Functional (psychogenic) movement disorders are seen in 3–5 % of all cases of most movement disorder clinics, and functional tremor (FT) is the most common presentation.

The natural history of the onset of FT is quite variable: FT often has an abrupt onset with rapid progression to maximum severity. Tremor may be episodic with spontaneous remissions in between episodes. Episodes in one limb may spontaneously resolve and then recur in another part of the body. Patients with FT and other functional movement disorders often have other unexplained somatic complaints such as fibromyalgia, atypical chest pain, and irritable bowel syndrome.

Two pathogenic mechanisms seem to play a role in FT.

1. Pathological tremor, such as tremor in Parkinson's disease (PD) is typically independent between different limbs, a phenomenon sometimes termed frequency dissociation (Orthostatic Tremor is an exception). Many patients with FT do not have independent oscillations, and have difficulty in maintaining two or more distinct rhythms in different body parts.  In PD and ET, the observed inter-limb frequency dissociation can be explained by simultaneous central oscillators. On the other hand, in FT, the motor system is intact and rhythmic contraction is likely to be mediated by top-down synchronization involving a common oscillator system at a higher cerebral level.
2. Rarely, independent rhythms can also be found in FT patients, e.g. in the case of lower limb tremors. They most likely represent oscillations on the basis of a clonus-like mechanism, which are enhanced by the ongoing co-contraction of antagonistic muscles.  This mechanism can be detected with the co- activation sign (voluntary-like co-contraction can be felt in both movement directions when passive limb movements are imposed).

WHAT FEATURES OF THE EXAMINATION WILL HELP TO ACHIEVE THE DIAGNOSIS?

Schwingenschuh et al. have proposed a set of EMG and accelerometry markers to be able to establish a positive diagnosis of psychogenic tremor, instead of a diagnosis of exclusion¹.

Markers included:

1. Incorrect tapping performance at 1, 3, and 5 Hz
2. Entrainment
3. Suppression
4. Pathological frequency shift at 1, 3, and 5 Hz
5. Pause or 50% reduction in amplitude of tremor with ballistic movements
6. Tonic co-activation before tremor onset
7. Coherence of bilateral tremors
8. Increase of tremor amplitude with loading

A standard approach to assessing the presence of FT is to examine the following:

• Variability and tremor frequency
• Entrainment 
• Distractibility
• Distractibility and the Ballistic Test
• Selective Disability
• Suggestibility
• Influence of adding weight to the limb
• Co-activation sign

VARIABILITY

Examine for variability of the tremor with respect to frequency, direction, and amplitude. The examiner should observe the patient’s tremor activity during history taking, a time when the patient may not be focusing on the tremor.   Most functional tremors have a similar amplitude at rest, during posture holding and on action; that is, FT can occur in all positions, which is unusual for other tremors. Note that pathological tremor, however, can have variable amplitude often increasing with anxiety. Furthermore, some pathological tremors, such as the head tremor in patients with essential tremor or cervical dystonia, may be quite irregular in rhythm and may change direction (e.g., changing from vertical to horizontal) which does not necessarily indicate FT.

With respect to variability, attentional mechanisms are clearly of great importance, as exemplified by an increase of tremor when the patient focuses on their tremulous limb, and a decrease of tremor with distraction.

The patient is a 70-year-old retired medical transcriptionist with severe depression who developed intermittent tremor in the right leg after a fall. The tremor then spread to the other leg and both arms but continues to be intermittent. She also has intermittent stuttering speech

(vv)4F.mp4 (tt)

.

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

Frequency

Recording tremor frequency is not directly helpful since the frequency of psychogenic tremor lies between 4 and 10 Hz, and there is a large overlap between psychogenic tremor and organic tremor frequencies, except for orthostatic tremor.

Low frequency (3 Hz) psychogenic tremors are rare. The upper frequency limit of voluntary mimicked tremor is 11 Hz, so that any tremor that is faster than 12 Hz is very likely to have an organic aetiology. As it is very difficult to maintain a voluntary oscillatory limb movement at the same frequency for any length of time, frequency variability in the same limb segment and in the same postural condition is strongly suggestive of a psychogenic tremor.

The patient is a 56-year-old female presenting with generalized tremor. Tremor started in the right leg, spreading shortly after to the left leg and both arms. Tremor is episodic (each episode lasts up to 20 minutes) and leaves her feeling exhausted. The frequency varies markedly in the same and in different body parts, alternates between the two legs, and involves different body parts or the whole body.

