Arachnoid cyst - thoracic

Case contributed by Frank Gaillard
Diagnosis certain

Presentation

Trauma. Back pain.

Patient Data

Age: 55 years
Gender: Male
mri

Prior CT (not shown) demonstrated a fracture through the T3 spinous process and left inferior articular process, with compression injuries deep to the superior endplates of the T3 and T4 vertebral bodies.

On MRI, there is marked posterior paraspinal muscle edema extending from the cervical region to T6. Abnormal marrow edema deep to the superior endplates of T2, T3 and T4 vertebral bodies is consistent with microtrabecular compression fractures. No evidence of intervertebral disc or longitudinal ligament disruption, although there is a trace of prevertebral hematoma anterior to the compression fractures.

There is an abnormal configuration of the thoracic cord, which at T4 level is focally displaced anteriorly to butt against the posterior aspect of the vertebral body. The posterior CSF space is enlarged, without evidence of epidural hematoma or CSF leak.

A targeted CISS sequence was performed which did not demonstrate cord rotation or ventral herniation. Instead, there is the impression of a dorsal arachnoid cyst whose superior margin is at the level of the mid T4 vertebral body and in axial dimension measures up to 14 x 8 mm. Craniocaudal extent is difficult to assess, but is thought to measure least 8.5 cm in length.

No abnormal cord hyperintensity is identified.

Case Discussion

Although in the setting of an acute fracture, ventral dural defect and cord herniation should be suspected, higher resolution imaging in this instance clearly demonstrates the upper margin of an an arachnoid cyst (or wind sock arachnoid web).  

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