Spinal Extradural Arachnoid Cyst (Nabors Type IA)

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Case: Spinal Extradural Arachnoid Cyst (Nabors Type IA)

Imaging Modality: MRI Thoracolumbar Spine Sequences Provided: Sagittal T2, Axial T2, Sagittal T1

Radiologic Findings: The MRI demonstrates a well-circumscribed, unilocular cystic lesion located within the posterior extradural space of the lower thoracic spine, at approximately the T11-T12 level. The lesion follows cerebrospinal fluid (CSF) signal characteristics across all pulse sequences, appearing uniformly hyperintense on T2-weighted images and hypointense on T1-weighted images.

Spanning approximately 24 mm in craniocaudal dimension, the cyst exerts focal mass effect upon the thecal sac. It causes significant anterior displacement and compression of the distal spinal cord. The axial T2-weighted sequence confirms its posterolateral extradural position, illustrating the resultant flattening of the dural sac. There are no internal septations, complex features, or solid components visible.

Impression: The location, morphology, and classic CSF-isointense signal behavior are diagnostic of a Spinal Extradural Arachnoid Cyst (SEAC). According to the Nabors classification system, this represents a Type IA lesion: an extradural meningeal cyst without the involvement of spinal nerve root fibers.

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