 
Optic nerve head
drusen are inherited dysplasia of the optic disc and blood supply (fig.
9) and may simulate papilledema. CT must be used to detect calcified
burried drusen and to exclude intracranial mass (ref. 5).
Spiral CT with reformations is an
important tool for the assessment of traumatic optic neuropathy in
emergency. Secondary to avulsion of the optic disc, intraorbitary
foreign bodies, optic canal fracture with impingement of the nerve or
hematoma of the optic sheath they need rapid surgical decompression or
steroïd therapy and so visual recovery may be obtained.
The most commun cause of the
demyelinating type of optic neuritis is multiple sclerosis (fig. 10)
and its particular form called Devic Syndrome (optic neuritis and
myelitis). Visual loss is typically unilateral whereas optic neuritis
is bilateral in acute disseminated encephalomyelitis. LEAD and other
vasculitis are responsable of auto-immune neuritis. Infective causes
include syphilis, toxoplasmosis, toxocariosis and Lyme desease. The
optic tract has a higher sensitivity to radiation than other cranial
nerves and radiation optic neuritis can still be observed (fig. 11). MR
is more sensitive than CT in the detection of optic neuritis (ref. 6).
66 % of all primary optic nerve
tumors are represented by gliomas (fig. 12). They are mostly seen in
children between 2 and 8 years old. Bilateral glioma is pathognomonic
of NF1 (10 to 30 % of optic gioma have NF1 and 15 to 40 % of NF1
patients have optic glioma). NF1 optic gliomas are characterized by
thickening of the optic nerve with moderate enhancement, possible
spontaneous regression and association with dysplasia or hamartoma of
the cerebellum and pallida. On the contrary, non NFgliomas have a
mass-like appearance, have a chiasmatic and hypothalamic involvement
and a poor prognosis despite their low grade histology : pilocytic
astrocytomas (ref. 7). Adult form is much less common and has a
malignant behaviour (high grade astrocytomas).
Meningiomas of the optic nerve arise
from meningothelial cells of the arachnoïd of the optic nerve sheath or
from extension of an intracranial meningioma into the orbit. They get a
distinct female predominence and bilateral meningioma may occur in
patient with or without neurofibromatosis. Retrobulbar lesions extend
extradurally early in their development, avoiding compression of the
nerve and cause progressive exophthalmos. Posterior meningioma, in the
contrary, confined within the dura cause chronic compression of the
optic nerve and vision loss (fig. 13). If MR is the study of choice, CT
may show calcifications highly suggestive of meningioma, whereas
“tram-track” sign can be observed in pseudo-tumors (fig. 17), neuritis
(fig.10 and 11), sarcoidosis and lymphoma.
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