Contact Lenses and Corrective Flying Spectacles in

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refractive correction worn by Royal Air Force (RAF) aircrew over the previous 12 mo. .... monthly disposable CL, 2 (1%) wore silicone hydrogel extended wear ...

RESEARCH ARTICLE

Contact Lenses and Corrective Flying Spectacles in Military Aircrew—Implications for Flight Safety Andrew M. Partner, Robert A. H. Scott, Penny Shaw, and William J. Coker

PARTNER AM, SCOTT RAH, SHAW P, COKER WJ. Contact lenses and corrective flying spectacles in military aircrew—implications for flight safety. Aviat Space Environ Med 2005; 76:661–5. Background: Refractive devices used by aviators need to suit the aerospace environment or their failure can have serious implications. A relatively minor visual disability can result in loss of life and aircraft. We surveyed commonly occurring problems with the different types of refractive correction worn by Royal Air Force (RAF) aircrew over the previous 12 mo. We also asked if they had experienced any flight safety incidents (FSI) relating to their refractive correction. Methods: A retrospective anonymous questionnaire survey was given to 700 active aircrew occupationally graded as requiring corrective flying spectacles (CFS) or contact lenses (CL) for flying. Results: 63% (443) of the questionnaires were completed. CL were worn by 53% of aircrew; 71% of them used daily disposable CL. CFS were worn by the remaining 47% of aircrew, 14% of whom used multifocal lenses. Of CFS wearers, 83% reported problems including misting, moving, discomfort, and conflict with helmet-mounted devices (HMD). CL-related ocular symptoms were reported in 67% of wearers including cloudy vision, dry eye, photophobia, red eyes, excessive mucus formation, CL movement, itching, and grittiness. No CL-related FSI were reported over the previous 12 mo compared with 5% CFS-related FSI (p ⬍ 0.001). The graded performance of CL for vision, comfort, handling, convenience, and overall satisfaction was significantly higher than for CFS. Conclusion: CFS are associated with problems in terms of comfort and safety. CL are well tolerated by aircrew, and deliver improved visual performance. Keywords: refractive correction, aviation environment, spectacles, eyeglasses, contact lenses, questionnaire, survey.

class of aircraft flown. FSI attributable to CFS were noted in 27% of fast jet aircrew and 17% overall. The incidents were related, for the most part, to lens misting or sweat accumulation on the lens. The majority of aircrew used CFS but showed a preference for CL if they had tried them. CL are well tolerated and give a greater field of view with no misting, sweat degradation, or conflict with HMD. Only 65% of aircrew CL wearers were under review by the RAF optometry department and some were using unsuitable CL types. After the survey, we took steps to address the problems that were highlighted. A new CFS frame was introduced to reduce discomfort and we improved the ease of access to RAF optometry departments by holding outreach CL clinics at larger air bases throughout the country. To close the audit loop, we repeated the survey after 3 yr. The aim of this survey was to observe changes in the refractive correction worn by RAF aircrew during this period and to compare the subjective aircrew experience of comfort and in-flight performance associated with CFS or CL. METHODS

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EFRACTIVE PROBLEMS in aviators have more implications than in the general population as a relatively minor visual disability can have catastrophic effects. Refractive correction in aircrew historically has been with corrective flying spectacles (CFS). Over the past 20 yr high water content soft contact lenses (CL) have also been successfully used for all Royal Air Force (RAF) aircrew groups (2). The visual requirements of aviators are changing with the increasing use of helmetmounted devices (HMD) and high performance aircraft. These factors along with developments in visual technology have increased the options for aircrew visual correction. We previously surveyed the different types of refractive correction worn by RAF aircrew for flying and assessed their subjective experience of comfort, in-flight performance, and the occurrence of a flight safety incident (FSI) associated with CFS or CL (15). We found that CFS were associated with significant problems in terms of comfort and safety; these differed between