(vv)3F.mp4(tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

 

ENTRAINMENT

The examiner should attempt to entrain the tremor by asking the patient to tap with the non-tremulous hand at various frequencies and in different directions to the hand that is believed to have FT.   The entrainment manoeuvre acts as a distracting manoeuvre affecting the attentional dysfunction which is a feature of FT (as also seen during contralateral ballistic movement)³.

This test is reported to be both sensitive and specific⁴.

Preferably, attempts at entrainment should be paced using a metronome and tested at different frequencies, attempting to avoid using a frequency that is harmonic to the abnormal movement studied. Slower speeds may be more effective.

If entrainment does occur, the entrained tremor will match the speed or the direction of the new task. The tremor may also become disrupted during these tasks, becoming jumpy and irregular, or become suppressed.  Frequency driving can be elicited in tremors of all the body parts including the legs and trunk.

It may be of importance to note that mirror movements are frequent in PD. They may occur in the tremulous hand, influencing the ongoing resting tremor, and can thus appear as “entrainment”⁵ as may be seen in the video below:

(vv)6EntrainmentMirrormovementPD.mp4(tt) 

From: Zach H, Dirkx M, Bloem BR, Helmich RC. The clinical evaluation of Parkinson’s tremor. J Parkinsons Dis. 2015;5(3):471-474. doi:10.3233/JPD-150650

Entraining other parts of the body³.

• Leg: leg tremors typically develop through a clonus mechanism, with the patient repetitively pushing down with the foot plantar flexed. Merely by moving the patient from this position the tremor, should stop, which is a useful sign. The entrainment test can also be performed in the legs by asking the patient to tap their unaffected or less affected foot in time to a rhythm set by the examiner with their foot, while looking for changes in the tremor or poor task performance.
• Head: head tremors are not easy to entrain by tapping of thumb and forefinger or the feet. Alternatively, patients can be asked to perform rotatory movements of the forearms 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.
• Palate: tapping movements of the hands are often sufficient to cause tremor frequency shift in such patients.

The patient is a 68-year-old female with sudden onset of right arm tremor while at church. Her right hand tremor is intermittent and accompanied by fluctuating truncal swaying movements, which later evolved into total body rhythmical but irregular movement.

(vv)5F.mp4 (tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

 

 

 

(vv)PractneurolVideo1.mp4(tt)

From: Roper LS, Saifee TA, Parees I, Rickards H, Edwards MJ. How to use the entrainment test in the diagnosis of functional tremor. Pract Neurol. 2013;13(6):396-398. doi:10.1136/practneurol-2013-00054

DISTRACTIBILITY

When the tremor is being examined directly, it may become more obvious, as compared to during history taking. However, when the examiner changes their focus to another limb or engages the patient in a specific task, the tremor in the affected limb may diminish. The examiner can create a distracting task such as asking the patient to perform serial sevens or other reasonably complex manoeuvres which engage the patient’s attention.

The entrainment manoeuvre acts as a distracting manoeuvre affecting the attentional dysfunction which is a feature of FT (as also seen during contralateral ballistic movement).

For example:

• Serial subtraction of 7 from 100 (sensitivity 72.7% specificity 73.3%)
• Tapping different fingers on to their thumbs in sequence (sensitivity 58.3%, specificity 84.4%)⁶.  Other studies have reported that distractibility is a very sensitive test, occurring in virtually all FT patients, and that it has high specificity⁴.

 

The patient is a 43-year-old female with a history of depression who presents with episodic tremor and unsteady gait. The tremor started in her legs while undergoing physical therapy postoperatively for cervical stenosis and progressed to episodes of generalized shaking.

(vv)1F.mp4(tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

The patient is a 39-year-old female with a history of chronic pain and anxiety presenting with sudden onset of right arm tremor. Tremor is mild in the morning and increases in intensity with increased activity. The tremor interferes with many activities and she is now dependent on her daughter’s assistance for activities of daily living.

(vv)2F.mp4(tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

 

Distractibility and the Ballistic Test

Patients are asked to perform a ballistic movement with the contralateral limb.  There is a significant reduction of tremor amplitude or cessation of contralateral tremor in psychogenic tremor patients. 

