DEMENTIA-RELATED MYOCLONUS

How do giant SEPs differentiate cortical from subcortical forms?

Giant somatosensory evoked potentials (SEPs) are a key neurophysiological marker used to identify the anatomical origin of myoclonus. They primarily serve to confirm cortical hyperexcitability, distinguishing cortical myoclonus from subcortical and peripheral forms (Latorre et al. 2023; Riva et al. 2024).

Diagnostic principle and differentiation

Giant SEPs are time- and phase-locked EEG responses that assess the function of the dorsal column–lemniscal system (Latorre et al. 2023).

Cortical myoclonus: This form is characterised by "enlarged" or "giant" cortical SEPs, which reflect an abnormal hypersensitivity of the primary somatosensory cortex (S1) (Latorre et al. 2023; Riva et al. 2024). In most patients, the initial N20 component is normal, indicating that thalamo-cortical input is processed correctly; however, the subsequent P25–N33 components are significantly enlarged, reflecting pathological processing within the sensory cortex (Latorre et al. 2023).
Subcortical myoclonus: In forms generated by the brainstem or spinal cord (subcortical-nonsegmental or segmental myoclonus), SEPs are typically normal or absent (Riva et al. 2024). Neurophysiological tests like SEPs are used in these cases specifically to exclude the presence of a cortical generator (Riva et al. 2024).

Amplitude and threshold data

While no universal definition of a "giant" SEP exists, researchers use several established thresholds to distinguish them from normal responses:

Standard thresholds: Early definitions by Shibasaki and colleagues identified SEPs as giant if the P25 was >8.6 µV or the N33 was >8.4 µV (Latorre et al. 2023).
Alternative definitions: Other studies define "giant" as an amplitude greater than the average of healthy subjects plus 2–3 standard deviations (SDs) (Latorre et al. 2023).
Extreme values: In some cortical myoclonus disorders, the response can be 5 to 10 times larger than that seen in healthy volunteers (Latorre et al. 2023).
Specific disorders: In patients with progressive myoclonus epilepsy (PME), the N33 component has reached an average of 41 µV, with ranges as high as 75 µV (Latorre et al. 2023).

Prevalence and sensitivity statistics

The sensitivity of giant SEPs for identifying cortical myoclonus varies significantly depending on the underlying pathology (Latorre et al. 2023):

Overall, two recent comprehensive studies found that giant SEPs were present in only 21% to 39% of patients with presumed cortical myoclonus, suggesting that while they are highly specific for cortical involvement when found, they have relatively low sensitivity (Latorre et al. 2023).

Diagnostic limitations

Specificity issues: Giant SEPs are not exclusive to cortical myoclonus; they have been observed in other conditions without jerks, such as multiple sclerosis, motor neuron disease, and functional neurological disorders (Latorre et al. 2023).
Subcortical false-positives: In rare instances, "enlarged" (though not necessarily giant) SEPs may occur in patients with "benign" epilepsies that are not truly myoclonic (Riva et al. 2024).
Treatment interference: Pharmacological treatments, such as perampanel, can reduce the amplitude of SEPs, potentially leading to false-negative results during diagnostic testing (Latorre et al. 2023).