Progressive Unilateral Foot Drop in a Child due to Neuromuscular Choristoma

An 8.5-year-old boy presented with a history of progressive unilateral foot drop. A contrast magnetic resonance imaging of the pelvic girdle and the lumbosacral plexus revealed a diffuse fusiform thickening of the right lumbosacral trunk and the sciatic nerve in the intrapelvic and gluteal regions, with findings characteristic of a choristoma. Clinical and radiologic features as well as complications of this unusual entity are described.