INTRODUCTION
Myoclonus is characterized by lightning-like muscle jerks. Electrophysiologically, these jerks are associated with electromyographic discharges that are relatively short in duration compared to voluntary jerks. Myoclonic jerks may be positive due to active muscle contractions, or negative in which jerks occur due to lapses of postural tone (the classic example of this is asterixis).
EPIDEMIOLOGY
Myoclonus has a prevalence rate of 8.6 per 100,000, based on a study in Olmsted County published in 1999.
PATHOBIOLOGY
Although the electrophysiology of myoclonus is well understood, the actual pathophysiology is not. Myoclonus can be classified based on either anatomic localization or etiology. Anatomically, myoclonus can originate from either the central or peripheral nervous system. In the central nervous system, the location is further subdivided into cortical, subcortical, brain stem, and spinal cord origin.
Myoclonus is usually classified based on anatomic localization (
Table 78.1). The etiology and clinical characteristics of myoclonus from each anatomic location will be discussed in the following text.
CORTICAL MYOCLONUS
Myoclonus originating in the cortex has unique electrophysiologic features, as described in
Table 78.2. The first three are unique to cortical myoclonus and are not seen in subcortical or spinal myoclonus.
Back-averaging by electroencephalography (EEG) is very helpful in identifying cortical spikes prior to the jerks, but the technique is not routinely available. It is done by averaging at least 150 to
200 myoclonic jerks and capturing their preceding cortical spikes.
A short duration between the spike and the jerk indicates fast conduction from the cortex to the muscles via the corticospinal pathway, usually less than 50 milliseconds.
Giant somatosensory-evoked potentials (SEPs) are very large cortical potentials seen by SEP recording techniques such as by stimulation of the median nerve while recording EEG. A typical SEP has a negative (upward) phase followed by positive (downward) and negative phases, respectively. Only positive and the second negative phases are enlarged in cortical myoclonus. These phases are motor volleys, as compared to the first negative phase which is a sensory volley.
The
C-reflex is a form of a long-latency reflex. When we stimulate muscle fibers, the afferent pathway is conducted through Ia sensory fibers, spinal cord, nucleus cuneatus, or gracilis and
ultimately to the primary sensory cortex. Then, the efferent pathway is conducted through the corticospinal tract to the alphamotoneuron. This typically takes about 40 to 50 seconds in the upper extremities. Therefore, when one electrically stimulates a muscle, the C-reflex will be seen on electromyography (EMG) recording about 40 to 50 milliseconds after the stimulation. Creflexes are typically enhanced in cortical myoclonus.
Multichannel EMG recording, also called
polymyography, can also be helpful in visualization of patterns of spread from one muscle to another, called the
recruitment pattern. Typical recruitment patterns in cortical myoclonus are described in
Table 78.2.
It is worth mentioning the Bereitschaftspotentials (BPs) here. The BP is a form of voluntary, movement-related potential seen on back-averaging EEG technique. It is also called a premovement or readiness potential. It is helpful in differentiating organic from psychogenic myoclonus; BPs are present in psychogenic movements but not in organic myoclonus.
SUBCORTICAL INCLUDING BRAIN STEM MYOCLONUS
The three main types of brain stem myoclonus are reticular reflex myoclonus, hyperekplexia, and palatal myoclonus. Myoclonus-dystonia syndrome or essential myoclonus also has a subcortical origin, as does myoclonus occurring after thalamic stroke (typically negative myoclonus or asterixis affecting one arm). The electrophysiologic findings are described in
Table 78.2.
Hyperekplexia, or exaggerated startle, has the nucleus gigantocellularis as a generator. It shows recruitment patterns, as described in
Table 78.2. Lack of habituation is a feature of hyperekplexia:
After repetitive stimuli, jerks will be less frequent and less severe clinically and electrophysiologically in the normal startle response but may fail to habituate in hyperekplexia.
SPINAL MYOCLONUS
Two major forms of spinal myoclonus exist: spinal segmental and propriospinal myoclonus. The typical recruitment pattern and the velocity of spread are described in
Table 78.2.
Spinal segmental myoclonus typically originates within a few or several adjacent spinal segments, typically cervical or lumbar.
Propriospinal myoclonus refers to axial jerks that originate in spinal cord segments, with spread up and down along the longitudinal axis of spinal cord. The very slow-conducting propriospinal pathway helps coordinate forelimb and hind limb movements in animals, such as cats, but the role of this pathway in humans is unclear.
It has been reported in the literature that BPs are associated with propriospinal myoclonus, and thus, a psychogenic cause has been proposed. However, there is evidence that there is disruption of fiber tracts in spinal cord seen on diffusion tensor imaging in patients with propriospinal myoclonus, suggesting that some forms of propriospinal myoclonus are organic.
PERIPHERAL MYOCLONUS
A typical example of peripheral myoclonus is hemifacial spasm, but peripheral myoclonus can also occur from irritation of spinal nerve roots, plexus, or peripheral nerves. The EMG burst duration varies, and there is lack of electrophysiologic patterns described earlier.
