Table 1

Potential diagnostic biomarkers in functional movement disorders

BiomarkerTechniqueGroup sizesProsConsSensitivitySpecificityPositive predictive value
Tremor
Test battery5 EMG and accelerometer recordings of upper limbs in relaxed condition, outstretched with and without weight loading, during tapping tasks and while performing ballistic movements.FMD: 38
PD: 24
ET: 19
Dystonic tremor: 19
Other type of organic tremor: 11
High sensitivity and specificity differentiating functional and organic tremor.
EMG/accelerometer are accessible techniques.
Unknown value for differentiating pure functional tremor from functional overlay; functional tremor can be diagnosed clinically with high-level confidence in most patients.90%96%92%
Myoclonus
Bereitschaftspotential and event-related desynchronisation15 EEGFMD: 29
Cortical myoclonus: 16
High specificity.
EEG is widely available.
76%100%100%
Paresis
EMG activity16 EMG of the affected hand while performing finger abduction of the non-affected hand.FMD: 10
Healthy controls: 36
Acute organic paralysis: 11
High sensitivity and specificity.
EMG is widely available.
100%100%100%
Quantified Hoover’s test17 18 Measuring force of involuntary and voluntary hip flexion in Hoover’s test. ‘Hoover’s index’—ratio of involuntary/voluntary pressure force.FMD: 9
Healthy controls: 9
Stroke: 9
Paresis due to pain (lumbar radiculopathy): 9
High sensitivity and specificity of ‘Hoover’s index’ (cut-off 1.4) in differentiating functional paresis from both organic paresis and feigners.Uncertain advantage in comparison to standard Hoover’s test.100%100%100%
Mixed
A model of functional connectivity19 Resting state fMRI. Hyperconnected right caudate, left amygdala and bilateral postcentral gyri. Decreased functional connectivity in the right temporoparietal junction and frontal areas.FMD: 23
Healthy controls: 25
Usable in a mixed group of FMD.Only compared with healthy controls and not organic movement disorders.
Expensive and not accessible in every hospital.
70%68%Diagnostic accuracy: 69%21
Body sway20 Trunk inclination in transverse plane and body angular velocity measured by accelerometers while performing distraction manoeuvre.FMD: 12
Healthy controls: 12
MS: 12
Can differentiate FMD from both organic disease and healthy controls.The equipment is not widely available.100%100%100%
  • The most promising potential biomarkers for different phenotypes of functional movement disorders.

  • EEG, electroencephalography; EMG, electromyography; FMD, functional movement disorder; fMRI, functional MRI.