Physiology-Based Treatment of Myoclonus

Abstract

Myoclonus can cause significant disability for patients. Myoclonus has a strikingly diverse array of underlying etiologies, clinical presentations, and pathophysiological mechanisms. Treatment of myoclonus is vital to improving the quality of life of patients with these disorders. The optimal treatment strategy for myoclonus is best determined based upon careful evaluation and consideration of the underlying etiology and neurophysiological classification. Electrophysiological testing including EEG (electroencephalogram) and EMG (electromyogram) data is helpful in determining the neurophysiological classification of myoclonus. The neurophysiological subtypes of myoclonus include cortical, cortical-subcortical, subcortical-nonsegmental, segmental, and peripheral. Levetiracetam, valproic acid, and clonazepam are often used to treat cortical myoclonus. In cortical-subcortical myoclonus, treatment of myoclonic seizures is prioritized, valproic acid being the mainstay of therapy. Subcortical-nonsegmental myoclonus may be treated with clonazepam, though numerous agents have been used depending on the etiology. Segmental and peripheral myoclonus are often resistant to treatment, but anticonvulsants and botulinum toxin injections may be of utility depending upon the case. Pharmacological treatments are often hampered by scarce evidence-based knowledge, adverse effects, and variable efficacy of medications.

Keywords: EEG; EMG; Myoclonus; neurophysiology; treatment.

Fig. 1

The morphological differences in surface EMG morphology between a voluntary movement and a myoclonus discharge. On the left is the surface EMG recording of a healthy subject contracting the wrist flexors as quickly and as briefly as possible, demonstrating a burst duration of greater than 100 ms and a gradual build-up of EMG activity. On the right is a myoclonus discharge obtained from the same muscle group, demonstrating a brief duration of less than 50 ms and sharp rise time [11]

Fig. 2

The morphological differences in surface EMG morphology between a voluntary movement and a myoclonus discharge. On the left is the surface EMG recording of a healthy subject contracting the wrist flexors as quickly and as briefly as possible, demonstrating a burst duration of greater than 100 ms and a gradual build-up of EMG activity. On the right is a myoclonus discharge obtained from the same muscle group, demonstrating a brief duration of less than 50 ms and sharp rise time [11]

Fig. 3

Surface EMG polygraphy is shown from the left upper extremity including the deltoids, biceps, triceps, wrist flexors, and wrist extensors. Myoclonus discharges are seen exhibiting cocontraction across flexor and extensor muscle groups as indicated by the arrows [3]

Fig. 4

Following EEG–EMG back-averaging, an averaged myoclonus discharge is pictured from the right wrist extensor along with the corresponding cortical transient at C3 [14]. The arrows indicate the onset of the positive deflection of the cortical transient and the onset of the myoclonus discharge occurring with a latency of ~ 20 ms