Definition

The myotonias are a group of hereditary degenerative muscle diseases that include myotonic dystrophy, myotonia congenita (Thomsen disease), and paramyotonia congenita. A symptom common to all myotonias is the inability of skeletal muscles to relax after chemical or physical stimulation.

Myotonic dystrophy, also known as Steinert disease, myotonia atrophica, or myotonia dystrophica, is the most common and the most severe form of the myotonias. It is characterized by skeletal muscles that are hypoplastic, dystrophic, and weak yet prone to persistent contraction. Although muscles are primarily affected, myotonic dystrophy is distinguished from nondystrophic myotonias by being a multisystem disease.

Incidence

Myotonic dystrophy is inherited as an autosomal dominant trait. In most cases, an affected person has one affected parent. The onset of symptoms can occur at any age but usually occurs in the second to third decade of life. A slow, progressive deterioration of skeletal, cardiac, and smooth muscle occurs, resulting in death by the sixth decade. An estimated one in 20,000 people worldwide have the disorder, with an equal occurrence in males and females. The severity of clinical symptoms usually increases with transmission to subsequent generations (genetic anticipation). Myotonic dystrophy is the most common and severe inherited muscular dystrophy of adulthood.

Pathophysiology and treatment

Myotonic dystrophy is a disorder of muscle membrane excitability that results in self-sustaining runs of depolarization. Electrophysiologic studies show a lowered resting membrane potential in muscle cells from patients with myotonic dystrophy. Therapeutic agents used to treat the myotonic contractures include quinine, procainamide, and phenytoin. These agents delay the return of membrane excitation by blocking rapid Na⁺ influx into muscle cells. Regional anesthesia and muscle relaxants do not prevent or relieve the recalcitrant contraction. Dantrolene has also been ineffective in reversing myotonia. Warming the ambient temperature or injecting local anesthetics into the involved muscles may induce relaxation. Steroids and inhalation anesthetic agents may also attenuate the contraction in some patients. No treatment is available for the muscle weakness that develops with myotonic dystrophy.