NORD gratefully acknowledges Matthew Kiernan, PhD, DSc, Bushell Chair of Neurology, Brain & Mind Research Institute, The University of Sydney, for assistance in the preparation of this report.
Acquired neuromyotonia is characterized by involuntary continuous muscle fiber activity (fasciculations) that cause stiffness and delayed relaxation in the affected muscles. Muscle twitching with a rippling appearance (myokymia) may occur along with these symptoms. Affected individuals may, at times, be unable to coordinate voluntary muscle movement and find difficulty in walking (ataxia). Other symptoms may include staggering and reeling (titubation), stiffness, and lack of balance in response to being startled. There may be diminished spontaneous gross motor activity.
The disorder is characterized by progressive stiffness, cramping, and weakness. Muscle activity is constant, and patients describe the feeling of continuous writhing or rippling of muscles under the skin. These movements continue during sleep. Diminished reflexes are also frequently a sign of this disorder. In some instances, muscle relaxation following voluntary muscle movement is delayed (grip myotonia) in the affected muscles. For example, affected individuals may be not be able to open their fists or eyes immediately after closing them tightly for a few seconds.
Affected individuals frequently have excessive sweating (hyperhidrosis), rapid heartbeats (tachycardia) and weight loss.
In slightly fewer than 20% of the cases, a set of symptoms, including arrhythmias, excessive salivation, memory loss, confusion, hallucinations, constipation, personality change and/or sleep disorders, are found. In such cases the disorder may be referred to as Morvan syndrome.
Approximately 20% of affected individuals have a tumor of the thymus gland (thymoma). The thymus glands are the source of a number of specialized cells associated with autoimmune functions. The disorder is also associated with peripheral neuropathies and autoimmune diseases including myasthenia gravis in some individuals. It has also been reported following infections and radiation therapy.
Acquired neuromyotonia is an autoimmune disease in which the immune system malfunctions so that it damages parts of one’s own body. Approximately 40% of affected individuals have antibodies to voltage-gated potassium channels (VGKC’s) that affect the points at which the signals from the nerve fiber meet the muscle cell (neuromuscular junction).
Aquired neuromyotonia is a rare disorder affecting males and females but is slightly more common among men. Disease onset is usually between the ages of 15 and 60 years but has also been reported in childhood.
The diagnosis of acquired neuromyotonia is based on the presence of continuous muscle contractions (myokymia), especially in the face and hands, rhythmic tics or twitches (fasciculations), and muscle cramps. The diagnosis is confirmed by studies of the electrical signs of muscle activity (electromyography).
Acquired neuromyotonia may be treated with anticonvulsant drugs such as phenytoin or carbamazepine, which may stop the abnormal impulses and prevent the symptoms from reoccurring. Plasma exchange (plasmapheresis) and intravenous immune globulin have been effective in a few cases but no long-term, controlled, clinical studies have been carried out.
Testing for acetylcholine receptor antibodies should be done if thymoma is suspected. The thymus gland should be surgically removed if thymoma is present.
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