Myoclonus

Classification of Myoclonus

A variety of classification schemes have been developed to categorize myoclonic movements. Appropriately identifying the features of myoclonus and classifying the movements are important because treatment and ultimately outcome vary considerably depending upon the underlying cause. Movements can be classified based upon their appearance, by their clinical and electrophysiologic features, and by their anatomic distribution and response to stimulus. They are most commonly classified by etiology.

According to Caviness1, the primary reasons to classify myoclonus according to its physiology include the following: (1) by localizing the physiologic properties, the clinician can more readily identify the underlying process causing the movements; (2) because certain types of myoclonus indicate specific disorders, identifying their basis again yields diagnostic information; (3) effective treatments are specific to physiology, so characterizing the movements often yields a better outcome; and (4) by classifying the movements, the clinician gains a greater understanding of the process that yields the movements.

Physiologic Classification: Cortical
Cortical myoclonus, arising from the sensorimotor cortex, is the most common type of myoclonus. The typically arrhythmic, focal or multifocal jerks may be spontaneous or induced by reflex or action. The movements are often associated with giant somatosensory evoked potentials (SEP) and electroencephalographic (EEG) discharges in the sensorimotor cortex, which are conducted rapidly in the pyramidal tracts, rostral to caudal. Examples of diseases in which cortical myoclonus is commonly found include progressive myoclonic epilepsy or PME (the 5 most common being Unverricht-Lundborg disease, Lafora disease, myoclonic epilepsy with ragged-red fibers [MERRF], neuronal ceroid lipofuscinoses, and sialidosis); celiac disease; Angelman syndrome; Huntington's disease; Rett syndrome; Creutzfeldt-Jakob disease; Gaucher disease; Alzheimer's disease; olivopontocerebellar atrophy; corticobasal ganglionic degeneration; and encephalopathy caused by HIV infection and metabolic or toxic conditions.

Physiologic Classification: Subcortical
Subcortical myoclonus may be a consequence of hypoxia and metabolic processes, such as renal or hepatic failure. The subcortical structures involved in the genesis of myoclonus are the thalamus and the brainstem. Myoclonus arising in the thalamus, whether from an infarct or a lesion, often causes asterixis in the arm. Subcortical myoclonus is typically generalized and stimulus sensitive. Examples include hyperexplexia and palatal myoclonus, which arise from the brainstem and may follow hypoxia (Lance Adams syndrome), Lyme disease, nonÐdopa-responsive parkinsonism, or a pontine demyelinating lesion.

Physiologic Classification: Spinal
Spinal myoclonus is typically associated with a focal lesion, with little spread of myoclonic activity from spinal-generator sites. The lesion may directly damage the cord or may cause changes in the afferent signaling from peripheral and supraspinal structures. The duration of the jerks is often longer and more variable than is seen in cortical or subcortical myoclonus. Segmental spinal myoclonus is usually rhythmic and not stimulus sensitive, and the movements are limited to those arising from contiguous segments of the cord. The spinal generator of propriospinal myoclonus is usually at the thoracic level and recruits axial muscles via slowly conducting polysynaptic propriospinal pathways both rostrally and caudally from the generator. The movements are usually more extensive than those seen with segmental spinal myoclonus and can be rhythmic or arrhythmic. They are typically slow, bilateral, synchronous jerks of flexion muscles of the trunk and lower limbs, have a frequency between 20 and 180 per minute, have a duration of 50 to 200 milliseconds, and persist during sleep. The usual cause is a focal spinal lesion, such as multiple sclerosis, syringomyelia, trauma, ischemic myelopathy, or an infection (for example, from herpes zoster, Lyme's disease, Escherichia coli, or HIV).

Physiologic Classification: Peripheral
Although some would argue that myoclonus can only result from the central nervous system, recent work has hypothesized that lesions of the peripheral nerves may alter sensory input and induce central reorganization, with abnormal sensory spinal afference causing loss of local inhibitory spinal interneurons.2 Peripheral myoclonus is typically not stimulus sensitive and usually results in arrhythmic jerks. The most commonly encountered peripheral myoclonus is hemifacial spasm, which can be idiopathic or caused by compression of the facial nerve. The movements are typically 200 to 400 milliseconds in duration and can last for only a few days or for weeks to months. Unlike most myoclonic movements, those of peripheral myoclonus persist during sleep.