A review of the eye at altitude
by Geoffrey C. Tabin, M.D.
In 1988, on the southwest face of Mt. Everest, two climbers died above 8000 meters; their last radio message was that they were both totally blind. In 1996, near Everest’s summit, a (highly publicized) case of blindness that was blamed on a popular surgical procedure done to correct refractive errors almost caused a climber’s death from exposure. These and other less dramatic episodes prompted this year’s review of mountain medicine to concentrate on the human visual system.
The eye is like a camera. First, lenses focus the light. The image is formed on a delicate and sensitive nerve layer called the retina, which is analogous to the film in the camera. The patterns that the eye observes are then transferred along a nerve to the brain. This optic nerve can be thought of as the messenger taking the film to the store to be developed. Finally, the film processing takes place in the visual cortex of the brain. The nerve impulses are interpreted into our visual world. Sight is dependent on the lenses, retina, optic nerve, and brain all functioning properly. All of these structures can be affected by high altitude.
The eye has two lenses that focus our visual world on the retina. Two-thirds of the power of the eye comes from the cornea, the clear window in the front of the eye. One-third of the power comes from the crystalline lens. It is important to note that the outer corneal surface and its interaction with the lubricating tear film is the most important refractive surface. If the cornea is too wet or too dry, vision will blur. In addition, the cornea has one of the highest densities of sensory nerve fibers in the body, making it exquisitely sensitive to pain.
The colored ring visible through the cornea is the iris, which serves as an aperture does in a camera. The iris will expand or contract to change the amount of light entering the eye. Immediately behind it is the lens that can be “focused” to see close objects (“near vision”). Everyone slowly loses their accommodating (“focusing”) power in this lens, which is why younger people with perfect distance vision require reading glasses after age 50. People who are “far-sighted” can use this focusing power in the lens to see well at a distance when they are young, but require glasses to see both near and far as they age and lose their accommodating power. “Near-sighted” people are unable to see at a distance without glasses, but can always see near objects as light is brought to a point in front of the retina when accommodation is totally relaxed.
The most common ocular problem in mountaineering is minor damage to the surface of the cornea. The top layer of cells, called the epithelium, can be damaged by a scratch, drying of the surface, or UV light causing a sunburn. Corneal scratches can occur in any climbing activity; they are painful and lead to a flooding of tears that blur the vision. “Snow blindness” from the intense direct and reflected UV light from the sun can quickly disable a mountaineer, even on a cloudy day. Dehydration, cold and wind can also damage the corneal surface. Fortunately, these injuries heal quickly, though antibiotics are needed to guard against corneal infections. Because blinking rubs the eyelids over a raw surface and exacerbates the pain, people with snow blindness are more comfortable with the eyes patched shut.
Virtually all cases of snow blindness, corneal freezing, and superficial abrasion heal completely, with restoration of normal vision occurring within 48 hours. During the acute period, however, a climber can be rendered totally disabled by blindness. Thus, the best treatment is prevention. Dark glasses, with side shields, must be worn at altitude, particularly when climbing snow and ice. Clouds do not block UV light energy, so it is imperative that glasses or goggles with UV blocking capability are worn on overcast days. Several cases of snow blindness have occurred when climbers’ glasses fogged because of condensation and they removed the lenses. This is a problem that is often worsened by the use of supplemental oxygen, when warm gas may leak around the mask. A good anti-fog preparation applied to the glass, prescription “no fog” ski goggles and being certain the oxygen mask fits well with no leakage of warm air are preventative measures that can preserve vision.
The retina is the second most common visual structure to be damaged when climbers venture to high altitude. The delicate nerve cells of this layer connect directly with the brain and share a similar physiology. Both the brain and the retina require an enormous and steady supply of oxygen. A specialized network of blood vessels feed the retinal cells. In order to allow clear focusing of light, a barrier prevents blood or fluid from escaping into the retinal spaces. Either decreased oxygen to the retinal cells or leakage of fluid or blood into the retina causes loss of vision.
