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Diagnosis of ET
- Bilateral action tremor of hands and forearms (but not rest tremor)
- Absence of other neurologic signs with the exception of the cogwheel phenomenon
- May have isolated head tremor with no signs of dystonia
- Long duration (>3 years)
- Positive family history
- Beneficial response to propranolol
Essential tremor is a common movement disorder and is up to 20 times more prevalent than the tremor caused by Parkinson's disease. It's incidence is 300-415 per 100,000 people. There is equal gender predliction, and an unknown racial predilection but it may favor whites. It has an autosomal dominant inheritance pattern with almost complete penetrance by age 65 (an example of variable expressivity). It affects from 5-10 million people in the United States.
The pathophysiology is generally poorly understood. Nor morphological changes have been identified in the brain. It has been attributed to a disturbance in the inferior olivary nucleus. In the inferior olivary nucleus, abnormal 4-12 Hz oscillations have been noted to occur. These are caused by Purkinje cells and the dentate and interpositus nuclei and distributed to thalamocortical and brainstem nuclei. Infarcts in these pathways have been reported to arrest ET.
The inferior olive is the "teacher of the cerebellum" and adjusts/modulates planned movements during execution in reponse to unconditioned afferent information. This is achieved by modulating cerebellar return to motor cortex by the Purkinje cells. In ET patients there is excessive recruitment of inferior olive neurons in reponse to afferent information.
- As these neurons oscillate synchronously in the 4-12 Hz range, the result is tremor
- Oscillations my result from excessive electrotonic coupling between the dendrites of inferior olivary neurons via GABA mediated gap junctions
- ET is not considered to be due to overactivity of this pathway
Options include medical and surgical treatment
Medications administered include Primidone or propranolol as first line agents. Primidone works by unknown means but significantly reduces tremor. Propranolol reduces tremor in 75% of patients and probably works through peripheral beta 2 antagonism. Alchohol also reduces the tremor. Other agents utilized with varying success include: clozapine, mirtazapine, gabapentin, benzodiazepines, Botulinum toxin (which weakens the limb).In general drug treatment is only efficaious in 50% of patients.
Surgical treatment includes thalamotomy and stimulation (see DBS for ET). The preferred target is the ventralis intermedius (Vim) nucleus of the thalamus. Subthalamic nucleus stimulation has been considered an attractive target as well. Vagus nerve stimulation has alos been used but with variable results.
Mov Disord. 2004 Oct;19(10):1163-8. Mov Disord. 2002;17 Suppl 3:S2-8. J Neurosurg. 2004 Jul;101(1):48-54. Neurosurgery. 2004 May;54(5):1120-29; discussion 1129-30.