Dorsal thoracic arachnoid web presenting as neuropathic pain: 'Scalpel' sign found on MRI.

Dorsal thoracic arachnoid webs are due to a deformity in the formation of arachnoid membrane in the spinal arachnoid space. These webs usually occur in the upper thoracic spine and are viewed on imaging as a pathogonomic 'scalpel' sign because of the resemblance on sagittal MRI to a surgical scalpel. We describe a case of a patient with a neuropathic pain presentation. After MR imaging, a focal dorsal indentation of the upper thoracic spinal cord at the T3-T4 level with the scalpel sign was found. This sign indicates that the patient's neuropathic pain was caused by the dorsal thoracic arachnoid web.