MRI and EEG findings differ significantly across myoclonus disorders, providing critical insights into the anatomical generator and underlying aetiology of the jerks. While EEG and polygraphy remain the "mainstay" for defining the origin of myoclonus, MRI is often used to identify specific structural abnormalities or neurodegenerative patterns associated with the symptom (Riva et al. 2024).

EEG and neurophysiological findings

EEG patterns are often highly suggestive of specific etiological conditions, though overlap exists between different disorders (Riva et al. 2024).

• Cortical myoclonus: This type is characterised by brief EMG bursts (typically <50 ms) that are time-locked to a preceding cortical transient (spike or sharp wave) on the EEG (Riva et al. 2024). The interval between the EEG spike and the muscle jerk ranges from 10–20 ms for arm muscles to 30 ms for leg muscles (Riva et al. 2024).
• Cortical-subcortical myoclonus: Observed in primary generalised epilepsies like Juvenile Myoclonic Epilepsy (JME), this pattern features spike/poly-spike and wave discharges at the EEG, with surface EMG bursts typically lasting 25–100 ms (Riva et al. 2024).
• Creutzfeldt-Jakob Disease (CJD): The classic EEG finding in CJD is diffuse periodic triphasic waves (Riva et al. 2024). It can also present as Radermecker complexes, which are high-voltage periodic repetitive polyphasic sharp and slow wave complexes lasting 0.5 to 2 seconds and recurring every 4 to 15 seconds (Ghosh et al. 2021).
• Neurodegenerative dementias:
  • Alzheimer's Disease (AD): EEG may show spike-wave (SW) complexes or central focal transients (Riva et al. 2024). Statistics show that subclinical epileptiform discharges occur in approximately 54% of AD patients (Zhang et al. 2022).
  • Dementia with Lewy Bodies (DLB): Patients with DLB have a significantly higher incidence of myoclonus, with a cumulative probability of 58.1% compared to 13.4% in AD and 3.0% in frontotemporal dementia (FTD) (Beagle et al. 2017).
• Metabolic and infectious disorders:
  • Hepatic encephalopathy: Typically shows diffuse triphasic waves often associated with negative myoclonus (Riva et al. 2024).
  • Subacute sclerosing panencephalitis (SSPE): Features symmetrical periodic delta wave complexes occurring at long intervals, associated with generalised jerks (Riva et al. 2024).

Diagnostic data and statistics

• Jerk-locked back-averaging (JLBA): This technique confirms a cortical origin in approximately 70% of clinically suspected cases (Latorre et al. 2023).
• Giant somatosensory evoked potentials (SEPs): Once considered a definitive marker, giant SEPs are only found in 21% to 39% of patients presumed to have cortical myoclonus (Latorre et al. 2023).
• Cortico-muscular coherence (CMC): High coherence (functional brain-muscle linkage) is found in approximately 85% of cortical myoclonus cases, predominantly in the beta and gamma frequency bands (Latorre et al. 2023).
• Burst duration: Research indicates the mean burst duration for cortical myoclonus is 31.1 ms, which is significantly shorter than non-cortical myoclonus at 56.7 ms (van der Veen et al. 2024). At a threshold of 45.0 ms, burst duration distinguishes the two with 100% sensitivity and 89.5% specificity (van der Veen et al. 2024).

MRI and neuroimaging findings

MRI findings range from nonspecific atrophic changes to pathognomonic signal abnormalities depending on the disorder.

• Prion diseases (CJD): MRI sensitivity for abnormalities ranges from 69% to 92% (Jurcau 2024). Common findings include basal ganglia hyperintensities (specifically in VV2 and MV2 subtypes) and cortical ribboning (common in MM subtypes) (Jurcau 2024). Some cases show bilateral symmetrical hyperintense lesions in the thalami and brainstem on T2 and FLAIR sequences (Ghosh et al. 2021).
• Alzheimer's Disease: While conventional MRI may only show diffuse atrophy (Pascarella et al. 2022), volumetric analysis in AD patients with subclinical epileptiform activity reveals increased volumes in the left frontal, temporal, and entorhinal cortex compared to those without such activity (Kalyvas et al. 2024).
• Autoimmune encephalitis: MRI often displays hyperintense signals in the medial temporal lobes (typical for LGI1 and GABAB receptor antibodies) or multiple cortical and subcortical areas (GABAA receptors) (Jurcau 2024).
• Progressive myoclonus epilepsies (PMEs): Neuroimaging usually reveals non-specific atrophic changes or features associated with specific genetic mutations (Riva et al. 2024).
• Myoclonus-dystonia: In cases of isolated myoclonus or myoclonus-dystonia, conventional MRI usually holds no real diagnostic value (Riva et al. 2024). However, advanced connectivity studies suggest cerebellar involvement in both cortical and subcortical forms (Riva et al. 2024).

Incidence and prevalence data

• General incidence: Pathological myoclonus has an estimated annual incidence of 1.3 cases per 100,000 persons per year (Riva et al. 2024).
• Dementia population: The cumulative probability of developing myoclonus in neurodegenerative dementias is 42.1% overall (Beagle et al. 2017).
• Myoclonus and seizures: In dementia patients, 33.3% of those with seizures also have myoclonus, whereas only 10.1% of those without seizures exhibit the symptom (Beagle et al. 2017).