Myotonia Congenita
Myotonia congenita (Thomsen's disease) presents in infancy and is inherited as an autosomal-dominant condition. There is an abnormality of the muscle chloride channel, and the disease is linked to the 7q35 loci.
There is variable penetrance. Symptoms may be present from birth, but usually develop later. The myotonia is relatively mild and may manifest as difficulty in releasing objects or difficulty walking or climbing stairs. Most patients do not show overt weakness. Functional difficulties in climbing stairs may be present. The myotonia is exacerbated by prolonged rest or inactivity. There is a “warm-up” phenomenon with reduced myotonia after repeated activity. Myotonia may be aggravated by cold, hunger, fatigue, and emotional upset. Patients may demonstrate grip myotonia or lid lag following upward gaze or squint and diplopia following sustained conjugate movement of the eyes in one direction. Nearly all have electrical myotonia by EMG, but there is a warm-up phenomenon with the myotonia reduced after a period of maximal contraction. Half of individuals have percussion myotonia. Patients may be symptom-free for weeks to months. The other common feature of myotonia congenita is muscle hypertrophy. Patients may exhibit a “Herculean” appearance. Patients have shown some benefit from treatment with quinine, mexiletine, dilantin, procainamide, carbamazepine, and acetazolamide.A recessive form of myotonia congenita (Becker form) also exists with later onset (ages 4 to 12), more marked myotonia, more striking hypertrophy of muscles, and associated weakness of muscles, particularly with short exercise. EMG shows myotonia in distal muscles and less myotonia after maximal contraction. On repetitive stimulation, there is a decremental CMAP response at high stimulation frequency (30 Hz) and following exercise. The dominant form seems more prone to aggravation of the myotonia by cold. Diagnosis is suspected based on clinical information and the presence of classical myotonic discharges on EMG. Diagnosis is confirmed with molecular genetic testing. Muscle biopsy is essentially normal, apart from the presence of hypertrophy of fibers and an absence of type II-B fibers.