Our study was a retrospective anonymous questionnaire survey of 700 active aircrew occupationally graded as requiring refractive correction for flying. They were grouped according to the class of aircraft flown—fast jet, heavy jet, C130, rotary aircraft, training aircraft, and specialist support aircrew, e.g., sentry aircrew, airmen aircrew. The questionnaire was clearly stated as voluntary and anonymous and was arranged From the Royal Centre for Defence Medicine, University Hospital Birmingham, Selly Oak Hospital, Birmingham, UK (A. M. Partner, R. A. H. Scott); the Department of Optometry, RAF Cranwell, Lincolnshire, UK (P. Shaw); and the Royal Air Force Centre for Aviation Medicine, RAF Henlow, Bedfordshire, UK (W. J. Coker). This manuscript was received for review in February 2005. It was accepted for publication in April 2005. Address reprint requests to: Flight Lieutenant Andrew M. Partner, Royal Centre for Defence Medicine, University Hospital Birmingham, Selly Oak Hospital, Raddlebarn Rd., Birmingham B29 6JD, UK; [email protected] Reprint & Copyright © by Aerospace Medical Association, Alexandria, VA.

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AIRCREW REFRACTIVE DEVICES—PARTNER ET AL.

Fig. 1. Percentage of aircrew using CL or CFS according to their different aircrew groups.

into three sections (General, CFS, and CL). Questions were forced choice, performance grading, or subjective. Both the CFS and CL groups were asked whether they had experienced a FSI. A FSI is a situation during the flight where the respondent felt their refractive device endangered the safe passage of the aircraft or its load. The results were collated on a database for statistical analysis and the Fischer exact test was used to compare data groups. RESULTS Of the 700 questionnaires sent, 443 (63%) were completed and returned. The average age of the aircrew was 39 yr (range 21– 60 yr); 97% were male. The average experience in the aircraft flown was 9.1 yr (range 2 mo–39 yr). CL had been tried by 255 (58%) at some stage during their flying career and, at the time of questioning, CL were used by 233 (53%); the remaining 210 (47%) used CFS (Fig. 1). Distance correction alone was used by 380 subjects (86%). The remaining 63 required a reading correction in the form of a multifocal lens. Multifocal lenses allow you to focus through different prescriptions for different distances through the same lens. There were 34 (54%) who wore D segment bifocals, 12 (19%) who wore executive bifocals, 16 (25%) who wore varifocals, and 1

who wore (2%) trifocals. Outside flying duties, 10 of the 63 aircrew using multifocal CFS wore varifocals instead of bifocals, 10 wore ‘half moon’ readers only, 2 wore executive bifocals instead of D segment bifocals, 2 wore D segments instead of executive bifocals, and 7 wore CL rather than CFS. Corrective Flying Spectacles A significant problem with CFS over the preceding 12 mo was reported by 83% of aircrew. There were 44% who reported misting of the lens, 36% who reported CFS movement, 63% who reported CFS discomfort, 10% who reported poor vision, 5% who reported bifocal problems, and 25% who reported conflict with their HMD. The proportion of the aircrew types experiencing misting, CFS movement, and discomfort by aircraft type is represented in Fig. 2. A FSI was reported by 5% of aircrew wearing CFS over the previous 12 mo. Contact Lenses Of the 255 aircrew who had tried CL, 233 (91%) continued to use them for flying. The RAF optometric CL fitting and review service was used by 222 (95%), the remainder arranged their own review. During flying duties, 165 (71%) wore daily disposable CL, 27

Fig. 2. Percentage of each aircrew group experiencing the common problems associated with CFS wear over the previous 12 mo.

