• A rapid, involuntary jerk of a muscle or group of muscles.
  NCI
  U.S. National Cancer Institute, 2021
• Involuntary shock-like contractions, irregular in rhythm and amplitude, followed by relaxation, of a muscle or a group of muscles. This condition may be a feature of some CENTRAL NERVOUS SYSTEM DISEASES; (e.g., EPILEPSY, MYOCLONIC). Nocturnal myoclonus is the principal feature of the NOCTURNAL MYOCLONUS SYNDROME. (From Adams et al., Principles of Neurology, 6th ed, pp102-3).
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2025
• Shock-like contraction of a portion of a muscle, an entire muscle, or a group of muscles; may be part of a disease process or a normal physiological response.
  CRISP Thesaurus
  National Institutes of Health, 2006
• Very brief, involuntary random muscular contractions occurring at rest, in response to sensory stimuli, or accompanying voluntary movements.
  Human Phenotype Ontology (HPO)
  The Human Phenotype Ontology Project, 2025
• (myoclonic jerk) A sudden, brief, strong contraction of a muscle or group of muscles that cannot be controlled.
  NCI Dictionary of Cancer Terms
  U.S. National Cancer Institute, 2021
• (myoclonic jerk) A sudden, involuntary contraction of a muscle or group of muscles; these movements may develop as a symptom of a number of neurological diseases, including epilepsy, Parkinson's disease, Alzheimer's disease, or Creutzfeldt-Jacob disease.
  NCI
  U.S. National Cancer Institute, 2021
• Myoclonus is a brief, shocklike contraction of a muscle or group of muscles. Diagnosis is clinical and sometimes confirmed by electromyographic testing. Treatment includes correction of reversible causes and, when necessary, oral drugs to relieve symptoms.
  Merck Manuals
  Merck & Co., Inc., 2025
• Myoclonus refers to a quick, involuntary muscle jerk. Hiccups are a form of myoclonus, as are the sudden jerks, or "sleep starts," you may feel just before falling asleep. These forms of myoclonus occur in healthy people and rarely present a problem.
  Mayo Clinic
  Mayo Foundation for Medical Education and Research, 2025