Posterior vertebral body erosion by arachnoid diverticula in cauda equina syndrome: an unusual manifestation of ankylosing spondylitis.

Cauda equina syndrome is an uncommon complication of longstanding ankylosing spondylitis. It is associated with dorsal arachnoid diverticula, which may erode the lamina and spinous processes of the bony lumbosacral spine. We describe a patient who developed cauda equina syndrome associated with the unusual finding of erosion of the posterior aspect of 2 vertebral bodies by arachnoid diverticula. This was clearly revealed by magnetic resonance imaging of the spine.