Myoclonus

Anoxic brain injuries may result in myoclonus, which, in turn, may be cortical, diencephalic, or reticular in origin; stimulus sensitive or action induced; and segmental, generalized, or multifocal in distribution. This type of myoclonus may be focal, preferentially affecting the distal limb muscles, or multifocal with spontaneous, reflexive or stimulus-sensitive jerks accentuated by movement. Frequently, anoxic-induced myoclonus is accompanied by secondary seizures, particularly after cardiopulmonary arrest. Status epilepticus is found in 32% of postanoxic patients, and in many, multifocal myoclonus alone or in combination with generalized tonic-clonic seizures is frequently observed. The incidence of myoclonic seizures is bimodal, with the majority of them occurring within 12 hours after cardiopulmonary resuscitation and the remaining occurring several days later. Electroencephalography (EEG) is useful when evaluating these patients, particularly when status epilepticus is suspected. The most frequent EEG findings include diffuse slowing with or without spike or polyspike complexes that are sometimes time locked to the myoclonic jerks. A burst-suppression EEG pattern, when recorded, has a poor prognostic significance. Magnetic resonance imaging of the brain may show diffusion restriction in the cortical and subcortical gray matter between 24 hours and 13 days. Isolated myoclonus generally does not require treatment unless it interferes with mechanical ventilation or nursing care. Myoclonus status is refractory to treatment, may require multiple antiepileptic drugs, and, when accompanied by convulsive status epilepticus, is best controlled with deep anesthesia.

Electrophysiologically, myoclonus is characterized by a muscle bursts that are less than 75 msec in duration. When the cerebral cortex is affected, a “giant” somatosensory evoked cortical response time locked to the onset of the jerk in back-averaged EEG may be obtained.

POSTANOXIC MYOCLONUS

In 1963, James Lance and Raymond D. Adams reported the first series of patients with the syndrome of intention or action myoclonus as a sequel to hypoxic encephalopathy. In postanoxic myoclonus, axial and proximal muscle groups are particularly affected, the myoclonus often occurring when patients perform an action, such as standing or reaching for an object (action myoclonus). Limb and truncal ataxia, cerebellar outflow tremor, and dysarthria are other common accompaniments. The exact substrate of postanoxic myoclonus generation is not clear. Postanoxic myoclonus may be the result of cortical or subcortical injury or be due to alterations in brainstem serotonergic pathways. The serotonergic raphe nuclei have frequently been implicated.

Some forms of myoclonus, particularly those of subcortical origin, are believed to arise from the reticular system primarily from the nucleus reticularis gigan-tocellularis. This reticular reflex myoclonus is characterized by a brief electromyographic burst lasting 10 to 30 msec, with generalized bilateral synchronous activation of muscles following a distribution suggesting spread up the brainstem and down to the cord.

Essential myoclonus may be idiopathic or familial, beginning in the first to second decade of life. In patients with essential myoclonus, the neurologic examination fails to demonstrate other deficits. In a few families, lower verbal scores have been reported and occasionally mental retardation. Similar to essential tremor, alcohol may help to ameliorate the symptoms, but the incidence of alcoholism is increased. In patients with myoclonus-dystonia, there is an autosomal pattern of inheritance, men are more affected than women, and there is a higher incidence of alcoholism and behavioral disturbances.

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