Vision Standards

doi: 10.1136/bjo.2008.156323 published online May 21, 2009Br J Ophthalmol

Gerry Clare, John A Pitts, Ken Edgington, et al. surgery

refractivevision standards and the implications of From beach lifeguard to astronaut: occupational

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From beach lifeguard to astronaut: occupational vision standards and the implications of refractive surgery G Clare1, J Pitts2, K Edgington3, BD Allan4

Corresponding author:

Gerry Clare Consultant Ophthalmologist, British Army University of Nottingham Queen’s Medical Centre Division of Ophthalmology and Visual Sciences Nottingham, NG7 2UH Email: [email protected] Telephone: 0115 924 9924 extension 62025 Fax: 0115 970 9963 John A Pitts Consultant Ophthalmologist, Bayview Hospital, Barbados Consultant Ophthalmologist to the UK Civil Aviation Authority Ken Edgington Consultant in Aviation and Occupational Medicine Airport Medical Services Ltd Horley, UK Bruce D Allan Consultant Ophthalmic Surgeon Moorfields Eye Hospital London, UK

Keywords: Vision; Diagnostic tests/Investigation; Treatment Surgery; Psychophysics

Word count: 3,000

BJO Online First, published on May 21, 2009 as 10.1136/bjo.2008.156323

Copyright Article author (or their employer) 2009. Produced by BMJ Publishing Group Ltd under licence.


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Minimum vision standards for employees are used in manufacturing,[1]transport industries,[2, 3]the emergency[4] and armed services1.[5]Traditionally, these have applied to colour vision, visual acuity and refractive error, with the later addition of visual fields for driving.[6]Vision standards are often historic and differ between countries, and their validity may be questioned as technological advances obviate some visual tasks.[7]Furthermore, entry standards applied to uncorrected acuity are being bypassed by advances in refractive surgery. This is of special significance in the armed services, where operational constraints must be taken into account. Our understanding of the interplay between visual demands at work and the effects of refractive surgery is evolving. Vision standards and official policies on refractive surgery should be understood in relation to the work environment, by both surgeons and patients. Performance-based and parametric tests are helping to define vision standards in a variety of occupations. Surgical correction of refractive errors can in many cases allow previously ineligible candidates to pursue their chosen occupation.

The rationale for visual standards

Visual standards are commonly based on concepts of public safety, selection for training, and competition. The public safety angle invokes a balance between the individual’s right to work and the right of society to expect a safe level of health, for example in its public transport workers. Selection reduces the cost of training by precluding unsuitable individuals who, if selected, would go on to fail. High standards of performance permit the competitive selection of highly able personnel.

Organizations frequently have separate vision standards for entry and retention, accommodating personnel whose vision has changed since joining. In accordance with modern precepts of equality of opportunity (e.g. Disability Discrimination Act 2005), exclusion requires justification based on evidence,[8]and if organizations lean too far towards very high standards, high-quality applicants may be denied employment. Thus, vision standards should serve both the individual worker and society at large.

Comprehensive vision standards for a variety of professions such as the armed forces and the police, as well as for motor sports and the offshore oil and gas industry are helpfully provided by the Association of Optometrists on their website, www.aop.org.uk. Abridged, updated versions are presented here (Table 1).

Police

Officer

Fire Officer Army soldier Ordinary

driver

Professional

driver

Motor

sports

Seafarer

(Merchant

Navy)

Bridge

watchkeeper

(Royal Navy)

UCVA 1st

eye

6/18 3/60 6/60 6/24

UCVA 2nd

eye

6/24 3/60 6/60 6/36

UCVA

binocular

6/36

BCVA 1st

eye

6/12 6/12 6/6 to 6/12

(right eye)*

6/12 6/9 6/6 6/6

BCVA 2nd

eye

6/12 6/12 6/36 6/12 6/12 6/9


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BCVA

binocular

6/6 6/9 6/6

Maximum

spherical

error

+3.00D -7.00D to

+8.00D

-2.50 to

+3.00D

Minimum

colour

vision

Dichromacy Anomalous

trichromacy

* Pass lantern

test

Pass lantern

test (low

brightness)

Visual

fields

Full Full Full

Table 1 Visual acuity and refractive error standards for a variety of UK occupations. Empty spaces represent unspecified standards.*Precise standard depends on trade, regiment or corps. Military ophthalmologists make final decisions on fitness to serve in borderline cases. Please refer to www.aop.org.uk for more detailed information. (UCVA: uncorrected visual acuity; BCVA: best corrected visual acuity)

