Myoklonus

Myoclonus refers to sudden brief, shock-like involuntary muscle contractions (positive myoclonus) or sudden suppression of muscle activity (negative myoclonus). The manifestations can be focal, multifocal, or generalized, and may occur at rest or with action. Myoclonic jerks are typically lasting only milliseconds but can occur in rapid succession, leading to a jerky or arrhythmic movement pattern. Negative myoclonus produces brief lapses in muscle activity resulting in a momentary lapse or cessation of movement.

Classification of myoclonus is clinical and etiological. It includes physiological myoclonus (e.g., sleep starts or hypnic jerks), essential myoclonus (often action-related and non-epileptic), epileptic myoclonus (associated with generalized or focal epilepsy), and reflex or stimulus-sensitive myoclonus (triggered by sounds, touch, or startling stimuli). Pathophysiologically, myoclonus may arise from cortical, subcortical, brainstem, spinal, or spinal cord generators, with cortical myoclonus typically EEG-correlated and stimulus-sensitive, and subcortical forms arising from brainstem networks.

Causes span metabolic and structural brain disease, neurodegenerative disorders, autoimmune or infectious processes, and drug or withdrawal effects. Common associations include metabolic derangements (uremia, hepatic failure, electrolyte disturbances), progressive myoclonic epilepsy syndromes (e.g., Unverricht-Lundborg, Lafora body disease), neurodegenerative conditions, and drug-induced or withdrawal phenomena (e.g., opioids, antidepressants, benzodiazepine withdrawal).

Diagnosis relies on history and video-EEG with EMG to characterize the timing and generator of the jerks, neuroimaging, and metabolic or autoimmune testing as indicated. Differential diagnoses include tics, chorea, tremor, and epileptic seizures.

Treatment targets the underlying cause where possible and may include antiseizure medications such as valproate, clonazepam, levetiracetam, or piracetam. Prognosis varies by etiology; benign, isolated myoclonus generally has a good outcome, whereas progressive myoclonus epilepsies often have a poorer prognosis.