EYE DISORDERS - Medicine for Mountaineering and Other Wilderness Activities - page 192

CHAPTER 15
EYE DISORDERS
Geoffrey C. Tabin, M.D.
Gilbert C. Wong, M.D.
Principal Contributor s
In1988,onthesouthwestfaceofMountEverest,twoclimbersdiedabove26,250feet(8000
m); their last radio message was that they were both totally blind. In 1996, near Everest's
summit, a highly publicized case of blindness, which was blamed on a popular surgical pro-
cedure done to correct refractive errors, almost caused a climber's death from hypothermia
and frostbite. These and other less dramatic episodes highlight the importance of under-
standing the basic anatomy and physiology of the eye, as well as knowing about the prob-
lems commonly encountered in wilderness or mountaineering environments.
The eye is like a digital camera. Lenses focus light entering through the pupil on a del-
icate and sensitive nerve layer at the back of the eye, the retina, which is analogous to the
light-sensitive sensor in a digital camera. The impulses from the nerve cells in the retina are
transmitted along the optic nerves to the brain. These nerves can be thought of as a messen-
ger taking film to be developed or the wiring that transmits a digital image to the viewing
screen. Finally,thenerveimpulses areprocessed inthevisual cortex ofthebrainwherethey
are converted to images that are interpreted in accord with the individual's experience with
the visual world. Sight is dependent on the lenses, retina, optic nerves, and brain all func-
tioning properly. All these structures can be adversely affected by high altitude or exposure
to the elements.
The eye has two lenses that focus light on the retina. Two-thirds of the focusing power
comes from the cornea, the clear window in the front of the eye. One-third of the focusing
comesfromthecrystallinelens.Theoutercornealsurfaceanditsinteractionwiththelubric-
atingfilmoftearsisthemostimportantrefractivesurface.Ifthecorneaistoowetortoodry,
vision blurs. In addition, the cornea has one of the highest densities of sensory nerve fibers
in the body, making it exquisitely sensitive to pain.
Thecoloredringvisiblethroughthecorneaistheiris,whichservesasanaperturedoesin
a camera. The iris expands or contracts the pupil to increase or limit the amount of light en-
teringtheeye.Immediatelybehinditisthelensthatcanbefocusedtoseecloseobjects(near
vision). Focusing is not needed for distant vision by individuals with normal vision. Every-
one slowly loses their accommodating (focusing) power in this lens, which is why people
with perfect vision when they are young require reading glasses after age fifty. People who
are farsighted 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.Nearsighted peopleareunabletoseeatadistancewithoutglassesbutcanalwayssee