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paramyotonia congenita

MIM.168300

Tuesday 5 August 2008

The cardinal symptom of paramyotonia congenita is paradoxical myotonia i.e., muscle stiffness initiated by activity, rather than rest.

This can be induced with a simple bedside test by asking patients to close their eyes repeatedly and forcefully.

Patients will experience progressive difficulty with relaxation of the eyelids. Myotonia may be more easily elicited, especially in a young child, by asking a patient to forcefully squeeze the examiner’s second and third fingers.

The onset is typically at birth, with signs of myotonia and weakness that usually remain unchanged throughout life. Both the myotonia and weakness are enhanced by exposure to cold. During attacks, the serum potassium level is variable (i.e., high, low, or relatively normal).

This disease is allelic to hyperkalemic periodic paralysis. Several mutations of the SCN4A gene on chromosome 17q were described in different families.

Signs are generally mild and infrequent, and typically may not need treatment. If cold-induced myotonia is significant, avoidance of cold is recommended. If pharmacologic treatment is necessary, sodium-channel blockers (e.g., mexiletine), chlorothiazide, or acetazolamide can be used.

See also

- channelopathies