Myotonia congenita is a congenital neuromuscular channelopathy that affects skeletal muscles (muscles used for movement). It is a genetic disorder. The hallmark of the disease is the failure of initiated contraction to terminate, often referred to as delayed relaxation of the muscles (myotonia) and rigidity. Symptoms include delayed relaxation of the muscles after voluntary contraction (myotonia), and may also include stiffness, hypertrophy (enlargement), transient weakness in some forms of the disorder (from certain genetic mutations), severe masseter spasm, and cramping. The condition is sometimes referred to as fainting goat syndrome, as it is responsible for the eponymous fainting seen in fainting goats when presented with a sudden stimulus. Of note, myotonia congenita has no association with malignant hyperthermia (MH). Symptoms and signs
The prolonged muscle contractions, which occur most commonly in the leg muscles in recessive mutations, and more commonly in the hands, face, and eyelids in dominant mutations, are often enhanced by inactivity, and in some forms are relieved by repetitive movement known as "the warm-up effect". This effect often diminishes quickly with rest. Some individuals with myotonia congenita are prone to falling as a result of hasty movements or an inability to stabilize themselves after a loss of balance. During a fall, a person with myotonia congenita may experience partial or complete rigid paralysis that will quickly resolve once the event is over. However, a fall into cold water may render the person unable to move for the duration of submergence.
The onset of skeletal muscle activity is associated with the initiation and propagation of action potentials again associated with an efflux of K+ to the extracellular fluid and transverse tubule system. When many action potentials are elicited subsequently more K+ is expelled from the cell into the transverse tubular system. As K+ accumulates in the transverse tubular system the equilibrium potential for K+ (EK+) normally around -80 mV, becomes more depolarized (depolarization), according to the Nernst equation. In skeletal muscle fibers the equilibrium potential for Cl− is around -80 mV, equal to that of K+ at rest. Cl− moves towards its equilibrium potential around -80 mV, while potassium moves towards its equilibrium potential more depolarized than -80 mV during activity. This results in a slightly more depolarized membrane potential of the fiber during repeated action potentials, see Goldman equation. The Na+ conductance is only elevated shortly compared to the K+ conductance during each action potential, which is why K+ largely determines the membrane potential (Cl− is passively distributed during rest).