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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 his 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, jerklike 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 33.1 ), (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 33.2 ). 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 33.1
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, tumors (neoplasms), or cerebral hypoxia due to temporary lack of oxygen (i.e., postanoxic 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 myoclonic epilepsy with ragged red fibers (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 |
TABLE 33.2
Classification of Myoclonus
| Classification Bases | Classifications |
|---|---|
| Affected body part | Focal Segmental Multifocal Generalized |
| Provoking symptom | Spontaneous Reflex Action |
| Neurophysiology | Cortical Subcortical Spinal |
| Etiology | Physiologic Essential Myoclonic epilepsy Secondary |
| Additional forms | Palatal myoclonus Periodic limb movements of sleep Psychogenic myoclonus |
A neurophysiologic classification links the myoclonus to the anatomic origin for the abnormal neuronal discharge within the central nervous system (CNS). Cortical myoclonus arises from the cerebral cortex and is considered epileptic, often being associated with other seizure types. Subcortical myoclonus usually arises from the brainstem. Spinal myoclonus originates within the spinal cord. Clinically, differentiation is often impossible, but electromyography may help.
Another classification, based on etiology, categorizes myoclonus into physiologic or pathologic forms. Common examples of “normal,” nonpathologic, physiologic myoclonus include hiccups or “sleep starts” occurring as one drifts into sleep. In pathologic myoclonus, the brief muscle jerks may occur infrequently or repeatedly. Examples include essential myoclonus, myoclonic epilepsy, and secondary myoclonus. Postanoxic encephalopathy and spongiform encephalopathy (i.e., Creutzfeldt-Jakob disease) are the best-known examples of pathologic myoclonus. Additional rare forms include (1) palatal myoclonus, (2) periodic limb movements of sleep, and (3) psychogenic myoclonus. Although pathologic myoclonus is always a sign of CNS dysfunction, its pathophysiologic mechanism often remains enigmatic. Myoclonus may be an important clinical indicator in determining the proper diagnosis. It is also sometimes a nonspecific feature within more widespread neurologic abnormalities.
Pathophysiology
The pathophysiologic mechanism leading to myoclonus is not well understood, thus complicating anatomic correlation. Cortical myoclonus is possibly a disorder of decreased cortical inhibition, although the reason for the reduced inhibition is unknown. Its frequent association with seizure disorders suggests a common pathophysiologic mechanism for myoclonus and some forms of epilepsy. Mechanisms for subcortical and spinal myoclonus are even less well appreciated.
Clinical Presentation
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