject who did not return to flight status at 1 mo had measurable haze, reduced LCVA and contrast sensitivity, and symptoms consistent with haze, including glare and halos around lights. It is not unusual for some subjects after PRK to develop greater levels of haze with concomitant symptoms which tend to resolve with the use of topical steroids and time (6,20). If recovery of LCVA to within expected limits (20/60 or better in both eyes) had been used instead of HCVA as the criteria for return to flight status, at 1 wk all LASIK and 9 of the 11 PRK subjects and at 1 mo all subjects would have met the criteria for return to flight 551

6 duty. All subjects were within expected limits on this test in the subsequent testing periods through 6 mo. If, however, contrast sensitivity to within expected limits on the SLCT (0.8 logcs or better in both eyes) had been used, return to flight status would have been delayed for more individuals. At 1 wk, 7 of the 9 LASIK subjects and 1 of the 11 PRK subjects and at 1 mo only 7 of 9 LASIK and 6 of the 11 PRK subjects would have met the criteria for flight status. By 6 mo, there was no further improvement among the LASIK subjects with seven of the nine LASIK subjects achieving expected outcomes on the SLCT. At 6 mo, eight of the nine PRK subjects met expected SLCT outcomes. This indicates that the SLCT may be the most sensitive of the visual performance tests presented in this paper for this population. Despite the statistically significant reduction in all measures of visual performance for PRK subjects at 1 wk, a concomitant reduction in flight performance was not seen. In fact, flight performance for PRK subjects was stable across the study period for all flight conditions and platforms. LASIK subjects maintained visual performance across the study period, experienced an improvement in night unaided and NVG flight performance in the simulator, but demonstrated stable performance in day simulator flight conditions and in the aircraft environment. The only time period where visual performance even weakly correlated with flight performance was in the early post-surgical period when PRK eyes had experienced a decrement and LASIK eyes had not. This increased spread in the visual performance data across the study sample at 1 wk and the improved flight performance measured in the LASIK subjects resulted in an improved correlation of absolute performance levels. The expected correlation of a decrement in vision resulting in a decrement in flight performance was not seen, however. Certainly factors other than central vision play a role in flight performance, such as peripheral field of view, visual information processing, and cognitive processes. Refractive surgery, like contact lenses, reduces the requirement for spectacles in the cockpit, thereby removing the interface issues experienced with spectacle frame edges and lens reflections and glare. The flight performance improvement in LASIK subjects and stability in PRK subjects measured in the early postsurgical timeframe might be explained by the improved field of view or the benefit of removing the problems of spectacle interface. There may also be a resilience of this performance task to changes in quality of vision. In other words, experienced aviators have the skill set and cognitive ability to overcome physiological decrements, at least for very controlled flight performance tasks. This study did not attempt to challenge the very extremes of flying capability that might be seen in emergency situations or other more visually challenging sets of operational tasks. The goal of this study was to determine differences in post-surgical visual and flight performance outcomes between PRK and LASIK in U.S. Army aviators. The results of this analysis favor LASIK in terms of the relatively quick visual recovery and overall level of visual performance after surgery. HCVA is less sensitive than contrast sensitivity to changes in visual performance in the recovery period after refractive surgery, as evidenced by the number of pilots who met HCVA standards but were outside expected limits on the SLCT after surgery. Additionally, performance on the SLCT had the greatest predictive value for flight performance in the early post-surgical period out to 1 mo. The current HCVA metrics may not be sufficient to verify a safe return to duty for all pilots. 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