and Peter Hedera2
(1)
Parkinson’s Clinic of Eastern Toronto, Toronto, ON, Canada
(2)
Department of Neurology Division of Movement Disorders, Vanderbilt University, Nashville, TN, USA
Abstract
Myoclonus is characterized by brief involuntary muscle jerks due to concise electromyographic bursts of 10–50 ms. Abnormal movements can be induced by both positive muscle contractions or negative symptoms with brief lapses of muscle contractions. This chapter reviews main clinical characteristics of myoclonus and its most common causes. We describe characteristic phenotypic features of myoclonus, classification based on clinical presentation and pathophysiology of myoclonus, and therapeutic options to control this abnormal movement.
Keywords
MyoclonusAsterixisMuscle contractionOpsoclonus-myoclonusCorticobasal degenerationMyoclonus refers to brief, lightning-like involuntary muscle jerks due to concise electromyographic bursts of 10–50 ms. These brief lightning-like jerks are rarely more than 100 ms in duration, and affect the extremities, face, and the trunk. The jerks are generally caused by positive muscle contractions, but can also be a result of sudden brief lapses of contraction, as seen in asterixis.
5.1 Classification
Myoclonus can be classified by site of origin, clinical features, response to stimulus, and etiology.
However, myoclonus can also be classified according to physiology which helps in identifying the underlying process and thus treatment.
5.1.1 Classification Based upon Site of Origin
Cortical Myoclonus
Cortical myoclonus is the most common form of myoclonus. It arises from the sensorimotor cortex and is generally arrhythmic. Focal or multifocal jerks may also be apparent due to spontaneity or induction by reflex or action. Following are some of the common conditions in which cortical myoclonus may be seen:
1.
Progressive myoclonic epilepsies include Unverricht-Lundborg disease, Lafora disease, MERRF, DRPLA, familial adult myoclonic epilepsy, idiopathic progressive myoclonic epilepsy, Angelman syndrome, and celiac disease.
2.
Encephalopathies caused by HIV metabolic or toxic conditions.
3.
Alzheimer’s disease.
4.
Gaucher’s disease.
5.
Creutzfeldt-Jakob disease.
Subcortical Myoclonus
Subcortical myoclonus is thought to be caused by metabolic and hypoxic dysfunction such as hepatic or renal failure. The brainstem and thalamus are usually involved in this type of myoclonus, which normally manifests itself as stimulus sensitive and generalized.
Spinal Myoclonus
Spinal myoclonus is usually associated with the presence of a focal lesion and is only rarely idiopathic. The lesion may compress the spinal cord or may have minimal mass effect. It is usually divided into two types, i.e., segmental spinal myoclonus and propriospinal myoclonus. Segmental spinal myoclonus may be rhythmic in nature and non-stimulus sensitive. In propriospinal myoclonus, the lesion is normally at the thoracic level and movements are more widespread than those observed in segmental spinal myoclonus. Propriospinal myoclonus may lead to episodic flexion of trunk with myoclonus. As mentioned above, the typical cause of spinal myoclonus is the presence of a focal spinal lesion such as trauma, syringomyelia, multiple sclerosis, an infection (e.g., herpes zoster, Lyme’s disease, or HIV), or ischemic myelopathy. Imaging of spinal cord, EMG as well as somatosensory evoke potentials (SSEP) may be helpful in localizing the site of pathology. Spinal myoclonus may cause significant interference in mobility in some cases.
Peripheral Myoclonus
The most frequently observed peripheral myoclonus is hemifacial spasm, which is idiopathic in nature and is believed to be caused by compression of the facial nerve. Hemifacial spasm is unilateral in most of the cases and is characterized by intermittent clonic contractions of the muscles of face.
Some of the studies have suggested that lesions of the peripheral nerves may change inhibitory spinal interneurons and sensory input.
5.1.2 Classification Based upon Clinical Features
Clinically, myoclonus can be classified by many different ways, e.g., site of involvement, amplitude of movements, frequency of movements, present with action, intended movements, or at rest. The jerks associated with myoclonus may occur repeatedly or individually. They may be generalized, focal, multifocal, or segmental.
1.
Generalized myoclonus involves synchronous jerks of one or more major muscle groups.
2.
Focal myoclonus involves movements restricted to defined body parts; most common is spinal myoclonus, propriospinal myoclonus, arm myoclonus, and hemifacial spasm.
3.
Multifocal myoclonus involves two or more nonadjacent areas of the body.
4.
Segmental myoclonus involves adjacent areas of the body usually due to trauma and inflammation or tumor.
The amplitude of the jerks observed in myoclonus ranges from mild contractions that do not move a joint to more severe larger contractions that move the head, limbs, or trunk. Myoclonic jerks vary in frequency from occasional isolated events to multiple episodes which may appear almost continuously. They may arise with indented movements, at rest, or with action. Myoclonic jerks may be stimulus sensitive, and can be aggravated by visual threat, noise, pinprick, movement, or light. Myoclonic jerks may occur sporadically and unpredictably. Some of the myoclonic jerks may occur in bursts of oscillations, whereas others are very intermittent, as in palatal myoclonus. In such a case, the jerks may resemble tremor.