Some of the most exacting and widely recognized visual standards are set for military pilots (Table 2). Vision standards for fighter pilots are based on distance and reading visual acuity, refractive error, colour vision, muscle balance, convergence, accommodation and stereopsis, but not contrast sensitivity or visual performance with night vision goggles (NVG). Current weapons systems requiring split visual tasks (e.g. Apache helicopter helmets) place further demands on pilots. Extended testing in these areas may help to define performance advantages relevant to modern combat flying such as target awareness in the peripheral visual field.[9] Arguments against introducing new functional tests for pilots include the relative lack of normative data and the possibility that pilots with contrast sensitivity at the low end of the scale may have neuro-adaptive mechanisms giving them unimpaired performance. Whilst new technology can assume tasks formerly reliant on human vision, such as monitoring targets, superior visual ability is still considered to be an essential survival advantage, and demanding entry standards for fighter pilots are likely to remain. The focus of debate moving forwards is likely to be determined by the extent to which refractive surgery can widen the pool of potential recruits.

Visual acuity Refractive error

UCVA each

eye

BSCVA 1st

eye

BSCVA

2nd

eye

BSCVA binocular Spherical error Cylindrical error

Pilot, Royal Navy,

Army

6/12 6/6 -0.75D to +1.75D +0.75D

Pilot, Royal Air Force 6/6 0D - +1.75D +0.75D

Commercial pilot,

JAR (entry)

6/9 6/9 6/6 -6.00D to +5.00D ±2.00D

Private pilot, JAR 6/12 6/12 6/6 -8.00D to +5.00D ±3.00D

Pilot astronaut,

NASA*

20/200 20/20 20/20

Canadian Forces

Pilot†

6/12;6/18 6/6 6/9 -5.00D to +5.00D

Group 1 Aviator, US

Navy

20/100 20/20 20/20 No limits


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Table 2 Visual acuity and refractive error minimum standards for military and civilian pilots. (JAR, Joint Aviation requirements; NASA, National Aeronautics and Space Administration). *From www.nasa.gov; †Department of National Defence, Canada (from Kumagai JK, Williams S, Kline D. Visual standards for aircrew: visual acuity for pilots. Contract Report 2005-142, Defence Research and Development Canada, Toronto, March 2005)

The history of visual standards

Historically, visual standards have been derived arbitrarily or intuitively. In 1917, a committee was appointed by the Council of the Ophthalmological Society to consider “The standards of vision desirable for the performance of different duties in the British Army”. Chaired by Edward Treacher Collins, the committee stated that “as late as 1837, ability to detect a person at ten paces was considered adequate in one continental army”.[10]However, it is the visual perception of colour, not form, which may first have been assessed against occupational requirements in a medical context.

The late industrial revolution heralded the introduction of colour vision standards in the rail and maritime industries as coloured signal lights came into use to prevent collisions.[11]Defects in colour vision had been recognized since the late eighteenth century, with one of the earliest descriptions provided by the English chemist John Dalton. He described his own colour deficiency, later discovered to be deuteranopia by DNA analysis of his preserved eyes in 1995.[12]

A contemporary of Dalton’s, Thomas Young postulated that normal colour vision depended on the presence of three photoreceptors, a theory developed by von Helmholtz in the mid-nineteenth century[13, 14]and confirmed by microspectrophotometry in 1983.[15] In 1855 Professor George Wilson of Edinburgh discovered the prevalence of colour defects to be relatively common.[11]He highlighted the dangers associated with defective red-green colour vision and advocated excluding colour defective sailors and railwaymen from certain jobs. These ideas were echoed by Frans Cornelis Donders in Holland, and other industrial nations followed suit.

Colour blindness was implicated in a train accident in Lagerlunda, Sweden, by Professor Holmgren of the University of Uppsala in 1877.[16]After a well-publicized enquiry, the Swedish State Railways introduced a requirement for normal colour vision, a criterion adopted for officers of the Swedish Navy. Holmgren developed the first standardized occupational test for colour blindness, based on the Young-Helmholtz theory of trichromatic vision, using skeins of coloured wool. Also in 1877 Donders described a lantern test for railroad workers, and the British Board of Trade introduced colour vision testing for officers in the Merchant Navy, eventually using coloured glass lanterns illuminated by oil lamps.[17]Electric lanterns followed in due course.

