Acquired neuromyotonia is characterized by involuntary continuous muscle fiber activity (fasciculations, doublet and triplet discharges on electromyography) 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 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. Symptoms of Pain are common.
In slightly fewer than 20% of patients, a set of symptoms, including arrhythmias, excessive salivation, memory loss, confusion, hallucinations, constipation, personality change and 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.