Keep Your Eye on the Individual's Visual Function, Part 1

Because of new surgical techniques, it may not be what you anticipated.

• By Bernard R. Blais, M.D., FAAO, FACOEM, FACS, Thomas Tredici, Sr., M.D.
• Oct 01, 2003

PREVENTION of work-related health complaints should be a top priority for occupational health professionals. Diagnosis and treatment of workers presenting with work-related problems represents an opportunity to prevent recurrences in those workers (tertiary prevention), to mitigate the effects of current work-related hazards in order to reduce the duration of the problem (secondary prevention), and to prevent the same problems in co-workers and those in similar jobs (primary prevention).

The discovery of significant work factors suggests work site intervention to prevent recurrences and hasten recovery may well be appropriate. The occupational health practitioner should be aware, however, that many musculoskeletal, psychological, and other problems are often caused by several work- and non-work-related factors in varying combinations. Many potentially work-related complaints result from more than one factor. Some factors are work-related, others are personal. Some personal factors are in reality a mismatch between the worker's abilities and job demands, or person-job fit. The work factors are necessary but not sufficient in many cases.

The practitioner's task in prevention is first to identify associated or causative workplace and personal factors. The practitioner should then suggest scientifically based selection and screening of personnel, personal protection, and task or job redesign, as well as treatment and disability management. Preventive efforts may include screening and placement, engineering controls, administrative changes, personal protective equipment, education and training at all levels of the company, close attention to the psychological needs of the employees, proper medical surveillance of the workplace, and the opportunity for contact with a health care provider if questions or complaints arise. The first avenue of investigation is to determine whether the job is a match with the capabilities of the individual worker.

Occupational health practitioners making recommendations for prevention of work-related complaints are faced with a lack of quantitative information for many common problems. An individual worker's symptoms themselves decrease productivity and cause discomfort. As such, clinicians are obligated by public health principles to mitigate the symptoms and to prevent a delay in recovery and recurrences in the individual, as well as occurrences in others. Such actions must often be taken on a case-by-case basis using information about worker-job fit, as well as preliminary or population data.

Almost all available studies either define exposure to work-related factors qualitatively or use job title as a proxy. Also important is the failure of many employers to assess in more than a cursory fashion whether employees can tolerate a given job or work environment, either at the time they are initially employed or on a routine basis subsequently. In circumstances when employees' physical or mental abilities are not compatible with job and work site requirements due to baseline factors, or as a result of aging or disease, it is not uncommon for them to experience physical or mental discomfort or the inability to perform their specific tasks without loss of production or decrease in safety in the course of their employment. Those employees who are invested in remaining at work will tend to try to overcome obstacles resulting from this intolerance; those who are experiencing other forms of job-related dissatisfaction are less likely to do so.

The biological reality is that although physical functional capacity varies from worker to worker, it invariably rises and falls over the lifespan. Thus, especially for those whose work entails significant physical labor, a change in career or work duties often will be necessary because of the natural decline of functional capacity with age. In addition, the decline of functional capacity with age can be delayed or accelerated by physical conditioning, overuse, illness, and injury. Also, the visual system undergoes predictable and anticipated changes during the lifespan of an individual. An infant has a hyperopia (farsighted) of +2 to +3 diopters. In adolescence, however, the hyperopia tends to lessen or give way to myopia (nearsighted); at approximately 20 years of age all eyes have a progressive decrease in accommodation (focusing at near point). At age 10 years, the amplitude of accommodation is 14D (diopters); at age 3, of 7 diopters; at 45, of 4 diopters; and at 60, 1 diopter. Parenthetically, it takes 3 diopters of near point accommodation to keep a target at 1 meter in focus.

Refractive Errors and Corrective Measures
The eye has potential for four types of refractive errors:

1. Hyperopia (distant vision)--farsightedness.

2. Myopia (sees at near)--nearsightedness.

3. Astigmatism--the refractive power is different in the various ocular meridians.

4. Presbyopia--the inability to read at near.

As noted above, the maturation of the eye may go through various refraction changes. Fortunately for the human race, first eyeglasses, then contact lenses, and now refractive surgery have corrected many of these errors of refraction. In primitive times, the hunters had to be young hyperopes (farsighted) or emmetropes. The myopes could stay back at the cave and build or fix things--up close. Convex spectacle lenses came into being about 700 years ago. The church/scholars used up most of the rare supply to write and copy documents.

About 25 percent of the U.S. population is myopic. About 25 percent has astigmatism of over 1D. About 60 percent are hypermetropic or emmetropic. After age 48, 100 percent of the population is presbyopic. Nearsightedness is more prevalent in certain population groups. Some military and college students become more myopic during study hours. Notice in photos of people in Third World nations, you rarely see anyone wearing glasses. In highly industrialized societies, spectacles are common. There is a relationship here.

