Approach

There are no validated serologic, radiologic, or pathologic markers of essential tremor (ET), and, therefore, the diagnosis remains a clinical one. The first step in evaluating the patient is to categorize the tremor. ET is the most common action tremor. History should provide a wealth of clues to support the diagnosis of ET.

There are now a number of commercially available products that can be used to perform tremor analysis. This is a dynamically developing area, with additional tremor studies, including longer-term monitoring with wearable devices, expected to enter clinical practice.

History

A slowly progressive clinical course is typical for ET, rather than an abrupt stepwise series of events.[2] The patient often describes difficulties with activities such as writing, eating, drinking, dressing, or any other activity that requires fine motor control (e.g., sewing, knitting, or drawing). The patient may also report that symptoms have abated following short-term use of alcohol, benzodiazepines, barbiturates, or gabapentin. The patient should be asked about use of prescription (particularly antipsychotics, lithium, and valproate) and/or over-the-counter medications, caffeine and alcohol consumption, and smoking habits.

Physical exam

Characterizing the tremor is of prime importance. The tremor of ET of moderate amplitude is very commonly present in posture (e.g., holding the arms out in front with elbows bent so that the fingers point at, but do not touch, each other), and is frequently present with goal-directed movement, and only rarely present at rest.[71] Rest tremor can occur in severe ET. The head and voice may be involved.

The presence of other tremors should be investigated and excluded. Asymmetry (ipsilateral arm or leg tremor), bradykinesia, and rigidity usually do not occur in ET. Similarly, signs of dystonia such as dystonic movements or posture, pain, a geste antagoniste, or muscular hypertrophy could sometimes occur in ET but should not be the predominant feature.[13] ET patients often have writing tremor, and spirals drawn by these patients show a distinct axis aligned with the orientation of tremor.[72]

Findings from studies in patients with ET suggest that several nontremor motor deficits (such as impairment of tandem gait and stance) and nonmotor deficits (such as hearing impairment, personality changes, and cognitive deficits) are frequently present.[3][73] However, the main finding on neurologic exam is always the tremor.

Laboratory or imaging studies

While there are no specific tests for ET, laboratory and imaging tests may be useful to exclude other conditions. For example, a low serum ceruloplasmin is useful to distinguish Wilson disease from ET, and if symptoms or signs of hyperthyroidism are present, then thyroid function tests should be performed. There is evidence to suggest that nuclear medicine studies on the striatal dopamine terminals may be useful in differentiating ET from Parkinson disease.[74] In addition, imaging studies (e.g., computed tomography scan or magnetic resonance imaging of head) may show unexpected mass and vascular lesions that contribute to the patient's presentation. However, the diagnostic validity of imaging studies has not been established for ET.[6]

Tremor physiology studies

Tremor analysis and tremor physiology studies should be performed by an experienced provider or center. The studies usually involve surface electromyography (EMG) recording from one or both hands, including agonist and antagonist muscles, together with a triaxial accelerometer. Testing usually includes at least 2-minute recordings of resting, postural, and kinetic conditions, and may also include weights or entrainment protocol (e.g., providing an audible metronome rhythm at varied frequencies; asking the patient to make rhythmic movements at a given frequency with the unrecorded limb). Alternative activation of agonist and antagonist muscles is a prerequisite of all tremor categories. Dystonic patients with tremor-mimicking “shakings” can be excluded by observing the co-contraction of both agonist and antagonist muscles, but not alternative contractions. Tremor frequencies can be obtained by spectrum analysis of the recordings from surface EMG and accelerometer. Clinical questions to which tremor physiology studies may provide useful answers include:

  • Identifying tremor and nontremor phenomena

  • Quantifying the tremor frequencies and their variabilities to distraction (entrainment protocol)

  • Excluding action tremor falsely identified as rest tremor

  • Identifying peripheral versus central origin of the tremor

  • Identifying psychogenic etiology.

Emerging tests

A tremor stability index (TSI), derived from the kinetic measurement of tremor, has been proposed to differentiate ET and Parkinson disease tremor. It has a maximum sensitivity of 95%, specificity of 95%, and diagnostic accuracy of 92%.[75]

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