What Is Essential Tremor?
Tremor, an oscillatory movement produced by rhythmic contractions of muscles, is the most common form of involuntary movement, but only a small fraction of those who shake seek medical attention. Essential tremor (ET) is the most common form of tremor, affecting about five percent of people over the age of 60 years. Although essential tremor is more common in the elderly and is often wrongly labeled as "senile tremor" it may start at any age. In contrast to rest tremor, which is most frequently secondary to Parkinson's disease, essential tremor is typically an action-postural tremor. Patients usually first become aware of the tremor when they are holding newspapers or utensils or when reaching for objects. Essential tremors can also affect the neck muscles and produce either "yes-yes" or "no-no" oscillation of the head, and the vocal cords, noted as a tremulous voice while singing or talking.
A review of the clinical features in 350 consecutive patients diagnosed with essential tremor referred to the Movement Disorders Clinic at Baylor Medicine has shown that hands are involved in 90 percent of all essential tremor patients, head in 41 percent, voice in 17 percent, and legs in 14 percent. Stress, overactive thyroid, and certain drugs can exacerbate the tremor. Despite the frequently used prefix, "benign" essential tremor can produce significant physical and psychosocial disability. In addition to social embarrassment, essential tremor can interfere with writing, eating, speaking, singing, and various other activities of daily living. Although the tremor frequency tends to decrease with age, the amplitude usually increases and the tremor may become more and more troublesome.
In some cases, tremors occur only during a particular task, such as writing, or while holding limbs in a certain position such as when bringing a glass to the mouth. These task- and position-specific tremors are considered variants of essential tremor, but in some cases, the clinical features overlap with another movement disorder called dystonia.
In some patients, dystonia produces rhythmical movements resembling essential tremor, but in contrast to essential tremor, dystonic tremor is more irregular and is more influenced by the position of the affected body part.
Some studies have suggested that there is an association between essential tremor and dystonia, and between essential tremor and Parkinson's disease. This is supported by the reports of well-studied families in which some members have typical essential tremor, while others have dystonia, parkinsonism, or a combination of all three disorders. Furthermore, some patients start with typical ET and several years or decades later develop associated parkinsonism or dystonia, or both. Although it should not be difficult to differentiate tremor of essential tremor and tremor typically seen in patients with PD, the occasional co-existence of the two disorders in the same individual may present a diagnostic challenge.
In addition to dystonia and parkinsonism, patients with essential tremor seem to have a higher frequency of deafness, hereditary neuropathy, cognitive problems, mild incoordination, and loss of balance.
Orthostatic tremor is a fast tremor, involving mainly legs and trunk. It is present chiefly on standing and may be associated with a feeling of unsteadiness and calf cramps, relieved by sitting or by assuming a supine position.
Frequently misdiagnosed as a stress-related condition or a natural consequence of aging, essential tremor is actually a manifestation of a genetic neurological disorder.
Family history of tremor is present in the vast majority of patients. Despite the overwhelming evidence that essential tremor is a genetic disorder, the gene for essential tremor have not yet been fully identified. A lot of genes were initially implicated in essential tremor, such as LINGO1, FUS, and TENM4. The difficulty on finding one gene despite multiple studies generated the idea that essential tremor is likely the result of the effect of multiple genes in combination with environmental factors. Beta-alkaloids (harmine and harmane) and lead were implicated in essential tremor.
Currently, essential tremor is considered the result of a dysfunction in the network that coordinates movements, which involves the cortex and deeper areas of the brain such as the thalamus, cerebellum, and olivary nucleus. Loss and swelling of specialized cells called Purkinje cells in the cerebellum and changes in the production of GABA were reported, but no clear mechanism was defined.
The treatment of tremor depends largely on its severity; many patients require nothing more than simple reassurance. Most patients who are referred to a neurologist, however, have troublesome tremors that require medication or surgical treatments. The large volume and slow frequency postural tremors usually do not respond well to pharmacologic therapy.
Since alcohol reduces the volume of essential tremor in about two-thirds of patients, some patients use it regularly for its "calming" effect and some use it before an important engagement where the presence of tremor could be a source of embarrassment. Regular use of alcohol to treat essential tremor is not recommended.
Propranolol and primidone are the most effective medications to treat essential tremor. Propranolol (Inderal) is a beta-adrenergic blocker, and it has immediate-release and extended-release formulations. It is usually well-tolerated, but patients can experience side effects, such as fatigability, sedation, depression, and erectile dysfunction. Propranolol is contraindicated in the presence of asthma and insulin-dependent diabetes. The anti-tremor effect of primidone (Mysoline) has been confirmed by several clinical trials. By starting primidone at a very low dose, the occasional side effects (nausea, vomiting, sedation, confusion, and loss of balance) can be prevented. A combination of the two drugs, propranolol, and primidone, may be more efficacious than either drug alone.
In addition to propranolol and primidone, other beta-blockers such as pindolol and nadolol, benzodiazepine drugs, such as lorazepam (Ativan), and clonazepam (Klonopin) also may also improve essential tremor and its variants. Gabapentin (Neurontin), topiramate (Topamax), and pregabalin (Lyrica), drugs approved for the treatment of epilepsy, may be also useful in the treatment of tremors. Gabapentin, levodopa, primidone, clonazepam, and phenobarbital may be useful in patients with orthostatic tremor, a possible variant of essential tremor.
Patients with head tremor do not usually respond to medications, but can be effectively treated with botulinum toxin (BTX) injections into the neck muscles. BTX is also effective in the treatment of hand tremor. Usually well-tolerated, BTX can produce transient weakness of the injected muscles.
Procedures and Surgery
Deep Brain Stimulation
Neurosurgical treatments, such as deep brain stimulation (DBS), are generally reserved for patients whose tremors continue to interfere with work or activities of daily living despite optimal medical therapy. DBS involves an incision in the scalp and drilling a hole in the skull. The surgeon then uses a "probe," a wire electrode, and advances the electrode into the portion of the brain that is thought to be functioning abnormally. The DBS lead, which contains four electrodes, is surgically inserted into the desired target and fixed to the skull with a ring and cap. An extension wire passes from the scalp area under the skin to the chest and is connected to a totally implantable pulse generator (IPG), a pacemaker-like device, which can deliver pulses with a variety of parameters, modes, and polarities into the target brain area. The IPG is placed under the skin in the upper chest area near the collar bone. The patient can activate or deactivate the DBS system by placing a hand-held device over the chest area that contains the IPG. The major advantage of DBS over the traditional ablative procedures is that the stimulating electrodes and parameters (frequency of stimulation, pulse width, and voltage) can be adjusted to the needs of the individual patients. Potential risks, such as hemorrhage, stroke or infection, are rare but should be considered when making a final decision about this treatment option. Side effects, if they occur, are usually reversible, but may include weakness, speech and swallowing difficulties, and abnormal sensations. The benefits of DBS persist even after more than 10 years of treatment.
MR-Guided Focused Ultrasound
Focused ultrasound thalamotomy reduces the abnormal brain activity that is responsible for the tremor, providing meaningful clinical benefits. FUS therapy is applied on a deep brain structure called the thalamus, with the goal of creating a tiny ablation or lesion reducing the abnormal brain activity responsible for the tremor. FUS is indicated when medications or local botulinum toxin injections do not provide adequate tremor control and/or there is poor tolerance to their side effects. FUS is not a cure for ET and does not necessarily improve any associated symptoms such as balance difficulties.