Myoclonus is a sudden, brief, involuntary muscle jerk. It is caused by abrupt muscle contraction, in the case of positive
myoclonus, or by sudden cessation of ongoing muscular activity, in the case of negative
myoclonus (NM).
Myoclonus may be classified in a number of ways, although classification based on the underlying physiology is the most useful from the therapeutic viewpoint. Given the large number of possible causes of
myoclonus, it is essential to take a good history, to clinically characterize
myoclonus and to look for additional findings on examination in order to limit the list of possible investigations. With regards to the history, the age of onset, the character of
myoclonus, precipitating or alleviating factors, family history and associated symptoms and signs are important. On examination, it is important to see whether the
myoclonus appears at rest, on keeping posture or during action, to note the distribution of jerks and to look for the stimulus sensitivity. Electrophysiological tests are very helpful in determining whether
myoclonus is cortical, subcortical or spinal. A single pharmacological agent rarely control
myoclonus and therefore polytherapy with a combination of drugs, often in large dosages, is usually needed. Generally,
antiepileptic drugs such as
valproate,
levetiracetam and
piracetam are effective in cortical
myoclonus, but less effective in other forms of
myoclonus.
Clonazepam may be helpful with all types of
myoclonus. Focal and
segmental myoclonus, irrespective of its origin, may be treated with
botulinum toxin injections, with variable success.