Paramyotonia Congenita

In persons with paramyotonia congenita, symptoms typically appear early in life, anywhere from infancy to adolescence. Patients complain of an inability to relax muscles and weakening that worsens with repeated activity or prolonged exercise [1]. This is exactly the difference between paramyotonia and classic myotonia in which symptoms are alleviated by physical activity. Symptoms last from minutes to days. The most commonly affected muscles are those in the face and upper extremities [2].

Patients may report worsened symptoms during activities involving cold temperatures such as shoveling snow or while handling frozen foods. Women may experience exacerbation of symptoms during menstruation or pregnancy [3]. Parents may state that their infant's eyes appear to "stick" or the infant has difficulty opening his/her eyes following crying spell [4].

Evaluation for paramyotonia congenita includes a medical history of the patient as well as his/her family, physical examination, and neuromuscular testing. Myotonia will worsen with repeated or prolonged movement and then ultimately resolve after a period of minutes to days [1]. Symptoms are worsened by exposure to cold temperatures [2]. Patients may provide a history of anesthetic complications [5]. The patient's family history will often reveal similar symptoms or "cold intolerance" in relatives. Hypertrophy of affected muscles may be present in adults.

Tests commonly used to establish a diagnosis of paramyotonia congenita include:

• Genetic analysis - Mutations of the SCN4A gene can be confirmed by genetic testing. A mutation of this gene confirms the diagnosis of paramyotonia congenita [6].
• Electromyography (EMG) - EMG findings facilitate differentiation between paramyotonia congenita and myotonia congenita [7] [8]. In patients with paramyotonia congenita, EMG will show diffuse myotonic discharges, which are more pronounced with cooling of the affected muscles, as well as a total absence of muscle contractures with progressive cooling [9].
• Muscle cooling - Lowering a temperature of affected muscles to 20-25°C induces muscle paralysis and stiffness, which persists even after rewarming. Exercise while the muscle is cooled results in exacerbation of muscle weakness and decreased compound muscle action potentials [10].
• Exercise test - Exercise test is performed to confirm periodic paralysis. A positive test is characterized by two key findings in affected muscles: compound muscle action potentials are decreased and weakening of the muscle with exercise [11].
• Muscle biopsy - A muscle biopsy showing vacuoles confirms the presence of non-specific myopathy [12] [6]; however, it does not confirm a diagnosis of paramyotonia congenita.

Laboratory tests do not confirm the diagnosis of paramyotonia congenita, but may help rule out other conditions. Patients with paramyotonia congenita may have a normal or slightly elevated creatine kinase level and/or normal or increased potassium level [13] [14] [15].

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