treatment may result in permanent dysfunction
of the eye–brain pathways, as these become
entrenched by age 9 or 10. After this time the
vision pathways are well established and ambly-
opia can no longer develop.
See also ASTIGMATISM; HYPEROPIA; MYOPIA; PTOSIS.
Amsler grid A basic test to detect or monitor the
progression of AGE-RELATED MACULAR DEGENERATION
(ARMD), a condition in which the macula, the area
on the RETINA responsible for fine detail vision,
deteriorates. The Amsler grid is a square with
evenly spaced horizontal and vertical lines, and a
dot in the center of the grid. The grid’s four corners
and lines should appear visible, straight, and intact.
Wavy lines, gaps in the lines, or missing segments
suggest damage to the macula. Such a result
requires further examination from an ophthalmol-
ogist who specializes in retinal disorders.
See also VISUAL ACUITY.
astigmatism A common refractive error of vision
that results from an irregularly shaped CORNEA.
Astigmatism may affect one EYEor both eyes. Typi-
cally the irregularity results in two focal points of
light that reach the RETINAinstead of a single focal
point, resulting in blurred or distorted images.
Astigmatism often coexists with HYPEROPIA (far-
sightedness) or MYOPIA (nearsightedness) and
tends to run in families. Corrective measures
include eyeglasses, contact lenses, and REFRACTIVE
SURGERY. Mild astigmatism may not produce
noticeable vision disturbances, in which case it
does not require correction. The success of correc-
tive measures depends on the extent and nature
of the corneal irregularities. Astigmatism often
accompanies age-related changes in the eyes and
vision, and is a common SIDE EFFECTof CORNEAL
TRANSPLANTATION.
Less commonly astigmatism results from irreg-
ularities in the surface of LENS, called lenticular
astigmatism. Options to correct for lenticular astig-
matism are CORRECTIVE LENSESor lens-replacement
surgery to implant an intraocular lens.
See also CATARACT EXTRACTION AND LENS REPLACE-
MENT; REFRACTION TEST; REFRACTIVE ERRORS.astigmatism 73