Neonatal dystrophia myotonica. Electrophysiologic studies.

The diagnosis of dystrophia myotonica was established in a boy 3 hours old, and confirmed by family study and electromyography (EMG) at 5 days. Clinical features included hypotonia, facial diparesis, "tented" upper lip, and arthrogryposis of both knees. Percussion myotonia was elicited. The EMG disclosed characteristic bursts of electrical activity that waxed and waned on muscle percussion or needle movement. The EMG may be valuable in supporting the diagnosis of dystrophia myotonica in neonates suspected of having the disease.