N

nearsightedness See MYOPIA.

night blindness Impaired dark adaptation result-
ing from slowed photochemical reactions in the
rods, the specialized cells of the RETINAthat per-
ceive contrast and detect visual images in low
light. Night blindness becomes increasingly com-
mon after middle age. The person with night
blindness may be unable to see at all in dim light
or may experience delayed adjustment when
going from a lighted environment to a dim or dark
environment. A diminished VISUAL FIELD with
restricted peripheral vision also contributes to
night blindness, as the outer edge of the retina
where peripheral vision takes place contains
mostly rods.
There are not many treatment options for night
blindness. Nutritional supplementation of vitamin
A and the antioxidants LUTEIN and ZEAXANTHIN,
which some studies show help maintain the
health of the eye and improve the functioning of
the rods, seem to aid some people. Adequate light-
ing when reading and especially when watching
television or movies reduces the need for the
eye to make accommodations for changing light.
Increased lighting can compensate for dimin-
ished dark adaptation in static settings such as
rooms and offices, though it is not possible to
make similar accommodations for functions such
as driving.
See also AGING, EYE AND VISION CHANGES THAT
OCCUR WITH; ANTIOXIDANT; PRESBYOPIA; VISION HEALTH.

nystagmus Involuntary movements of the eyes,
usually rapid and repetitive. Nystagmus can be
congenital or acquired; in either circumstance it is
a symptom of underlying disorders rather than

itself a condition. Nystagmus nearly always indi-

cates VISION IMPAIRMENT; if congenital, the impair-

ment may improve or completely resolve with

age. Vision impairment in adults depends on the

underlying cause of the nystagmus. Temporary

induced nystagmus, such as may occur with

caloric testing (warm or cool water infused into

the auditory canal) to assess disorders of the

vestibular system, does not affect vision, although

vestibular disorders can cause nystagmus.

Causes of Causes of

Congenital Nystagmus Acquired Nystagmus

ALBINISM(absence of retinal vestibular disorders

pigmentation) brainstem or cerebellum

congenital macular defects damage or tumor

congenitalCATARACT MULTIPLE SCLEROSIS

absence of iris TRAUMATIC BRAIN INJURY(TBI)

anomalies of theOPTIC NERVE TRAUMATIC BRAIN INJURY(TBI)

chronic ALCOHOLabuse

The diagnostic path includes a comprehensive

OPHTHALMIC EXAMINATIONand NEUROLOGIC EXAMINA-

TION. Treatment targets the underlying cause.

Some adults who have acquired nystagmus

receive relief from the muscle relaxant medication

baclofen (Lioresal), which interrupts NERVEsignals

from the BRAIN to the muscles that control the

eyes. The long-term consequences for vision

depend on the cause and duration of the nystag-

mus. Occasionally nystagmus occurs as an unde-

sired SIDE EFFECTof antiseizure medications, and

typically goes away with switching to another

medication.

See also BENIGN PAROXYSMAL POSITIONAL VERTIGO

(BPPV); DIPLOPIA; MUSCLE RELAXANT MEDICATIONS;PHO-

TOPHOBIA; STRABISMUS.

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