21st June 2023

Myoclonus

Myoclonus is the involuntary muscular jerking caused by a cluster of muscle or muscle group contractions lasting milliseconds. Myoclonus is typically positive, causing jerking, but it can also be negative, causing muscle tone loss.

The causes of myoclonus are:

• Cortical – intracranial causes such as epilepsy, multiple sclerosis, Parkinson’s disease, stroke, traumatic brain injury, or CNS tumours – EMG – <50ms burst potentials. EEG-EMG polygraphy – EEG discharges are low amplitude and precede myoclonic EMG activity. This confirms cortical myoclonus.
• Subcortical – brainstem myoclonus and propriospinal myoclonus. EMG – bursts of 25-300ms
• Segmental – spinal tumour or spinal cord injury. EMG – discharges of 50-1,000ms.
• Peripheral – A peripheral neuropathy with a motor component. EMG – discharges of 50-200ms in the affected muscles.
• Secondary – e.g. drug toxicity, metabolic disturbances, and liver or renal failure.

Myoclonus is diagnosed with the following tests:

• Examination – its distribution, whether or not it happens at rest, when the arms were outstretched, or during actions.
• Blood tests to rule out metabolic disorders – glucose, electrolytes, thyroid function tests, and drug and toxin testing
• MRI head or spine
• EEG
• EMG
• Somatosensory evoked potentials
• EEG-EMG polygraphy on pairs of agonist-antagonist muscles
• CSF analysis

Treatment of myoclonus depends on the cause:

• If secondary, treating the underlying cause by treating the drug toxicity or correcting the metabolic disturbance
• AEDs, e.g. levetiracetam or valproate– used for cortical myoclonus
• Clonazepam – first line for symptomatic relief
• Botulinum toxin A