At high altitude the pressure in the atmosphere decreases, leading to less oxygen diffusing into the bloodstream. Because the retina and brain require a constant amount of oxygen, one of the responses to a lower saturation of oxygen in the blood is an increase in the flow of blood to these vital structures. Elsewhere in the body, arteries have muscular coats to deal with flow increases, but the retinal vessels are designed to remain clear and not obstruct the light signals with big beefy muscles. Instead, they have delicate support cells called pericytes that surround the vessels. These pericytes are sensitive to damage from lack of oxygen and can also rupture in high-flow situations. The result is a leaking of blood into the retina known as retinal hemorrhaging.
High altitude retinal hemorrhages (HARH) occur in a majority of people who have summited 8000-meter peaks, but they have also been reported in visitors to ski areas in Colorado. Most high altitude retinal hemorrhages do not affect vision. However, leakage into the area of nerve cells that carry signals of central vision, called the macula, can cause a profound loss of vision. Upon return to lower elevation the hemorrhages resorb completely, but they can lead to a disorganization of fine neural connections and leave the climber with a permanent decrease in vision in the affected eye.
As noted above, the brain and the retina have similar physiologies. It therefore makes sense that similar events happen in the retina and brain. High altitude cerebral edema has been well described. A similar swelling of the retina without the frank bleeding of high altitude retinal hemorrhages may occur, accounting for the blurred vision that is often reported by people with high altitude cerebral edema. In addition, some scientists believe that tiny hemorrhages similar to those in the retina occur in the brain. This thought has led some to advise climbers with HARH to descend, but since HARH are so common this advice is largely ignored. Persons with a macular hemorrhage probably should descend in case the bleed spreads.
The worst retinal problem climbers encounter is damage to the retinal cells from lack of oxygen. This is similar to a stroke, and the visual loss can be extensive and permanent. The central retinal vasculature is particularly susceptible to occlusion when the blood becomes more viscous from the combined effect of increased red blood cells and dehydration. The retinal cells that are responsive to dim light are called rods and are especially sensitive to lack of oxygen. The result is a sharp decrease in night vision even as low as 2000 meters.
The next structure essential to vision is the optic nerve. This vital connection links the eye to the brain. Increased pressure in the brain from swelling of the nerve cells, increased blood volume and edema is transmitted to the optic nerve. This leads to papilledema, a swelling of both optic nerves that causes transient blackouts of vision. Blockage of the blood flow to part of the optic nerve can cause a permanent visual loss.
Climbers who have glaucoma must be careful when ascending to high altitude. Glaucoma is damage to the optic nerve associated with increased pressure within the eye. The decrease in oxygen at altitude makes the optic nerve much more vulnerable to glaucomatous damage. Diamox is often taken to help the kidneys correct the problem of respiratory alkalosis caused by rapid breathing at altitude. Diamox is also a potent drug for glaucoma that decreases the amount of aqueous fluid in the eye. Glaucoma patients should take all of their current medications and also consider taking Diamox when climbing high.
An increasing number of climbers are describing brief blurring or, rarely, complete blindness at altitude, sometimes no higher than 4500 meters. The symptoms disappear promptly with descent. A similar condition called amaurosis is rare at sea level. The blurring is sometimes accompanied by flashes of light (scintillating scotomata). Because such events have been described in migraine victims, with or without headache, this temporary blindness may be due to a migraine equivalent triggered by hypoxia.
Finally, the brain can be damaged by the lack of oxygen either with cerebral edema or strokes. Unlike retinal or optic nerve problems that affect one eye only, brain injuries always affect both eyes. The visual cortex is the area of the brain where sight is interpreted. With prolonged lack of oxygen or an acute blockage of blood flow, “cortical” blindness can develop, ending all vision in both eyes.
Fortunately, blindness from the retina, optic nerve and brain are relatively uncommon. The most common ocular problem (on flat land as well as in the mountains) is a refractive error that requires glasses. Perfect vision requires that the image be focused on the retinal surface. In myopic (near-sighted) people, the combined power of the cornea and lens brings light to a point in front of the retina. Without corrective lenses, the image spreads out and is blurred on the retina. Far-sighted people, on the other hand, focus behind the retina. Far-sighted people can bring light into focus by using the accommodating power of their lens. With age the amount of extra lens power to overcome far-sightedness declines.
