Sphenoid pneumosinus dilatans. A sign of meningioma originating in the optic canal.

Surgical exploration of three patients who had a radiological diagnosis of sphenoid pneumosinus dilatans disclosed an intracanalicular meningioma of the sheath of the optic nerve (MSON), bilaterally in two patients and unilaterally in the other. Two cases of sphenoid pneumosinus dilatans, previously reported, are reviewed, and a new case is presented. The relationship of sphenoid pneumosinus to MSON is discussed. Sphenoid pneumosinus dilatans may be an early sign of MSON in the optic canals or at the optic foramens.