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Incidence

Paramyotonia congenita is an uncommon disorder; it is estimated to affect fewer than 1 in 100,000 people. This condition is inherited in an autosomal dominant pattern. PMC is a very rare disorder that affects males and females in equal numbers. Mutations in the SCN4A gene cause paramyotonia congenita. PMC is classified as a form of periodic paralyses, a group of muscle disorders characterized by irregular episodes of muscle weakness or stiffness.

Clinical Characteristics

Paramyotonia congenita is a disorder that affects skeletal muscles. Beginning in infancy or early childhood, people with this condition experience bouts of sustained muscle tensing (myotonia) that prevent muscles from relaxing normally. Myotonia causes muscle stiffness that typically appears after exercise and can be induced by muscle cooling. This stiffness chiefly affects muscles in the face, neck, arms, and hands, although it can also affect muscles used for breathing and muscles in the lower body. Mothers may note their infants\' inability to open their eyes easily after crying, facial distortion after washing the face with cold water, or hand stiffness in their child while playing in the snow. Unlike many other forms of myotonia, the muscle stiffness associated with paramyotonia congenita tends to worsen with repeated movements. In paramyotonia congenita, repeated muscle contractions may worsen the myotonia; this paradoxical myotonic (hence paramyotonia) response is opposite that usually seen with myotonia. Mutations in the SCN4A gene alter the usual structure and function of sodium channels. The altered channels cannot effectively regulate the flow of sodium ions into skeletal muscle cells. The resulting increase in ion flow interferes with normal muscle contraction and relaxation, leading to episodes of muscle stiffness and weakness. Unlike chloride-channel myotonic conditions, muscle stiffness associated with PMC worsens rather than improves after exercise or repeated muscle contraction (paramyotonia) and is extremely cold sensitive, which is opposite to the so-called warm-up phenomenon.

Precipitants

Exposure to cold initially causes muscle stiffness in these individuals, and prolonged cold exposure leads to temporary episodes of mild to severe muscle weakness that may last for several hours at a time. Cold induces stiffness followed by flaccid paralysis. Other precipitating factors include rest followed by exercise, potassium ingestion, and prolonged fasting. Individuals must be cautious to sudden exposures to very cold weather, as well as avoiding sudden heavy physical activity.

Provocation Tests

During brief exercise, overexcitation of muscles can cause stiffness, and with prolonged exercise, the overexcitation can occasionally lead to a fatigue-like weakness or even complete paralysis. The hallmark of physical findings is the inability to immediately open the eyes after repeated sustained eyelid closure. Grip and percussion myotonia and lid lag are commonly elicited.

Diagnostic Procedures

When PMC is suspected, electromyography helps. During the test, the muscles are chilled and electrical signals are recorded before and after the muscle is cooled. The electromyography (EMG) will show rapid repetitive electrical discharges and myotonic discharges. EMG cannot always diagnose PMC definitively. Genetic testing on a blood sample will result in a definitive diagnosis by showing the presence of a characteristic mutation in the SCN4A gene.