What is myoclonus?
Myoclonus (say: my-oh-KLOH-nus) is the sudden and rapid twitching of a muscle or a group of muscles. This is often described as a "shock-like" movement and can be repetitive or rhythmic. The twitching is caused by uncontrolled muscle contraction or relaxation.
Myoclonus twitches often occur when children start a movement like walking or kicking a soccer ball. They may happen in response to external stimuli such as flashing lights or touch. More than one twitch could happen at the same time. Twitches may occur in a series or even follow a distinct pattern. The twitches can occur in the neck, back, face, arms, legs or the entire body.
Many types of myoclonus
Mild and severe
In milder cases of myoclonus, children have a single muscle contraction followed by relaxation and a loss of muscle tone. This usually happens while the child begins to fall asleep and in this context, the movement is called "hypnic jerk". This type of myoclonus happens in healthy children and adults.
In more severe cases of myoclonus, groups of muscles contract involuntarily throughout the body. This can prevent children from carrying out normal activities such as gripping objects.
Myoclonus: named by cause
The different types of myoclonus are named depending on their cause.
- Action myoclonus is triggered or made worse when a child starts a movement. This type of myoclonus is often seen after an injury caused by a lack of oxygen to the brain.
- Sleep myoclonus is a mild form that occurs as the person falls asleep and happens in healthy children. Sleep myoclonus can also take more severe forms, including restless leg syndrome.
- Stimulus-sensitive myoclonus is triggered by external stimuli such as light, noise and movement.
- Asterixis (negative myoclonus) is a sudden involuntary relaxation of a muscle. This type of myoclonus is usually seen in children with liver or kidney disease.
- Palatal myoclonus is the rhythmic contraction of the muscles at the roof of the mouth (the soft palate).
- Epileptic myoclonus is a specific type of seizure that can be seen in children with epilepsy.
Myoclonus types are also named depending on the area of the brain that is affecting movement:
- Cortical myoclonus is caused by abnormalities in the cerebral cortex, the outer layer of the brain.
- Subcortical myoclonus is caused by abnormalities in the deep portions of the brain or brainstem.
- Spinal myoclonus is caused by abnormalities within the spinal cord.
Myoclonus is caused by conditions that affect the brain
Myoclonus is a sign of a condition that affects the brain and spinal cord. For example, it is a common symptom in epilepsy. Myoclonus can also be caused by brain tumours, head or spinal cord injuries and stroke. Other causes include infections, kidney or liver failure, lipid storage disease, lack of oxygen to the brain and chemical and drug poisoning.
Exactly how these conditions cause myoclonus is not fully known. Parts of the brain control muscles throughout the body, so myoclonus may be linked to abnormal chemical levels in the brain.
Treatment of myoclonus with medicines
Myoclonus is treated with different medications.
- Benzodiazepines: This class of drugs helps to reduce symptoms by slowing down the central nervous system. Side effects include drowsiness and loss of coordination. Clonazepam is an example of a benzodiazepine usually used for treatment.
- Anti-seizure medications, such as valproic acid and levetiracetam.
A doctor will prescribe a specific drug depending on the type of myoclonus and its cause.
Sometime a doctor may prescribe more than one drug. This is because myoclonus can be caused by abnormalities in different areas of the brain. Different drugs target different brain areas.