The optic canal, in particular its intracranial end, represents a "locus
minoris resistentiae" for optic nerve compression in a variety of
pathologic conditions. The intracranial optic nerve shares the limited
space within this narrow passage with the carotid and ophthalmic artery,
all being surrounded by bone and rigid dura. Any pathological condition
going along with an increase of soft tissue volume, such as in optic
nerve sheath tumors, parasellar neoplasms, dolichoectasia of the carotid
and/ or ophthalmic artery, hematomas, etc., or reduction of the lumen of
the bony optic canal by hyperpneumatization of the sphenoid sinus,
hyperostosis or developmental abnormalities must act as a
space-occupying lesion causing optic nerve compression either by
pressing the nerve against the vessel or the neighboring dura or bone.
The spectrum of clinical signs and symptoms of optic nerve compression
in this area is rather wide and includes acute as well as slowly
progressive visual loss and all kinds of visual field defects in the
presence of a normal disk, papilledema, pri mary optic atrophy or
cavernous optic atrophy mimicking var ious clinical disease entities
such as retrobulbar optic neuritis, anterior and posterior ischemic
optic neuropathy, soft glaucoma and others. Some of the lesions causing
optic nerve compression in this area are rather small and need to be
visualized or excluded by thin section CT such as pneumosinus dilatans
of the sphenoid bone, dolichoectasia of the internal carotid artery,
small men ingiomas around the optic foramen and others."