38 Myoclonus

Clinical Vignette

A 73-year-old right-handed man with history of coronary artery disease, myocardial infarction, peripheral vascular disease, and hypertension presented with cardiac arrest followed by resuscitation with subsequent development of anoxic brain injury. He was seen by a neurologist in the intensive care unit (ICU) setting 1 week after cardiac arrest. His family was very concerned about constant “jerkiness” of his body. Those movements seemed to worsen when people were touching the patient, which affected all four extremities as well as his trunk. He appeared very uncomfortable because of these jerks.

Neurologic examination demonstrated an elderly man who was intubated without any sedation. He was restless, inconsistently able to open his eyes to verbal commands, and followed very simple yes-and-no questions. His examination was notable for multiple, irregular, large-amplitude, brief shocklike jerks of the trunk, arms, and legs. These movements occurred randomly and many of them were stimulus sensitive. His magnetic resonance image (MRI) was unremarkable and electroencephalography (EEG) showed multifocal spike discharges. A diagnosis of postanoxic generalized myoclonus (Lance–Adams syndrome) was made and the patient was started on sodium valproate, which significantly diminished those movements.

Myoclonus is characterized by sudden, abrupt, brief, involuntary, jerk-like contractions of a single muscle or muscle group. They are related to involuntary muscle contractions (positive myoclonus) or sudden inhibition of voluntary muscular contraction, with lapses of sustained posture (negative myoclonus or asterixis). Myoclonus may affect any bodily region, multiple bodily regions, or the entire body, interfering with normal movements and posture.

There are various classifications of myoclonus; these include (1) etiology (Table 38-1), (2) affected body region (focal, segmental, multifocal, or generalized forms), (3) the presence or absence of specific provocative factors, and (4) specific site of nervous system origin of the abnormal neuronal discharges (Table 38-2). Spontaneous myoclonus has no clinically identifiable mechanism. Reflex myoclonus occurs in response to specific external sensory stimuli. Voluntary movement or attempts to perform specific movements induce action or intention myoclonus.

Table 38-1 Etiologies of Pathologic Myoclonus

Type of Myoclonus	Etiologies
Essential	Autosomal dominant trait with reduced penetrance and variable expressivity
Myoclonic epilepsy	Juvenile myoclonic epilepsy, benign myoclonus of infancy
Secondary	Brain trauma, infection, inflammation (encephalitis, Creutzfeldt–Jakob disease), tumors (neoplasms), or cerebral hypoxia due to temporary lack of oxygen (i.e., posthypoxic myoclonus or Lance–Adams syndrome)
Spinal	Spinal cord trauma, infection, inflammation, or lesions may produce segmental myoclonus
Inborn biochemical errors	Inborn errors of metabolism (lysosomal storage diseases: Tay–Sachs disease, Sandhoff disease, sialidosis)
Infectious	Creutzfeldt–Jakob disease
	Subacute sclerosing panencephalitis (SSPE)
	Whipple disease (facial myoclonus—oculofacial masticatory monorhythmia)
Neuroimmunologic	Stiffman variant: Encephalomyelitis with rigidity
Neurodegenerative	Parkinsonism, Huntington disease, Alzheimer disease, Lafora disease, corticobasal degeneration, progressive supranuclear palsy, or olivopontocerebellar atrophy
Metabolic	Metabolic conditions, such as kidney, liver, or respiratory failure, hypokalemia, hyperglycemia, etc.
Mitochondrial	Mitochondrial encephalomyopathy, particularly MERFF syndrome (myoclonus epilepsy with ragged-red fibers), or other progressive myoclonic encephalopathies, including those characterized by epilepsy and dementia (e.g., Lafora disease) or epilepsy and ataxia (e.g., Unverricht–Lundborg disease)
Medications: Drug-induced myoclonus	Serotonin receptor inhibitors: serotonin syndrome; toxic levels of anticonvulsants, levodopa, and certain antipsychotic agents (tardive myoclonus)
Toxins	Exposure to toxic agents, such as bismuth or other metals