Essential tremor
Essential tremor, also called benign tremor, familial tremor, and idiopathic tremor, is a medical condition characterized by involuntary rhythmic contractions and relaxations of certain muscle groups in one or more body parts of unknown cause. It is typically symmetrical, and affects the arms, hands, or fingers; but sometimes involves the head, vocal cords, or other body parts. Essential tremor is either an action tremor—it intensifies when one tries to use the affected muscles during voluntary movements such as eating and writing—or it is a postural tremor, which occurs when holding arms outstretched and against gravity. This means that it is distinct from a resting tremor, such as that caused by Parkinson's disease, which is not correlated with movement. Unlike Parkinson's disease, essential tremor may worsen with action.
Essential tremor is a progressive neurological disorder, and the most common movement disorder. Though not life-threatening, it can certainly be debilitating. Its onset is usually between 40 and 50 years of age, but it can occur at any age. The cause is poorly understood. Diagnosis is made by observing the typical pattern of the tremor coupled with the exclusion of known causes of such a tremor. There is currently no medical test available to identify an essential tremor.
While essential tremor is distinct from Parkinson's disease, which causes a resting tremor, essential tremor is nevertheless sometimes misdiagnosed as Parkinson's disease. Some patients have been found to have both essential tremors and resting tremors.
Treatments for essential tremor include medications, typically given sequentially to determine which provides the most efficacy with least side effects. Clostridium botulinum toxin injections and ultrasound are also sometimes used for cases refractory to medications.
Signs and symptoms
The tremors linked with essential tremor are generally action tremors, which means they occur during intentional movements rather than when the body is at rest.In mild cases, ET can manifest as the inability to stop the tongue or hands from shaking, the ability to sing only in vibrato, and difficulty doing small, precise tasks such as threading a needle. Even simple tasks such as cutting in a straight line or using a ruler can range from difficult to impossible, depending on the severity of the condition. In disabling cases, ET can interfere with a person's activities of daily living, including feeding, dressing, and taking care of personal hygiene. Essential tremor generally presents as a rhythmic tremor that occurs only when the affected muscle is exerting effort. Any sort of physical or mental stress tends to make the tremor worse.
The tremor may also occur in the head, jaw, and voice, as well as other body regions, with the general pattern being that the tremor begins in the arms and then spreads to these other regions in some people. Women are more likely to develop the head tremor than are men, and it is also found to be more severe in women than men. In people with essential tremor, the head tremor can be either vertical or horizontal and is typically accompanied by tremors in the hands or voice. Other types of tremor may also occur, including postural tremor of the outstretched arms, intention tremor of the arms, and rest tremor in the arms. In one study, men had more severe postural hand tremor when compared to women. Some people may have unsteadiness and problems with gait and balance. Abnormal tandem gait was more commonly observed in older ET people and those with more than 5 years of disease duration.
ET-related tremors do not occur during sleep, but people with ET sometimes complain of an especially coarse tremor upon awakening that becomes noticeably less coarse within the first few minutes of wakefulness. Tremor and disease activity can intensify in response to fatigue, strong emotions, low blood sugar, extreme cold and heat, caffeine, lithium salts, some antidepressants, stress, and other factors.
Parkinson's disease and parkinsonism can also occur simultaneously with ET. The degree of tremor, rigidity, and functional disability did not differ from patients with idiopathic Parkinson's disease. Hand tremor predominated, and occurred in nearly all cases, followed by head tremor, voice tremor, neck, face, leg, tongue, and trunk tremor. Most other tremors occurred in association with hand tremor. More severe tremors, a lower sleep disorder frequency, and a similar prevalence of other non-motor symptoms also can occur. A non-motor feature such as hearing impairment has been shown to have higher prevalence in those with ET compared with those that are healthy or with Parkinson's. Cochlear pathologies which affect the inner ear are proposed as the potential cause, but there is still insufficient information regarding retrocochlear pathologies and central auditory processing.
Walking difficulties in essential tremor are common. About half of patients have associated dystonia, including cervical dystonia, writer's cramp, spasmodic dysphonia, and cranial dystonia, and 20% of the patients had associated parkinsonism. Olfactory dysfunction is common in Parkinson's disease, and has also been reported to occur in patients with essential tremor. A number of patients with essential tremor also exhibit many of the same neuropsychiatric disturbances seen in idiopathic Parkinson's disease. The similarity in characteristics between parkinsonism and essential tremor can make it difficult to distinguish between the two at times.
Essential tremor with tremor onset after the age of 65 has been associated with mild cognitive impairment, as well as dementia, although the link between these conditions, if any, is still not understood. Familial cases of ET tend to present at younger ages, whereas older ages of onset are linked to faster progression of the disease.
Essential tremor has two tremor components, central and peripheral. These two tremor components were identified by measuring the tremor of ET patients once with no weights on their hands and then with 1-pound weights on their hands. The addition of the weights resulted in a tremor spectrum with two peaks, one that maintained the same frequency and one that decreased in frequency. Only with the addition of the weights was the peripheral tremor distinguishable from the central tremor.
The frequency of essential tremor is 4 to 12 Hz, depending on which body segment is affected. Previously, it was 4 to 11 Hz according to the American Family Physician on Classification of Tremors and Treatment Update. Proximal segments are affected at lower frequencies, and distal segments are affected at higher frequencies.
Cause
Essential tremor was once thought to be a single disease state, however, research shows that there are multiple factors that are associated with causing essential tremor. This leads to the consideration that essential tremor is more akin to a family of diseases, due to the presence of both genetic and sporadic essential tremors. Currently, there are multiple main hypotheses behind essential tremor, being the degeneration of the cerebellum, inheriting the tremor, ingestion of toxins, or the presence of Lewy Bodies in the brainstem. However, post mortem studies showed that only a small number of patients had Lewy Bodies, and was more common for patients not to exhibit them.Cerebellar
It is unknown how the degeneration of the cerebellum leads to essential tremor, however, it is hypothesized that it may be due to the loss of Purkinje cells, as they release gamma-aminobutyric acid, which is an inhibitory neurotransmitter meant to control the firing of neurons in the cerebellum. In certain essential tremor clinical studies which augment the GABA pathway, only some participants exhibited a reduction of tremor. Some patients have responded to alcohol, claiming alcohol has reduced the tremor, however the reduction is only short term. However, alcohol might only be beneficial for those with an impacted GABA pathway, and may not be benefit patients who developed the tremor via other pathways.