In 1862, the systematic testing of visual acuity was made possible by the invention of a chart consisting of capital letters designed to subtend an angle of five minutes of arc at a given distance.[18]This advance, made in Utrecht by Herman Snellen, would form the basis of all future visual acuity. Snellen’s optotypes were first presented by


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Donders at the 2nd International Congress of Ophthalmology in 1862.[18]Donders first understood the basis of refractive errors[19] and is reported to have been the first to introduce an occupational visual acuity standard, for Dutch Railroad workers in 1877.[11]

It is difficult to date the introduction of vision standards relating to refractive error, but the Treacher Collins’ 1917 committee recommended visual acuity for general Army service should be “at least 6/24 with either right or left eye without glasses, and at least 6/12 with the right eye, aided, if necessary, by glasses” and that “in the case of sphericals refractive error should not exceed 8 dioptres. Simple cylindricals should not exceed 4 dioptres, and of the highest meridian in combined sphero-cylindricals should not exceed 8 dioptres”.[10]How this was arrived at is unknown. In contrast to other European armies, the British Army had previously not “taken into account”[10] men with glasses but had been forced to do so by the sheer demand for front line troops. The question of vision standards vexed the committee, which concluded that further investigation and consultation with the War Office were necessary to determine vision standards of the many occupations within the army. At around the same time, it was recognized that visual standards in military aviation would have to be higher than that of “the ordinary soldier” (better eye 6/12, 6/6 with correction, worse eye 6/18, 6/12 with correction).[20]

Significant strides were made in the Industrial Revolution and the First World War concerning vision standards, which have continued to evolve to the present day.

Current Testing of Visual Performance

Task-based visual tests

Vision standards using high-contrast photopic acuity criteria may miss more subtle anomalies of peripheral vision, stereopsis and contrast acuity. Testing of these aspects of vision by conventional methods (e.g. Pelli-Robson chart, stereoacuity tests) may not be sensitive enough to detect potentially adverse effects on ability to function. Visual tests devised to document eye disease may not be the best indicators of visual performance in health, although screening tests such as the Keystone visual-skills test can pick up deficiencies. Moreover, it is difficult to specify what the minimum level of visual acuity should be in order to shoot a rifle, fly a plane safely or perform retinal surgery. In-depth observational and questionnaire-based analyses of the various requirements of a particular occupation, such as fire fighting[21]and policing,[22]have emphasized these areas of uncertainty. This has led to a trend to develop visual task-based tests with specific functional relevance.

Simulators test higher skills which rely on visual performance. They include night driving with and without glare, the tasks being to detect and identify road hazards.[23]In ophthalmic surgery, virtual reality simulation of complex procedures is becoming increasingly popular. Correlations between performance on surgical simulators, performance in tests of visual function including stereopsis, and later surgical performance will form an important area for future research.


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Computerized tasks associated with occupational performance include tests of the ability to detect and identify peripheral stimuli. The useful field of view test attempts to analyze attention span within the visual field, and may be a better predictor of driving performance than conventional screening tests.[24]Similarly, the contrast acuity assessment, designed for pilots who have undergone refractive surgery, assesses the ability to detect contrast changes within the minimum spatial vision requirements.[25]

Empirical task-based tests simulate working conditions to determine minimum vision requirements. The ability of air traffic controllers to carry out safety-critical tasks requiring colour discrimination can be tested in simulated control-room scenarios, excluding some anomalous trichromats.[26]One study of police applicants showed that the uncorrected visual acuity (UCVA) required to identify a weapon in a room and to find dislodged glasses was at least 20/125.[27]For train crews viewing slide-projected images to identify a hazardous scene at 285 feet, a best corrected visual acuity (BCVA) of 20/20 was needed.[28]In a study of the visual requirements of beach lifeguards, the angle subtended by a human head at 300m was calculated to be 6/17.[29]However, the BCVA standard, determined by incremental blurring with spherical lenses, was at least 6/9 in one eye and 6/18 in the other, because of the additional requirements to search for and identify the visual target.

Occupational colour vision testing has been subjected to scrutiny recently. In 2002, a colour defective pilot who crashed in Tallahassee airport was found to have had difficulty distinguishing the Precision Approach Path Indicator lights despite having passed a Farnsworth lantern test.[30]The Optec 900, said to be more stringent than the Farnsworth test, has since come into use.[31]Lantern tests are secondary colour vision tests which do not exclude all anomalous trichromats,[32]but pick up difficulties in colour signal light recognition better than clinical tests such as the Nagel anomaloscope.[33]The Holmes-Wright Lantern, devised in the UK in 1974, is no longer produced and is being superseded by the improved Fletcher-Evans CAM Lantern Test (www.evansinstruments.co.uk). There is some variability between the lantern tests used in different European countries to meet Joint Aviation Requirements[34]as well as inconsistency of the pass criteria;[35]moreover, these tests do not identify individuals with superior colour discrimination.

Tests of visual parameters

Although not dependent upon task performance, other objective tests of the limits of human vision continue to provide more information and may make the characterization of occupational visual standards more comprehensive.