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AIRCREW REFRACTIVE DEVICES—PARTNER ET AL. TABLE I. VISUAL PERFORMANCE OF CL AND CFS WITH PERCENTAGE ATTAINED FOR EACH CATEGORY (1 ⫽ VERY POOR, 2 ⫽ POOR, 3 ⫽ NEITHER GOOD NOR BAD, 4 ⫽ GOOD, AND 5 ⫽ EXCELLENT). Grade 1

2

3

4

5

Factors

CL

CFS

CL

CFS

CL

CFS

CL

CFS

CL

CFS

Vision Comfort Handling Convenience Overall Satisfaction

0 0 1.3 4.5 0

2.3 25.4 7.4 14.4 9.7

1.3 1.3 5.1 1.9 1.9

5.8 35.6 24.5 27.3 31.0

7.7 10.3 26.3 16.0 9.6

17.6 25.9 43.3 36.7 37.3

34.6 39.7 39.7 31.4 46.2

46.5 11.3 22.2 17.9 20.0

56.4 48.7 27.6 46.2 41.7

27.6 1.6 2.7 3.9 2.1

(12%) wore twice disposable weekly CL, 38 (16%) wore monthly disposable CL, 2 (1%) wore silicone hydrogel extended wear CL, and 1 (0.5%) wore rigid gas-permeable CL. All subjects were compliant with approved cleaning regimens. Only the 2 subjects who wore extended wear CL regularly slept overnight in them, although 71 (30%) with other types of CL slept overnight in them from time to time. No incidents of CL-related corneal ulcers were reported. CL-related ocular symptoms during flight were reported in 67% of aircrew. Dry eyes were the most common side effect, occurring in 104 (45%). Other problems included grittiness in 55 (24%) airmen, CL movement in 49 (21%) airmen, red eyes in 37 (16%) airmen, itching in 29 (12%) airmen, cloudy vision in 26 (11%) airmen, excessive mucus formation in 4 (2%) airmen, and photophobia in 1 (0.5%) airman. No FSI related to CL wear were reported over the previous 12 mo, significantly less than with CFS (p ⬍ 0.001). Performance of CFS and CL The graded performance of CFS as good or excellent was 74% for vision, 13% for comfort, 25% for handling, 22% for convenience and 22% for overall satisfaction. For CL it was graded as good or excellent in 91% for vision, 88% for comfort, 67% for handling, 78% for convenience, and 88% for overall satisfaction. The performance of CL was rated as good or excellent signifi-

cantly more than CFS, for all parameters (p ⬍ 0.001). A performance summary is represented in Table I. DISCUSSION Compared with the previous RAF aircrew refractive survey, there has been a marked increase in the use of CL from 31% to 53% (Fig. 3). The superior visual performance of CL suggests operational and safety advantages that outweigh the disincentive of personal expenditure on CL (only CFS are issued at public expense). Exposure to CL use in aircrew was increased after the previous survey and 91% continued to use them as their preferred choice of refractive correction for flying duties, especially fast jet aircrew. While 5% of CFS-wearing aircrew (equating to 52 per thousand aircrew per year) reported an FSI that endangered the flight or resulted in a near miss, it is of note that no actual incident of loss or endangerment of RAF aircraft has been attributed to defective CFS, either wholly or in part. In civilian aviation, Nakagawara et al. attempted to report a prevalence of FSI where refractive devices used by the pilots were deemed a contributing factor (10). Unfortunately, the available resources to link refractive correction to a FSI are limited, and many investigations highlighting human error are restricted by liability and employment concerns. Indeed, since 80% of all aviation accidents result from human error, one would expect vision, arguably the most important special sense in control of the aircraft, to be a significant

Fig. 3. Change of contact lens usage between the first (2000) and second (2003) survey according to aircrew group.

