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Myotonia Congenita

Myotonia congenita (Thomsens 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 be manifest as difficulty in releasing objects or by 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 décrémentai 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.

Paramyotonia Congenita

Paramyotonia congenita is an autosomal-dominant myotonic condition with at least two distinct genetic etiologies involving the sodium channel—a subunit (SCN4A) located at chromosome 17q35 and a muscle chloride channel (CLCN1) located at chromosome 7q35. The worsening of the myotonia with exercise is referred to as paradoxical myotonia. Weakness or stiffness may occur together or separately, there is cold and exercise aggravation, hypertrophy of musculature, and more severe involvement of hands and muscles of the face and neck. Myotonic episodes usually subside within a matter of hours but may last days. Some patients become worse with a potassium load. On electrodiagnostic studies there is a drop in CMAP amplitude with cooling. Dense fibrillations disappear below 28°C, myotonic bursts disappear below 20°C, and electrical silence may occur below 20°C. Treatment has involved mexiletine or tocainide.

Schwartz-Jampel Syndrome (Chondrodystrophic Myotonia)

Schwartz-Jampel syndrome is an autosomal-recessive disorder with myotonia, dwarfism, diffuse bone disease, narrow palpebral fissures, blepharospasm, micrognathia, and flattened fades. Onset is usually before age 3. Patients have respiratory and feeding difficulties with impaired swallowing. Limitation of joint movement may be present along with skeletal abnormalities, including short neck and kyphoscoliosis. Muscles are typically hypertrophic and clinically stiff. There is a characteristic fades with pursed lips, micrognathia, and small mouth. Patients may be difficult to intubate. Ocular changes include myopia and cataracts. There may be hirsutism and small testes. The symptoms are not progressive. The protein perlecan with gene loci at chromosome Ip34-p36 has been implicated.

Electrodiagnostic studies show continuous electrical activity with electrical silence being difficult to obtain. There is relatively little waxing and waning in either amplitude or frequency of complex repetitive discharges. Abnormal sodium channel kinetics in the sarcolemma of muscle has been demonstrated. Some therapeutic benefit has been reported with procainamide and carbamazepine.

 
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