The median age of professional pilots, like that of the general population, is increasing. Pilots from the World War II era are retiring in great numbers and Vietnam aviators are now in their 30s, 40s and 50s. Granted, there are younger recruits to fill their slots, but most of the older group still have quite a few good years left to them. If they take care of their eyes.
As the human body ages it gradually changes. It becomes less flexible, less resilient, more sensitive to changes in temperature, etc. Aging alters the structures of the eye in subtle and not so-subtle ways that affect vision quality. Pilots aged 40 and older should be aware of these changes and their consequences.
In the normal process of aging, the tears become less viscous, more watery, and it takes more tears to lubricate the eye. Tearing is a common complaint in the elderly. Over-the-counter tear-substitute drops may be used to restore the normal oily layer. Replenishment of the oily layer decreases the need for increased tear volume and the symptom of excessive tearing disappears. The cornea is the primary focusing structure of the eye. It has five layers which are carefully aligned to optimize light transmission and focus on the light-sensitive, seeing portion of the eye-the retina. A number of age related physiological changes affect the cornea and its transparency. The clouding of the cornea induced by dystrophies is one example. Dystrophies are degenerative conditions that affect the inner and outer layers of the cornea and disrupt the normal optics. Arcus senilis (white ring at the base of the cornea) is one of the most frequently observed dystrophies, or degenerative changes, of the cornea. However, it does not cause visual impairment because it is limited to the periphery of the cornea, away from the central visual axis. It is also important to mention that it is normal for the cornea to yellow with age.
It is very important that pilots (particularly age 40 and older) have regular eye exams to monitor the integrity of the cornea, and rule out any serious problems that may potentially cloud the cornea and adversely influence visual performance. A routine eye exam includes careful microscopic assessment of the cornea and the entire front segment of the eye.
Subtle vision loss
The anterior chamber is filled with a clear fluid (aqueous) that is constantly manufactured and drained into narrow channels at the base of the iris. The aqueous also yellows slightly with age, but the most widely publicized problem occurring in the anterior chamber is glaucoma.
Glaucoma is a blinding eye disease which is caused by a mechanical blockage of the aqueous drainage system. With no route for fluid drainage and new fluid constantly being produced, an increase in the internal (intraocular) pressure of the eye results. This exerts pressure on the optic nerve and eventually causes blindness. There are many types of glaucoma, including the very painful “angle closure” variety, but “open angle” is what most eye doctors screen for because there are no symptoms until the patient starts losing sight.
Glaucoma may occur at any age, but it is more commonly seen in individuals over the age of 40. Around the age of 80 the incidence goes up dramatically. It’s important that a glaucoma evaluation be performed annually to rule out any subtle loss in the pilot’s visual field which may hinder aircraft operation and flight safety. Although glaucoma cannot be cured, if it is detected at an early stage it can be arrested by simply administering eye drops.
The iris is the colored part of the eye and the hole in the center of the iris is called the pupil. The pupil plays an important function by taking all the available light and reducing it to a “pencil” of light rays. The “pencil” of light creates a clearly focused image on the retina. Under reduced levels of illumination the pupil dilates to allow more light to come into the eye, and under high levels of illumination the pupil constricts and reduces the amount of light entering the eye.
The pupil also works as an “f-stop” just like a camera lens. When the pupil constricts, the depth of focus increases in the eye. It is possible then to focus on objects within a larger range. This is why people 40 years and older like to use bright lights when they read-their eyes work better. With age, the pupil I tends to get constricted (“pinpoint”) and the iris muscles do not react as readily to changes in illumination. As a result the depth of focus becomes relatively fixed, and does not provide the flexibility that it did earlier in life.
The lens is a wonderful and unique structure. All the cells formed in the lens are retained in the lens capsule (unless the lens is extracted during cataract surgery). The lens cells are not shed like other body cells. This means that the old cells are pushed to the center as the new cells wrap around them, giving the lens the appearance of an onion. As the eye ages, the lens becomes so dense that it no longer flexes and its accommodation (focusing ability) drops off. This is termed presbyopia. This is when we notice that our “arms are getting too short” for us to read. The rigid lens also increases the time it takes for us to refocus from far to near or near to far, hindering the ability to concurrently scan cockpit instruments and the horizon.
