Table 2 Listing of diagnostic methods in functional tremor

Test battery

The EMG based tests: tapping performance, tapping response at different frequencies, testing of the ballistic movement response, tonic coactivation, coherence testing, loading tests have been shown as an effective tool to differentiate different tremor forms in a neurophysiological test battery

[27]

Electromyography (EMG)

EMG can help to differentiate between different tremor forms, by using surface EMG and time-frequency analysis. Variation in tremor frequency and between different muscles can be recorded. In functional tremor fewer limbs and limb segments are involved than in essential or Parkinson tremor. Further, the variability of the frequency is higher. The term “frequency dissociation” describes frequency differences higher than 0.1 Hz between two extremities. This can usually not be detected in functional tremor but in organic forms.

[20]

somatosensory evoked potentials (SEP)

During self-initiated movements a reduction in the SEP amplitude can be found at the onset of the movement. This is called sensory attenuation (SA) and is reduced in functional movement disorders

([20, 22])

EMG coherence and cumulant analysis

Recording EMG of muscle pairs and performing coherence and synchronicity analysis shows different patterns in postural upper limb tremor occurring in different tremor associated movement disorders

[32]

Wavelet coherence analysis

Method that can detect coherence variations and phase differences between EMG signals. This can help to differentiate between organic and functional tremor forms and is more precise than conventional coherence analysis.

[21]

transcranial magnetic stimulation (TMS)

In a study comparing organic and functional dystonia the motor cortical inhibition (short interval intracortical inhibition, SICI) measured by TMS showed abnormal values in both organic and non-organic groups. Further, paired associative stimulation (PAS) was abnormally high in the organic patient group and normal in the functional disorder group. They also found enhanced facilitation of the motor evoked potentials (MEP) in organic dystonia patients, but not in the functional controls. This could be validated in TMS recordings using cortical inhibition (SICI, long interval intracortical inhibition, LICI and cortical silent period, cSP). A reduced cortical inhibition is discussed as a precipitating factor for both organic and non-organic dystonia.

[20]

pre-movement potential

By using back averaging, in simultaneous recordings of the EMG and EEG it can be analysed if the EMG activity is preceded by EEG activity. This so-called movement related cortical potentials (MRCP) precede the onset of voluntary movement that are self-initiated. Two components of the Bereitschaftspotential (BP) and the reafferent potential recorded after the EMG activity can be differentiated. The involuntary movements in patients with FMD have a BP with appearance similar to that of normal voluntary movements.

[20]

functional brain imaging (SPECT)

Distinct patterns of cerebral perfusion at rest and during motor tasks could distinguish between functional and essential tremor from ET. A study showed a deactivation of the default mode network

[8]

PET scan

Possibly, abnormal sensory integration is a part of the pathogenesis of functional tremor. PET showed a hypermetabolism of the posterior parietal lobes bilaterally in patients

[19]

Polymyography coupled to accelerometry

Multiple recordings of muscle activity coupled with frequency analysis can be used to differentiate between different tremor forms in experimental study designs. They characterize the typical features like spontaneous variability of the tremor frequency and frequency entrainment induced by contralateral rhythmic tasks

[2]