intensity of the light to enter the inner eye. The
examination takes only a few minutes.
See also OPHTHALMIC EXAMINATION; OPHTHAL-
MOSCOPY.
Snellen chart The familiar eye chart, featuring
lines of letters and numbers that progressively
decrease in size, to assess VISUAL ACUITY. Dutch
ophthalmologist Herman Snellen (1834–1908)
developed the chart in the 1860s. The top letter is
typically E. A person with normal vision can read
a one-inch letter from 20 feet away, designated as
20/20 vision. Each line on the chart represents a
ratio of normal vision. A person who can see at 20
feet what someone who has normal vision could
see at 60 feet has 20/60 vision. A REFRACTION TEST
then determines the precise correction necessary
to bring visual acuity as close to 20/20 as possible.
To take a Snellen test, a person reads the smallest
line possible with each eye separately (one eye
covered) and both eyes together.
See also AMSLER GRID; OPHTHALMIC EXAMINATION;
VISION IMPAIRMENT.
strabismus A condition, also called tropia, in
which the eyes do not focus simultaneously on
the same object. One EYEmay turn inward (“cross-
eye” or esotropia), or one eye may turn outward
(“walleye” or exotropia). Congenital strabismus
may occur with RETINOBLASTOMAor RETINOPATHYof
prematurity and becomes apparent in the first few
months after birth. Most strabismus develops in
children between the ages of one and five. About
half of the time the cause of strabismus in children
is unknown (idiopathic) and not associated with
any underlying condition.
In adults, strabismus may develop as a conse-
quence of TRAUMA TO THE EYE,TRAUMATIC BRAIN
INJURY (TBI), or STROKE. Adults may experience
double vision (DIPLOPIA) or uncoordinated move-
ments of the eyes. Adults also may acquire strabis-
mus as a consequence of vision loss in one eye,
which results in lack of visual signals that cue the
BRAINfor MUSCLEmovements of the eye. Common
causes of acquired strabismus in adults include
stroke, trauma, GRAVES’S DISEASE, and other sur-
gery.
The diagnostic path includes comprehensive
ophthalmic and NEUROLOGIC EXAMINATIONS. Timely
treatment in children is essential to prevent AMBLY-
OPIA, in which the brain learns to perceive images
from only one eye. This learning establishes the
brain’s vision pathways, and if untreated becomes
a form of permanent vision loss. Strabismus treat-
ment may include exercises or surgery to
strengthen the eye muscles of the weak eye.
See also GRAVES’S OPTHALMOPATHY; OPHTHALMIC
EXAMINATION; VISION IMPAIRMENT.stye See HORDEOLUM.118 The Eyes