Paramyotonia congenita of Von Eulenburg

Paramyotonia congenita of Von Eulenburg (PMC) is a rare inherited skeletal muscle disorder belonging to the group of non-dystrophic myotonias caused by mutations in the SCN4A gene, which encodes the voltage-gated sodium channel Nav1.4 in skeletal muscle. The condition is characterized by myotonia (difficulty relaxing muscles after voluntary contraction) that is paradoxically worsened by repeated muscle use — hence the term 'paramyotonia,' which distinguishes it from classical myotonia where repeated contractions typically improve stiffness. A hallmark feature of PMC is that symptoms are prominently triggered or aggravated by cold exposure. The disease primarily affects skeletal muscles, with the face, hands, and forearms being most commonly involved. Patients may experience muscle stiffness, difficulty opening the eyes or releasing grip in cold environments, and episodes of muscle weakness that can follow prolonged myotonia or cold exposure. Symptoms are typically present from birth or early infancy and remain relatively stable throughout life, though they do not usually worsen progressively. During cold-induced episodes, patients may develop flaccid weakness lasting minutes to hours. The severity of symptoms varies among affected individuals, even within the same family. Electromyography (EMG) typically demonstrates myotonic discharges, and genetic testing of the SCN4A gene confirms the diagnosis. There is no cure for paramyotonia congenita, but treatment focuses on symptom management. Sodium channel blockers such as mexiletine are the primary pharmacological treatment and can effectively reduce myotonia in many patients. Avoidance of cold exposure and careful management of triggers are important non-pharmacological strategies. Other medications that have been used include lamotrigine, flecainide, and acetazolamide, though evidence is more limited. The condition does not typically affect life expectancy, and most patients maintain a good quality of life with appropriate management.

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