L–M

lens The primary focusing structure of the EYE,
located in the center at the front of the eye. The
lens is transparent, convex (rounded outward on
each side), round, and flexible. A thin membrane
encloses the lens. Tiny muscles at the front edges
of its sides, the ciliary muscles, contract to flatten
the lens and relax to thicken the lens. These
adjustments alter light refraction (the angle at
which the lens bends lightwaves entering the eye)
to accommodate near and distant vision. The most
common health conditions that affect the lens are
PRESBYOPIA, in which the FLEXIBILITY of the lens
diminishes with aging, and CATARACT, in which
protein deposits cloud the lens and obscure vision.
The lens is also vulnerable to accidental injury,
particularly from blunt force (such as a baseball)
or puncture.
For further discussion of the lens within the
context of ophthalmologic structure and function
please see the overview section “The Eyes.”
See also CATARACT EXTRACTION AND LENS REPLACE-
MENT; CORNEA; HYPEROPIA; MYOPIA; REFRACTIVE ERRORS;
RETINA; VISION IMPAIRMENT.

mydriasis Excessive or persistent dilation of the
pupil that is a symptom of ophthalmic or systemic
conditions. The ophthalmologist may induce
mydriasis, using topical atropine, to examine the
inner EYE. Therapeutic mydriasis using atropine-
based ophthalmic drops is an alternate treatment
for AMBLYOPIA. Two eye conditions can cause
mydriasis: GLAUCOMAand damage to the iris. In the
healthy eye the iris, a muscular membrane, con-
trols the opening of the pupil. INFLAMMATIONof or
tears in the iris affect its ability to function, which
also can result in mydriasis.
Other causes of mydriasis are systemic, involv-
ing damage to NERVOUS SYSTEMstructures and func-

tions, and may include TRAUMATIC BRAIN INJURY

(TBI), STROKE, and medication response such as

with narcotic use, which causes the muscles to

relax. Eye disorders often affect only one eye (uni-

lateral mydriasis), whereas systemic conditions

typically affect both eyes (bilateral mydriasis).

PHOTOPHOBIA (sensitivity to bright light) often

accompanies mydriasis as the dilated pupil cannot

limit light from entering the eye. VISION IMPAIRMENT

depends on the extend of the dilation; a fully

dilated pupil prevents focus on near objects.

The diagnostic path begins with a basic OPH-

THALMIC EXAMINATIONincluding SLIT LAMP EXAMINA-

TION and OPHTHALMOSCOPY, unless there is clear

evidence that the mydriasis results from systemic

causes. TONOMETRY, which measures the pressure

within the eye (INTRAOCULAR PRESSURE), determines

whether glaucoma is present. Tears of the iris are

typically apparent when looking at the eye as they

distort the iris (the colored portion of the eye).

Inflammation of the iris (IRITIS) often reddens the

eye and is apparent with ophthalmoscopy. Further

diagnostic measures turn to NEUROLOGIC EXAMINA-

TION. Treatment targets the causative condition.

See also EYE PAIN; NARCOTICS; TRAUMA TO THE EYE.

myopia A refractive error commonly called

nearsightedness, in which the EYEhas difficulty

focusing on distant objects. Myopia results when

the focal point of lightwaves entering the eye falls

short of the RETINA, causing the images the retina

registers to be blurred. The refractive error occurs

because the distance from the front to the back of

the eye is longer than normal. Symptoms of

myopia include

• squinting when looking at distant objects


• straining to see when driving

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