Myoclonus


Myoclonus is a brief, involuntary, irregular twitching of a muscle, a joint, or a group of muscles, different from clonus, which is rhythmic or regular. Myoclonus describes a medical sign and, generally, is not a diagnosis of a disease. It belongs to the hyperkinetic movement disorders, among tremor and chorea for example. These myoclonic twitches, jerks, or seizures are usually caused by sudden muscle contractions or brief lapses of contraction. The most common circumstance under which they occur is while falling asleep. Myoclonic jerks occur in healthy people and are experienced occasionally by everyone. However, when they appear with more persistence and become more widespread they can be a sign of various neurological disorders. Hiccups are a kind of myoclonic jerk specifically affecting the diaphragm. When a spasm is caused by another person it is known as a provoked spasm. Shuddering attacks in babies fall in this category.
Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Most often, myoclonus is one of several signs in a wide variety of nervous system disorders such as multiple sclerosis, Parkinson's disease, dystonia, cerebral palsy, Alzheimer's disease, Gaucher's disease, subacute sclerosing panencephalitis, Creutzfeldt–Jakob disease, serotonin toxicity, some cases of Huntington's disease, some forms of epilepsy, and occasionally in intracranial hypotension.
In almost all instances in which myoclonus is caused by central nervous system disease it is preceded by other symptoms; for instance, in CJD it is generally a late-stage clinical feature that appears after the patient has already started to exhibit gross neurological deficits.
Anatomically, myoclonus may originate from lesions of the cortex, subcortex or spinal cord. The presence of myoclonus above the foramen magnum effectively excludes spinal myoclonus; further localisation relies on further investigation with electromyography and electroencephalography.

Types

The most common types of myoclonus include action, cortical reflex, essential, palatal, those seen in the progressive myoclonus epilepsies, reticular reflex, sleep and stimulus-sensitive.

Epilepsy forms

  • Cortical reflex myoclonus is thought to be a type of epilepsy that originates in the cerebral cortex – the outer layer, or "gray matter", of the brain, responsible for much of the information processing that takes place in the brain. In this type of myoclonus, jerks usually involve only a few muscles in one part of the body, but jerks involving many muscles may occur. Cortical reflex myoclonus can be intensified when patients attempt to move in a certain way or perceive a particular sensation.
  • Essential myoclonus occurs in the absence of epilepsy or other apparent abnormalities in the brain or nerves. It can occur randomly in people with no family history or among members of the same family, indicating that it sometimes may be an inherited disorder. Essential myoclonus tends to be stable without increasing in severity over time. Some scientists speculate that some forms of essential myoclonus may be a type of epilepsy with no known cause.
  • Juvenile myoclonic epilepsy usually consists of jerking and muscle twitches of the upper extremities. This may include the arms, shoulders, elbows, and very rarely, the legs. JME is among the most common types of epilepsy and can affect one of every 14 people with the disease. These seizures typically occur shortly after waking up. Onset for JME can be seen around puberty for most patients. Administration of medications that also treat multiple seizure types is usually the most effective form of treatment.
  • Lennox–Gastaut syndrome , or childhood epileptic encephalopathy, is a rare epileptic disorder accounting for 1–4% of childhood epilepsies. The syndrome has much more severe symptoms ranging from multiple seizures daily, learning disabilities, and abnormal findings in electroencephalograms. Earlier age of seizure onset is correlated with a higher risk of cognitive impairment.
  • Progressive myoclonus epilepsy is a group of diseases characterized by myoclonus, epileptic seizures, tonic–clonic seizures, and other serious symptoms such as trouble walking or speaking. These rare disorders often get worse over time and can be fatal. Studies have identified at least three forms of PME. Lafora disease is inherited as an autosomal recessive disorder, meaning that the disease occurs only when a child inherits two copies of a defective gene, one from each parent. Lafora disease is characterized by myoclonus, epileptic seizures, and dementia. A second group of PME diseases belonging to the class of cerebral storage diseases usually involves myoclonus, visual problems, dementia, and dystonia. Another group of PME disorders in the class of system degenerations often is accompanied by action myoclonus, seizures, and problems with balance and walking. Many of these PME diseases begin in childhood or adolescence. Treatment is not normally successful for any extended period of time.
  • Reticular reflex myoclonus is thought to be a type of generalized epilepsy that originates in the brainstem, the part of the brain that connects to the spinal cord and controls vital functions such as breathing and heartbeat. Myoclonic jerks usually affect the whole body, with muscles on both sides of the body affected simultaneously. In some people, myoclonic jerks occur in only a part of the body, such as the legs, with all the muscles in that part being involved in each jerk. Reticular reflex myoclonus can be triggered by either a voluntary movement or an external stimulus.

