Neuromuscular choristoma (hamartoma) with smooth and striated muscle component: case report with immunohistochemical and ultrastructural analysis.

Benign peripheral nerve tumors with a mesenchymal component are rare and are represented principally by the neuromuscular choristoma. We describe an 11-month-old male infant who presented with a mass arising from the left brachial plexus. The lesion was bound firmly to the involved nerves and consisted histologically and ultrastructurally of bundles of well-differentiated smooth muscle cells intermingled with striated muscle fibers and unmyelinated nerve fibers. In the 13 previously published cases of neuromuscular choristoma, no smooth muscle component was observed. This unique peripheral nerve choristoma with not only striated but also smooth muscle closely resembled neuromuscular choristoma and was considered a variant of it.