Physiologic Classification
Cortical
Cortical myoclonus, arising from an area of the brain known as the sensorimotor cortex, is the most common type of myoclonus. The jerky movements usually have a regular rhythm and may be limited to one muscle or muscle group (focal) or several different muscle groups (multifocal). They may occur without an obvious cause (spontaneously) or be brought on by a voluntary action or a reflex response. Examples of diseases in which cortical myoclonus is commonly found include
- Progressive myoclonic epilepsy, or PME, with the five most common forms being
- Unverricht-Lundborg disease
- Lafora disease
- Myoclonic epilepsy with ragged-red fibers or MERRF
- Neuronal ceroid lipofuscinoses
- Sialidosis
- Celiac disease
- Angelman syndrome
- Huntington's disease
- Rett syndrome
- Creutzfeldt-Jakob disease
- Gaucher disease
- Alzheimer's disease
- Olivopontocerebellar atrophy
- Corticobasal ganglionic degeneration
- Encephalopathy caused by HIV infection and metabolic or toxic conditions
Subcortical
Subcortical myoclonus may be the result of the person experiencing abnormally low levels of oxygen in the brain (hypoxia) or a metabolic process, such as kidney or liver failure. The structures in the brain involved in this form of myoclonus include the thalamus and the brainstem. Myoclonus, which may arise when the thalamus is damaged by a lack of blood supply or oxygen (infarct), often causes involuntary, jerking or flapping movements (asterixis) in the arm. Subcortical myoclonus typically affects many muscle groups (generalized) and is sensitive to outside stimuli. Examples include an exaggerated startle response, also called hyperexplexia, and palatal myoclonus, which arises from the brainstem and may follow hypoxia (Lance Adams syndrome), Lyme disease, or nonÐdopa-responsive parkinsonism.
Spinal
Spinal myoclonus is typically associated with a localized area of damaged tissue (focal lesion). The injured area may include direct damage of the spinal cord or may cause abnormal changes in the function of the spinal cord. The duration of the jerks is often longer and more variable than are the jerks of cortical or subcortical myoclonus. Segmental spinal myoclonus is usually rhythmic and not stimulus sensitive. The movements of propriospinal myoclonus (involving all of the spinal cord and not a specific segment) are usually more extensive than those seen with segmental spinal myoclonus and may have a regular rhythm or no rhythm (arrhythmic). The movements are typically slow, affecting both sides of the body (bilateral), with flexing jerks of the trunk and lower limbs. These movements continue 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 disease, E. coli, or HIV).
Peripheral
The most commonly encountered peripheral myoclonus is hemifacial spasm, which may occur for no apparent reason or be 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.
