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Myoclonus — sudden, shocklike contractions of a single muscle or muscle group — occurs with various neurologic disorders and may herald the onset of a seizure. These contractions may be isolated or repetitive, rhythmic or arrhythmic, symmetrical or asymmetrical, synchronous or asynchronous, and generalized or focal. They may be precipitated by bright flickering lights, a loud sound, or unexpected physical contact. One type, intention myoclonus, is evoked by intentional muscle movement.
Myoclonus occurs normally just before falling asleep and as a part of the natural startle reaction. It also occurs with some poisonings and, rarely, as a complication of hemodialysis.
If the patient has a seizure, gently help him lie down. Place a pillow or a rolled-up towel under his head to prevent concussion. Loosen constrictive clothing, especially around the neck, and turn his head (gently, if possible) to one side to prevent airway occlusion or aspiration of secretions.
If the patient is stable, evaluate his level of consciousness (LOC) and mental status. Ask about the frequency, severity, location, and circumstances of myoclonus. Has he ever had a seizure? If so, did myoclonus precede it? Is myoclonus ever precipitated by a sensory stimulus? During the physical examination, check for muscle rigidity and wasting, and test deep tendon reflexes.
Generalized myoclonus may occur in advanced stages of Alzheimer’s disease, a slowly progressive dementia. Other late findings include mild choreoathetoid movements, muscle rigidity, bowel and bladder incontinence, delusions, and hallucinations.
Diffuse myoclonic jerks appear early in Creutzfeldt-Jakob disease, a rapidly progressive dementia. Initially random, they gradually become more rhythmic and symmetrical, usually occurring in response to sensory stimuli. Associated effects include ataxia, aphasia, hearing loss, muscle rigidity and wasting, fasciculations, hemiplegia, and vision disturbances, or possibly, blindness.
With viral encephalitis, myoclonus is usually intermittent and either localized or generalized. Associated findings vary, but may include a rapidly decreasing LOC, a fever, a headache, irritability, nuchal rigidity, vomiting, seizures, aphasia, ataxia, hemiparesis, facial muscle weakness, nystagmus, ocular palsies, and dysphagia.
Hepatic encephalopathy occasionally produces myoclonic jerks in association with asterixis and focal or generalized seizures.
Hypoxic encephalopathy may produce generalized myoclonus or seizures almost immediately after restoration of cardiopulmonary function. The patient may also have a residual intention myoclonus.
Uremic encephalopathy commonly produces myoclonic jerks and seizures. Other signs and symptoms include apathy, fatigue, irritability, a headache, confusion, a gradually decreasing LOC, nausea, vomiting, oliguria, edema, and papilledema. The patient may also exhibit elevated blood pressure, dyspnea, arrhythmias, and abnormal respirations.
With idiopathic epilepsy, localized myoclonus is usually confined to an arm or leg and occurs singly or in short bursts, usually upon awakening. It’s usually more frequent and severe during the prodromal stage of a major generalized seizure, after which it diminishes in frequency and intensity.
Myoclonic jerks are usually the first signs of myoclonic epilepsy, the most common cause of progressive myoclonus. At first, myoclonus is infrequent and localized, but over a period of months, it becomes more frequent and involves the entire body, disrupting voluntary movement (intention myoclonus). As the disease progresses, myoclonus is accompanied by generalized seizures and dementia.
Myoclonus may be seen in patients with alcohol, opioid, or sedative withdrawal or delirium tremens.
Acute intoxication with methyl bromide, bismuth, or strychnine may produce an acute onset of myoclonus and confusion.
If the patient’s myoclonus is progressive, take seizure precautions. Keep an oral airway and suction equipment at his bedside, and pad the side rails. Because myoclonus may cause falls, remove potentially harmful objects from the patient’s environment, and remain with him while he walks. Be sure to instruct the patient and his family about the need for safety precautions.
As needed, administer drugs that suppress myoclonus: ethosuximide, L-5-hydroxytryptophan, phenobarbital, clonazepam, or carbidopa. An EEG may be needed to evaluate myoclonus and related brain activity.
Although myoclonus is relatively uncommon in infants and children, it can result from subacute sclerosing panencephalitis, severe meningitis, progressive poliodystrophy, childhood myoclonic epilepsy, and encephalopathies such as Reye’s syndrome.
Read excerpts from these other book chapters related to Seizures:
Copyright Details: Handbook of Signs & Symptoms (Third Edition), Copyright © 2008 Williams & Wilkins.
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More About This Book:
Title: Handbook of Signs & Symptoms (Third Edition) Authors: Springhouse Publisher: Lippincott Williams & Wilkins Copyright: 2006 ISBN: 1-58255-402-1 
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