REFERENCE · DIAGNOSTIC TECHNIQUES

Functional Jerks: Diagnostic Technique Inventory

These nine entries include observations, signal-analysis methods and differential investigations. They are not nine independently validated FND tests. The [EEG–EMG and back-averaging page](01-eeg-emg-and-jerk-locked-back-averaging.md) now explains the featured assessment, with its muscle-recording context. Quantified potentials and beta analysis remain brief adjunct discussions, not separate completed protocols.

Diagnostic techniques at a glance

Clinical variability and distractibility

The clinician compares the timing, distribution and task dependence of spontaneous jerks. Clear inconsistency can support a functional formulation, but irregularity also occurs in other myoclonus. Clinical observations guide laboratory testing rather than replace it. [2]

Surface EMG burst analysis

Electrodes record the duration of muscle activation during jerks. Longer or variable bursts can be supportive, while very brief bursts may suggest cortical myoclonus. There is overlap; no single burst-duration threshold establishes FND. [2][3]

Recruitment order and propagation

Recordings from several muscles identify where a jerk begins and how activation spreads. Variable recruitment can support functional axial jerks. A stereotyped spinal-looking pattern alone is insufficient because it can also occur in functional presentations. [4][5]

Stimulus latency and habituation

In a justified specialist protocol, variable or long response delays and changing responses to repeated stimuli may support a functional pattern. These are adjuncts with overlap, not standalone tests. Do not startle someone or repeat distressing stimuli for demonstration. [2]

Jerk-locked back-averaging and Bereitschaftspotential

EEG is aligned to repeated EMG-defined jerks to look for a slowly developing premovement signal. Its presence can support functional jerks against certain myoclonus comparators. Absence does not exclude FND, and its presence does not mean conscious intention. [6][7]

Quantified Bereitschaftspotential

Computer-assisted measurement supplements visual inspection of the premovement signal. A small comparative study suggests diagnostic benefit when combined with other measures. Artifact, too few jerks and analysis choices matter; this is not a separate bedside test. [7]

Time–frequency analysis looks for a decrease in beta activity before jerks. Original and subsequent comparative studies suggest that it can add information when the conventional premovement potential is absent. It remains a specialist adjunct with limited study populations. [7][8]

Cortical-myoclonus investigations

Jerk-linked cortical spikes, somatosensory evoked potentials and long-loop reflex testing help identify alternative causes. These are differential investigations, not positive FND tests. Normal results do not automatically establish functional jerks. [2]

Comparison with motor tics

Urges and temporary suppressibility can occur in both tics and functional jerks. Premovement potentials also overlap. The clinical course and tic history remain necessary; jerk laboratory findings cannot simply be transferred to functional tic-like symptoms. [1][6]

Research and Sources

The original source numbering is retained. This preservation does not constitute a new full-text review of every inventory entry.

Citation	Figure	Full citation
[1]	—	Edwards MJ, Koens LH, Liepert J, Nonnekes J, Schwingenschuh P, van de Stouwe AMM, Morgante F. Clinical neurophysiology of functional motor disorders: IFCN Handbook Chapter. Clinical Neurophysiology Practice. 2024;9:69–77. FND-CIT-0022. https://doi.org/10.1016/j.cnp.2023.12.006
[2]	—	van der Veen S, Caviness JN, Dreissen YEM, et al. Myoclonus and other jerky movement disorders. Clinical neurophysiology practice. 2022;7:285-316. DOI. PMID: 36324989. FND-CIT-0151.
[3]	—	Zutt R, Elting JW, van der Hoeven JH, et al. Myoclonus subtypes in tertiary referral center. Cortical myoclonus and functional jerks are common. Clinical neurophysiology : official journal of the International Federation of Clinical Neurophysiology. 2017;128(1):253-259. DOI. PMID: 27940047. FND-CIT-0186.
[4]	—	van der Salm SM, Koelman JH, Henneke S, et al. Axial jerks: a clinical spectrum ranging from propriospinal to psychogenic myoclonus. Journal of neurology. 2010;257(8):1349-1355. DOI. PMID: 20352254. FND-CIT-0118.
[5]	—	Erro R, Bhatia KP, Edwards MJ, et al. Clinical diagnosis of propriospinal myoclonus is unreliable: an electrophysiologic study. Movement disorders : official journal of the Movement Disorder Society. 2013;28(13):1868-1873. DOI. PMID: 24105950. FND-CIT-0138.
[6]	—	van der Salm SM, Tijssen MA, Koelman JH, van Rootselaar AF. The bereitschaftspotential in jerky movement disorders. Journal of neurology, neurosurgery, and psychiatry. 2012;83(12):1162-1167. DOI. PMID: 22952323. FND-CIT-0125.
[7]	—	Beudel M, Zutt R, Meppelink AM, et al. Improving neurophysiological biomarkers for functional myoclonic movements. Parkinsonism & related disorders. 2018;51:3-8. DOI. PMID: 29653908. FND-CIT-0122.
[8]	—	Meppelink AM, Little S, Oswal A, et al. Event related desynchronisation predicts functional propriospinal myoclonus. Parkinsonism & related disorders. 2016;31:116-118. DOI. PMID: 27477621. FND-CIT-0137.