Tredici's U.S. Air Force experiences were with correcting refractive errors in an occupation--aviation--in which good vision plays a dominant role. Refractive errors (mainly myopia) and astigmatism are the main causes for rejection for flying training. Through World War II, 20/20 uncorrected visual acuity was needed. Lower standards were acceptable for navigators, observers, and flight surgeons. Class II (trained) aviators may have lower standards. All aviators above age 45 (presbyopia) need spectacles. In the 1970s and 80s, contact lenses began to be used in place of spectacles to correct refractive errors in aviation.

Today, all of these refractive errors are being surgically corrected to make the individual have normal vision (estimated 20/20 or 6/6). La Haye and Sustello, in the October 2001 article on the older worker ("Safety Eyewear & The Older Worker," page 38, Occupational Health & Safety, Vol. 70, No. 10), stated that you may recall as a child being able to hold an object within about 3 inches of your nose before it became blurred. In contrast, at age 50 the closest you can focus may be more than 30 inches away. Although the nearest point at which you can focus moves steadily outward from childhood, it is not considered to be presbyopia until the loss of near focus (the closest distance at which you can read clearly) passes the critical 14- to 20-inch distance. At this critical point, people need magnifying reading glasses for close-up work and consider themselves presbyopic.

Newer surgical techniques will be discussed, but all need to be identified and followed to see whether complications will arise (for example, a myopic person becoming hyperopic).

Lens Structure and Accommodation
Positioned between the aqueous humor and the vitreous body, the transparent crystalline lens forms one of the refractive media of the eye. The lens plays a passive role in the process of accommodation, permitting light rays that have passed through the cornea and aqueous humor to be focused onto the retina.

The lens is composed entirely of epithelial cells in different stages of maturation. The cell mass is enclosed in an acellular, elastic capsule. Cell division and, thus, growth of the lens continue throughout life; as new lens cells are formed, the older cells are displaced toward the interior of the lens. At an early stage in its development, the lens becomes isolated from a direct blood supply and is therefore dependent on the aqueous and vitreous humors as a source of nutrition and a pathway for eliminating the waste products of metabolism.

The lens of the fully developed eye is a biconvex, transparent structure located immediately in front of the vitreous body and behind the iris. The lens of the young eye is generally colorless, but a yellowish to amber color develops with age. The front surface of the lens, facing the cornea, is bathed with aqueous humor, which flows through the posterior chamber, through the pupil, and into the anterior chamber. The entire cellular mass of the lens (the epithelium, cortex, and nucleus) is completely contained within the elastic capsule, which has a smooth outer surface. The lens is held in place by suspensory ligaments (zonules) running from the ciliary body and inserting into the superficial lens capsule around the equator. The lens is also supported by its immediate apposition to the vitreous. The lens grows in size and weight throughout life.

Accommodation refers to the process whereby changes in the dioptric power of the crystalline lens occur so that an in-focus retinal image of an object of regard is obtained and maintained at the high-resolution fovea (Ciuffreda, 1991). Accommodation is measured in units called diopters. The average person's range of focus from longest to shortest focal point decreases by 85 percent from age 5 to age 65, a process often referred to as accommodative regression.

During accommodation, the principal change in the lens shape is seen at the anterior surface. The anterior surface of the unaccommodated lens has a spherical radius of curvature of about 12 mm. When the eye accommodates, the anterior surface tends to bulge centrally, attaining a radius of curvature of about 3 mm. The more peripheral anterior surface shows relatively little or no increase in curvature. To a large extent, the shape of the accommodated lens is determined by the elastic characteristics and dimensions of the lens capsule. The anterior lens surface bulges at the center during accommodation because the central portion of the anterior capsule is thinner.

Presbyopia ("aged eye") refers to the slow, normal, naturally occurring, age-related, irreversible reduction in maximal accommodative amplitude (i.e., recession of the near point) sufficient to cause symptoms of blur and ocular discomfort or asthenopia (tired eye) at the customary near working distance. Essentially, the near point approaches and then becomes coincident with the far point.

Presbyopia is generally first reported between 40 and 45 years of age, with its highest onset between ages 42 and 44 (Kleinstein, 1987), although its onset may occur at any time from 38 to 48 years, depending on a variety of factors. Although its prevalence across all ages in the population is 31 percent, from approximately age 52 on, the prevalence of presbyopia is considered to be essentially 100 percent (Kleinstein, 1987). Yet the eye's ability to focus at a very near focal point starts in early childhood. The condition is made worse when accommodation is utilized to keep the distant target in focus. In essence, presbyopia may develop at any age. This emphasizes the need to examine the accommodative power when one has his or her distant point fully corrected.

