Severe resting clonus caused by thyrotoxicosis in a 16-year-old girl with hereditary spastic paraparesis: a case report.

A 16-year-old girl with a history of hereditary spastic paraparesis developed the subacute onset of severe clonus and new upper-extremity spasticity due to thyrotoxicosis. This case demonstrates the ability of hyperthyroid states to worsen symptoms of spasticity in a child with an underlying spastic disorder. It also demonstrates the importance of investigating for common systemic diseases as a cause of movement disorders even in children with underlying known genetic disorders.