Discussion Author: Trudi Kim Nguyen
Intracranial dermoids are slow growing, congenital, cystic masses that contain not only squamous epithelium, as found in an epidermoid, but also sweat glands, hair, and sebaceous glands. Dermoid cysts(and epidermoid cysts) are not true neoplasms but are inclusions of ectoderm within the neural tube during its closure from the third to fifth week of embryonic development.
Intracranial dermoids are rare. The posterior fossa is the most common location for intracranial dermoids, which can be found anywhere in the CNS. The lesions are most often midline and can have an intra- or extra-axial location. When in the posterior fossa dermoids often have a sinus tract to the skin(dermal sinus) along with a defect of the overlying skull. The dermal sinus can allow the dermoid to become infected by providing direct access to overlying bacteria on the skin.
Presenting symptoms can include headache, seizures, and cranial nerve deficits. The most common presentation is meningitis. There can be a painless lump under the scalp. Dermoid cysts are usually discovered during the first three decades of life with posterior fossa lesions often presenting in infancy and early childhood. Infection or rupture of a dermoid will lead to acute presentation of symptoms.
The CT appearance of a dermoid cyst is a unilocular, low attenuation, usually hetergeneous, well circumscribed mass. Hounsfeld Units below 0 help comfirm the presence of lipid. A heterogeneous nodule within the mass may represent matted hair. On MR a dermoid will be a hetergeneous mass with multiple regions of high T1 signal due to the liquified fat. If the dermoid has ruptured the high T1 signal will be scattered within the subarachnoid space.
Treatment consists of surgical excision.
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