The optic neurogram: evaluation of CSF "block" caused by compressive lesions at the optic canal.

Patients with progressive blindness resulting from bony proliferative diseases such as osteopetrosis may benefit from optic canal decompression. A radiologic technique is described whereby the practicality of surgical optic canal widening is evaluated preoperatively using the intrathecal water-soluble contrast agent lopamidol. Conceptually, if a patient who is losing vision demonstrates a block on the optic neurogram, then likely there is still sufficient neural tissue within the optic canal and sheath to cause obstruction, and therefore decompression may be indicated. However, if preoperatively there is no block to lopamidol, then surgical intervention is not indicated, as the nerve has already undergone severe atrophy to the point of "autodecompression." Cases of differing etiologies, with and without blocks, illustrate the utility of the method and point out the importance of early surgical management to prevent irreversible damage to the optic nerve.