Skeletal Muscle Channelopathies: familial periodic paralysis and non-dystrophic myotonias
Familial periodic paralyses include hypokalemic and hyperkalemic forms, autosomal dominant hereditary conditions due to mutations in ion channels, pores present in the skeletal muscle membrane which allow the regular flow of sodium and potassium for example to and from the cells. An affected parent has a 50% chance of transmitting the disease to his/her siblings. These disorders are characterized by sudden episodes of muscle weakness which occur when serum potassium levels either decrease below normal values (hypokalemic forms) or increase (hyperkalemic forms) above normal values. Triggers may be exercise or carbohydrate rich meals according to the different forms. Weakness may affect upper and lower limbs and recovery may occur spontaneously over 2-3 days in the hypokalemic form. Most often iv potassium supplement is needed in the hypokalemic variants. Permanent myopathic weakness may occur in patients with recurring periodic paralysis episodes. Cardiac and respiratory muscles are usually unimpaired. Nevertheless, Andersen-Tawil syndrome represents an uncommon periodic paralysis causing arrhythmia and long QT syndrome (cardiac muscles take longer than usual to recharge between beats).
Periodic paralysis is considered as a “benign” myopathy, yet patients live in the worry of experiencing a crisis: several studies suggested how quality of life in patients with periodic paralysis is comparable to patients with severe diseases such as muscular dystrophies.
Non-dystrophic myotonias are a heterogeneous set of rare diseases including congenital paramyotonia and myotonia congenita (Thomsen disease and Becker type). These diseases may also result from mutations in ion channels, in this case sodium or chloride channels and cause myotonia. This is experienced by the patient as if muscles ‘lock’ and is characterized by delayed relaxation after voluntary contraction (e.g. patients with myotonia have difficulty in re-opening eyes after sneezing, in squeezing hands, in opening a bottle of water, in grabbing something).
A customized rehabilitation program may provide dietetic, neuromotor, functional and therapeutic directions. Pharmacological treatment can contrast myotonia or reduce the frequency and severity of the paralytic attacks, alleviating symptoms and improving quality of life.