Anterior dural ectasia mimicking a lytic lesion in the posterior vertebral body in ankylosing spondylitis.

Anterior dural ectasia is an extremely rare finding in ankylosing spondylitis (AS). The authors describe a unique case of AS in which the patient presented with cauda equina syndrome as well as an unusual imaging finding of erosion of the posterior aspect of the L-1 (predominantly) and L-2 vertebral bodies due to anterior dural ectasia. Symptomatic patients with long-standing AS should be monitored for the presence of dural ectasia, which can be anterior in location, as is demonstrated in the present case.