 

(vv)PractneurolVideo2.mp4(tt)

 

Note that in some PD patients, the tremor may cease at the onset of a ballistic movement, incorrectly suggesting a FT, as shown in the video below⁵.

(vv)7BallisticmovementPD.mp4(tt)

From: Zach H, Dirkx M, Bloem BR, Helmich RC. The clinical evaluation of Parkinson’s tremor. J Parkinsons Dis. 2015;5(3):471-474. doi:10.3233/JPD-150650 

SELECTIVE DISABILITY

Selective disability refers to when a patient demonstrates severe tremor on finger-to-nose testing but then is able to hold a cell phone to his ear to have a conversation.

 

SUGGESTIBILITY

The tremor may be suggestible, changing in severity based on cue; for example, the tremor may increase with hyperventilation or decrease with vibration of a tuning fork, when the examiner suggests these responses before using these techniques. In general, physicians should be cautious when applying such an approach, and consent may be indicated to avoid conflict, which may arise if patients feel that their examiner has misled them.

The patient is a 55-year-old male with sudden onset of truncal tremor after standing up from the dinner table. This episode lasted for several hours. He continued to have episodes of truncal and limb tremor. Tremor causes unsteady gait and he now uses a walker.

 

(vv)7F.mp4(tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

 

 

The patient is a 52-year-old medical billing and coding specialist with a history of reflex sympathetic dystrophy who developed tremor the weekend after a hurricane. The tremor occurs in episodes, varies in intensity, at times causing total body tremor. She has intermittent stuttering. She is separated from her husband and lives with her mother.

(vv)8F.mp4(tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

INFLUENCE OF ADDING WEIGHT TO THE LIMB

A functional tremor may worsen when the patient is provided a weighted object to hold; an unusual finding for most organic tremors of central origin. This may be seen in as many as 70% of patients with functional tremor, who show worsening with addition of a weight. However, the sign is not specific, since it may also be seen in essential tremor and parkinsonian tremor (20% and 5% of the patients, respectively), particularly in severe tremors. Other than the frequency, a paradoxical increase of both amplitude and frequency or a change of the location (i.e. from proximal to distal part of the limb or from one limb to another) or a sudden shift of the direction (i.e. from flexion/extension to pronation/supination) of the tremor should be searched for, as indicators of FT.

See: Weight-loading

CO-ACTIVATION SIGN

Co-contraction of the muscles of the tremulous joint is frequent in FT. On clinical examination, as the limb is moved passively, the examiner may feel voluntary co-contraction (rhythmic trembling).  In addition, the examiner experiences variability: the patient may allow passive movements within a limited range, but the range may subsequently increase, and the apparent increase in tone diminishes or breaks down, accompanied by disappearance of tremor.

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 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: if tremor onset is captured, co-contraction is seen as increased background EMG signal prior to the re-emergence of oscillatory activity (see figure below). The co-contraction is thought to allow the triggering of tremor initiation. By contrast, organic tremor occurs without such co-activation, the figure below showing a the typical example of parkinsonian rest tremor.

Cogwheeling in the setting of Parkinson’s disease or essential tremor differs from the coactivation sign because cogwheeling is present over the whole range of movement of a particular joint.

 WRITING AND DRAWING

Marked intertask and intratask variability when performing the pen and paper assessments suggests functional tremor.

 

 

 

 

 

 

 

 

 

 

  

  
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The patient is a 37-year-old female with bilateral hand tremor for eight years and a family history of tremor. Tremor is alcohol-responsive and worse with exercise, fatigue, and stress. It has worsened over the past few years. On examination she has some atypical features, including distractibility, suggestibility, and an irregular spiral. She has a combination of essential tremor and FT.

(vv)9F.mp4(tt)

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

ELECTROPHYSIOLOGICAL STUDIES

Recording of the limb affected by tremor using surface electromyography and an accelerometer of a few relevant muscles allows for measurement of:

1. Tremor frequency (Distractibility, Entrainment and Variability)
2. Ballistic Test
3. Pattern of cocontraction between antagonist muscles.
4. EMG burst duration
5. Coherence

1. Tremor frequency

Distractibility: While tremor is recorded, the patient performs a distracting task, either mental or motor.  Record baseline tremor prior to the distraction, and continue to record following the distraction task, attempting to identify changes in tremor pattern.

Recordings may show brief interruptions of the tremor, change in frequency, or transient decrease of amplitude.  Variation in the EMG pattern (i.e. from rhythmic to shuddering pattern) may be seen. 

