Mystery Case: Scalpel sign

Abstract

A 44-year-old man presented with 1 year of gradually progressive lower limb weakness and numbness and bowel and bladder incontinence. Neurologic examination revealed a T8 spinal sensory level and spastic paraparesis. Preoperative imaging (figure) of the thoracic spine revealed the scalpel sign, a radiologic entity diagnostic of a dorsal thoracic arachnoid web.1 The lesion was surgically and histologically confirmed as an arachnoid web. Clinical deficits and the cord signal abnormality resolved postoperatively. Arachnoid webs are bands of arachnoid tissue extending to the pial surface, causing focal dorsal indentation of the thoracic cord. Early identification followed by surgery appears to result in good functional recovery.

Spinal arachnoid web—a distinct entity of focal arachnopathy with favorable long-term outcome after surgical resection: analysis of a multicenter patient population

“…Accurate diagnosis, in particular, the differentiation from other arachnopathies is crucial, as progression can result in severe disability in untreated patients. In conventional MRI studies, diagnostic criteria rely on the so-called "scalpel sign" [18][19][20][21][22] which is a characteristic focal dorsal indentation of the spinal cord resembling a scalpel blade. This finding, first described by Reardon et al in 2013 22 , is considered pathognomonic of SAW.…”