Eyeglasses are the first option for correcting refractive errors. Anti-fog treatments, prescription glacier glasses or goggles can improve their adaptation for mountaineering. However, in misty conditions and rain, glasses can be a real handicap. Contact lenses provide better vision in inclement weather. New extended-wear disposable soft contact lenses can give added comfort and convenience on multi-day climbs where care and cleaning of the lenses is difficult. However, on extended climbs when the contact lenses are worn too long, they can predispose the climber to corneal infections. Moreover, any contact lens will decrease the flow of oxygen to the cornea, thereby compromising the corneal health. And at high altitude, contact lenses can lead to considerable eye discomfort. Thus, climbers have eagerly embraced new developments in refractive surgery to eliminate the need for eyeglasses.
The first refractive procedure to decrease near-sightedness was radial keratotomy (RK). In this technique, radial incisions are made in a spoke-like fashion around the cornea, allowing the corneal curvature to relax and decreasing the amount of focusing power of the cornea and thus decreasing the near-sightedness. This procedure, which was wildly popular for several years, has many disadvantages for climbers. First and foremost, the radial incisions permanently weaken the cornea. A normal eye that receives blunt trauma will retain its integrity as the structures behind the globe give way prior to rupturing of the eye. When one has had a radial keratotomy, the 90 percent thickness cuts of the surgery become the weakest component of the eye, and even relatively minor trauma can result in a vision-ending rupture of the globe. This is a concern for climbers when you consider the risk of ice- or rock-fall or how frequently an ice axe bounces out of the ice and strikes the eye.
A second problem of the radial keratotomy is an irregular astigmatism that can cause a slight distortion of vision if the cuts are not made perfectly. And, even when the cuts are made perfectly, the scars can still affect vision when the pupil is widely dilated as occurs in both dim light and with maximal adrenalin (climbers). These scars near the visual axis will produce halos, ghost images and blurring.
A final concern of radial keratotomy is fluctuating refractions, particularly at high altitude. Recent research has suggested that the lack of oxygen on the outer corneal surface causes a transient swelling around the incisions, resulting in even more flattening of the cornea. This leads to a shift of the refraction, making the radial keratotomy patient more far-sighted at high altitude. A young climber with radial keratotomy may notice no ill effects as the accommodative power of the lens can easily overcome the change. Thus, many climbers who have had RK have spent considerable periods of time above 8000 meters noting no problems with their vision. However, in older climbers who have lost their accommodative faculties, the change toward far-sightedness causes blurry vision at altitude.
In the last few years, great improvements have occurred in the realm of refractive surgery with the development of the excimer laser. The excimer laser is a very short wave length laser that is able to carve the prescription into the cornea without damaging other structures. This is much more accurate than the radial keratotomy, does not weaken the structure of the eye and is stable at high altitude. The first excimer laser procedure to be developed was photo refractive keratectomy (PRK), whereby a laser carves the prescription into the cornea from the top surface. This results in a total abrasion of the top surface of the cornea that is painful for several days. It also takes several days to heal. Finally, PRK is only able to treat a moderate amount of myopia.
In order to expand the range of PRK, a new procedure, known as Lasik (laser assisted in situ keratomileusis), has been developed. In Lasik, a small flap is cut into the top corneal surface beneath the layer where scarring cells are located. This flap is lifted up and the prescription is carved into the bed underneath. This allows correction of larger degrees of short-sightedness with great accuracy. It also causes minimal discomfort and quicker visual rehabilitation. Like PRK, the post-surgical refraction will not shift with altitude.
The one disadvantage of the Lasik procedure is that Lasik is a surgical procedure: Complications can occur in both cutting the flap and repositioning the flap after the laser procedure. These complications are extremely rare but can result in a decrease in best-corrected visual acuity. My current recommendation is that people with small amounts of myopia have the relatively less-risky PRK and people with larger amounts of myopia have Lasik. In 1998 no climber should still be getting radial keratotomy.
A second question is what refraction to aim for. The laser can leave any refractive error that is desired in the eye. It is worth remembering that a climber with no refractive error will start to need reading glasses when they are around 50. This can mean difficulty in seeing well at arms’ length—that is, in seeing a distance useful in slotting protection. If the laser leaves a small amount of residual myopia, vision will remain adequate at a distance and allow clear focus at near distances for a longer period of time. Thus, if one aims for a refractive error of approximately -0.50, a climber will be able to see his or her footholds until a very advanced age.