• Michael M Hakky, Almamoon I Justaniah, Carlos David, Robert J French, Dann Martin, Nathan Kwok, and Sami H Erbay.
• Department of Radiology, Lahey Clinic Medical Center, Burlington, Massachusetts.
• J Neuroimaging. 2015 Sep 1; 25 (5): 818-23.

BackgroundArachnoid cysts and meningeal membranes are among the differential diagnostic considerations of extra-medullary causes of thoracic myelopathy. In this case series of 7 patients, we present compressive meningeal membranes mimicking dorsal arachnoid cyst. The propensity of the meningeal membranes for the dorsal aspect of upper thoracic spine may reflect derangements of the septum posticum.ObjectiveTo provide the spectrum of imaging appearances and clinical presentations of pathology of the septum posticum to improve imaging utilization and to better guide treatment planning.MethodsSeven patients aged 40 to 75 with MRI findings of ventral displacement and dorsal cord compression in the upper thoracic spine were further evaluated with CT-myelograms. The primary indication was to exclude dorsal arachnoid cyst. Two patients with progressive symptoms and lower extremity weakness were operated for decompression.ResultsCT-myelogram excluded space occupying lesions and cord herniation in all cases. Intradural dorsal meningeal webs and membranes were inconsistently visualized. In the 2 operated cases, thick coalescing membranes and hyperdynamic turbulent CSF flow were severely compressing the thoracic cord.ConclusionDerangements of septum posticum may present a spectrum of findings that should be considered in the differential of thoracic myelopathy. Flattening of the posterior cord margin is a reliable imaging clue for a dorsal extra-medullary compressive lesion. Cord compression results from combination of adhesive membranes and turbulent CSF flow. The clinical course may be difficult to predict. Periodic imaging follow up can be helpful to confirm stability of findings in expectantly managed cases.Copyright © 2015 by the American Society of Neuroimaging.

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