Clonus: Symptoms, Causes and Treatment

Clonus is a neurological condition marked by involuntary, rhythmic muscle contractions and relaxations, often triggered by a sudden stretch or stimulation. It is commonly seen in individuals with neurological injuries or diseases affecting the central nervous system. Understanding clonus—its symptoms, underlying causes, and available treatments—can improve quality of life and clinical outcomes for those affected. In this comprehensive article, we'll explore the clinical manifestations, root mechanisms, and current approaches to managing clonus, synthesizing the latest research and clinical insights.

Symptoms of Clonus

Clonus is most recognizable by its distinctive muscular "beats"—a rapid, rhythmic oscillation typically observed at joints such as the ankle or wrist. For healthcare providers and patients alike, recognizing these symptoms is crucial for timely diagnosis and intervention. Let’s break down the key features and clinical presentation of clonus.

Symptom	Description	Common Locations	Source(s)
Rhythmic Jerks	Rapid, involuntary muscle contractions and relaxations	Ankle, wrist	2, 6, 10
Provoked by Stretch	Triggered by sudden muscle or tendon stretch	Ankles, sometimes wrists	2, 4, 6
Sustained Oscillation	Repetitive, continuing movements after trigger is removed	Distal joints	2, 6
Associated with Spasticity	Often occurs with increased muscle tone	Limbs	4, 10

Table 1: Key Symptoms

What Does Clonus Look Like?

Clonus typically presents as a series of rapid, rhythmic muscle contractions that can be visibly observed, most commonly at the ankle (ankle clonus) but also at other joints, such as the wrist. When a healthcare provider quickly dorsiflexes (bends upwards) the foot at the ankle, the foot may then "beat" up and down several times involuntarily—a hallmark of ankle clonus 2, 6, 10. In some cases, similar repetitive jerks can be seen at the wrist or other joints.

Triggering and Sustaining Factors

• Provocation by Stretch: Clonus is often triggered when a muscle is stretched suddenly, such as when the foot is forced upwards at the ankle. This rapid stretch activates abnormal reflex pathways that lead to the oscillatory movement 2, 4, 6.
• Persistence after Trigger: Remarkably, once started, clonus can continue for several seconds, even after the external force is removed. This sustained oscillation is a key distinguishing feature 2, 6.
• Location: While ankle clonus is the most common, wrist clonus and clonus at other distal joints can also occur, especially in severe neurological disease 6, 10.

Associated Neurological Signs

Clonus rarely occurs in isolation. It is usually accompanied by other signs of upper motor neuron lesions, such as:

• Spasticity (increased muscle tone)
• Hyperreflexia (exaggerated reflexes)
• Muscle stiffness or weakness

These symptoms reflect underlying disturbances in the central nervous system, often due to injury or disease affecting the brain or spinal cord 4, 10.

Causes of Clonus

Understanding the causes of clonus is essential for effective management. While the visible symptoms arise in the muscles, the root causes lie in disruptions to neurological pathways, particularly those that control muscle reflexes and tone.

Cause	Mechanism/Trigger	Typical Contexts	Source(s)
Spinal Cord Injury	Disrupted descending motor control	Trauma, chronic disease	2, 4, 3
Central Nervous System Lesions	Loss of inhibitory signals, increased excitability	Stroke, MS, brain injury	4, 3, 6
Drug Toxicity	Overstimulation of reflex pathways	Tramadol overdose	1
Peripheral Inputs	Sustained muscle/tendon stretch	Reflex hyperactivity	2, 3, 4, 6

Table 2: Primary Causes

Neurological Injury and Disease

Most cases of clonus are secondary to injuries or diseases that damage the central nervous system:

• Spinal Cord Injury (SCI): Both clinically complete and incomplete spinal cord injuries can lead to clonus, as they disrupt normal inhibitory controls from the brain to the spinal cord, resulting in hyperexcitable motor pathways 2, 4.
• Stroke and Brain Injury: Lesions in the brain's motor pathways, such as those caused by stroke or trauma, can similarly remove inhibitory influences, leading to spasticity and clonus 4, 3.
• Multiple Sclerosis (MS): Demyelinating diseases like MS can affect the pathways that regulate muscle tone and reflexes, predisposing patients to clonus 4.

Mechanisms Underlying Clonus

The origins of clonus have been debated, but current research points to a complex interplay of central and peripheral mechanisms:

• Reflex Instability: Clonus appears when the stretch reflex loop becomes unstable, often due to increased excitability of spinal motoneurons and prolonged reflex delays in distal muscles (like those of the ankle) 4. Computer models show that when motoneuron firing thresholds are reduced and reflex pathway delays are long, oscillatory (clonic) movements occur 4.
• Central Oscillators vs. Peripheral Reflexes: Some evidence suggests that clonus is not solely the result of muscle stretch or peripheral reflexes. Central mechanisms—possibly involving spinal generators—may play a key role, especially since clonic activity can persist even after the initial stimulus is removed 2, 3, 6. However, the interplay between central and peripheral factors is likely responsible for the clinical presentation 2, 3, 4, 6.

