Orbit and optic nerve.

The optic nerve is a fiber tract of the brain, and is not a true cranial nerve. Primary pathologic processes seen in the central nervous system and its linings can therefore affect the optic nerve. Along their course the fibers of the visual pathways are also susceptible to secondary extrinsic processes. The MR appearance of many of the primary and secondary processes that affect the optic nerve are characteristic. Where the imaging features of a process are not characteristic, a fairly narrow differential diagnosis can be suggested depending on the imaging appearance and anatomic location of the pathologic entity. This article is a categorical overview of the disease entities that may affect the optic nerve and tracts. Pathologic processes are organized according to the typical sites of involvement along the visual pathways, from the retina to the occipital cortex. Medical imaging, pathologic, clinical, and epidemiological highlights are presented.