What Is Clonus a Sign Of? Causes and Conditions

Clonus is a sign of disrupted signaling between the brain and the spinal cord, almost always pointing to damage or disease in what neurologists call the upper motor neuron pathway. When you see or feel involuntary, rhythmic jerking at a joint, usually the ankle, it means the nervous system’s normal braking mechanism on reflexes has been compromised. The list of conditions that can cause this ranges from stroke and spinal cord injury to multiple sclerosis, serotonin syndrome, and even severe preeclampsia during pregnancy, so context matters enormously when interpreting what clonus means for any individual person.

How Clonus Happens

Under normal circumstances, the brain sends signals down the spinal cord that keep stretch reflexes in check. When you tap a tendon with a reflex hammer, the muscle fires once and then quiets down because descending signals dampen any further activity. Clonus occurs when that dampening is lost. A quick stretch of the muscle, like pushing the foot upward at the ankle, triggers a reflex contraction. Without the brain’s inhibitory control, the contraction itself stretches the opposing muscle, which fires its own reflex, and the cycle repeats. The result is a series of involuntary, rhythmic jerks that can last anywhere from a few beats to many seconds.

Spasticity and clonus share the same root cause, an upper motor neuron lesion that removes the normal inhibition of stretch reflexes, but they look different. Spasticity shows up as a velocity-dependent tightness in the muscle: the faster you try to move the limb, the more it resists. Clonus, by contrast, produces repetitive, uncontrollable jerks rather than sustained stiffness.¹ Both can be present at the same time in the same limb, and they often are. But clonus is the more visually dramatic of the two and tends to be the symptom that brings people to a clinician’s attention because it can feel alarming: a foot bouncing on its own, sometimes vigorously enough to shake the whole leg.

Research on people with chronic spinal cord injuries has refined our understanding of how clonus sustains itself. Rather than being driven purely by the repeated stretch of muscle, clonus appears to involve intrinsic oscillating networks of neurons in the spinal cord that become active once they lose the brain’s supervisory input.² In other words, the spinal cord itself starts generating a rhythmic pattern once it is cut off from higher control. This is why clonus can be so persistent and why it sometimes appears even without an obvious triggering stretch.

Stroke

Stroke is one of the most common reasons a person develops clonus. When a stroke damages the motor pathways running from the brain through the brainstem and down the spinal cord, the stretch reflexes below the level of injury lose their normal regulation. Lower limb spasticity and ankle clonus are particularly common after stroke, often emerging in the weeks or months following the initial event as the acute phase resolves and the chronic upper motor neuron pattern sets in.³ The clonus can interfere substantially with walking, balance, and daily activities. In people with hemiplegic stroke, both spasticity scores and clonus scores tend to be quite elevated on the affected side, and treating one often helps the other.⁴

Spinal Cord Injury

After spinal cord injury, clonus is extremely common and can be one of the most frustrating long-term symptoms. Because the injury physically interrupts the descending fibers that keep reflexes in check, the spinal circuits below the lesion become hyperexcitable. Clonus in this population can interfere with self-care, wheelchair use, transfers, and rehabilitation exercises.⁵ Electromyographic recordings during clonus episodes after cervical spinal cord injury show that the rhythmic activity does not stay confined to one muscle. Activity can spread to multiple muscles in the same limb and even to the opposite limb, which underscores how broadly the spinal cord’s excitability is altered.

Not all spinal cord pathology that causes clonus involves traumatic injury. Tumors compressing the spinal cord, such as a cervical schwannoma, can produce clonus by depriving the stretch reflex circuits of their normal supraspinal control.² In that scenario, the clonus can actually mimic other movement disorders like action tremor, making diagnosis tricky until imaging reveals the underlying compression. This is one reason clinicians do not treat clonus as a diagnosis in its own right but as a signal that something upstream is wrong.

Multiple Sclerosis

Multiple sclerosis damages the myelin insulation that wraps nerve fibers in the brain and spinal cord. When the demyelinated fibers happen to be in the motor pathways that descend from the brain, the result is the same loss of reflex inhibition seen in stroke or spinal cord injury, just produced by a different mechanism. Clonus is recognized as a disabling symptom of upper motor neuron dysfunction in people living with MS.⁶ It tends to fluctuate with disease activity: a new relapse or a worsening plaque in the right location can intensify clonus, while effective disease-modifying therapy and rehabilitation may quiet it down.

For many people with MS, clonus at the ankle is among the symptoms that most directly affects quality of life. The involuntary jerking can make it difficult to keep a foot on a car pedal, stand steadily in a shower, or sleep comfortably. Because MS is a chronic and often progressive condition, managing clonus is typically an ongoing effort rather than a one-time fix.

Metabolic and Systemic Causes

Not every case of clonus traces back to a structural brain or spinal cord lesion. Severe hepatic failure is a metabolic condition that can produce clonus, likely because accumulated toxins in the bloodstream disrupt normal neuronal signaling in the central nervous system.⁷ In liver failure, the clonus may appear alongside other neurological signs like confusion, asterixis (a flapping tremor of the hands), and altered consciousness. Treating the underlying metabolic derangement, for instance by managing ammonia levels, can reduce or resolve the clonus.