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AIRCREW REFRACTIVE DEVICES—PARTNER ET AL. contributing factor. However, the National Transportation Safety Board found only 17 out of a total 46,462 (⬍ 0.0004%) FSI were attributed to refractive correction. If this figure only represents FSI in the air transport sector, we would expect this number to be higher in the very different military aviation environment. This implies that either the RAF aircrew FSI have not been properly reported or that aircrew concerns have not stood up to objective scrutiny. CFS problems have decreased from 97% to 83% compared with the previous survey (15). Preferential use of CL over CFS may have contributed to this reduction. In addition, specific measures to improve CFS comfort over the nose and ears have resulted in a significant drop in discomfort problems from 84% to 63%. CFS conflict with HMD remained at approximately 25% in both surveys. It has previously been reported that there is an increased incidence of aircraft accidents due to spatial disorientation with HMD (especially night vision devices) and conflict with CFS may add to the problem (1). CL wearers reported fewer problems during flight than CFS wearers (67% vs. 83%) and only 11% of these were related to poor vision. Near correction was required for 16% of aircrew. The majority of presbyopic aircrew (54%) used D segment bifocals for flying. These incorporate a small reading segment devoting the majority of the field of view to distance vision. The near vision requirements in the air appeared to differ from those on the ground with 49% choosing a different near vision lens type outside flying duties. To help CL wearing in presbyopic aircrew, we have developed a halfmoon reader with a frame that is suitable for aircrew use (13). These are intended for intermittent use, especially in heavy jets and multi-engine transport aircraft where aircrew have opportunity to remove their spectacles during flight. They are, however, incompatible with HMD usage. Efforts to make the RAF optometric CL fitting and follow-up service more accessible have increased attendance from 65% to 95%. In the general population, soft contact lenses now represent 95% of new fits and 80% of refits; 55% of soft CL wearers prefer monthly disposables and 45% prefer daily disposables (9). Daily disposable CL use in aircrew has increased from 35% to 71% over the past 3 yr. They offer good visual performance and comfort while being hygienic and convenient (11,14). Silicone hydrogel extended wear CL were successfully used in two aircrew members and represent one in six soft CL refits in the general population. They are a new type of soft CL with sufficient oxygen transmission to prevent corneal edema seen with conventional CL if the lenses are worn overnight. Users of CL are at increased risk of microbial keratitis if they fail to wash their hands prior to insertion or over-wear their CL. Blepharitis and dry eyes also increase the risk of infection and there has been research demonstrating a change in the normal ocular biota in the lids and conjunctiva favoring extended wear CL users, something to consider when treating CL-related infections (16). Currently extended wear CL are licensed for up to 30 d 664

continuous wear, but are associated with allergic reactions in some individuals and should, therefore, be prescribed to aircrew with caution (4). Other CL-related ocular symptoms of dryness, photophobia, red eyes, excessive mucus formation, CL displacement, and itching during flight all reduced from our previous survey. The most common CL-related symptom, dry eye, reduced from 73% to 45%. This symptom is more prevalent among aircrew and probably relates to a combination of dehydration in a high altitude environment associated with a reduced blink rate in CL users. Appropriate CL fitting and advice by optometrists experienced in the management of aircrew is likely to have contributed to this improvement. A similar experience of successful CL usage was reported in United States Air Force and Marine Corps aircrew (3,8). Corneal refractive surgery has the potential to produce the benefits of CL wear without the side effects. The U.S. Army, Navy, and Air Force perform photorefractive keratectomy (PRK) on certain personnel under audited conditions with a relatively low complication rate and good visual results (5,12). Postoperative night vision abnormalities and reduced contrast sensitivity have not been significant problems. Simulated cockpit studies after PRK found that while appreciation of low-contrast targets decreased, the operation did not decrease head-up-display readability. The reduced low-contrast sensitivity does not appear to be clinically or operationally significant. High altitude or G forces do not affect post-operative vision. In summary, flight performance after PRK has been found to improve with better aircraft detection, better use of night vision goggles, improved instrument appreciation, and higher aircraft carrier landing scores. More recently, Levy et al. recommend further research into the appropriateness of laser in situ keratomileusis in aircrew following an uneventful recovery in an Israeli fast jet pilot (6). CONCLUSION This survey highlights the increased use of CL by the aircrew population over the past 3 yr. For many, CL use has advantages in terms of comfort and visual performance over CFS. Refractive correction in aircrew needs to address the unique visual environment and offer a range of suitable and safe options, which possibly includes corneal refractive surgery (7,17).

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