Primary orthostatic tremor is a rare movement disorder characterized by a rapid tremor in the legs that occurs when standing. The tremor disappears partially or completely when an affected person is walking or sitting. Individuals with primary orthostatic tremor experience feelings of unsteadiness or imbalance. The tremor is sometimes described as causing "shaky legs" and can cause affected individuals to immediately attempt to sit or walk because of a fear of falling. In many cases, the tremors become more severe over time. Primary orthostatic tremor is a constant problem that can affect the quality of life of affected individuals. The exact cause of primary orthostatic tremor is unknown.
Primary orthostatic tremor was first described in 1984 by Heilman. There is controversy within the medical literature regarding whether primary orthostatic tremor is a variant of essential tremor, an exaggerated physiological response to standing still or a distinct clinical entity.
The main symptom of primary orthostatic tremor is the occurrence of a rapid tremor affecting both legs while standing. A tremor is involuntary, rhythmic contractions of various muscles. Orthostatic tremor causes feelings of unsteadiness or imbalance in the legs. The tremor associated with primary orthostatic tremor has such high frequency that it may not visible to the naked eye but can be palpated by touching the thighs or calves or can be detected by listening to these muscles with a stethoscope. The tremor is position-specific (standing) and disappears partially or completely when an affected individual walks, sits or lies down. In many cases, the tremor becomes progressively more severe and feelings of unsteadiness become more intense. Some affected individuals can stand for several minutes before the tremor begins; others can only stand momentarily. Eventually, affected individuals may experience stiffness, weakness and, in rare cases, pain in the legs. Orthostatic tremor, despite usually becoming progressively more pronounced, does not develop into other conditions or affect other systems of the body.
In some cases, affected individuals may also have a tremor affecting the arms. In one case reported in the medical literature, overgrowth of the affected muscles (muscular hypertrophy) occurred in association with primary orthostatic tremor.
The exact cause of primary orthostatic tremor is unknown (idiopathic). Some researchers believe that the disorder is a variant or subtype of essential tremor. Other researchers believe the disorder is a separate entity. Some individuals with primary orthostatic tremor have had a family history of tremor suggesting that in these cases genetic factors may play a role in the development of the disorder. However, more research is necessary to determine the exact, underlying cause(s) of primary orthostatic tremor.
Primary orthostatic tremor affects males and females in equal numbers. Because many cases of primary orthostatic tremor often go unrecognized or misdiagnosed, the disorder is believed by some to be under-diagnosed, making it difficult to determine the true frequency of this disorder in the general population.
A diagnosis of primary orthostatic tremor is based upon a thorough clinical evaluation, a detailed patient history, and supported by specialized tests such as a surface electromyogram (EMG). Misdiagnosis is common. Many individuals may be initially suspected of having a psychogenic disorder (a disorder caused by a psychological cause rather than a physical one).
Clinical Testing and Work-Up
A surface EMG measures the electrical impulses of muscles at rest and during contraction. A surface electromyogram can often rapidly establish a diagnosis of primary orthostatic tremor by reproducing the characteristic tremor in the legs. With a surface electromyogram electrodes are placed on the skin overlying the muscles that are to be tested.
Various medications may help relieve symptoms associated with primary orthostatic tremor. In most cases, affected individuals are treated with a drug called clonazepam (Klonopin). However, some affected individuals do not respond to this first-line drug treatment.
Some affected individuals responded favorably after being treated with an anti-seizure (anticonvulsant) drug called gabapentin (Neurontin). A very small double-blind, placebo controlled study demonstrated that affected individuals experienced a sustained improvement when treated with the drug. Authors of the study suggested that gabapentin be considered a first-line therapy for individuals with primary orthostatic tremor.
Additional drug therapies that have been used to treat individuals with primary orthostatic tremor include primidone (Mysoline), chlordiazepoxide (Librium), pregabalin (Lyrica), pramipexole (Mirapex), phenobarbital and valproic acid (Depakote). Drugs commonly used to treat people with Parkinson's disease (levodopa or pramipexole) may also be prescribed for individuals with primary orthostatic tremor. Additional treatment is symptomatic and supportive.
The medical literature details several cases of individuals with primary orthostatic tremor who have been treated by thalamic deep brain stimulation. Thalamic refers to the thalamus, a portion of the brain that relays sensory information. It is believed that some types of tremors occur due to abnormal brain activity that is processed in the thalamus. During this procedure, an electrode is placed into the thalamus and a thin wire is passed under the skin is connected to a small battery pack (which is also placed underneath the skin. The electrode is used to send electrical impulses (stimulate) to the brain and interrupt aberrant nerve signals that contribute to tremors. In some cases reported in the medical literature, tremors of individuals with primary orthostatic tremor were successfully controlled with thalamic deep brain stimulation. However, more research is necessary to determine the long-term safety and effectiveness of this potential therapy for primary orthostatic tremor.
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