Myoclonus that originates from the cerebral cortex is termed cortical myoclonus and is usually reflex induced and focal. The cortical source of myoclonus can be established by the observation of spikes in the electroencephalogram associated with electromyographic correlated jerks.
Myoclonus that originates from the brainstem can be either segmental (e.g., oculo–palatal–pharyngeal myoclonus) or generalized (e.g., reticular myoclonus). Palatal myoclonus is rhythmical (around 2 Hz) and can be classified as primary or secondary. Secondary palatal myoclonus is more common and is caused by a lesion within the Guillain-Mollaret triangle encompassing the red nucleus, inferior olivary nuclei, and dentate. This causes an interruption of the dentatoolivary pathway, leading to the denervation of the olives, which can become hypertrophic. Multiple sclerosis and vascular lesions are frequent causes of secondary palatal myoclonus which may persist during sleep.
Myoclonus originating from the spinal cord consists of two clinical forms. Propriospinal myoclonus causes truncal flexion along with jerks, usually caused by a stimulus. The other clinical form of myoclonus originating from the spinal cord is spinal segmental myoclonus, which is rhythmic and may also occur during sleep. In propriospinal myoclonus, the first muscles to be activated are generally from the thoracic cord, with slow downward and upward spread. Myoclonic jerks can sometimes occur from a peripheral nerve, spinal root, or plexus.
Rasmussen encephalitis is a disorder of childhood or adolescence characterized by unilateral focal seizures, including epilepsia partialis continua and a progressive hemiplegia.
5.1.3 Classification Based upon Provocative Factors
Myoclonic movement can be classified based on response to movements or certain stimuli. The movements can be either spontaneous, in which no stimulus is needed to induce it, or it may appear in response to stimulus. Common causes of spontaneous myoclonus are metabolic encephalopathies of Creutzfeldt-Jakob disease.
Reflex myoclonus occurs in response to verbal, somesthetic, or auditory stimulus. Hyperekplexia is an exaggerated startle reflex which can be idiopathic or genetic in terms of etiology. Action myoclonus is very disabling as it causes disruption of volitional movements that are associated with provocative factors.
5.1.4 Classification Based upon Etiology
Myoclonus can be classified into the following etiologic categories:
1.
Physiologic myoclonus
2.
Essential myoclonus
3.
Epileptic myoclonus
4.
Symptomatic myoclonus
Physiologic myoclonus is a normal phenomenon with no associated disability. Hypnic jerks, benign sleep myoclonus of infancy generally occurring in the immediate postpartum period, and hiccoughs are some of the examples of physiologic myoclonus.
Essential myoclonus is idiopathic, without any progressive course, and is not linked with other neurologic conditions. It is generally multifocal in distribution and coexists with features of dystonia in some patients. Some patients may have a positive family history.
Epileptic myoclonus occurs in patients with generalized convulsions but patients also have myoclonus. Most patients have cognitive dysfunction such as seen in Lafora disease and neuronal ceroid-lipofuscinosis.
Symptomatic myoclonus occurs as part of an encephalopathic process due to many different neurological conditions such as dementias, spinocerebellar degenerations, storage diseases, focal brain damage, and toxic, infectious, metabolic, and physical encephalopathies. Whipple disease is an infectious condition which has facial myoclonus known as oculofacial-masticatory myorhythmia. Serotonin syndrome is a type of toxic encephalopathy and is caused by medications that produce extreme serotonergic stimulation.
5.2 Investigations
Serum glucose levels, liver function tests, electrolytes, and screening studies for antibodies, drugs, and toxins may be helpful in assessment of different causes of myoclonus. Imaging studies such as MRI of the brain may be required in some cases.
Electromyography may help to localize the site of pathology in spinal myoclonus, and electroencephalography is used to assess the cortical involvement in myoclonus. Genetic testing can also be done where a familial cause, such as dentatorubral-pallidoluysian atrophy, and myoclonic epilepsy are suspected.
5.3 Causes
5.3.1 Opsoclounus Myoclonus Syndrome
Opsoclonus-myoclonus syndrome or polyminimyoclonus is a rare type of myoclonic syndrome in which the myoclonic jerks are of minimal amplitude. Patients have irregular, chaotic, and spontaneous saccadic movements of eyes; hence, the term “dancing eyes,” or opsoclonus, is used to describe the ocular movements. Although initially described in children in association with a neuroblastoma, it is also observed in adults in postviral or paraneoplastic syndrome.
5.3.2 Exaggerated Startle Syndromes
Patients with exaggerated startle syndromes present with abnormal myoclonic movements in response to unexpected tactile, visual, or auditory stimuli. These movements are believed to originate from the brainstem. Examples of such stimuli are a blink, flexion and abduction of the arms, contraction of the face, and flexion of the trunk and neck.
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