A myoclonus is a sudden, jerky contraction of a muscle or a group of muscles. Most people are familiar with these movements. For example, just before falling asleep you feel a sudden jerk, so-called sleep myoclonus. Another example is the hiccups, a myoclonus of the diaphragm. These are physiological myoclonuses.

A myoclonus is a sudden, jerky contraction of a muscle or a group of muscles. Most people are familiar with these movements. For example, just before falling asleep you feel a sudden jerk, so-called sleep myoclonus. Another example is the hiccups, a myoclonus of the diaphragm. These are physiological myoclonuses.

There are many drugs that cause myoclonuses as a side effect. An overview article contains a comprehensive list of medicines describing myoclonuses as a side effect (antibiotics, antidepressants, antipsychotics, opiates, cytostatics, anti-dementia agents, anti-Parkinson’s drugs, anxiolytics, lithium, antiepileptic drugs, and others). Sometimes the myoclonuses occur after starting these drugs, sometimes after increasing the dose or suddenly discontinuing the drug. 1  Clozapine is a known cause of myoclonuses in psychiatry, which is often associated with an excessively high blood level of clozapine. This can be caused by suddenly stopping smoking or interaction with another drug (e.g., fluvoxamine)2,3. Lithium and antidepressants can also cause myoclonuses, often at a higher dosage. 4.5

Myoclonuses are classified into positive, negative and action myoclonuses. With a positive myoclonus, muscle jerks are involved, while negative myoclonus appears as a shock-like involuntary jerky movement caused by a sudden, brief interruption of muscle activity.  In action myoclonus, this occurs with a movement or is aggravated by it. A negative  myoclonus  is described, among other things, in  clozapine as sinking through the  knees 6.

Myoclonuses induced by drugs are almost always completely reversible after discontinuation of the causative agent.

Movies

Example of positive myoclonus

Example of negative myoclonus

Publications

  1. Janssen S, Bloem BR, van de Warrenburg BP. The clinical heterogeneity of drug-induced myoclonus: an illustrated review. J Neurol 2017; 264(8): 1559-66.
  2. Chochol MD, Kataria L, O’Rourke MC, Lamotte G. Clozapine-Associated Myoclonus and Stuttering Secondary to Smoking Cessation and Drug Interaction: A Case Report. J Clin Psychopharmacol 2019; 39(3): 275-7.
  3. Wigard ME, van Gool AR, Schulte PF. [Addition of fluvoxamine to clozapine: theory and practice]. Tijdschr Psychiatr2013; 55(2): 113-21.
  4. Caviness JN, Evidente VG. Cortical myoclonus during lithium exposure. Arch Neurol 2003; 60(3): 401-4.
  5. Spigset O, Hedenmalm K, Dahl ML, Wiholm BE, Dahlqvist R. Seizures and myoclonus associated with antidepressant treatment: assessment of potential risk factors, including CYP2D6 and CYP2C19 polymorphisms, and treatment with CYP2D6 inhibitors. Acta Psychiatr Scand 1997; 96(5): 379-84.
  6. Uzun O, Bolu A, Tasci AB, Ogur B. Knee Buckling (Negative Myoclonus) Associated With Clozapine: Reports on 3 Cases. Clin Neuropharmacol 2020; 43(1): 26-7.

Example of an action myoclonus