On accelerometry, abrupt interruptions of the signal may be distinguished from the typical waxing and waning modulations seen in PD. This is an important point, given that rest tremor in PD is mainly intermittent, and highly modulated by the emotional state and mental activation level.

Entrainment: Recording of tremor frequency can confirm entrainment.  Simultaneous recording of the contralateral limb is critical to demonstrate the frequency driving induced by a variety of manoeuvres such as finger tapping or other movements driven by a metronome.

 

At (a) when the left arm is at rest, the spontaneous tremor frequency of the right arm is 5.8 Hz, as shown by EMG and also by the accelerometer at (b).

The patient is then asked to tap with the left hand at a 3.5 Hz rhythm paced by a metronome, as shown at (c). The tremor frequency of the FT in the right arm switches to the voluntarily paced frequency of 3.5 Hz, as shown at (d).

After the test (B), the newly acquired frequency of 3.5 Hz persists in the right arm, as seen at (f).

ECR: extensor carpi radialis; FCR: flexor carpi radialis⁸.

 

 

Variability: In FT, while attempting to produce two rhythms with distant body parts, either the externally paced or the self-generated rhythm is typically poorly maintained. By contrast, voluntary motor tasks are not affected by the presence of an organic tremor in the opposite arm or leg.

Bilateral recording allows for evaluation of the effect of tremor on motor performance of the contralateral limb. Contralateral voluntary motor tasks may be significantly impaired when tremor is present, but are preserved when FT ceases.

The effect of unilateral FT on contralateral motor performance can be assessed using bilateral recordings: the patient is instructed to generate a rhythm in one hand, while the arm affected by FT is held at rest.  This regular rhythm can be contrasted to a different state during which the FT arm is lifted up, inducing the FT, and often resulting in the development of a disorganized movement in the arm not affected by the FT.

2. Ballistic test

As described above, the patient is asked to perform a rapid movement with one hand, for example, touching the examiner's finger, or a fast wrist extension. During this movement an interruption of the tremor in the contralateral hand (usually for more than 300 ms) is a sign compatible with functional tremor.
 

3. Pattern of cocontraction at tremor onset

If tremor onset is captured, co-contraction is seen as increased background EMG signal prior to the re-emergence of oscillatory activity (see figure below). The co-contraction is thought to allow the triggering of tremor initiation. By contrast, organic tremor occurs without such co-activation, the figure below showing a the typical example of parkinsonian rest tremor.

 

Muscle activation starts with a tonic contraction for approximately 300 ms of the wrist flexors and extensors, before reciprocal alternating tremor bursts develop.  This initial contraction can be shown in many patients with functional tremor. By contrast, a patient with PD shows a reciprocal alternating pattern of tremor which is present from the onset of movement (B)⁹.

 

4. EMG burst duration

Burst length is typically greater than 50 ms in psychogenic tremor. Tremor bursts of very short duration are likely to be organic, and, in particular, burst durations less than 50 ms are characteristic of cortical myoclonic tremor and primary orthostatic tremor.

5. Coherence

Tremor in multiple body parts, at the same frequency, is an example of coherence of tremor. Tremor coherence in multiple body parts is suggestive of FT. In particular, episodes of ‘‘total body tremor’’ are most consistent with FT. However, tremor coherence between limbs is not seen in all FT patients.

Inter-limb EMG coherency demonstrates the temporal link between two tremors occurring in different body parts. When a high degree of coherence is found it is likely that both tremors have a common generator in the brain or depend on two coupled systems. For example, when a control subject voluntarily beats a drum at the same frequency with both hands, right-left coherency is high. By contrast, there is no right-left coherence in essential or parkinsonian tremors, since in these conditions there are distinct central uncoupled oscillators involved for the different tremulous limbs.

 

The patient is a 79-year-old male with severe depression and sudden-onset upper body tremor. The tremor was first episodic with normal periods in between. Tremor worsened during a cardiac stress test and is now constant with intermittent exacerbations. Tremor improves when lying down.

(vv)6F.mp4(tt)

 

From: Thenganatt MA, Jankovic J. Psychogenic tremor: a video guide to its distinguishing features. Tremor Other Hyperkinet Mov (N Y). 2014;4:253

 

Downloadable PDF:   Assessment of Functional Tremor 

EXAMINATION FUNCTIONAL TREMOR (FT)

 

---