Introduction

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):

1. a shift in the tremor frequency, with pauses or other disruption to the tremor when tapping at a different frequency with the other limb;

2. poor task performance—a paradoxical and inexplicable difficulty to tap at a different frequency to the tremor with the other (usually unaffected) limb;

3. pure entrainment, when the tremor frequency shifts to match the frequency of tapping.

How to do it

This technique is for the most common clinical situation: asymmetric or unilateral tremor of the arm or hand. (see online supplementary videos and box 1).