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The optic nerve is the structure at the back of the eye that
carries visual information from the eye to the central nervous system
(CNS). Approximately 120 million rods and cones sense light in the outer
retina. This information is transmitted to approximately 1 million
ganglion cells in the inner retina. These ganglion cells converge
to form the optic nerve. The electrical impulses generated in the
optic nerve are transmitted to the occipital lobe via the lateral
geniculate nucleus. Posterior to the eye, the optic nerve is covered
by a dural sheath and arachnoid membrane. This structure surrounds
the nerve until it enters the brain, where it is contiguous with
the subdural space (Figure 33–1).
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The only portion of the optic nerve that is visible on examination
is the site where the nerve attaches to the eye at the posterior
retina (Figure 33–2). The normal nerve has a central area through
which the central retinal artery travels, branching into vessels
lining the inner layer of the retina. The central portion is called
the cup, and the entire area is called the disc. The cup:disc ratio describes the relationship
between these 2 structures. The normal ratio ranges from 0.1 to
0.4, although larger cups may be normal in certain ethnic populations,
particularly African Americans. Increased intraocular pressure may
cause enlargement of the cup, and increased intracranial pressure
(ICP) may cause edema and obscuration of the cup.
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Optic nerve problems in children occur infrequently. However,
bilateral optic nerve hypoplasia (ONH) is one of the common causes
of profound visual loss in infancy.
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The optic nerves form from the optic stalk, the structure that
connects the embryonic forebrain and optic vesicle. It contains
an outer layer of cells from the neural crest, which differentiates
into the structures of the sheath that surround the optic nerve
(pia, arachnoid, and dura). An inner neuroectodermal layer regresses
and is replaced by ganglion cells that originate in the inner layer
of the retina and travel through the optic nerve to the lateral
geniculate body. At 4 months of gestation, almost 4 million ganglion
cells are present within the nerve. This number decreases to 1 million
by the time of birth. Myelinization of the optic nerve occurs ...