Near vision is dependent on accommodative power as well as distance acuity and therefore cannot easily be expressed as an angle. In terms of occupational testing, it is useful to know whether tasks requiring excellent near vision, such as cartographic drawing or searching for cracks in a metal fuselage, can be performed. Backlit Landolt C optotypes can be used for extra-fine acuity measurements.[36]

The Snellen chart measures uneven increments in visual acuity, and is less reliable and reproducible test than standardized logarithmic visual acuity charts,[37]casting


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doubt over its suitability for measuring vision standards. Contrast sensitivity function (CSF) has been demonstrated to be a better predictor of target detection in low-contrast conditions.[38]The Small Letter Contrast Test offered an improvement over standard tests by picking up more subtle visual defects, and was promoted for use in evaluating visual performance after refractive surgery and in the selection of military aviators.[39-41]However, the contrast of the letters faded with time. Photopic and mesopic contrast acuity can be measured with backlit charts using linear sine-wave gratings and image-processing software;[42]this test is now preferred by U.S. military ophthalmologists.[43]

In addition to testing in the contrast domain, aberrometry[44, 45]and sophisticated measures of intraocular light scatter[46, 47]are evolving which could in future help determine the suitability of candidates for occupations with specific, task related demands on visual performance.

The impact of refractive surgery

Modern refractive surgery has blurred the boundaries of occupational vision standards, with many candidates previously excluded from a job now able to undergo a corrective procedure in order to qualify. However, the effects of refractive surgery on vision in low contrast or low luminance have not yet been fully defined. Peripheral vision may be adversely affected by some procedures.[48] Low contrast logMAR charts and CSF measurements at mesopic luminance levels may help to improve our understanding.

Open-minded policies on refractive surgery have a number of advantages. A prohibitive stance may deter candidates from disclosing a history of refractive surgery (surgery-induced changes can be detected by imaging techniques), or may lead recruits to choose the wrong procedure. For example, radial keratotomy may result in unstable refraction, making it unsuitable for military and other occupations.[49, 50]The availability of refractive surgery expands the pool of potential recruits, which may be especially significant in countries with a high prevalence of myopia.

The United States Military has identified benefits associated with refractive surgery including cost, military readiness and improved morale.[23, 51, 52]Wearing of glasses is problematic functionally for the military (and other occupations) because of incompatibility with equipment and the inherent risk of loss, breakage, fogging and glare. Contact lenses are prohibited on operations because of hygiene difficulties and serious infection risks.[53]The potential to enhance military capability by refractive surgery has been recognized since the safety of photorefractive keratectomy (PRK) was demonstrated in a cohort of special operations personnel in 1993.[54]The launch of the Warfighter Refractive Eye Surgery Program in 2000 has resulted in treatment centres being established worldwide, with over one hundred thousand personnel receiving treatment.

Current and future areas of research


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Initially, PRK was considered the gold standard in the U.S. Navy,[55]but LASIK has since gained favour and is increasingly advocated for naval personnel.[23]The U.S. Army, in contrast, has retained a preference for surface treatments because of the risk of traumatic LASIK flap dislocations during combat.[51]If sustained on operations, this rare complication would severely debilitate a soldier and require repatriation. Conversely, late corneal haze has been observed in troops serving in the Middle East, due to fibrosis following surface treatment (pers. comm., Captain Steven Schallhorn, US Navy (retired), 2008).

While PRK has been found to be safe following aircraft ejection,[56]it has been argued that the high gravitational forces sustained could dislodge a LASIK flap. Extensive animal tests using a rabbit model found the corneal flap to be stable during ejection[57]and windblast.[58]The effects of changes in ocular biomechanical properties following refractive surgery, such as reduced corneal hysteresis,[59]are not fully understood in the context of physiological extremes.

The view that LASIK and fast jets were not mutually incompatible was reinforced by an Israeli Air Force pilot who had an uneventful return to full duties 2 weeks after refractive surgery.[60]Refraction in post-LASIK eyes has been found to be stable in conditions of prolonged exposure to altitude and hypoxia, and LASIK is now considered suitable for U.S. Air Force and Navy pilots and National Aeronautics and Space Administration (NASA) astronauts (Table 3).[23]

Refractive procedures permitted Pre-operative refractive error

LASIK Surface

treatment

Intracorneal

ring segments

Radial

keratotomy

Phakic

IOL

Maximum

spherical

equivalent

Minimum

postoperative

probation

British Army (entry) � � � � � ±6.00D 6-12 months

Royal Air Force aircrew

(entry)

� � � � �

Royal Air Force aircrew

(retention)

� � � � 6-12 months

Joint Aviation

Requirements

� � � -6.00D to +5.00D 3 months

(LASIK)

US naval aviator (trained) �* � � 3-6 months

Police (UK) � � � � 6 weeks

Fire Brigade � � � � 12 months

NASA � � �

Republic of Singapore Air

Force (pilot applicants)

� � � -5.00D (-2.00 cyl)

Table 3 Refractive surgery waivers, armed and emergency services UK, US Navy and NASA, and Republic of Singapore Air Force. Empty spaces are either unspecified or waivers determined on a case-by-case basis. *LASIK waiverable only as part of LASIK in Designated Aviators Study. Minimum postoperative period assumed no complications and is generally lower for surface treatment.

