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Background
Dorsal thoracic arachnoid webs are rare entities that arise over the dorsal aspect of the spinal cord, and exert extrinsic compression upon the cord. This exhibit presents two patients who were imaged one year apart at a community-based hospital and diagnosed with dorsal thoracic arachnoid webs. Clinical history, findings, and recommended surgical intervention. This exhibit highlights the early and later changes associated with an arachnoid web and educates clinicians on its presence. Few case reports have been described in the literature, and a knowledge gap still persists potentially due to under-recognition of such a rare clinical entity.

Educational Goals / Teaching Points
Educate radiology residents and attendings and nonradiology providers on a rare radiologic entity known as a dorsal thoracic arachnoid web. Discuss radiculopathy symptoms and potential curative treatments. Identify the associated distinct radiologic sign. Discuss a short differential diagnosis. Discuss short/long-term outcomes status after resection of an arachnoid web.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Dorsal thoracic arachnoid webs arise over the dorsal aspect of the spinal cord usually involving the thoracic cord level and exert focal extrinsic compression upon the spinal cord. There may or may not be an association with syrinx formation, depending on the degree of compression and chronicity. If there is longstanding distortion and compression of the cord, expansion of the cord secondary to an enlarging syrinx may also be seen. A distinct radiologic sign associated with this entity includes the scalpel sign due to focal dorsal indentation of the thoracic cord with ventral displacement of the spinal cord at that level. Findings best visualized on MRI T2 weighted sagittal and axial scans of the thoracic spine. The arachnoid web itself is not demonstrated on imaging, however, its resulting mass effect/distortion of the spinal cord are what direct radiologists to its presence. Normal or increased pulsation artifact may be present along the dorsal thecal sac, possibly secondary to altered CSF dynamics.

Conclusion
Dorsal thoracic arachnoid webs are less well-known entities in the field of radiology, though they are becoming more appreciated with increased education and awareness of the finding amongst neuroradiology specialists. In the appropriate clinical setting and supporting imaging findings, radiologists are advised to alert the patient’s provider regarding the likelihood of this entity, and as such, can have profound and meaningful impact on the patient’s health if addressed via neurosurgical intervention. Once diagnosis is suspected and/or confirmed, neurosurgical intervention can be curative, even in the setting of long-standing symptoms. Any associated syrinx may also likely resolve on follow-up MRI. The exact etiology of an arachnoid web remains to be unclear and further research in that regard will need to be pursued. Continued education into the myriad of entities in the field of spine radiology, both benign and/or malignant etiologies, will continue to help radiologists keen on advancing their knowledge base.