Dorsal Dermal Sinuses

      A form of occult spinal dysraphism, dorsal dermal sinuses are thin, epithelium-lined channels that open on the skin posteriorly in a hyperpigmented patch or a hairy nevus. The sinus tracts extend deep into the subcutaneous tissues, reaching the spinal canal in one-half to two-thirds of cases. The sinuses may be attached to the dura, causing tenting of the thecal sac. When they pass intradurally, they may end in the subarachnoid space, conus medullaris, filum terminale, a nerve root, a fibrous nodule on the surface of the cord, or a dermoid or epidermoid cyst. Roughly 50% of the dermal sinuses end in dermoid or epidermoid cysts. Conversely, 20% to 30% of dermoid cysts and dermoid tumors have associated dermal sinus tracts. The course of the sinus tract may be short or long, and varies from patient to patient. The tract may be lined by fat. The dermatome level of the sinus opening correlates with the metameric level of the cord where it attaches. Endnote

      Bony abnormalities vary from none when the tract penetrates at the level of a posterior ligament, to focal or multilevel spina bifida. When a dermoid or epidermoid is present, the nerve roots are frequently bound down to it. There may be a history of meningitis from extension of bacteria along the tract or from chemical irritation if the cyst ruptures.

      T1 or T2-weighted sequences often demonstrate the sinus tract coursing obliquely through the subcutaneous tissues. Dermoids are identified by their fatty components. Although epidermoids can be isointense with CSF on all MR pulse sequences, usually they have a more heterogeneous internal texture than CSF. Flow-sensitive sequences can increase the contrast between free-flowing CSF and the solid epidermoid tumor. In equivocal cases, myelography with CT can help clarify the diagnosis. 

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