Disorder causing difficulty relaxing contracted voluntary muscles. All or only a few may be affected. Myotonia seems to originate in the muscles (myopathy) rather than the nervous system. Certain toxins can cause it. A hereditary form, myotonia congenita (Thomsen disease), can affect eyelid and eye motion, swallowing, or talking. Quick movements cause muscle stiffening. There is also a myotonic form of muscular dystrophy. Analgesics, anesthetics, and anticonvulsant drugs can alleviate the symptoms.

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Myotonia is a symptom of a small handful of certain neuromuscular disorders characterized by the slow relaxation of the muscles after voluntary contraction or electrical stimulation. Generally, repeated effort is needed to relax the muscles, and the condition improves after the muscles have warmed-up. However, prolonged, rigorous exercise may also trigger the condition. Individuals with the disorder may have trouble releasing their grip on objects or may have difficulty rising from a sitting position and a stiff, awkward gait. Symptoms of myotonia are more frequently experienced in women during pregnancy.

Myotonia can affect all muscle groups, however the pattern of affected muscles can vary depending on the specific disorder involved. It may be acquired or inherited, and is caused by an abnormality in the muscle membrane. Specifically, the ion channels. Myotonia is a symptom commonly seen in patients with myotonic muscular dystrophy, of which two documented types and one speculated type exist, and in a group of disorders called channelopathies (hereditary diseases that are caused by mutations in the chloride, sodium or potassium ion transport channels in the muscle membrane), such as Myotonia Congenita (Congenital Myotonia) of which two types called Becker's Disease and Thomsen's Disease exist. There is also a disorder called Paramyotonia Congenita. Myotonia arising from channelopathies, and myotonic muscular dystrophy can be exacerbated by exposure to cold (and occasionally heat), by eating foods that are potassium-rich (such as bananas), with exertion, especially after long periods of inactivity, sudden surprises, and stressful situations.

Myotonia is not always a disease-related or abnormal phenomenon. Humans and other animals (such as the fainting goat) often display myotonia when placed in situations of extreme stress or fear; a resultant increase in 'fight-or-flight' hormones such as epinephrine and cortisol may cause increased muscle tension throughout the body.

People suffering from disorders involving myotonia can have a life threatening reaction to certain anaesthetics, one of these conditions occurs when the patient is under anaesthetic and is termed " Malignant Hyperthermia ". Anaesthesiologists cannot diagnose this condition until the patient is under anaesthetic so this condition is very life threatening.

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