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Functional (psychogenic) tremor is the commonest functional movement disorder.1 As with other functional neurological symptoms, a key diagnostic component is to search for positive physical signs.1 ,2
A key positive sign in a patient with functional tremor is a change in the tremor with distraction of attention.1 There are several potential distraction tasks, but the ‘entrainment test’ is the most important.2 ,3 Here, the clinician asks the patient to tap another limb at a different frequency to the tremor. We present a method of performing and interpreting the entrainment test.
What are we looking for?
The term ‘entrainment test’ suggests that we are looking for ‘pure’ entrainment. This is where the tremor frequency switches to match exactly the frequency of a voluntary rhythmical movement performed by the unaffected limb. In reality, this is not that common—tremor recordings found pure entrainment in only 5 of 13 patients2 and 2 of 6 patients.3 In practice, therefore, we are looking for three broader responses to the entrainment (see online supplementary videos):
a shift in the tremor frequency, with pauses or other disruption to the tremor when tapping at a different frequency with the other limb;
poor task performance—a paradoxical and inexplicable difficulty to tap at a different frequency to the tremor with the other (usually unaffected) limb;
pure entrainment, when the tremor frequency shifts to match the frequency of tapping.
How to do it
Aide Mémoire for Performing and Interpreting the ‘Entrainment Test’.
What to do
Place the patient in the position in which there are the most tremors (usually arms out in front).
Move the least tremoring or non-tremoring limb slightly away from the other and ask the patient to focus on it.
Ask the patient to tap with their thumb and forefinger to a rhythm set by you, which you should vary, making it faster and slower. Encourage the patient to keep following you.
What to look for
The frequency of the tremor shifts to match the frequency of the tapping (pure entrainment)
The frequency and rhythmicity of the tremor changes; for example, pausing, speeding and then slowing.
The patient cannot keep up accurately with the tapping task with their unaffected or less affected hand (poor task performance).
Sit opposite the patient and perform a ‘normal’ tremor examination. This includes looking for tremor at rest, on posture and during action. This allows you to assess the tremor and to determine the position in which it is most prominent (usually with the arms held straight out in front).
Put the patient in the position with the most prominent tremor. Take the limb with the least tremor and ask the patient to focus visually on this. This is important as visual attention towards the tremoring limb plays an important role in generating functional tremor.4 It can help to move the non-tremoring limb slightly away from the tremoring limb.
Ask the patient to look at your hand and to tap their thumb and forefinger together in time with the frequency you are setting with your thumb and forefinger. This is a crucial component of the test, as a simple self-paced tapping does not sufficiently distract attention. Vary your tapping speed, asking the patient to try to match you exactly. Watch the other limb for a change in tremor frequency including pauses. You should also monitor the performance of the tapping task itself.2 Poor task performance—inexplicable inability to follow a simple tapping movement with the non-tremoring hand—suggests that both distractibility and entrainment are contributing equally to the tremor.
Sensitivity and specificity
In a study using tremor recordings to assess specificity and sensitivity of various examinations, manoeuvres to diagnose functional tremor, there was pure entrainment in only 5 out of 13 patients with functional tremor and in none of the 25 patients with organic tremor.2 This suggests that pure entrainment has low sensitivity (39%) but high specificity (100% in this study). However, the entrainment test's sensitivity increased markedly when taking into account ‘abnormal tremor frequency shifts’. ‘Abnormal’ changes in tremor frequency were those that were less than 2 SDs above the mean baseline frequency of the organic tremor patients, giving a sensitivity of 77% and specificity of 88%.2
Using the entrainment test for tremor outside the arms
There are limited data to suggest the best method for examining such patients. In our experience, leg tremors typically develop through a clonus mechanism, with the patient pushing down with the foot plantar flexed. Just moving the patient from this position should stop the tremor, which is a useful sign. The entrainment test can be performed in the legs by asking the patient to tap their unaffected or less affected foot in time to a rhythm set by us with our foot, while looking for changes in the tremor or poor task performance as described above. Head tremors are not readily interfered with by tapping of thumb and forefinger or the feet. In this situation, we ask patients to perform rotatory movements of the forearms (similar to the pronation/supination test for bradykinesia on the Unified Parkinson's Disease Rating Scale5) in time to a rhythm set by the examiner, or to perform rapid side-to-side movements of the tongue. Again, one is looking for a change in the tremor or inexplicable poor task performance. Functional tremors of the palate can also occur.6 Tapping movements of the hands are often sufficient to cause tremor frequency shift in such patients.
Putting the entrainment test in context
The entrainment test is just one part, yet an important part, of the assessment of a patient with suspected functional tremor. The assessment begins with historical features that are common in functional tremor but uncommon in organic tremors, including sudden onset, remissions and relapses, unusual triggers and relieving manoeuvres. Functional tremor is often present at rest, on posture and during action, something that is unusual in organic tremor (outside the setting of Holmes’ tremor). Aside from the features of the entrainment test discussed above, another useful test (see online supplementary video) is to ask patients to perform a ballistic movement to a target (such as the examiner's finger) with the non-tremoring hand. This causes a brief pause in functional tremor (‘pause with ballistic movement’—see online supplementary video).2 ,8 Some patients generate functional tremor primarily by cocontraction and, on occasion, this may be appreciated clinically. When patients are asked to make quick movements of the tremoring limb—for example, to move quickly from arms outstretched to arms bent at the elbows—the tremor will stop briefly before restarting in the new position: this is because cocontraction and rapid movement cannot occur together. However, it may be hard to recognise this sign in a clinical setting and thus this may be a situation where formal tremor studies can help, for example, in looking for cocontraction in the electromyography recording just before the onset of tremor (the coactivation sign). Tremors produced by cocontraction may well produce a negative entrainment test, even when the test is performed carefully.4 Weight loading of the tremoring limb typically suppresses the amplitude of an organic tremor (as it changes the natural oscillatory dynamics of the arm). Conversely, loading often causes a marked exacerbation of functional tremors.2 Similarly, restraint of the affected limb by the examiner usually stops or markedly suppresses an organic tremor, while in some patients with functional tremor, this considerably increases the tremor, sometimes causing it to spread, inexplicably, to other body parts. However, no single test or historical feature is diagnostic in isolation, and the whole clinical picture is needed for a thorough assessment of each patient.
Some patients have a functional tremor on top of an organic tremor or other organic neurological disorder. This is always difficult and emphasises the importance of careful assessment and follow-up to monitor for new symptoms and signs.
Using the entrainment test beyond diagnosis
The entrainment test is an excellent, positive method to separate functional tremor from organic tremors, but is that its only use? Positive signs in patients with functional motor disorders, including a positive entrainment test, can help in explaining to the patient how the diagnosis was reached and how this might inform treatment.9 This form of ‘rational persuasion’ may well be better than a more negative explanation based on normal scans and other tests.10 Also, some patients can use distraction techniques similar to those used in the entrainment test to reduce and stop their own tremor as part of a process for gaining control over the tremor in everyday life.
The entrainment test is a sensitive and specific test for functional tremors if used in its broadest sense (not just ‘pure entrainment’). It is relatively easy to learn and is a useful addition to the diagnostic toolbox for functional motor disorders.
Competing interests None.
Provenance and peer review Commissioned; externally peer reviewed. This paper was reviewed by Jon Stone, Edinburgh, UK
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