Myoclonus

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38 Myoclonus

Diana Apetauerova

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

Table 38-2 Classification of Myoclonus

Classification Bases Classifications
Affected body part
Focal
Segmental
Multifocal
Generalized
Provoking symptom
Spontaneous
Reflex
Action
Neurophysiology
Cortical
Subcortical
Spinal
Etiology
Physiological
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, that is, 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

Physiologic Nonpathologic Myoclonus

Shocklike contractions of the arms or legs during sleep or as individuals drift off to sleep are a common form of physiologic myoclonus, sometimes described as physiologic sleep myoclonus.

Pathologic Myoclonus

It is essential to distinguish the various forms of presumed pathologic myoclonus (Table 38-1).

Essential myoclonus is an isolated neurologic finding that has no association with seizures, dementia, or ataxia. It is nonprogressive, usually multifocal in distribution, typically induced by voluntary movements (action myoclonus), and usually responds to alcohol (Fig. 38-1).

image

Figure 38-1 Myoclonus (Essential and Posthypoxic).

Although often familial, essential myoclonus can occur sporadically. Familial essential myoclonus appears to be an autosomal dominant trait with reduced penetrance and variable expressivity. Symptoms typically begin before the age of 20 years. Essential myoclonus often occurs in association with other movement disorders, particularly tremor and dystonia.

Various forms of epilepsy may be accompanied by myoclonus. For example, in forms of idiopathic epilepsy, such as juvenile myoclonic epilepsy and benign myoclonus of infancy, myoclonus may be a primary finding.

Any underlying disease process or cause of CNS dysfunction may lead to secondary myoclonus (Fig. 38-1).

Additional Forms of Myoclonus

Palatal myoclonus has rapid, rhythmic jerking of muscle of one or both sides of the soft palate. It is more appropriately classified as a form of tremor despite continued use of the term palatal myoclonus.

Periodic limb movements of sleep differ from physiologic sleep myoclonus in that they typically consist of repeated, stereotypic, upward extension of the great toe and foot, possibly followed by flexion of the hip, knee, or ankle. These usually involve both legs, tend to occur in repeated episodes lasting a few minutes to several hours, and occur during non-REM sleep. An association with restless legs syndrome is common.

In psychogenic myoclonus, symptoms have a mental or emotional basis rather than an organic origin. In most patients, the condition worsens with stress or anxiety. The myoclonus can be segmental or generalized.

Differential Diagnosis

Myoclonus must be differentiated from other movement disorders, including tics, tremors, ataxia, and chorea. When the jerks are single or repetitive but arrhythmic, a tic diagnosis should be considered. A history of an urge associated with tics is helpful in the diagnosis. In contrast, organic myoclonus is usually briefer and less coordinated or patterned than tics.

Rhythmic forms of myoclonus may be confused with tremors. The pattern of myoclonus is more repetitive, abrupt-onset, square-wave movements, unlike the smoother sinusoidal activity of tremor. Rhythmic myoclonus usually ranges from 1 to 4 Hz, differing from faster tremor frequencies.

Myoclonus, particularly action (intention) myoclonus, is often confused with cerebellar ataxia. Myoclonic jerking occurs during voluntary motor activity, especially when patients attempt to perform a fine motor task, such as reaching for a target.

Diagnostic Evaluation

A diagnosis of myoclonus is based on a thorough clinical assessment, evaluation of the nature of the myoclonus (e.g., electrophysiologic characteristics), bodily distribution, provocative factors, and a careful family history. Examination and observation of patients with myoclonus are important diagnostic steps. However, patients with myoclonus can have entirely normal examination results, particularly with physiologic and essential myoclonus. When myoclonus is present during examination, characterization of its rhythm, repetitiveness, onset, and frequency is important. Because myoclonus may occur with other movement disorders, it is important to look for evidence of dystonia, tremor, ataxia, or spasticity.

The clinical distribution of the myoclonus is also helpful. Focal myoclonus is more commonly associated with CNS lesions. Segmental involvement may suggest brainstem or spinal cord lesions. Multifocal or generalized myoclonus suggests a more diffuse disorder, as seen in diffuse postanoxic insults. This particularly involves the reticular substance of the brainstem. Precipitating factors are important for stimulus-sensitive myoclonus. Therefore, somesthetic sensory input testing is needed. It is important to determine whether the myoclonus occurs spontaneously and whether symptoms improve or worsen with voluntary activity.

During testing for negative myoclonus (asterixis), patients are asked to extend their arms with the wrists back or to perform another movement that requires holding the limb against gravity. In this way, a sudden loss of muscle contraction causes the hand or the arm to fall downward.

Specialized testing can be used to determine the site of the abnormal neuronal discharge within the CNS (e.g., cerebral cortex, brainstem, or spinal cord) and establish the underlying cause. These studies typically primarily include EEG, and less commonly electromyography, or somatosensory evoked potential testing. Neuroimaging studies such as MRI or computed tomography can on rare occasions demonstrate structural lesions. Other specialized diagnostic tests may help to exclude particular conditions such as hereditary, metabolic, mitochondrial, infectious, vascular, neoplastic, toxic, or neurodegenerative processes.

Treatment and Prognosis

The treatment of myoclonus varies depending on the type. If a specific cause is found, myoclonus usually resolves with effective treatment of the underlying disease. A good example is juvenile myoclonic epilepsy. This usually responds to valproate and may require lifelong treatment. Less specific symptomatic therapy typically includes medications to reduce the severity of the myoclonus, such as benzodiazepines. Cortical myoclonus may respond to valproate, piracetam, levetiracetam, or lamotrigine. Myoclonus from a hypoxic event may respond to 5-hydroxy-tryptophan, and this may help in other causes. Carbamazepine may worsen myoclonus and should be avoided.

Prognosis depends on the form of myoclonus. Generally, although myoclonus is not a life-threatening condition, it may be secondary to serious, debilitating fatal diseases such as individuals with Creutzfeldt–Jakob disease. Postanoxic myoclonus is another disorder that is associated with an extremely poor prognosis in the post–cardiac arrest individual.

Researchers are attempting to clarify the genetic and molecular aspects of myoclonus. Newer physiologic techniques, such as magnetoencephalography, are being used to study cortical activity in cortical reflex myoclonus.

Additional Resources

Caviness JN. Myoclonus. Parkinsonism & related disorders. 2007;13(Suppl. 3)):S375-S384.

Gordon MF. Toxin and drug-induced myoclonus. Adv Neurol. 2002;89:49-76.

Hallet M. Neurophysiology of brainstem myoclonus. Adv Neurol. 2002;89:99-102.

Obeso JA. Therapy of myoclonus. Clin Neurosci. 1995-1996;3:253-257.

Rubboli G, Tassinari CA. Negative Myoclonus. An overview of its clinical features, pathophysiological mechanism, and management. Nerurophysiol Clin. 2006 Sep-Dec;36(5-6):337-343. Epub 2007 Jan 23

Shafiq M, Lang AE. Myoclonus in parkinsonian disorders. Adv Neurol. 2002;89:77-83.

Watts RL, Koller WC, editors. Movement Disorders: Neurologic Principles and Practice. New York, NY: McGraw-Hill Companies, Inc, 1997.

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