Psychogenic tremor may well be the most common psychogenic movement disorder. Prognosis is poor because the tremor persists despite treatment attempts in up to 90% of the patients on long-term follow-up (1) and more than 50% of the patients are unable to follow their professional careers (2,3). The epidemiology has never been formally investigated. There was an epidemic outbreak of psychogenic tremor during the First World War (4), but during more recent wars like the Vietnam War (5), tremor was found less frequently. The reasons for this peculiar difference have never been elucidated. Currently, the data from movement disorder referral centers show a prevalence of psychogenic tremor of about 2% to 5% of all referred patients with tremor (3,6).

The diagnosis requires careful neurologic assessment, but objective and more reliable tests are necessary. Some neurophysiologic tests have been proposed to confirm the diagnosis (3,7, 8, 9, 10, 11, 12, 13, 14, 15), but the sensitivity and specificity of these tests have not yet been sufficiently tested. The treatment of psychogenic tremor is not yet elaborated and only some small, uncontrolled cohort studies have been published.

The current paper summarizes the clinical diagnosis, electrophysiologic tools that may assist the diagnosis, and hypotheses on the pathophysiology of tremor.

Psychogenic tremor is not a diagnosis of exclusion, but can be separated on clinical grounds (see Table 31.1), and the diagnosis can be confirmed by additional electrophysiologic tests. Several studies have revealed very similar findings when assessing patients with psychogenic tremors clinically (3,6, 7, 8,16, 17, 18, 19). We will summarize these findings here.

Almost all of the papers refer to a past medical history for conversion disorders. However, this is not found in all the cases, and the patient’s report may also be incomplete.

Most psychogenic tremors start suddenly and may have one or several spontaneous remissions and relapses. This is usually reported by the patients. Sometimes patients present in the emergency room with the first manifestation of their tremor. We saw this in 15% of our own series.

Another clinical feature which is only rarely mentioned is the fatigue of tremor (8). When patients are observed for extended time periods, the tremor may cease or at least diminish in amplitude. This fatigue can be clearly distinguished from the “waxing and waning” of many largeamplitude organic tremors.

All the descriptions mention that patients sometimes have unusual combinations of rest, postural, and action tremors. The detection of such special clinical features strongly depends on the experience of the investigating physician, and this is not an easily applicable clinical test. The important aspect is probably the reproducibility of tremor during various maneuvers: Organic tremors usually have a very similar movement performance when specific motor acts are repeatedly performed and the tremor always interferes mainly with specific parts of the movement. This may vary in psychogenic tremors very much. However, again, no data are available on the reliability of this criterion.

The variability of tremor frequency is usually easier to assess. Tremor frequencies can be separated clinically when they differ more than 1.5 to 2 Hz. In patients with psychogenic tremor, larger changes are sometimes observed that can then be regarded as a sign of psychogenic tremor. Organic tremors do not show such large variations (11,14,20).

An important part of the clinical investigation is the assessment of distractibility. Such distraction can be loading patients mentally (counting backwards) or asking them to perform complicated motor tasks. The expected result is then a loss of regularity of tremor, a change of frequency, a meaningful amplitude reduction, or even a cessation of tremor. A very elegant and easily applicable test is the entrainment test of tremor frequency, which is a formalized way to test distractibility clinically. The patient is asked to perform rhythmic voluntary movements with the less affected hand. The frequency of the imposed rhythmic movements should clearly differ from the tremor frequency and is usually chosen between 2.5 and 4 Hz. If patients cannot produce such a rhythm internally, the investigator can pace them externally. The observed parameter is the change of frequency in the contralateral extremity affected by tremor. It will be explained later why the entrainment test may be meaningfully explained in terms of pathophysiology.

The cocontraction sign is another important sign for the diagnosis of psychogenic tremor. The physician has to move the distal extremity around one joint (usually the wrist joint) while the tremor is present. It is the same testing procedure as for rigidity. The physician feels the resistance in both directions during tremor. This enhanced resistance persists as long as the tremor is present and ceases when the tremor stops. It is a kind of give-way relaxation when the tremor stops, and is one of the clinically reliable signs for the diagnosis of psychogenic tremor.

Finger tremor is mostly absent in psychogenic tremor. Finger tremor can be found when patients make a tremor by voluntary alternating movements, but not if they use the cocontraction mechanism for the production of the tremor.

The separation of tremors is based on various clinical features and can be assisted by electrophysiologic parameters like the frequency or amplitude of tremor, and more complex measures like the coherence between different muscles. Recently, these electrophysiologic techniques were applied to patients with psychogenic tremors. Some of these tests may even have a high specificity. The following summarizes the tests which have been reported to be helpful.