Rare dorsal thoracic arachnoid web mimics spinal cord herniation on imaging

Case summary

A 35-year-old female presented with a 1-year history of progressive upper back pain, bilateral lower extremity weakness, and numbness. The thoracic MRI images showed a focal area of ventral cord displacement (e.g. the “scalpel sign” at T4) and a T2-T4 syringomyelia. The CT myelogram documented a possible ventral dural defect supporting the diagnosis of ventral cord herniation [Figures 1 and 2]. Intraoperatively, the patient was found to have a dorsal thoracic web; there was no cord herniation [Figure 3]. Postoperatively, the patient remained neurologically stable and the MR scan 8 weeks later showed a significant reduction in the syrinx size.

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Figure 1:: Magnetic resonance imaging of the thoracic spine (sagittal view) shows the syrinx with dorsal indentation of the cord (arrow).

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Figure 2:: Computed tomography myelogram of the thoracic spine (sagittal view) shows the ventral displacement of the cord (arrow).

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Figure 3:: Intraoperative microscopic view (50×) shows dilation of the dorsal arachnoid with the dorsal arachnoid web.

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Magnetic resonance imaging

For DAW, SAC, and VSCH, thoracic MRI’s usually shows an enlarged dorsal subarachnoid space with ventral shifting of the spinal cord. With DAWs, there may also be accompanying attenuation of the CSF signal dorsal to the cord on T2-weighted images (e.g. CSF flow artifact). Also, a dorsal indentation of cord (e.g., the “scalpel sign”) and/ or an abnormal lesion on T-2 axial images in the dorsal subarachnoid space.[1-3,6] Further supporting, the diagnosis of DAW is CSF flow quantitative study using a cardiac- gated phase-contrast cine-mode that can show a marked reduction in rostral CSF flow in the dorsal subarachnoid space.[1] Alternatively, the diagnosis of VSCH would be better confirmed by an MR or myelo-CT documenting a segment of cord parenchyma extending outside the dura mater, while for SAC, there would be no such pathognomonic MR signs.[1,5-7]

Computed tomography myelogram

For DAW, there are no pathognomonic signs on CT myelogram.[1-7] Alternatively, with VSCH, the CT myelogram shows herniation of the cord through a dural defect with no contrast seen anterior to the cord itself while for SAC; the flow of contrast on a myelo-CT study is interrupted by a space-occupying lesion in the dorsal subarachnoid space.[1,7]

Combined MR and Myelo-CT in the Diagnosis of DAW, SAC, and VSCH

Combined, MR and myelo-CT studies better help differentiate DAW from SAC and VSCH. Reardon et al. indicated that the presence of the positive “scalpel sign” on MRI was highly suggestive for DAWs, as was the accompanying documentation of a syrinx.[5] For VSCH, the preoperative MRI alone usually proved diagnostic in 30% of patients, while SAC, MRI, and CT myelography combined may be diagnostic in 68% of cases.[1-7]

Here, although the MRI showed a positive “scalpel sign” consistent with a DAW, the CT myelogram was interpreted as showing a VSCH. Notably, this “misdiagnosis” could have significantly impacted surgery, as DAWs are usually approached posteriorly, while VSCH is more frequently approached anteriorly.