Recent refinements to refractive techniques include advanced surface ablation and sub-Bowman’s keratomileusis. Comparing the two, Schallhorn et al found no


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difference in visual acuity and photopic contrast sensitivity at one year.[23]Haloes, glare disability and loss of contrast sensitivity may affect vision, particularly in the early postoperative period.[61, 62]Recent evidence suggests that wavefront-guided (aberrometry based) laser retreatment is effective in treating persistent night vision symptoms, principally by reducing spherical aberration.[63]Wavefront-guided primary laser treatments produce fewer higher order aberrations than conventional techniques, with improved night vision and higher levels of patient satisfaction.[64]

The use of NVGs, so essential to the military, has prompted a number of studies to ascertain the visual performance in NVGs after refractive surgery. High-contrast visual acuity through NVGs was found to be equal or better three months after PRK.[65]Night firing range performance, with and without NVGs, was demonstrated to improve following both PRK and LASIK.[66]Another study compared one group of U.S. Army helicopter pilots who had undergone LASIK to another who had PRK.[41]LASIK patients had an advantage at one week, in terms of overall visual performance, but this was no longer present at 1 or 6 months postoperatively. Post-surgical flight performance was assessed in a Black Hawk simulator under night and NVG flight conditions. Overall, flight performance was stable or improved.

For patients in the presbyopic age group, monovision is an increasingly popular option.[67]Monovision relies on binocular blur suppression to provide an enhanced range of focus with emmetropia in one eye and low myopia in the other. Reduced spectacle dependence is achieved at the price of reduced stereopsis and reduced suppression of blur in high-contrast mesopic conditions (for example, driving at night).[68]The optimal monovisual correction, in one study using contact lenses in emmetropic presbyopes, was determined to be 1.5D.[69]Monovision correction is currently disallowed by Joint Aviation Requirements (Class 1), but the arguments against monovision for pilots are poorly defined. In the event of loss of vision in the distance-corrected eye, a commercial or private pilot may still be able to land a plane with myopia between 1 and 1.5D using cockpit instruments. Conversely, the inability to read cockpit instruments clearly may impair flying ability in presbyopic emmetropes who lose their reading correction whilst flying.

Compromises in the quality of vision of associated with multifocal lens implants and their impact on flying ability have also not yet been fully evaluated. Problems with haloes are more common than with monofocal implants. Again, however, the advantages of an improved range of focus may be useful functionally, and the impact of visual side effects on flying ability may be determined by interindividual differences in neural adaptation.[70]

Conclusions

Military and other organizations around the world are recognizing the value of refractive surgery in recruiting, suitably employing and thus retaining personnel. Often, individuals with relatively poor eyesight need no longer be excluded from their dream jobs. Advice given in practice by the independent clinician should reflect the changing trends. In all cases, the official standards should be consulted. A regularly updated website could provide all the relevant information. The principle of understanding the work environment, engendered by Bernardino Ramazzini in 1700,


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remains as valid today,[71]and research should continue to challenge and refine occupational visual standards.

1The definitive source of occupational vision standards for the Services is Joint Service Publication 346 for general guidance and the individual service manning authority documents which have more detail by trade.

Acknowledgements: Ms. Chetna Lakhanpal, Lt Col Andrew Jacks RAMC, Wg Cdr Malcolm Woodcock RAF, Cdr Elizabeth Hofmeister USN, Lt Col Gerard Nah RSAF, Lt Col Mark Adams RAMC, Lt Col Fiona Foulkes RAMC, Capt Steven Schallhorn USN (retired).

"The Corresponding Author has the right to grant on behalf of all authors and does grant on behalf of all authors, an exclusive licence (or non-exclusive for government employees) on a worldwide basis to the BMJ Publishing Group Ltd and its Licensees to permit this article to be published in British Journal of Ophthalmology editions and any other BMJPGL products to exploit all subsidiary rights, as set out in our licence http://bjo.bmjjournals.com/ifora/licence.pdf "

Competing Interest: None declared.


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