Myoclonus

Original Editor - Natalie Patterson Top Contributors - Natalie Patterson, Lucinda hampton, Chrysolite Jyothi Kommu and Aminat Abolade

Introduction

Myoclonus is a sudden, brief involuntary twitching or jerking of a muscle or group of muscles. It is a clinical sign and is not itself a disease. The twitching cannot be stopped or controlled by the person experiencing it. Myoclonus can begin in childhood or adulthood, symptoms ranging from mild to severe.

Etiology

Myoclonus has many causes:

1. Most commonly by a disturbance of the brain or spinal cord (the central nervous system, or CNS), or
2. Seldom by an injury to the peripheral nerves.

Myoclonus is one of the signs in a wide variety of nervous system disorders for example:

• Dystonia
• Multiple sclerosis
• Parkinson's disease
• Huntington disease
• Alzheimer's disease.
• Also seen in conjunction with infection, head or spinal cord injury, stroke, brain tumors, kidney or liver failure, chemical or drug intoxication, or metabolic disorders.
• Prolonged oxygen deprivation to the brain, called hypoxia, may lead to post-hypoxic myoclonus.

Subdivisions

Similar to most movement disorders, myoclonus can be focal, multifocal, segmental, or generalized.

Myoclonus can be subdivided by its anatomical origin into the following types:

1. Cortical myoclonus: caused by a hyperexcitable focus within the sensory-motor cortex. It typically involves a limb or the face and is triggered by action of intention.
2. Subcortical-reticular myoclonus: may occur spontaneously, in response to various peripheral stimuli of during voluntary action. The myoclonic jerks and tend to be generalised myoclonic jerks. Axial and proximal muscles are mainly involved, causing neck flexion, shoulder elevation with trunk and knee extension.
3. Spinal (includes propriospinal myoclonus and segmental spinal myoclonus)
4. Peripheral (includes hemifacial spasm)

Myoclonus can be termed positive or negative, when caused by

1. A sudden muscle contraction, it is known as "positive myoclonus,"
2. A brief loss of muscular tone, it is known as "negative myoclonus".

Clinical Presentation

Myoclonic jerks are usually arrhythmic (without rhythm or regularity) and can be described as action myoclonus (activated by voluntary movement), reflex myoclonus (activated by sensory stimulation).

Rhythmic segmental myoclonus and brainstem myoclonus persisted during sleep. Myoclonic jerks usually represent brief muscle contractions (positive myoclonus) but may also be produced by equally brief lapses of muscle contraction (negative myoclonus or asterixis (a tremor of the hand when the wrist is extended)). In other words, positive myoclonus jerks originate from rapid, active contractions of a muscle or group of muscles.

Myoclonus can cause significant disability with impairment in activities of daily living and possibly depression. The jerky movements occurring during rest, with muscle activation or by external stimuli as sound resulting in interference with performing or starting the desired correct movement for a specific task may result in embarrassment.

This less than 2 minute video is silent and shows some basic movements and gait of a person with myoclonus.

Physical Examination

It is important to check whether myoclonus appears at rest, on posture (keeping the arms outstretched) or during action and to note its distribution.

Myoclonus at rest indicates a spinal or brainstem source, whereas action-induced myoclonus points to a cortical origin. Focal and multifocal jerks, occurring during voluntary action, are typical of cortical myoclonus. Spinal segmental myoclonus is also focal, although contrary to cortical myoclonus, it is not action-induced and is occasionally stimulus sensitive. Generalized myoclonus is usually subcortical (brainstem or propriospinal myoclonus) or less frequently cortical. The amplitude of myoclonus varies considerably. Very small, hardly visible distal myoclonic jerks (mini polymyoclonus) are typical for Multiple System Atrophy (MSA), whereas very large amplitudes are typical for progressive myoclonic epilepsies (PME).

Diagnosis

After reviewing of the person’s medical history and physical exam, the following may be ordered to confirm the diagnosis of myoclonus:

• Electromyography (EMG)
• Electroencephalography (EEG)
• Evoked potential studies capture the electrical activity in the brain, brain stem, and spinal cord evoked by specific stimuli (i.e., tactile, auditory, visual stimulation)
• Laboratory urine or blood tests for possible causes and to rule out other conditions that may cause symptoms similar to myoclonus
• Magnetic resonance imaging (MRI)

Treatment

Classification of myoclonus into cortical and subcortical helps in the guidance of pharmacotherapy since spinal myoclonus fails to respond to medications effective for cortical myoclonus and vice versa. Treatment of myoclonus is most effective when a reversible underlying cause can be found that can be treated, such as another condition, a medication or a toxin. If the underlying cause can't be cured or eliminated, then treatment is aimed at easing myoclonus symptoms, especially when they're disabling. There are no drugs specifically designed to treat myoclonus. More than one drug may be needed to control the symptoms.

Medications

Medications that doctors commonly prescribe for myoclonus include:

• Clonazepam (Klonopin), a tranquilizer, is the most common drug used to combat myoclonus symptoms. Clonazepam may cause side effects such as loss of coordination and drowsiness.
• Anticonvulsant have proved helpful in reducing myoclonus symptoms.
• Botulinum toxin can be offered as a treatment option in focal peripheral myoclonus.
• Deep brain stimulation (DBS) has been reported in myoclonus–dystonia syndrome, with the target being both internal globus pallidus (GPi) and thalamic nuclei

Differential Diagnosis

Rhythmic myoclonus may be confused with tremor. Its frequency is often slower than the commonly observed tremors, it is present at rest, is not modified significantly by voluntary movements and often persists during sleep.

Myoclonus may also be confused with chorea, especially if multifocal and asynchronous, but in Chorea the movements continue in a constant flow, randomly distributed over the body and randomly distributed in time.