Table 1.
Characteristic electrophysiological findings of neurodegenerative and infectious disorders that can presenting with myoclonus.
| Disorder | Prevalence of myoclonus | Surface electromyography | Electro-encephalography | Additional investigations |
|---|---|---|---|---|
| Alzheimer’s disease | Common | Different presentation of myoclonus across different cases and disease stages: 1) Multifocal myoclonus, with brief discharges in action and rest is most common. 2) Polyminimyoclonus, small, multifocal twitched in fingers and hands. 3) Rapid progression with rest and generalized myoclonus with < 100 ms discharges, agonist-only pattern or with cocontraction with other muscles. |
Progressive decrease in background rhythm and increased slower frequencies. 1) Variable JLBA with focal contralateral central negativity in EEG with onset 20–40 ms pre-myoclonus and 40–80 ms duration. 2) JLBA with bifrontal EEG negativity with onset 50–170 ms pre-myoclonus and 100–180 ms duration. 3) Periodic sharp waves similar to CJD have been described. |
Enlarged SEP can be found. Variable presence of long latency responses to median nerve stimulation. |
| Creutzfeldt-Jakob disease | Common | Focal, multifocal, bilateral, or generalized distribution. Discharges around 60 ms. Negative myoclonus can be present. |
Periodic synchronous discharge time-locked to myoclonus with 100–160 ms duration and latency of 50–85 ms to myoclonus. | Enlarged SEP can be found. |
| Subacute Sclerosing Panencephalitis | Unknown | Quasiperiodic jerks with sustained ‘dystonic’ posture lasting up to approx. 1 s in upper extremities. Burst duration > 200 ms. Fairly resistant to stimuli. |
Associated to generalized complex EEG discharges: high voltage (300–1500 μV), repetitive, polyphasic and sharp and slow wave complexes with 500–2000 ms in duration, occurring every 4–15 s. | – |
| Lewy Body dementia | 15–25 % | Multifocal or generalized myoclonus during action and rest. Larger amplitude compared to myoclonus in Parkinson’s disease with duration of 20–40 ms. |
Focal short latency EEG transient prior to EMG discharge. | – |
| Corticobasal degeneration | Unknown | Synchronous rhythmic repetitive trains of 25–50 ms discharges in agonist–antagonist pairs. | EMG-EEG transient is usually elusive. Cortical correlate back-averaged from magneto-encephalography. |
SEP is unremarkable or altered without enlargement. Enhanced long latency EMG reflexes. Short C-reflex |
| Multiple System Atrophy (MSA) | Most studied in MSA cerebellar type. | Polyminimyoclonus during postural activation with duration < 100 ms or action myoclonus. Somatosensory stimulus-sensitive jerks. |
No back-averaged cortical correlate in few patients with polyminimyoclonus, but EEG transient prior to EMG discharge was present in patients with action myoclonus. | Enlarged SEP can be seen. Enhanced long latency EMG responses. |
| Progressive supranuclear palsy | Rare | Action myoclonus with discharges < 50 ms. | Gross correlation between EEG epileptiform activity and EMG discharges. | – |
CJD, Creutzfeldt-Jakob disease; EEG, electro-encephalography; EMG, electromyography; JLBA, jerk-locked back-averaging; SEP, sensory evoked potential.