Myoclonus

What myoclonus is

Myoclonus is a description of a movement, not a single disease. A jerk caused by a sudden muscle contraction is called positive myoclonus. A brief loss of muscle tone that makes a limb drop or the body sag is negative myoclonus, also called asterixis, the "flapping tremor" seen in liver and kidney failure.

The jerks can be:

• focal (one body part), segmental (adjacent parts) or generalized (the whole body)
• spontaneous, or triggered by movement (action myoclonus), noise, light or touch (reflex myoclonus)
• single or in rhythmic runs

Myoclonus differs from a tremor, which is a regular back-and-forth shaking, and from a tic, which can be briefly held back and is often preceded by an urge.

Harmless (physiological) myoclonus

These jerks happen in healthy people and need no treatment:

• Hypnic jerks or sleep starts: a sudden jolt, often with a falling sensation, as you fall asleep. They are more common with caffeine, stress, sleep deprivation and late exercise.
• Hiccups: myoclonus of the diaphragm.
• Startle jerks in response to a sudden noise.
• Jerks after exercise or anxiety: after heavy exercise, tired muscles can twitch and jerk briefly.
• Infant sleep myoclonus: jerks in sleeping newborns that stop when the baby wakes.

Myoclonus after heavy exercise

Brief jerks or twitches after intense exercise are usually from muscle fatigue, dehydration, electrolyte shifts, caffeine or pre-workout stimulants, and settle within hours with rest, fluids and food. Severe, whole-body jerks after exercise are different. They need assessment, because exercise and sleep deprivation can trigger seizures in juvenile myoclonic epilepsy, and severe exertion can cause muscle breakdown (rhabdomyolysis) with dark urine and electrolyte problems. Very rarely, post-exercise jerking points to a metabolic muscle disorder.

Medical causes of myoclonus

Epilepsy

Myoclonic seizures are brief jerks, often of both arms, typically within an hour of waking. Juvenile myoclonic epilepsy starts in the teenage years and is triggered by sleep deprivation, alcohol and flashing lights. Other childhood epilepsy syndromes also cause myoclonus.

Metabolic and organ failure

• Kidney failure and uremia
• Liver failure (with asterixis)
• Low blood sugar, low sodium, low calcium or magnesium
• Severe infection or sepsis

Medicines and toxins

Common culprits include opioids (especially at high doses or in kidney failure), gabapentin and pregabalin, some antidepressants, lithium, levodopa, some antibiotics and anesthetic agents. Serotonin syndrome causes jerks with fever, agitation and sweating. Withdrawal from alcohol or benzodiazepines can also trigger it.

Brain injury

• Post-hypoxic myoclonus: after the brain is starved of oxygen, for example after a cardiac arrest. Early, severe myoclonus suggests serious brain injury. A later, action-triggered form (Lance-Adams syndrome) can appear after recovery.
• Head injury, stroke, brain tumors, encephalitis.

Degenerative and other brain diseases

• Alzheimer's disease and Lewy body dementia, mostly in later stages
• Parkinson's disease, multiple system atrophy and corticobasal degeneration
• Creutzfeldt-Jakob disease: rapidly progressive dementia with startle-sensitive myoclonus
• Progressive myoclonic epilepsies, rare inherited disorders
• Multiple sclerosis

Spinal myoclonus

Jerks of the trunk muscles, often when lying down, from spinal cord injury or inflammation, or with no cause found (propriospinal myoclonus).

Functional myoclonus

Jerks generated by the brain's movement networks without structural damage. They are real and involuntary, often vary over time, and may be triggered by stress. They respond to specialized physical therapy and psychological treatment.

Symptoms

• sudden jerks or twitches, like an electric shock
• dropping objects or spilling drinks
• jerks that worsen with movement, emotion or stimulation
• difficulty walking, speaking or eating in severe action myoclonus
• in negative myoclonus, sudden lapses of posture, such as the knees buckling

Diagnosis

A neurologist will ask when jerks occur, what triggers them, whether anyone in the family has similar problems, and review all medicines. Tests may include:

• blood tests: kidney and liver function, electrolytes, glucose, thyroid, and drug levels
• EEG to look for epileptic activity linked to jerks
• EMG to measure the duration and pattern of muscle bursts
• MRI of the brain or spine
• genetic tests or spinal fluid tests in selected cases

Treatment

The first step is removing the cause: stopping or reducing a responsible medicine, correcting electrolytes or treating organ failure. Myoclonus from metabolic causes and drugs usually resolves once the cause is fixed.

Medicines to reduce jerks include:

• levetiracetam and valproate, used for many types including myoclonic epilepsy
• clonazepam, a benzodiazepine, which can cause drowsiness
• piracetam in some countries, and zonisamide
• botulinum toxin injections for focal myoclonus

Some antiseizure medicines, such as carbamazepine and phenytoin, can worsen myoclonic epilepsy, so the choice of drug matters. Physical and occupational therapy help with daily tasks. People with juvenile myoclonic epilepsy are advised to keep a regular sleep schedule and limit alcohol.

Outlook

Physiological myoclonus is harmless. Drug-induced and metabolic myoclonus usually clears once the trigger is removed. Juvenile myoclonic epilepsy is usually well controlled with medicine, though often lifelong. Myoclonus from degenerative brain diseases tends to progress, but medicines can ease it.

Frequently asked questions

What is myoclonus?

Myoclonus is a brief, involuntary, shock-like jerk of a muscle or group of muscles. Hiccups and the jolts you feel falling asleep are common harmless forms. Persistent or widespread jerks can be caused by epilepsy, medicines, organ failure or brain disease and should be checked by a doctor.

Is myoclonus serious?

Usually not. Sleep starts, hiccups and occasional jerks after exercise are normal. Myoclonus is more concerning when it is frequent, widespread, worsening, comes with confusion or seizures, or appears with kidney or liver disease or after a new medicine.

Why do I get muscle jerks after heavy exercise?

Brief jerks after intense exercise are usually due to muscle fatigue, dehydration, electrolyte loss or stimulants such as caffeine. Rest, fluids and food usually settle them. Severe whole-body jerks, loss of awareness or dark urine after exercise need medical assessment.

What medicines cause myoclonus?

Common causes include opioids, gabapentin, pregabalin, some antidepressants, lithium, levodopa and certain antibiotics, especially at high doses or when the kidneys are not working well. Drug-induced myoclonus usually resolves when the medicine is reduced or stopped under medical advice.

How is myoclonus treated?

Treating the cause comes first. When jerks persist, medicines such as levetiracetam, valproate or clonazepam often reduce them. Botulinum toxin injections can help jerks confined to one area. The best treatment depends on the type of myoclonus, so a neurologist's assessment is important.