Mucous cysts of the external popliteal sciatic nerve during childhood: presentation of two cases and a review of the literature.

Mucous cysts of the peroneal head constitute the most frequent cause of compression of the external popliteal sciatic nerve (EPS). Depending on the site in which it occurs, we distinguish between intranervous cysts, which are very rare, and extranervous ones, which are more frequent. The two varieties, which constitute anatomical variants of the same lesion and, probably, of a metaplasia of the connective tissue, are, from an anatomical point of view, true cysts, with their own connective walls, endothelial covering, and mucoid contents. Clinically, there is pain, paresthesia, a palpable mass, and motor deficit, all occurring at an early stage of the disease, depending on its site and size. At times, when there is no palpable mass, there may be problems with differential diagnosis with regard to disc compression. Ultrasound examination, electromyography, and, above all, MRI, have all proven to be useful in the study of the features of the neoformation; nonetheless, in our experience, they were not able to reveal either intra- or extranervous localization.