Neuro-Behçe Disease Causing Nuclear/Fascicular Forth Nerve Palsy.

Neuro-Behçe disease (NBD) has a predilection for affecting the parenchyma of the upper brainstem; however, involvement of the fourth nerve nucleus or fascicle by NBD has not been previously described. We report a case of a young man with acute right fourth nerve palsy and history of Behçet disease with an enhancing lesion in the left caudal midbrain corresponding to the left trochlear nerve nucleus/fascicle. This is the first described case of NBD producing nuclear/fascicular fourth nerve palsy. It also demonstrates an important clinicoanatomical correlate of decussation of fourth nerve fibers to the opposite side after exiting the midbrain.