Other Triggers

• Drug Toxicity: Certain medications, notably tramadol in overdose, can provoke clonus. In these cases, clonus may be an early warning sign of impending seizures, and preventive interventions (like magnesium sulfate) are recommended 1.
• Peripheral Stimulation: Although less common, clonus can sometimes be triggered by non-neurological factors, such as repetitive cutaneous (skin) inputs 3.

Treatment of Clonus

Treating clonus requires a multifaceted approach, tailored to the underlying cause, severity, and functional impact on the patient. Management strategies range from pharmacological interventions to physical therapies and innovative neurostimulation techniques.

Treatment	Mechanism/Approach	Efficacy/Notes	Source(s)
Medications	Reduce nerve/muscle excitability	Magnesium sulfate, benzodiazepines, anticonvulsants	1, 5, 9
Nerve Blocks	Interrupt nerve conduction	Phenol nerve block more effective than botulinum toxin	10
Botulinum Toxin	Inhibits neurotransmitter release	Reduces clonus, less effective than phenol block	10
Electrical Nerve Stimulation	Inhibits reflex pathways	Suppresses clonus for hours	7, 8
Laser Photostimulation	Modulates neural activity	Temporary suppression of clonus	7
Physical Therapy	Stretching, strengthening, splinting	Supportive, symptom reduction	4, 10

Table 3: Treatment Options

Pharmacological Therapies

• Magnesium Sulfate: Especially in cases of drug-induced clonus (like tramadol toxicity), magnesium sulfate can prevent seizures and reduce clonus 1.
• Benzodiazepines and Anticonvulsants: These medications act on the central nervous system to dampen neuronal excitability and are sometimes used for generalized myoclonus and clonus 5, 9. However, some forms of clonus may be resistant to standard anticonvulsant therapy 5, 9.

Local Interventions

• Phenol Nerve Block: Injection of phenol into specific nerves (such as the tibialis posterior nerve) can significantly reduce ankle clonus by blocking nerve conduction and reducing alpha motoneuron excitability. Phenol nerve block has shown greater efficacy and longer-lasting effects compared to botulinum toxin in clinical studies 10.
• Botulinum Toxin: Injection into overactive muscles (e.g., triceps surae in the calf) inhibits acetylcholine release at the neuromuscular junction, thereby reducing muscle contractions. While effective, its effect may be less pronounced and shorter in duration than nerve blocks 10.

Neuromodulation Techniques

• Electrical Nerve Stimulation: Subcutaneous electrical stimulation of peripheral nerves (e.g., radial, median, or saphenous nerves) can suppress clonus and spasticity for several hours after treatment. This approach works by inducing centrifugal inhibition of spinal reflexes 8.
• Laser Photostimulation: Targeted laser irradiation over nerves can temporarily suppress clonus, likely through photochemical neural modulation. This effect is rapid and has been observed in patients with spastic paraparesis 7.

Supportive and Physical Therapies

• Physical Therapy: Regular stretching, strengthening, and splinting can help manage symptoms, improve joint mobility, and reduce the functional impact of clonus 4, 10.
• Patient Education: Teaching patients and caregivers how to recognize triggers and manage symptoms is essential for long-term self-management.

Emerging and Investigational Strategies

Research into the mechanisms of clonus continues to inform new therapeutic approaches, including advanced neuromodulation, targeted drug therapies, and personalized rehabilitation protocols 2, 4, 7, 8.

Conclusion

Clonus is a complex neurological phenomenon, often a sign of underlying central nervous system dysfunction. Its distinctive rhythmic muscle oscillations are both diagnostically important and functionally disabling for many patients. Through a better understanding of its symptoms, causes, and treatment options, clinicians can provide more effective and personalized care.

Key points covered:

• Symptoms: Clonus is characterized by rapid, rhythmic, involuntary muscle contractions—most often at the ankle—triggered by sudden stretch and often associated with spasticity and hyperreflexia 2, 4, 6, 10.
• Causes: The main causes are lesions of the central nervous system (spinal cord injury, stroke, MS) leading to reflex hyperexcitability. Both central and peripheral mechanisms interplay to produce the oscillatory movement, with drug toxicity (e.g., tramadol overdose) as an additional trigger 1, 2, 4, 6.
• Treatment: Management includes medications (magnesium sulfate, benzodiazepines), localized interventions (phenol nerve block, botulinum toxin), neuromodulation (electrical/laser stimulation), and supportive therapies. Phenol nerve block appears more effective than botulinum toxin for ankle clonus 1, 7, 8, 10.
• Ongoing Research: Continued investigation into the neurological underpinnings of clonus is expanding treatment options and improving outcomes for those affected.

By staying informed about the latest research and integrating multi-faceted therapeutic strategies, both patients and healthcare providers can work together to better manage and mitigate the impact of clonus.