Postoperative spinal subdural hygroma without incidental durotomy: illustrative cases

BACKGROUND: Spinal subdural hygroma (SSH) is a rare pathological entity occurring as a complication of spinal surgery. It is different from spinal subdural hematoma due to blunt trauma, anticoagulation therapy, spinal puncture, and rupture of vascular malformations. OBSERVATIONS: The authors presented five patients with SSH who received decompression for lumbar stenosis. None had incidental durotomy. All presented postoperatively with unexpectedly severe symptoms, including back and leg pain and weakness. Postoperative magnetic resonance imaging (MRI) revealed SSH with a characteristic imaging finding termed the “flying bat” sign. Four patients underwent evacuation of SSH, with immediate and complete resolution of symptoms in three patients and improvement in one patient. One patient improved without additional surgery. At surgery, subdural collections were found to be xanthochromic fluid in three patients and plain cerebrospinal fluid (CSF) in one patient. LESSONS: Unexpectedly severe back and leg pain and weakness after lumbar or thoracic spine surgery should raise suspicion of SSH. MRI and/or computed tomography myelography shows the characteristic findings termed the flying bat sign. Surgical evacuation is successful although spontaneous resolution can also occur. The authors hypothesized that SSH is due to CSF entering the subdural space from the subarachnoid space via a one-way valve effect.