Myoclonic disorders: a practical approach for diagnosis and treatment

Abstract

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.

Keywords: classification; clinical approach; myoclonus; treatment.

Figure 1.

Negative myoclonus: cortical negative myoclonus. There is a sudden interruption of the muscle activity when the patient is holding the left leg up against gravity. Duration of EMG silent period is 50–100 ms.

Figure 2.

Cortical myoclonus: EMG and EEG trace in a case of cortical myoclonus. (A) A magnification of a segment (20 ms/division) where myoclonic jerks are observed. Surface EMG shows brief bursts of activity (of approximately 20 ms duration) with a typical rostrocaudal pattern of muscle activation in the right upper limb. (B) EEG back averaging of the right first dorsal interossei muscle. EMG burst demonstrates cortical spikes in C3 derivation, starting 22 ms before the EMG myoclonic burst.

Figure 3.

Brainstem reticular myoclonus. Multichannel EMG recording: Following acoustic stimulation there was an initial activation of the right sternocleidomastoid muscle with a latency of 68 ms, followed by the spread to rostral and caudal muscles.

Figure 4.

Propriospinal myoclonus. With the patient in a recumbent position, surface multichannel EMG from right-sided muscles shows a jerk of approximately 400 ms duration. This jerk is electrically evoked, starts with a latency of 200 ms in the rectus abdominis muscle and is followed by activation of rostral and caudal muscles.

Figure 5.

Spinal segmental myoclonus. Multisurface EMG shows myoclonic bursts confined mainly to the right triceps but affecting also a few adjacent myotomes.

Figure 6.

Psychogenic myoclonus. A slow rising wave called the Bereitschafts potential is seen in EEG back averaging of the right triceps jerk (duration of the triceps jerk is 200 ms).