Potential diagnostic biomarkers in functional movement disorders
| Biomarker | Technique | Group sizes | Pros | Cons | Sensitivity | Specificity | Positive 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 | EEG | FMD: 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.