C h a p t e r 3 8
Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
961
of an asymmetric bowing of the cornea, but it also can
result from defects in the cornea, lens, or retina.
Both hyperopia and myopia, as well as astigmatism,
can be corrected with the appropriate lens. Refractive
corneal surgeries such as laser in situ keratomileusis
(LASIK), photorefractive keratectomy, and radial kera-
totomy can be performed to correct the corneal curva-
ture in persons with myopia.
17
Disorders of Accommodation
The ability of the eye to adjust its focus from far to
near objects by changing the shape of its lens is called
accommodation.
Accommodation is associated with
convergence of the eyes (focusing the vision on a near
point), pupillary constriction, and thickening of the
lens through contraction of the ciliary muscle of the
eye.
16
Contraction of muscle fibers in the ciliary body
is controlled mainly by parasympathetic stimulation
of the oculomotor nerve (CN III). Paralysis of the cili-
ary muscle resulting in pupillary dilation, with loss of
accommodation, is called
cycloplegia.
Pharmacologic
cycloplegia through the use of a topical anticholinergic
agent is sometimes used to aid in refractive examination
of the eye, particularly in small children.
The term
presbyopia
refers to a decrease in accommo-
dation that occurs because of aging.
16
The normal prolif-
erative process in the lens nucleus that occurs throughout
life causes the lens to thicken and its capsule to become
less elastic, so that the range of focus or accommodation
is diminished. As a result, the person begins to notice
the inability to read small print or discriminate close
objects. This is usually worse in dim light, on arising in
the morning, or when the person is fatigued.
Cataracts
A cataract is a lens opacity that interferes with the trans-
mission of light to the retina. Aging is the most common
cause of cataracts, and most people over age 60 have
some degree of lens opacity. Although aging is the most
important cause of cataracts, other factors such as hered-
ity, environmental influences, systemic diseases (e.g., dia-
betes mellitus), drugs, and trauma may be involved.
15,18
Long-term exposure to sunlight (ultraviolet radiation)
has been associated with increased risk of cataract for-
mation. Corticosteroids have been implicated as caus-
ative agents in cataract formation. Both systemic and
inhaled corticosteroids have been cited as risk factors.
19
Clinical Manifestations.
The manifestations of cata-
racts depend on the extent of opacity and whether the
defect is bilateral or unilateral. Cataracts are usually
bilateral (with the exception of traumatic or congenital
cataracts). Age-related cataracts, which are typically the
result of opacification of the lens nucleus (nuclear scle-
rosis), are characterized by increasingly blurred vision
and visual distortion.
15,20
Dilation of the pupil in dim
light improves vision. In addition to decreased visual
acuity, cataracts tend to cause light entering the eye to
be scattered, thereby producing glare or the abnormal
presence of light in the visual field (see Fig. 38-1C).
Diagnosis and Treatment.
Diagnosis of cataracts is
based on ophthalmoscopic examination and the degree
of visual impairment on the Snellen vision test. On oph-
thalmoscopic examination, a cataract may appear as
a gross opacity filling the pupillary aperture, or as an
opacity silhouetted against the red background of the
fundus. Other tests that determine the potential ability
to see well after surgery, such as electrophysiologic test-
ing in which the response to visual stimuli is measured
electronically, may be done.
There is no effective medical treatment for cataract.
Although strong bifocal lenses, magnification, appropri-
ate lighting, and visual aids may help, surgery is the only
treatment for cataract.
15,20
It is commonly performed
on an outpatient basis with the use of local anesthesia.
The cataract lens is typically fragmented into fine pieces,
which then are aspirated from the eye, and an intraocu-
lar lens implanted. Monofocal intraocular lenses that
correct for distance vision are available. Although eye-
glasses may still be needed for near vision, the recent
introduction of multifocal intraocular lenses has reduced
the need for both distance and near vision corrective
eyeglasses.
Childhood Cataract.
Childhood cataracts can be
divided into two groups: congenital (infantile)
cataracts, which are present at birth or appear shortly
thereafter, and acquired cataracts, which occur later and
are usually related to a specific cause.
15,21
About one third
of childhood cataracts are hereditary; another third are
secondary to metabolic or infectious processes or
associated with a variety of syndromes; and the final third
results from undetermined causes. The development of
congenital or infantile cataracts depend on the total dose
of the agent and the stage of development at the time of
exposure. It is during the last trimester of fetal life,
genetically or environmentally influenced malformation
of the superficial lens fibers can occur. Congenital lens
opacities may occur in children of diabetic mothers. The
presence of bilateral cataracts is often inherited and
associated with other disease conditions, so genetic testing
is required.
Most congenital cataracts are not progressive and
are not dense enough to cause significant visual impair-
ment.
15
However, cataracts that are dense, central, and
larger than 2 mm in diameter require surgical manage-
ment on an urgent basis. If not treated within the first
2 months of life, visual deprivation produces irreversible
loss of vision
15,21
(i.e., ambylopia, to be discussed).
Disorders ofThe Retina
The retina is the most complex structure of the eye. It
receives visual images produced by the optical system
of the eye and converts the light energy into an electri-
cal signal that is transmitted by way of the optic nerve
to the visual cortex, where the image is perceived.
16,22,23
Disorders of the retina and its function include disorders
of the retinal vessels such as retinopathies that cause
