Facts on File Encyclopedia of Health and Medicine

focused beam of light that illuminates the struc-
tures of the eye. Ophthalmoscopy is the essential
introductory examination of the eye and can
determine what, if any, further diagnostic proce-
dures are necessary. Ophthalmoscopy allows the
doctor to examine the inner surfaces of the eye-
lids, general surface of the eye (sclera and
CORNEA), pupil response, and iris. It also allows the
doctor to visualize the inner structures at the back
of the eye, notably the RETINA, optic disk, and
macula.
See also OPHTHALMIC EXAMINATION; OTOSCOPY; SLIT
LAMP EXAMINATION; TONOMETRY.

optical coherence tomography (OCT) An imag-
ing procedure that noninvasively and painlessly
permits the ophthalmologist to visualize the layers
of the RETINA. OCT can provide a “virtual biopsy”
of retinal tissue, helping diagnose or monitorAGE-
RELATED MACULAR DEGENERATION (ARMD), macular
holes, retinal tears, and OPTIC NERVEinflammation
or damage such as can result from GLAUCOMA. The
ophthalmologist can perform OCT in the office; no
preparation or recovery is necessary.
See also ELECTRORETINOGRAPHY.

optic nerve The second cranial NERVE, which
conveys nerve impulses from theEYEto the BRAIN.
There are two optic nerves, one from each eye.
The fibers that become the optic nerve originate in
the occipital lobes of the cerebrum, in an area
called the visual cortex. Each extends along struc-
tures called the optic tracts that pass through the
temporal lobes and the center of the brain, con-
verging in the optic chiasm. At this point the optic
tracts cross, such that the one originating in the
left visual cortex goes to the right eye and the one
originating in the right visual cortex goes to the
left eye. Each optic nerve enters the back of the
eye, terminating in the RETINA.
The ophthalmologist can see through the oph-
thalmoscope the end of the optic nerve, called the
optic disk. It appears as a pale circle, about the size
of a pencil eraser, against the dark background of
the retina. The retina’s network of nerves extends
from the optic nerve, gathering nerve impulses
from the rods, cones, and other nerve cells in the
retina.

CONDITIONS THAT CAN AFFECT THE OPTIC NERVE

aging GLAUCOMA

ISCHEMIC OPTIC NEUROPATHY TOXIC OPTIC NEUROPATHY

PAPILLEDEMA PAPILLITIS

RETINAL DETACHMENT RETINITIS PIGMENTOSA

RETROBULBAR OPTIC NEURITIS RETINOPATHY

For further discussion of the optic nerve within

the context of ophthalmologic structure and func-

tion please see the overview section “The Eyes.”

See also AGING, VISION AND EYE CHANGES THAT

OCCUR WITH; CRANIAL NERVES; ENUCLEATION; OPHTHAL-

MOSCOPY.

optic nerve atrophy Death of NERVEcells within

the OPTIC NERVE, affecting the optic nerve’s ability

to convey nerve signals from the EYEto the BRAIN.

Optic nerve atrophy can be partial or complete;

when complete there is total loss of vision. Condi-

tions of the eye or systemic neurologic disorders

can cause optic nerve atrophy. Symptoms include

diminished VISUAL ACUITYandVISUAL FIELD.

CAUSES OF OPTIC NERVE ATROPHY

Eye Conditions Neurologic or

Systemic Conditions

congenitalOPTIC NERVE HYPOPLASIA MULTIPLE SCLEROSIS

ISCHEMIC OPTIC NEUROPATHY TRAUMATIC BRAIN INJURY(TBI)

congenitalCATARACT STROKE

RETINITIS PIGMENTOSA methanol poisoning

GLAUCOMA untreatedSYPHILIS

The diagnostic path begins with OPHTHAL-

MOSCOPY, which allows the ophthalmologist to see

the visual changes in the optic disk (end point of

the optic nerve where it joins the RETINA) that

denote its atrophy. Further assessment to deter-

mine the cause may include diagnostic imaging

procedures such as COMPUTED TOMOGRAPHY (CT)

SCANor MAGNETIC RESONANCE IMAGING(MRI) and a

comprehensive NEUROLOGIC EXAMINATION. Treat-

ment targets the underlying cause, though it can-

not recover vision already lost. Treatment that can

halt the causative condition can prevent further

loss of vision, though when the cause is a degen-

erative disorder such as MULTIPLE SCLEROSISvision

loss is likely to continue.

optic nerve atrophy 105