Midline septa in the lumbo-sacral thecal sac: acquired abnormality or developmental anomaly? The equivalent of diastematomyelia occurring below the spinal cord?

Careful analysis of over 400 patients radiologically diagnosed with arachnoiditis showed a subgroup of eight patients with a specific set of imaging signs that suggest a pre-existing developmental abnormality mimicking the appearance of arachnoiditis. These cases have been collected over a 15-year period. No autopsy or surgical proof is available. All patients have (i) the presence of a well-defined midline septum in the sagittal plane that splits the thecal sac, usually symmetrically, in the lower lumbar or sacral region, below the level of the spinal cord; (ii) the absence of adjacent epidural fibrosis that one would expect if the septation had been caused by surgical intervention or trauma; (iii) the preservation of cerebrospinal fluid within the nerve root sheaths and around the nerve roots at the level of the septation; and in four cases (iv) the presence of either calcification or bone in this midline septum, yet nowhere else in the theca or epidural space. These cases suggest a developmental variant called, for want of a better term, 'split lumbo-sacral thecal sac' analogous to diastematomyelia but not involving the spinal cord. This variant is potentially confused with arachnoiditis or thecal scarring in the lumbo-sacral sac.