Paramyotonia congenita (PMC) is a rare genetic disorder manifesting itself in infancy or early childhood and characterized by paradoxical myotonia or muscle tensing. As in other cases of myotonia, affected individuals experience sustained periods of muscle stiffness, reduced reflexes, and muscular paralysis, usually affecting muscles in the face, neck, arms, and hands. However, unlike the typical myotonia, which usually occurs during rest after exercise, muscle stiffness in PMC appears during the period of exercising. In addition, PMC is also induced by cold temperature. These distinguishing features lead to the paradoxical nature of myotonia in PMC.
A thorough evaluation of the patient's clinical symptoms and history is necessary for evaluation. The myotonia should worsen upon exercise and upon exposure to cold temperature for a definite diagnosis. Differential diagnosis includes hypokalemic periodic paralysis (HPP). The most effective method of managing the condition is to avoid the triggers that cause the myotonic attacks. Thus, patients are advised to take care while exercising, and to avoid cold temperature. Some patients may require medications. Certain patients with PMC may have a specific form that is sensitive to serum potassium levels. Such patients may need to avoid potassium in their diets.
PMC is inherited in an autosomal dominant manner with 100% penetrance. Mutations in the SCN4A gene have been shown to be responsible for the condition. The SCN4A gene codes for a protein that functions as a sodium channel. Proper functioning of this sodium channel is important for a smooth contraction and relaxation of the skeletal muscles. Defects in the protein cause disruption in the ion balance, thereby interfering with the process of muscle contraction and relaxation. This process leads to the muscle stiffness observed in PMC.
Certain other mutations in the same SCN4A gene have been shown to be responsible for HPP. Some researchers, therefore, argue that PMC is not distinct from HPP.