Essential tremor is a neurological disorder characterized by rhythmic shaking of part or all of the body. This disorder often is inherited, in which case it is termed familial tremor.
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Unlike the tremor of Parkinson’s disease, essential tremor is not accompanied by rigidity or bradykinesia (slowness of movement). The tremor most commonly affects the hands and arms but may also affect the voice, head, trunk or legs. The tremor is predominantly an action tremor, meaning it becomes most prominent during the active movement of the limb, such as while eating or drinking from a cup.
Essential tremor often responds well to medications, such as propanolol. Many patients will also notice improvement in their symptoms after consumption of an alcoholic beverage.
For patients whose tremor no longer responds adequately to medication, surgical treatment can provide a dramatic restoration of function. Both thalamotomy and thalamic deep brain stimulation can provide excellent relief from disabling tremor.
Thalamotomy – involves the creation of a lesion within an area of the brain’s thalamus called the nucleus ventralis intermedius (Vim). Although the precise physiology of tremor is not yet fully understood, this nucleus appears to serve as a relay between the brain pathways responsible for smooth, coordinated movements. Microelectrode recording of neurons within this nucleus in patients with tremor demonstrates electrical activity that is synchronized with the tremor itself. Thalamotomy destroys these neurons and eliminates tremor. A thalamotomy on one side of the brain will only control the tremor on the opposite side of the body.
In patients with essential tremor or Parkinson’s disease, thalamotomy is approximately 90 percent effective in controlling tremor initially, with about 80 percent of patients experiencing effective control over the long term. For other types of tremor, such as intention tremor as a result of brain injury, stroke or multiple sclerosis, the overall rate of effectiveness is much lower, although some of these patients may still benefit from the procedure.
Potential complications of thalamotomy include weakness, clumsiness, difficulty swallowing and slurred speech. With unilateral (single-sided) thalamotomy, the risks are low, making thalamotomy an attractive option for treating tremor on one side. Since many patients will be satisfied with the restoration of function to their dominant hand, single-sided treatment is often adequate.
Patients with severe bilateral or midline (head, trunk or voice) tremor will usually want treatment on both sides. When thalamotomy is performed on the second side, the risk of complications, especially speech or swallowing problems, is somewhat higher. Also, because thalamotomy involves destruction of brain tissue, any undesired side effects of surgery may be permanent.
Thalamic deep brain stimulation – provides a non-destructive alternative to thalamotomy. In this procedure, instead of creating a lesion, a stimulating electrode is implanted within the nucleus ventralis intermedius (Vim) of the thalamus. This electrode is connected to a pacemaker-like device called an implantable pulse generator, which is capable of delivering an electrical current to the electrode. The electrical current shuts down the surrounding brain tissue, producing the same beneficial effect on tremor as a thalamotomy.
Potential side effects are similar to those seen with thalamotomy; however, the pulse generator can be programmed with different electrode combinations and stimulation parameters to maximize the suppression of tremor and minimize these side effects. More importantly, the effect is reversible, occurring only when the pulse generator is on. For this reason, thalamic stimulation is safer for treatment of both sides than bilateral thalamotomy.