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contains not only astrocytes but also specialized cells associated with the
lamina cribrosa (
Hernandez, Igoe, &Neufeld, 1988
). The effects of ischemia
on them and the geometry of the optic nerve head axons, vasculature, and
supporting cells are likely different from the posterior optic nerve.
2.3. Optic neuritis
The most common acute optic neuropathy in young adults is optic neuritis
or inflammatory optic neuropathy. In the majority of cases, optic neuritis is
associated with multiple sclerosis, either as the harbinger or occurring in pa-
tients with diagnosed disease. Optic neuritis can also be idiopathic or asso-
ciated with other inflammatory diseases such as sarcoidosis and systemic
lupus erythematosus. Clinically, patients with optic neuritis usually have vi-
sual loss affecting some or all of the central visual field, with consequent
decreased visual acuity. Dyschromatopsia is common, most likely the result
of relatively greater involvement of axons of color-coding retinal ganglion
cells. The degree of color vision loss in optic neuritis is greater than that in
ischemic optic neuropathy and glaucoma, suggesting that there is differential
involvement of subgroups of axons among the three diseases. Presumably,
the effects of inflammation, ischemia, and glaucoma are different enough
to cause different kinds of axonopathies in these three diseases and are a clue
to the complexity of the axonal response to injury.
Optic neuritis also differs significantly from glaucoma and ischemic optic
neuropathy in that it is associated with significant recovery of visual func-
tion, unlike the usually permanent visual loss associated with the latter
two. Recovery begins within a few weeks of disease onset, and in many
cases, recovery of vision is virtually complete. The reasons for the excellent
recovery are unclear. One possibility is that the demyelination associated
with the inflammation results in conduction block, which would be ame-
liorated by remyelination. Another is that the inflammation itself causes con-
duction block. Neither of these reasons explains the absence of significant
visual defects despite sometimes considerable axonal loss from the acute
episode (
Costello et al., 2006
).
Neuromyelitis optica is a disease that includes an inflammatory optic
neuropathy closely related to typical optic neuritis, but where recovery is
uncommon without treatment. In many cases, there are antibodies to the
water transporter aquaporin-4 which could inhibit or increase its activity
and/or reflect the misexpression of the transporter, in either case raising
the possibility that the intracellular water content of axons or nearby glia