    Diaphragmatic flutter

A very rare form includes the diaphragmatic flutter, the Belly Dancer's Syndrome, or Van Leeuwenhoek's disease. It was first described by Antonie van Leeuwenhoek in 1723, who had it. The condition characterizes spoken communication that sounds like a short-breathed hiccup. These muscle spasms can recur dozens of times per day. Rate of diaphragmatic contraction ranges between 35 and 480 contractions per minute, with the average rate found to be 150. Studies show that possible causes include disruptions within the central or peripheral nervous systems, anxiety, nutritional disorder, and certain pharmaceuticals. No single treatment has proven effective, though blocking or crushing of the phrenic nerve can provide instantaneous relief when pharmacologic treatment has proven ineffective.
Only about 50 people in the world have been diagnosed with diaphragmatic flutter.

Other forms

  • Action myoclonus is characterized by muscular jerking triggered or intensified by voluntary movement or even the intention to move. It may be made worse by attempts at precise, coordinated movements. Action myoclonus is the most disabling form of myoclonus and can affect the arms, legs, face, and even the voice. It is often associated with tonic-clonic seizures and diffuse neuronal disease such as post-hypoxic encephalopathy, uremia, and the various forms of PME, although, in the case of focal cerebral damage, the disease may be restricted to one limb. This type of myoclonus often is caused by brain damage that results from a lack of oxygen and blood flow to the brain when breathing or heartbeat is temporarily stopped. Over-excitement of the sensorimotor cortex or reticular formation is also a cause of action myoclonus. Serotonin and GABA neurotransmitters are thought to cause this lack of inhibition, which is a possible explanation as to why improvements are made with the administration of serotonin precursors. Systems involved include the cerebellodentatorubral, pyramidal, extrapyramidal, optic, auditory, posterior columns and gracile and cuneate nuclei, spinocerebellar tracts, motor neurons of cranial nerves and anterior horns, and muscle fibers.
  • Palatal myoclonus is a regular, rhythmic contraction of one or both sides of the rear of the roof of the mouth, called the soft palate. These contractions may be accompanied by myoclonus in other muscles, including those in the face, tongue, throat, and diaphragm. The contractions are very rapid, occurring as often as 150 times a minute, and may persist during sleep. The condition usually appears in adults and can last indefinitely. People with palatal myoclonus usually regard it as a minor problem; some complain of an occasional "clicking" sound, a noise made as the soft palate muscles contract.
  • Middle ear myoclonus occurs in the muscles of the middle ear. These muscles may include the tensor tympani and stapedius muscles. It can involve the muscles surrounding the Eustachian tube, which include the tensor veli palatini, levator veli palatini, and salpingopharyngeus. Those affected describe it as a thumping sound or sensation in the ear.
  • Spinal myoclonus is myoclonus originating in the spinal cord, including segmental and propriospinal myoclonus. The latter is usually due to a thoracic generator producing truncal flexion jerk. It is often stimulus-induced with a delay due to the slow conducting propriospinal nerve fibers.
  • Stimulus-sensitive myoclonus is triggered by a variety of external events, including noise, movement, and light. Surprise may increase the sensitivity of the patient.
  • Sleep myoclonus occurs during the initial phases of sleep, especially at the moment of dropping off to sleep, and include familiar examples of myoclonus such as the hypnic jerk. Some forms appear to be stimulus-sensitive. Some people with sleep myoclonus are rarely troubled by it, or need treatment. If it is a symptom of more complex and disturbing sleep disorders, such as restless legs syndrome, it may require medical treatment. Myoclonus can be associated with patients with Tourette syndrome.