A sacral arachnoid cyst causing holocord syringomyelia.

Spinal extradural arachnoid cysts (ACs) have an infrequent predilection for the sacrum. As with their counterparts in other regions of the spine, cysts in this location are mostly asymptomatic. Common presentations in symptomatic cases include pain in the low back or perineum, radiculopathy, and sphincteric dysfunction. The authors report a hitherto undescribed presentation in which the predominant symptoms are those related to an associated holocord syrinx. This 15-year-old boy presented with fluctuating, spastic paraparesis and a dissociated sensory loss in the trunk. Admission MR imaging of the spine showed an extradural AC from S-2 to S-4 and a holocord, nonenhancing syrinx. The patient underwent S-2 laminectomy, fenestration of the cyst, and partial excision of its wall. Intradural exploration revealed a normal-looking filum terminale and the absence of any dural communication with the cyst. At a follow-up visit 6 months after surgery, his motor and sensory deficits had resolved. Follow-up MR imaging showed complete resolution of the syrinx in the absence of the sacral AC. This is the first report of a sacral extradural AC causing holocord syringomyelia. Because conventional theories of syrinx formation were not helpful in elucidating this case, a hypothesis is postulated to explain the clinicoradiological oddity.