Clonus is a rhythmic, repeated jerking of a muscle after it has been stretched, most often felt at the ankle. It is a sign about the nervous system rather than the muscle itself.1Neuroscience Online. Motor System Disorders A few beats that fade quickly can show up in healthy people, so whether it keeps going matters more than whether it appears at all.2Neuro Signs. What Is Clonus a Sign Of? Causes and Conditions When it does keep going, it usually points to one of a handful of brain or spinal cord conditions, including stroke, multiple sclerosis, spinal cord injury, ALS, and cerebral palsy. Occasionally it signals an emergency such as serotonin syndrome, preeclampsia, liver failure, or a sudden stop of baclofen.
Why Clonus Happens
Every muscle has a stretch reflex that makes it tighten when it is suddenly pulled. It’s the same reflex a doctor tests with a tap on the knee. Nerve pathways running from the brain down the spinal cord normally keep that reflex calm.3Neuroscience Online. Motor System Disorders When those pathways are damaged, the calming signal is lost and reflexes become exaggerated.4Neuroscience Online. Motor System Disorders Sometimes the reflex is then strong enough that the muscle contracts several times in a row, at about 5 to 7 jerks per second, when stretched and held.5Neuroscience Online. Motor System Disorders As each contraction relaxes, the held stretch pulls the muscle long again, which restarts the loop until it fades or the stretch is released.
In daily life, this often looks like a foot that bounces up and down when the ball of the foot presses on a stair, a car pedal, or the floor. Lifting the foot or changing its angle often stops it. The beat is steady and regular, nothing like a random twitch. It can make walking, driving, or standing feel unsteady.
Other kinds of shaking behave differently. A tremor happens without any stretch, often while you hold still or move. Muscle twitching is small and scattered. A cramp is one sustained, painful tightening rather than repeated beats. Clonus is set off by a quick stretch and repeats at a steady pace.
Where the problem sits matters. Doctors use exaggerated reflexes to tell whether weakness comes from the brain and spinal cord or from nerves out in the limbs.6Neuroscience Online. Motor System Disorders In people with long-term spinal cord injury, signals from the brainstem appear to help keep clonus going.7Descending Brainstem Systems Contribute to Ankle Clonus in Humans with Spinal Cord Injury. Descending Brainstem Systems Contribute to Ankle Clonus in Humans with Spinal Cord Injury
When Clonus Is Normal and When It Isn’t
To check for ankle clonus, a doctor moves the foot up and down a few times, then pushes the toes sharply up toward the shin and holds.8Patient Examination Videos. Video – Ankle Clonus – Patient Examination Videos Usually nothing happens, though a few back-and-forth beats can occur.9Patient Examination Videos. Video – Ankle Clonus – Patient Examination Videos Short clonus that fades, seen equally on both sides, can occur in healthy people who simply have brisk reflexes.10Neuro Signs. What Is Clonus a Sign Of? Causes and Conditions Sustained clonus, meaning about five or more beats or beats that keep going, is never normal and points to damage in the brain-to-spinal-cord pathways.11Neuro Signs. What Is Clonus a Sign Of? Causes and Conditions12BMJ. Testing the reflexes
Doctors usually test with you relaxed and the knee slightly bent, because tension in the leg can set off or hide beats. A reading taken while you are nervous or holding the leg stiff may not reflect your usual state.
Clonus on only one side is more concerning than mild, matching clonus on both sides. Doctors may test the kneecap as well as the ankle.13BMJ. Testing the reflexes Even so, judging clonus depends largely on the examiner’s experience, since it is usually rated by eye and feel.14IDEALS. Mechatronic and biomechanical considerations toward the design of an ankle clonus simulator Doctors compare it with your strength, your sensation, and the reflexes at your knees and elsewhere before deciding what it means.
If you notice clonus yourself, tell the doctor when it started and whether it is getting worse, and whether it affects one side or both. Mention any weakness, stiffness, numbness, balance trouble, or bladder changes. Also mention new medicines, especially antidepressants or migraine drugs.
Long-Term Conditions That Cause Clonus
Stiff leg muscles and clonus are common after a stroke.15PubMed Central. The effect of fascial manipulation therapy on lower limb spasticity and ankle clonus in stroke patients A brain injury upsets the balance of calming and exciting nerve signals,16PubMed. A literature review of the pathophysiology and onset of post-stroke spasticity so the size and location of the damage shape how stiff or jerky the limbs become.17PubMed. A literature review of the pathophysiology and onset of post-stroke spasticity Right after a stroke the affected limb is often weak and floppy. Stiffness and clonus tend to develop over the following weeks to months, usually in the leg or ankle on the weak side, often when the person puts weight on the foot or tries to walk.
In multiple sclerosis, clonus is a disabling symptom caused by damage to the brain-to-spinal-cord pathways.18PubMed. Percutaneous epidural spinal cord stimulation immediately reduces clonus in an individual with multiple sclerosis MS damages the protective coating on nerve fibers in the brain and spinal cord, which slows or scrambles the calming signals traveling down to the legs. Clonus can appear alongside stiffness, weakness, or numbness, and vision changes are common in MS too. Symptoms often come and go in episodes, so clonus may be more noticeable in some weeks than others. Heat, tiredness, or an infection can temporarily worsen leg stiffness and jerking. That does not mean the disease itself has advanced.
After spinal cord injury, stiff, tight muscles often develop once an early period of limp paralysis passes.19PubMed. Spasticity after spinal cord injury So clonus may not appear right away but can emerge weeks later.
