neurologic disorders
Myoclonus
Classification of myoclonus
Myoclonus may be classified as physiologic (benign) or pathologic ( Classification reference ). Physiologic myoclonusmay occur when a person is falling asleep and during early sleep phases (called hypnic myoclonus). Hypnic myoclonus can be focal, multifocal, segmental, or generalized (see below) and may resemble a startle reaction. Another type of physiologic myoclonus is hiccuping (diaphragmatic myoclonus). can result from various disorders and drugs (see table Some Causes of Myoclonus ). The most common causes are Hypoxia Drug toxicity Metabolic disturbances Other causes of pathologic myoclonus include degenerative disorders affecting the basal ganglia and some dementias. Myoclonus may also be classified as follows: By its distribution: Focal, segmental (contiguous areas), multifocal (noncontiguous areas), or generalized By its site of origin: Cortical, subcortical, segmental, or peripheral By clinical presentation: Positive or negative By its etiology: Essential (primary), acquired, or idiopathic By its trigger: Sensory or spontaneous Myoclonus classified by site of origin is as follows: Cortical:Cortical myoclonus is associated with cerebral cortex damage or epilepsy. Photic visual stimuli or touching may trigger myoclonic jerks, which may cause abnormalities on an electroencephalogram (eg, focal or generalized spike-and-wave or polyspike-and-wave epileptiform discharges, giant somatosensory evoked potentials). The myoclonic jerks may be less evident at rest but aggravated during motor action. This type of myoclonus can severely impair speech and gait. Subcortical myoclonus is associated with disorders that affect the basal ganglia or other subcortical structures. Its effects are similar to those of cortical myoclonus. However, there are no electroencephalogram abnormalities or giant somatosensory evoked potentials, and photic visual stimuli are not a trigger. The category of subcortical myoclonus includes essential myoclonus, myoclonus-dystonia, reticular reflex myoclonus, startle syndromes, Creutzfeldt-Jakob Disease (CJD) , and Forms of segmental or peripheral myoclonus are relatively rare. Segmental myoclonus includes spinal segmental and propriospinal myoclonus. Spinal segmental myoclonus refers to myoclonus in spinal muscles of one or several contiguous segments of the spinal cord. Propriospinal myoclonus is characterized by slowly propagated movements that spare the face, often with a burst duration incompatible with other types of myoclonus. Palatal myoclonus, now mainly considered a misnomer, has been reclassified as palatal tremor. The most common peripheral myoclonus is Hemifacial Spasm ; it results primarily from vascular compression of the facial nerve as it exits from the brain stem or from compression by cerebellopontine angle tumors. Hemimasticatory spasm is much rarer; it is characterized by unilateral, paroxysmal contractions of the jaw muscles. It may be caused by compression of the motor branch of the trigeminal nerve. Classifying myoclonus based on site of origin is thought to be the most helpful when choosing the most effective treatment. Clinical presentation of patients with myoclonus may be classified as positive or negative: Positive:Patients have an active muscle twitch leading to a jerk. Negative:Muscle tone suddenly lapses (with electrographic silence on electromyography); when antigravity muscles lose muscle tone, the patient may fall. Negative myoclonus includes asterixis (eg, flapping of the hands that occurs in patients with severe liver failure). Positive and negative myoclonus often occur in the same patient. Etiology of myoclonus may be essential (primary), acquired (most common), or idiopathic. Essential (primary) myoclonushas no identifiable cause and/or is suspected to involve genetic factors. has multiple causes, including many metabolic disorders (see table Some Causes of Myoclonus ). Most cases of myoclonus are acquired. Idiopathic myoclonusis myoclonus whose presence is completely unexplained. Myoclonus may have a trigger or not: Sensory sensitive:Myoclonus is triggered by a stimulus (eg, sudden noise, movement, light, visual threat), as can occur when a person is suddenly startled (startle response). Spontaneous:Myoclonus occurs without a trigger, as often occurs when the cause is metabolic. 