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U N I T 1 0
Nervous System
reflex test performed to check for abnormalities in the
back of the eye (posterior segment) and opacities in the
visual axis, such as cataracts or corneal opacity.
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Visual
examinations should then be performed during all
well-child visits. These should include age-appropriate
evaluation of visual acuity, ocular alignment, and ocular
media clarity (cataracts, tumors).
The treatment of children at risk for development
of amblyopia must be instituted well before the age of
6 years to avoid the suppression phenomenon. Surgery
for congenital cataracts and ptosis should be done
early. Severe refractive errors should be corrected. In
children with strabismus, the alternate blocking of the
vision in one eye and then the other with a patch forces
the child to use both eyes for form discrimination. The
duration of occlusion of vision in the good eye must
be short (2 to 4 hours per day) and closely monitored,
or deprivation amblyopia can develop in the good eye
as well.
or farsightedness, the image falls behind the
retina, and in myopia, or nearsightedness, it
falls in front of the retina. Accommodation is the
process by which a clear image is maintained
as the gaze is shifted from a far object to a near
object. Presbyopia is a change in the lens that
occurs because of aging such that the lens
becomes thicker and less able to change shape
and accommodate for near vision. A cataract is
characterized by increased lens opacity.
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The retina covers the inner aspect of the
posterior two thirds of the eyeball and is
continuous with the optic nerve. It contains
the photoreceptors for vision: the rods, for
black and white discrimination, and the cones,
for color vision.The retinopathies are visual
disorders involving the small blood vessels of
the retina, which can result from a number of
local and systemic disorders, including diabetes
mellitus and hypertension. Retinal detachment
involves separation of the sensory receptors
from their blood supply; it causes blindness
unless reattachment is accomplished promptly.
Macular degeneration, which is a leading cause
of blindness in the elderly, is characterized by
loss of central vision resulting from destructive
changes in the central fovea.
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Glaucoma is a chronic, degenerative visual
field loss usually associated with an increase in
intraocular pressure due to impaired outflow of
aqueous humor from the anterior chamber of
the eye. Glaucoma is commonly classified as
open-angle glaucoma, in which impaired outflow
of aqueous humor is due to dysfunction of the
drainage system, and angle-closure glaucoma,
in which occlusion of the anterior chamber angle
impairs access to the drainage system.
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Eye movement, which is controlled by the
extraocular muscles, provides for alignment
of the eyes and binocular vision. Strabismus,
which involves abnormalities in coordination of
eye movements, can be caused by weakness or
paralysis of the extraocular muscles (paralytic
strabismus), inappropriate length or insertion
of the extraocular muscles (nonparalytic
strabismus), or accommodation disorders.
The neural pathways for vision develop during
infancy. Amblyopia is a condition of diminished
vision that results from inadequately developed
CNS circuitry due to visual deprivation or
abnormal binocular interactions during the
period of visual immaturity.
SUMMARY CONCEPTS
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The eyeball, or optic globe, which is a complex
sensory organ that provides the sense of vision,
is protected posteriorly by the bony structures of
the orbital cavity and anteriorly by the eyelids.
The eyeball is composed of three layers: an outer
layer, which includes the white, opaque sclera
and the transparent cornea; a middle vascular
layer called the uvea, which includes the choroid,
ciliary body, and iris; and an inner neural layer,
which includes an outer pigment epithelium
and the inner, light-sensitive retina, which is
connected to the optic nerve.
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A conjunctiva lines the inner surface of the
eyelids and covers the eyeball to the junction of
the cornea and sclera. Conjunctivitis, also called
red eye or pink eye, may result from bacterial or
viral infection, allergic reactions, or the injurious
effects of chemical agents, physical agents, or
radiant energy.
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Keratitis, or inflammation of the cornea, can be
caused by infections, hypersensitivity reactions,
ischemia, defects in tearing, or trauma.Trauma
or disease that involves the stromal layer of the
cornea heals with scar formation and permanent
opacification.These opacities interfere with the
transmission of light and may impair vision.
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The lens is a biconvex, transparent, flexible
structure that can change shape to allow precise
focusing of light on the retina. Refraction refers to
the ability of the lens to bend light rays in order
to focus an object on the retina. In hyperopia,