Other systemic conditions that disrupt brain chemistry can theoretically provoke clonus, though structural lesions remain far more common. The practical lesson here is that clonus in a person who has no known history of stroke, spinal cord injury, or neurological disease should prompt a broader medical workup rather than an assumption that it must be a spinal problem.

Serotonin Syndrome and Medications

Clonus has a special diagnostic role in serotonin syndrome, a potentially life-threatening reaction to excess serotonin activity in the brain. When a person taking a serotonin-affecting medication, such as an SSRI, SNRI, triptan, or certain pain medications, develops spontaneous clonus, the Hunter Serotonin Toxicity Criteria treat that as a strong indicator. The diagnostic framework considers a case positive when there is a serotonergic agent plus any of the following: spontaneous clonus on its own, inducible clonus paired with agitation or heavy sweating, or ocular clonus (rapid rhythmic eye movements) paired with agitation or sweating.⁸

In serotonin syndrome, clonus tends to appear alongside a cluster of other symptoms: agitation, rapid heart rate, elevated temperature, dilated pupils, and diffuse hyperreflexia throughout the body. A case report documented a patient presenting with severe bilateral lower extremity clonus, diffuse hyperreflexia, persistent tachycardia, and ophthalmoplegia in the setting of overlapping serotonin toxicity and another neurological condition.⁹ The distinguishing feature of serotonin syndrome clonus is that it develops acutely, often within hours of a medication change or overdose, and resolves once the serotonergic excess is addressed. This is fundamentally different from the chronic, structural clonus of stroke or spinal cord injury.

If you or someone you know is on a serotonin-affecting medication and develops new-onset ankle jerking along with agitation, sweating, or rapid heart rate, that combination warrants urgent medical attention. Serotonin syndrome can escalate quickly, and recognizing clonus as part of the picture can speed up the diagnosis.

Preeclampsia and Eclampsia

In obstetrics, ankle clonus takes on a very specific meaning. During pregnancy, clinicians routinely check deep tendon reflexes and look for clonus as part of monitoring for preeclampsia, a condition involving dangerously elevated blood pressure and organ dysfunction. Clonus at the ankle is considered one of the premonitory signs of eclampsia, the seizure complication of preeclampsia. Other warning signs that tend to cluster alongside it include severe hypertension, headache, visual disturbances, and upper abdominal pain.¹⁰

The mechanism in preeclampsia is not a permanent structural lesion. Rather, the severe hypertension and associated vascular changes appear to cause enough cerebral irritability that the upper motor neuron pathways become hyperexcitable. Once the pregnancy ends and blood pressure normalizes, the clonus typically resolves. This makes it quite different from the clonus of stroke or MS, where the underlying damage is lasting. For pregnant women, though, the appearance of clonus during a hypertensive episode is a red flag that needs immediate clinical evaluation because it suggests the nervous system is under significant stress.

Clonus in Infants

Parents and pediatricians sometimes notice ankle clonus in babies during the first year of life, and the question of what it means is not always straightforward. In newborns, the nervous system is still maturing, and a few beats of clonus at the ankle can be a normal finding that resolves as myelination of the motor tracts progresses. Sustained or pronounced clonus, however, is more concerning.

A study tracking infants who exhibited ankle clonus within their first year found that roughly 57% turned out to be neurologically normal on follow-up. The remaining children had diverse outcomes: cerebral palsy was the most common pathological diagnosis, followed by intellectual disability, borderline cognitive scores, and motor delay.¹¹ Those numbers mean that clonus in an infant is not automatically an ominous sign, but it does warrant monitoring. A baby with a few beats of ankle clonus at two months who is otherwise developing normally is in a very different situation from a baby with sustained clonus, poor head control, and abnormal tone.

When Clonus Looks Like Something Else

One of the practical difficulties with clonus is that the rhythmic jerking can resemble other movement disorders, particularly tremor. Action-induced clonus, for instance, can look nearly identical to an action tremor: the limb shakes rhythmically when the person tries to use it. In at least one documented case, what appeared to be an action tremor in a patient’s hands turned out to be clonus caused by a tumor compressing the cervical spinal cord. The distinction only became clear after imaging revealed the schwannoma and the “tremor” resolved with treatment of the underlying compression.²

Myoclonus is another condition sometimes confused with clonus, despite the similar-sounding names. Myoclonus refers to sudden, brief, shock-like involuntary movements that can originate from the brain, spinal cord, or peripheral nerves and do not follow the rhythmic, sustained pattern typical of clonus. The two conditions can co-exist, but they arise from different mechanisms and have different diagnostic implications. If a movement is truly rhythmic, sustained, and triggered by stretching a joint, it is much more likely to be clonus than myoclonus.