Clinically, when the near-work distance equals half of an individual's residual accommodative amplitude--which occurs, on average, at 40 years of age (Bennett & Rabbetts, 1989)--the gradual onset of symptoms will become manifest (Millodot & Millodot, 1989). These symptoms are as follows (Borish, 1970; Morgan, 1960; Patorgis, 1987):

1. Vision at the customary near-work distance is blurred or can be sustained only with excessive accommodative effort and some ocular discomfort.

2. Drowsiness occurs after a short period of reading or near work.

3. Reading material must be held farther away (e.g., closer to the receding near point and surrounding depth of field) to be seen more clearly. Thus, on average, smaller individuals with proportionally shorter arms develop presbyopic symptoms at an earlier age than do age-matched but proportionally taller persons. Some patients may actually complain, "My arms aren't long enough to see up close anymore." What they are describing is the fact they can no longer keep the object of interest within the proximal edge of the depth of focus of their progressively receding near point.

4. Occasionally, especially in very early or incipient presbyopia, asthenopia related to attempts at excessive accommodative effort is reported. It may even lead to an accommodative spasm and pseudomyopia (as keeping the biceps muscle in contraction for an extended period of time might cause it to go into a spasm).

5. Transient diplopia may be experienced as a result of the increased accommodative response/effort and the consequent synkinetically overdriven accommodative convergence that may be difficult to control consistently using compensatory negative fusional vergence (Ciuffreda, 1992; Ciuffreda & Tannen, 1995). The most effective way to estimate the accommodative power is by performing an ocular visual screening with and without full correction, although the techniques described by La Haye and Sustello do provide a gross appreciation of the accommodative diopter power and not the exact measurement needed for a prescription.

Visual Task Analysis
Before the vision screening for a worker can be recommended, there should be an analysis of the visual tasks required by the job. Analyzing the visual factors required for a task is of crucial importance. Ideally, the analysis should be performed at the workplace.

Factors such as distance and size of the critical details of the task should be assessed, along with the need for color discrimination; depth perception; body, head, or eye postures; field of vision; eye movement requirements; and the contrast and illumination at the job site. Important visual factors can be identified in this manner, and this analysis fulfills regulatory requirements of the Americans with Disabilities Act and OSHA's 29 CFR 1910.132.

North developed a useful job checklist for the analysis of visual requirements for each job in the workplace. It entails a careful survey of each component of a given job in relation to the employee's visual skills used in the performance of his or her tasks. This analysis requires a broad knowledge of visual abilities and limitations (problems of accommodation, convergence, presbyopia, coordination, muscle balance, etc.) lighting, physical factors, and the host of eye hazards of the particular operation.

Proper vision is an important factor in industrial efficiency and has a marked bearing on output and on safety. Adequate measurement and classification of visual requirements of a job is one of the most effective means of determining the potential efficiency of applicants for jobs. By the same means, it is possible to increase the efficiency of employees on the job.

Visual symptoms usually can be resolved with a combination of ergonomic changes in the environment and the provision of appropriate visual care to the computer worker. Studies indicate visual complaints occur in 50 to 90 percent of workers who use VDTs. These problems result from visual inefficiencies or from eye-related symptoms caused by a combination of individual visual abnormalities and poor visual ergonomics. The problems occur whenever the visual demands of the task exceed the abilities of the individual. Such difficulties are real and prevalent, and the basis for most of the problems is understood.

The Visibility of Tasks
The ability to perform most tasks depends on many visual and non-visual variables. Factors that influence visual performance include the visual capability of the individual, the visibility of the task, and psychological and general physiological factors. Naturally, the better the visibility, the easier it is to perform the task.

The choice of safety glasses is especially critical for individuals who have a loss of accommodation (presbyopia), use contact lenses in the workplace, and have had or are having a refractive change. A presbyopic lens should provide for the lack of accommodation so the user can perform visual tasks efficiently and effectively in accordance with the essential functions of the position. The occupational presbyopic lens should attempt to mimic the normal accommodative process and provide a physiologic amplitude of accommodation.

The occupational eye care provider should prescribe lenses to allow the performance of essential visual tasks comfortably with good visual and body ergonomics. The electrician, for example, works 96 percent of the time looking up and at multiple focal lengths. Paste-on bifocal lenses will not satisfy the need for a safe, functional work area.

This article originally appeared in the October 2003 issue of Occupational Health & Safety.