Presentation
Three weeks of worsening back pain, ataxia and lower limb sensory changes.
Patient Data
At the T6 level, there is prominence of the dorsal arachnoid space 6 mm in AP diameter, with pulsation artifact. There is an associated deformity of the cord, which is displaced ventrally. The cord is thinned to an AP diameter of 3 mm at this level (normal cord measures 5 mm in AP diameter above this level).
Below the lesion, there is T2 hyperintensity of the cord measuring 7 cm in length, extending to the T8-9 level. No post-contrast enhancement to suggest tumor.
Conclusion:
T6 cord deformity with ventral displacement of the cord. Differential includes a dorsal arachnoid web or ventral cord herniation. This is associated with T2 hyperintensity of the cord inferior to this. No evidence of a spinal cord tumor.
Intrathecal contrast is most marked within the thoracic spine. Deformity and ventral displacement of the spinal cord which is most marked at the level of T6. Prominence of the CSF space posteriorly at this level which measures 8 mm in AP diameter. Small volume CSF is evident anterior to the spinal cord at this level.
Multilevel degenerative change of the spine with prominent bridging anterior osteophytes in the thoracic spine as well as facet joint arthropathy with partial ankylosis of the upper thoracic facet joints.
Conclusion:
Deformity and ventral displacement of the spinal cord at the level of T6. The presence of CSF anterior to the cord at this level excludes spinal cord ventral herniation and favors a thoracic arachnoid web.
Intra-operative longitudinal ultrasound of the spinal cord at the level of the lesion confirms the presence of intra-dural tissue indenting the cord (red arrow). Post-adhesiolysis, the cord deformity has resolved and the spinal cord is pulsating freely within the CSF.
US images courtesy of Mr Jin Tee.
Case Discussion
The patient went on to have surgery that confirmed the diagnosis of a dorsal arachnoid web.
Histology:
Macroscopic description: Labeled "intradural lesion". A sheet of grey-pink tissue 15x7x<1mm.
Microscopic diagnostic opinion: Dural connective tissue with adherent arachnoid tissue. No significant pathology.
Discussion:
Dorsal arachnoid webs are rare entities that can present with a variety of symptoms including pain and lower limb neurological symptoms 1. Findings on MRI include a focal cord deformity with expanded dorsal CSF space. Differentials of this appearance include ventral cord herniation or an arachnoid cyst. However, the presence of pulsation artifacts makes arachnoid cysts less likely 1,2.
Differentiating between a dorsal arachnoid web and ventral cord herniation can be difficult. In a dorsal arachnoid web, the expanded CSF is said to resemble the shape of a scalpel, giving rise to the aptly named "scalpel sign" 2.
In equivocal cases (as in this case), a CT myelogram is useful to differentiate between these two conditions by looking for the presence of contrast dorsal to the cord 1.
An informal intra-operative US further corroborated the CT findings and histology confirmed the lesion to be an arachnoid web.
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