How Clinicians Test for Clonus

The bedside test for clonus is straightforward. With the patient’s leg relaxed, the examiner briskly pushes the foot upward (dorsiflexes the ankle), stretching the calf muscles. If clonus is present, the foot starts bouncing rhythmically. Clinicians count the beats: fewer than three is usually considered within normal range or equivocal, while sustained clonus (more than three beats, or clonus that continues as long as the stretch is held) is abnormal and suggests an upper motor neuron problem.

In research settings and specialized clinics, clonus can be characterized more precisely using electromyography, which records the electrical activity of the muscles during the rhythmic contractions. Automated analysis of these recordings can identify the frequency, intensity, and duration of each contraction within a clonus episode. An algorithm developed for this purpose matched human analysts in accuracy while working hundreds of times faster, which highlights how consistent and measurable the rhythmic pattern of clonus actually is.¹² In clinical practice, though, the bedside test is usually sufficient to establish whether clonus is present. The harder question is always what is causing it.

Managing Clonus

Because clonus is a symptom rather than a disease, treatment depends heavily on the underlying cause. In serotonin syndrome, stopping the offending medication and providing supportive care usually resolves the clonus within hours to days. In preeclampsia, delivering the baby and managing blood pressure addresses the root problem. In metabolic conditions like hepatic failure, treating the metabolic derangement is the priority.

For chronic clonus from stroke, spinal cord injury, or MS, management is more complex and typically ongoing. The same medications used for spasticity are the first-line options. Oral drugs used for this purpose include centrally acting agents like baclofen, clonidine, and tizanidine, as well as anticonvulsants like benzodiazepines and gabapentin, and the peripherally acting drug dantrolene.¹³ When oral medications are insufficient, interventional options include focal injections of botulinum toxin or phenol into the offending muscles, and implantation of an intrathecal baclofen pump that delivers medication directly to the spinal fluid.

Physical rehabilitation also plays a role. Stretching programs targeting the ankle muscles have shown improvements in range of motion, stiffness, and walking speed in people with post-stroke spasticity and clonus. Fascial manipulation, a hands-on physical therapy technique, has been studied specifically for lower limb spasticity and clonus in stroke patients.³ More experimental approaches include epidural spinal cord stimulation, which has shown promise in immediately reducing clonus in a person with MS, though this remains an investigational technique.⁶

Practical Signals Worth Knowing

Whether clonus should worry you depends entirely on the clinical context. A few beats of ankle clonus in a healthy, anxious person being examined for the first time may mean nothing. Sustained clonus in someone with known MS is a familiar, if frustrating, part of their condition. But new-onset clonus in someone who was previously neurologically normal is always worth investigating, because it suggests that something has recently changed in the brain or spinal cord.

The urgency varies by scenario. New clonus with fever, agitation, and a recent medication change raises the possibility of serotonin syndrome, which can deteriorate rapidly. Clonus in a pregnant woman with high blood pressure and headache puts eclampsia on the table, and that is an obstetric emergency. Gradual-onset clonus with slowly worsening leg stiffness and difficulty walking could point toward an undiagnosed spinal cord lesion, MS plaque, or slowly growing tumor compressing the spinal cord, all of which need imaging to sort out.

One common misconception is that clonus always means something catastrophic has happened. In the infant population, as noted, the majority of babies with ankle clonus in their first year turn out to be neurologically normal.¹¹ Similarly, mild clonus in an otherwise well adult may reflect a benign variation in reflex excitability rather than a serious lesion. The beat count matters: a couple of beats that quickly fatigue are much less concerning than sustained, vigorous clonus that persists as long as the stretch is maintained. And the company the clonus keeps, whether it appears alongside weakness, sensory changes, bowel or bladder dysfunction, or systemic symptoms like fever and tachycardia, shapes the clinical picture far more than the clonus alone.

1
PubMed Central. Clinical understanding of spasticity: implications for practice
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Journal of Movement Disorders. A Case of Action-Induced Clonus that Mimicked Action Tremors and was Associated with Cervical Schwannoma
3
PubMed Central. The effect of fascial manipulation therapy on lower limb spasticity and ankle clonus in stroke patients
4
PubMed. Alcohol neurolysis of tibial nerve motor branches to the gastrocnemius muscle to treat ankle spasticity in patients with hemiplegic stroke
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PubMed Central. Characteristics of lower extremity clonus after human cervical spinal cord injury
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PubMed. Percutaneous epidural spinal cord stimulation immediately reduces clonus in an individual with multiple sclerosis
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PubMed Central. Ankle clonus
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Psychopharmacology Institute. What Is Clonus a Sign Of? Causes and Conditions
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PubMed Central. Wernicke’s Encephalopathy and Serotonin Syndrome: A Case Report of Overlapping Pathologies
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PubMed Central. Risk factors for eclampsia in pregnant women with preeclampsia and positive neurosensory signs
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PubMed. Prognosis of infants with ankle clonus within the first year of life
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PubMed Central. Automatic analysis of EMG during clonus
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PubMed Central. A Review of Spasticity Treatments: Pharmacological and Interventional Approaches