Essential tremor (ET) is classified as a neurological disorder. It is commonly characterized as an uncontrollable shaking that occurs in various body parts, such as the head, chin, hands, arms, and also presents with a shaking voice due to affected vocal cords. ET is different from a resting tremor, which is associated with Parkinson’s, and is commonly classified as an action tremor.
Mechanism of Injury / Pathological Process
The likelihood of acquiring ET has been shown to increase with age, in different ethnicities, and in persons with a family history of ET. The risk of acquiring ET has been shown to be more prevalent in older populations. An increased risk has also been found in Caucasians as opposed to African-American populations.
Genetic factors have been linked to increased prevalence of ET with some studies suggesting potential linkages identified at the 3q13 and 2p22 chromosomes. However, there is a large discrepancy if genetic factors are a cause of ET since other studies found only a moderate link (60-63%) in monozygotic twins and a low link (27-42%) in dizygotic twins. This is indicative that environmental factors likely play a role in the prevalence of ET's. A link in β-carboline alkaloids (harmine and harmane) has been found in subjects with ET since high concentrations of β-carboline alkaloids were found in their blood.
β-carboline alkaloids are found in high meat diets and increased meat consumption has been linked with prevalence in persons with ET. An increased concentration of lead was also found in subjects with ET's. These environmental factors increase the risk for developing ET, and the risk of aquiring ET increases when genetics, age, and ethnicity of the person favourable ET development.
ET is considered to be one of the most, if not the most, common adult movement disorders. It is estimated to affect 0.9% of the population worldwide. The prevalence increases greatly with age, it is estimated to reach an average of 4.6% in the population over 65 years of age. Additionally, more recent studies indicate the potential of a slightly higher prominence of the condition in men than women. Finally, it is thought that further research could indicate differences in prevalence among different ethnic groups.
The literature is somewhat variable on the method of diagnosis of ET. This is due to varied clinical presentations and a lack of agreement among specialists on the definition of ET. Despite variability and an inability to test for serological, pathological or radiological markers as a method of diagnosis, there are several clinical criteria which are used to differentiate tremor types. This clinical examination is very comprehensive and typically includes the following observations or tests:
- Observe the patient at rest and note any resting tremor or the head, hands or leg
- Ask the patient to stretch out arms to observe for postural tremor
- Check finger-nose-finger movements to observe kinetic tremor
On assessment of ET, there should be no findings of any other focal neurological condition. A diagnosis of definite ET includes the following clinical criteria:
- Postural tremor of moderate amplitude is present in a least one arm (although usually bilateral)
- Tremor of moderate amplitude is present in at least one arm during at least four tasks
a. Pouring water
b. Using a spoon to drink water
c. Finger-nose-finger maneuver
d. Drawing a spiral
- Tremor must interfere with at least one activity of daily living (ADL)
- Medications, hypothyroidism, alcohol use and other neurological conditions are not the cause of the tremor
The clinical criteria used for a “probably ET” are listed as follows:
- Tremor of moderate amplitude is present in at least one arm during at least four tasks, or head tremor is present
- Medications, hyperthyroidism, alcohol use and other neurological conditions are not the cause of the tremor
Any of the following objective findings which would indicate a tremor diagnosis other than ET: abnormal neurological exam findings, isolated voice tremor, isolated positon-specific or task-specific tremor and isolated tongue, chin or leg tremor. Tremors associated with Parkinson’s can be difficult to differentiate from ET. Key features such as asymmetric onset, resting tremor and a tremor while walking would indicate a parkinsonian origin and not ET.
Screening questionnaires can be used for diagnosis of ET but they generally only have a moderate (60-70%) sensitivity  Responses to these questionnaires generally correlated well to the clinical examination findings for those with definite or probably ET. Subjects with a diagnosis of mild ET from a clinical examination were most often screened as negative on the questionnaires .
Essential tremor primarily presents as a kinetic tremor of the upper extremities, meaning it appears when we move throughout our various daily activities. The tremor typically oscillates between 4 and 12 Hz, with the frequency of the tremor being inversely proportional to one’s age . The tremor may also present as intention tremor, increasing in magnitude and frequency as the patient voluntarily moves their arm or hand closer to specific objects or points.
Although primarily located in the arms, essential tremor may gradually spread in a somatotrophic fashion to other parts of the body such as the head or voice, or less commonly the lower extremities. Patients experiencing more severe or farther progressed essential tremor may present with additional postural tremor or may be disabled in their ability to perform activities of daily living (ADLs) such as eating and dressing. Essential tremor presents with a larger set of additional characteristics than initially thought. These include motor features such as signs of cerebellar dysfunction and tremor at rest, as well as non-motor features such as cognitive deficits and personality changes.
Pharmacological treatment for ET has shown some promising results. Propranolol and primidone are the most commonly used drugs. They have around a 50 - 70% effectiveness in patients. However, there are some side effects such as dizziness, nausea, fast or slow heart beat, and drowsiness among others. Topiramate is an alternative medication used for ET and is often used when the previous two are ineffective. It is effective in about 22-37% of patients. A recent systematic review by Bruno et al 2017 found that while effective, more studies are still needed to determine its efficacy and safety.
Two main types of surgical techniques are used for essential tremor, deep brain stimulation and thalamotomy. Deep brain stimulation (DBS) was originally used for treating Parkinson’s Disease but has been us to treat ET as well. It involves implanting electrodes in the thalamus in the ventral intermediate nucleus. These are then connected to a pulse generator typically located behind the clavicle. When it is turned on it stimulates the thalamus reducing the tremor. It can be turned on and off at the patients discression. A systematic review in 2010 found that DBS significantly improved outcomes in patients with ET. Moreover, any mild adverse effects they produced were reduced by modulating the pulse of the deep brain stimulation.
Thalamotomy involves lesioning the portions of the thalamus that cause the tremors. Different types include radiofrequency thalamotomy, laser induced thermal therapy, magnetic resonance guided focused ultrasound, focused thermolesions and gamma knife thalamotomy. They essentially do the same thing but due to different procedures some are associated with higher risks.
Physical Therapy Management
Physical therapy treatment for ET primarily focuses on resistance training. Typically, this is done on the upper extremity to improve strength and coordination, and to reduce tremor severity. Six weeks of resistance training using bicep curls, wrist flexion and extension has been shown to improve these outcome measures. Even performing isometric loads on the index finger reduces pointing variability for patients with ET. In addition to resistance training, behavioral relaxation therapy has been shown to decrease the tremor severity. Behavioral relaxation involves putting the patient into a relaxed position using a ten-step progression. This is done because stress has been shown to increase the severity of tremors.
- Resting tremor
- Postural tremor
- Kinetic tremor
- Isometric tremor
- Task-specific tremor
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