Case: A 64-year-old male presents with a three-year history of progressive back pain, lower limb weakness and unstable gait. He experiences no bowel or bladder difficulty and there is no history of prior trauma or spinal surgery. Physical examination revealed lower limb weakness with spasticity.
Findings: A focal dorsal indentation of the thoracic spinal cord is present at the T6 level, with associated compression and deformity of the cord which is displaced anteriorly (Images A-C).
Extensive abnormal cord signal abnormality is present below the level of the indentation extending to the T8 level (Image B).
No abnormal enhancement is present on the post contrast image (Image D).
The CT myelogram shows the “upside down scalpel sign” with the characteristic focal indentation of the dorsal thoracic spinal cord with widening of the dorsal cerebrospinal fluid space, and cord expansion below the level of the dorsal indentation (Image E, pink arrow).
Corresponding axial CT myelogram image at the level of the dorsal indentation (Image F, green arrow) demonstrates a remaining thin CSF space between the anterior cord and the ventral theca. This subtle but key finding helps exclude a diagnosis which is almost identical visual on MRI, namely spinal cord hernia through an anterior rent in the dura. Image G is an axial image below the level of the indentation.
Diagnosis: Dorsal thoracic arachnoid web with spinal cord compression.
Differential: Ventral cord herniation; Dorsal arachnoid cyst (simulates thoracic cord hernia).
Discussion: Spinal arachnoid webs represent intradural extramedullary bands of arachnoid tissue that extend to the pial surface of the spinal cord. They typically occur in the thoracic spine producing a focal indentation on the dorsal spinal cord.
The characteristic dorsal indentation of the cord has been described as the “scalpel sign,” due to the resemblance on sagittal imaging to a scalpel, with the blade pointing posteriorly.
The differential in this case is that of ventral spinal cord herniation, or a dorsal arachnoid cyst. In ventral spinal cord herniation, there is deformity on the ventral surface of the cord as it protrudes through a ventral dural defect, and there is NO space between the cord and the ventral theca. Arachnoid cysts can be identified on imaging by their marginated thin but low signal walls, and they produce a relatively smooth scalloping on the cord surface. On CT myelography an intraspinal filling defect or delayed filling of the arachnoid cyst is sometimes present.
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Editor’s note: This case was submitted by Dr. Tiffany So, who was selected for a scholarship for our upcoming Melbourne Case Review course. Congratulations to Dr. So!