As the lens cells get packed tighter and tighter they soon form dense spots and these spots are called “cataracts.” Cataracts are a normal consequence of aging and if we live long enough everybody gets them.
Cataracts generally form in the center of the lens and, as the pupil constricts with age, it directs all of the light entering the eye into the area of the dense cataract. Thus pilots with cataracts find vision becoming worse when they try to read sectional charts or approach plates. Also, lens opacities act to spread the light entering the eye and add to the glare sensitivity in individuals suffering from cataracts. Cataracts also decrease the ability to distinguish low contrast targets. Low contrast targets are common in aviation where there is high intensity light which acts to blend a target in with its background.
Fortunately, today’s surgical procedures are so advanced that cataract extractions are handled on an out-patient or office-visit basis, and the patient is permitted to go home the same day. Further, the human lens is now replaced with a plastic (intraocular) lens so that the patient sees well after surgery without having to wear thick glasses.
The virtuous vitreous
The vitreous is the Jello-like fluid that fills the inside of the eyeball and contributes to its shape and firmness. The vitreous is formed during embryonic development and is not replaced or replenished like the fluid in the anterior chamber. However, the vitreous does change consistency; it becomes more fluid, more watery, with age and with near-sightedness. As a result, there is an increase in the number of free-floating cells in the eye which the individual sees as “spots” or “cobwebs.” The medical term for these loose cells is “muscae volitantes” (flittering flies). This condition is annoying to the individual, but it is not serious and doesn’t lead to serious visual impairment. However it may give rise to the perception of artificial “bogeys.,,
A more serious complication of the “fluid vitreous” is that it tugs on the retina at key attachment sites and may lead to retinal detachment. In this case the floaters may be accompanied by flashes of light, the appearance of a “curtain” going down and covering the vision, and a decrease in vision in the affected eye. Retinal detachment is an emergency condition and requires immediate medical care.
The yellowing of the vitreous, cornea, aqueous and lens in the elderly gives rise to a decrease in sensitivity to shades of blue.
The retina is composed of the rods and cones, the “seeing cells,” of the eye. The cones are responsible for providing optimal vision (20/20) and allowing us to differentiate colors. The cones comprise only 5% of the retina and are concentrated in an area called the macula. The optics of the eye are focused on the macula.
The rods comprise the remainder of the retina. They allow us to see under low levels of illumination and to detect moving or flickering objects. The rods do not provide good vision. A 10′ shift off the macula of the eye will result in a tenfold decrease in target-detection performance. The area around the macula, where the rods are concentrated, is called the peripheral retina. It is by far the larger portion of the eye’s “seeing area” but it is the least understood by laymen-including pilots.
The peripheral retina plays a key role in locomotion. It allows us to navigate through narrow hallways and doorways and in crowds without bumping into objects or people. To lose peripheral vision is extremely debilitating. Besides locomotion, the peripheral retina provides for vision under reduced levels of illumination (night vision). At very low levels of illumination the macula becomes a blindspot and we depend entirely upon the peripheral retina for our safety and survival.
The most common age-related condition which affects the macula is “age- related maculophathy.” This condition causes blindness in about 10% of Loss of central or peripheral vision, impacting target detection performance the population over age 60. It is due to a reorganization of the macula that interferes with the way the cones function.
Conversely, the peripheral retina may be affected by almost any systemic condition (eg, high blood pressure, arteriosclerosis, diabetes) as well as drugs that affect the diameter of the blood vessels; this includes coffee and cigarettes. The result is a decrease in the blood supply and nourishment to the peripheral retina and an impairment of visual function. During the day this may translate into poor target detection performance and at night an overall reduction in visual capability- particularly at altitudes above 5000 feet. It is imperative that senior pilots have an eye exam in which the pupil is dilated and the peripheral retina carefully assessed.
No aviation professional needs to be told what a crucial role vision plays in pilot performance, but it doesn’t hurt to be reminded how that performance may be adversely affected by overlooking the basic changes in vision that come with age and ignoring the simple remedies for most of these physiologically induced problems.
The visual system is unique and if we understand its limitations and make appropriate compensations, it is possible to insure continued safety and optimal performance. Good vision is a gift and, if properly cared for, can last a lifetime.
Dr. William Monaco