ALS destroys nerve cells in both the brain and the spinal cord.20PubMed Central. Amyotrophic lateral sclerosis It causes weakness, muscle wasting, twitching, stiffness, and exaggerated reflexes together.21PubMed Central. Amyotrophic lateral sclerosis
Ankle clonus in a baby’s first year does not always mean a problem.22PubMed. Prognosis of infants with ankle clonus within the first year of life Among 169 babies who had ankle clonus at least once, about 57% developed normally.23PubMed. Prognosis of infants with ankle clonus within the first year of life Many of the rest were later diagnosed with cerebral palsy or developmental delays.24PubMed. Prognosis of infants with ankle clonus within the first year of life Doctors therefore look at the baby’s age and other neurological signs before drawing conclusions.25PubMed. Prognosis of infants with ankle clonus within the first year of life
At a visit for new clonus, expect a full reflex and strength exam. An MRI of the brain or spine is often the next step, blood tests may check for other causes, and a referral to a neurologist is common.
Sudden Clonus That Needs Urgent Care
Clonus is the main sign doctors use to recognize serotonin syndrome, a reaction to too much serotonin from certain medicines.26Journal of Palliative Medicine. What Is Clonus a Sign Of? Causes and Conditions It counts if clonus appears on its own, or with agitation or heavy sweating.27Journal of Palliative Medicine. What Is Clonus a Sign Of? Causes and Conditions Rhythmic eye jerking with agitation, or stiffness with a temperature above 38°C (100.4°F), also point to it.28Journal of Palliative Medicine. What Is Clonus a Sign Of? Causes and Conditions New clonus after starting or combining antidepressants, migraine drugs, or some painkillers needs urgent attention. Serotonin syndrome often starts within hours of a new medicine or a dose increase, so timing is a major clue. Early signs can feel like a bad flu or anxiety: restlessness, sweating, a racing heart, or shaking.
Overactive reflexes and clonus are a sign of preeclampsia, a dangerous high blood pressure condition of pregnancy.29London Health Sciences Centre. Obstetrical Assessment and Common Complications Clonus may warn that seizures, called eclampsia, could follow, and can signal that preeclampsia is affecting the nervous system.30London Health Sciences Centre. Obstetrical Assessment and Common Complications31PubMed Central. Preeclampsia and Related Problems A pregnant person with shaking legs along with headache, vision changes, or swelling should seek care the same day.
Sudden liver failure can quickly cause confusion, flapping hands, exaggerated reflexes, clonus, seizures, and coma.32PubMed. The neurological manifestations of acute liver failure Yellow skin together with confusion is a reason to go to the emergency room.
Baclofen calms stiff muscles and clonus.33PubMed Central. Intrathecal baclofen withdrawal syndrome- a life-threatening complication of baclofen pump: A case report Stopping it suddenly lets the overactive signals return in force.34PubMed Central. Intrathecal baclofen withdrawal syndrome- a life-threatening complication of baclofen pump: A case report This often happens when an implanted baclofen pump or its tube fails.35PubMed. Cyproheptadine for intrathecal baclofen withdrawal Early signs are itching and increased stiffness,36PubMed. Cyproheptadine for intrathecal baclofen withdrawal but it can quickly progress to high fever, confusion, and seizures, and can be fatal.37PubMed. Cyproheptadine for intrathecal baclofen withdrawal Never stop baclofen suddenly, and call the pump team at once for these signs.
Go to the emergency room for clonus with fever or heavy sweating, or with confusion, agitation, or seizures. Tell the staff about every antidepressant and any dose change, plus triptans, tramadol, and supplements like St John’s wort. Mention a baclofen pump and its refill date, and bring the medicine bottles if you can.
How Clonus Is Treated
Treatment starts with the cause. If a medicine is behind the clonus, as in serotonin syndrome, the first step is stopping or changing that drug under medical supervision, and treating the cause usually eases the clonus. For long-term conditions like stroke or MS, treatment aims to reduce stiffness and improve function rather than cure the underlying damage.
Baclofen, diazepam, tizanidine, and dantrolene are approved to treat stiff muscles.38PubMed. Drugs used to treat spasticity Clonidine, gabapentin, and botulinum toxin have shown benefit but need more study.39PubMed. Drugs used to treat spasticity Botulinum toxin injections improved clonus in studies of 181 patients, though the evidence is still early.40PubMed. Effect of Botulinum Toxin on Clonus: A Systematic Review Injections relax only the muscle they are given into, while pills act throughout the body, so doctors choose based on whether the problem is mainly in one muscle group or spread across the body.
Baclofen can also be delivered by an implanted pump straight to the spinal fluid.41PubMed. Cyproheptadine for intrathecal baclofen withdrawal In one person with MS, spinal cord stimulation reduced clonus right away.42PubMed. Percutaneous epidural spinal cord stimulation immediately reduces clonus in an individual with multiple sclerosis Soft-tissue therapy has been tried to ease ankle clonus after stroke.43PubMed Central. The effect of fascial manipulation therapy on lower limb spasticity and ankle clonus in stroke patients
Muscle relaxants come with trade-offs. Drowsiness and weakness are common. Some stiffness actually helps people stand, so overtreatment can hurt walking. Doses are raised slowly, and they should never be stopped suddenly without advice.
Day to day, pressing the heel down firmly or lifting the foot often stops a bout. Regular stretching keeps calf muscles from tightening, and physical therapists can suggest splints or footwear. Noting triggers like cold, tiredness, or infection helps. It also helps to count beats and note the side and time, and short phone videos let the doctor see what happens. Bring the record to appointments.