Rudimentary meningocele of the skin. Clinicopathologic features and differential diagnosis.

Although "rudimentary meningocele" (RM) or "meningothelial hamartoma" of the skin is a seemingly recently described entity, it has been included in past reports as a variant of primary cutaneous meningioma. We document our experience with four such lesions and compare the histologic and immunohistochemical features of these cases with those of seven classic meningoceles (CM) and four giant cell fibroblastomas (GCF). Although all of these entities share significant points of microscopic similarity, RM and CM are lesions composed of meningothelial cells, whereas GCF is probably of myofibroblastic origin. Rudimentary and classic meningoceles demonstrate cellular immunoreactivity for vimentin and epithelial membrane antigen, whereas the cells of GCF lack the latter determinant and may express muscle-specific actin. Our observations suggest that RM and CM represent closely related developmental malformations; however, RM becomes clinically apparent in a somewhat older patient population than CM and is not associated with major skeletal anomalies as may be found with CM. The distinction between various cutaneous meningothelial proliferations has prognostic importance, as does their separation from GCF. RM and CM are adequately treated by simple excision, whereas GCF, a probable form of fibromatosis, has the potential for local recurrence.