"On the other hand": a case of hereditary, congenital mirror movement.

We describe a 20-year-old male Singaporean army recruit with hereditary, congenital mirror movements who presented with difficulties in military training because releasing the grip of one hand resulted in a similar release by the other hand. His father has mirror movements with a significant decrease in symptoms with time, a phenomenon that has not been previously described. Both father and son have no associated neurological abnormalities. Bilateral cortical representation for hand muscles and the presence of active ipsilateral corticospinal projections have been postulated as the mechanisms responsible for this condition.