Myotonia congenita

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Genetic testing

NCV/EMG

Laboratory

Imaging

Biopsy

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Fig. 33. Myotonia congenita. A Muscle myotonia in the hypoth-enar muscles. B Myotonic discharges in the EMG from affected muscle

Fig. 34. Thomson's myotonia congenita. A Increased muscle bulk in the arms and chest in a patient with Thomson's disease. B Hypertrophy of the extensor digitorum brevis muscle

Percussion Myotonia Congenita

Variable, may affect both limb and facial muscles.

Progresses very slowly over a lifetime. Usually strength is spared.

- Myotonia congenita (Thomsen): onset in infancy.

- Myotonia congenita (Becker): onset is usually in early childhood.

Distribution/anatomy

Time course Onset/age

Myotonia is usually mild, approximately 50% may have percussion myotonia. The myotonia (Fig. 33) is associated with fluctuations, and may be worsened by cold, hunger, fatigue and emotional upset. Muscle hypertrophy is seen in many patients (Fig. 34), and occasionally patients may complain of myalgias. Patients may report a "warm-up" phenomenon, in which the myotonia decreases after repeated activity. Muscle strength is usually normal.

Patients may also have a "warm-up" phenomenon. The disease is more severe than Thomsen's, and although strength is usually normal in childhood, there is often mild distal weakness in older individuals. Strength often deteriorates after short periods of exercise. Hypertrophy may also be observed in the leg muscles, although it is less common than in Thomsen's disease.

Mild myotonia occurring late in life, with less muscle hypertrophy.

Clinical syndrome

Myotonia congenita (Thomsen)

Myotonia congenita (Becker)

Myotonia levior

Thomsen's disease is due to a defect of the muscle chloride channel (CLCN1). Pathogenesis Thomsen's disease is an autosomal dominant disorder, with the gene abnormality localized on chromosome 7q35. The mutation interferes with the normal tetramer formation on the chloride channel. Chloride conductance through the channel is eliminated or reduced. Normal chloride conduction is necessary to stabilize the membrane potential. Without chloride conductance there is increased cation conductance after depolarization, and spontaneous triggering of action potentials. In missense mutations of the chloride channel there is a partial defect in normal conductance of chloride. In contrast, with frame shift mutations there is complete loss of chloride conductance. In Becker's disease there is likewise a defect of the muscle chloride channel (CLCN1), with a recessive mode of inheritance linked to chromosome 7q35. A variety of genetic defects have been described including more than 20 missense mutations, and deletions. Depending on the type of mutation there may be low or reduced opening of chloride channels, or there may be chloride efflux but not influx. A final type of congenital myotonia, myotonia levior, is autosomal dominant and again is related to a mutation of the CLCN1 channel.

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