Myotonic dystrophy is an autosomal dominant, inherited, progressive disorder affecting the muscles. The disorder is characterized by muscle wasting, weakness, and myotonia (delayed relaxation of muscle). Wasting of the temporalis and masseter muscles combined with frontal balding results in its characteristic appearance. Patients have other abnormalities including cataracts, heart conduction defects, dysphagia, excessive daytime somnolence, and mild cognitive impairment.

Myotonic dystrophy type 1 has distal weakness, with myotonia prominent in the hands; type 2 has proximal muscle weakness (also called PROMM or proximal myotonic myopathy). The signs and symptoms of myotonic dystrophy typically manifest in the third or fourth decade, although they can occur at any age.

Congenital myotonic dystrophy can occur in neonates born to affected mothers. These children have a severe phenotype with prominent weakness, hypotonia, skeletal abnormalities, feeding difficulties, and respiratory distress at birth.