Clonus | Video demonstration & Explanation (in 3 mins) | Neurology | OSCE

Clonus is an important neurological sign that reflects upper motor neuron dysfunction. Among its common sites, the ankle is most frequently tested in clinical practice. Recognizing clonus, its causes, and how to elicit it correctly is vital for clinicians and students alike.

This article explains what ankle clonus is, why it occurs, conditions where it is found, and a step-by-step approach to performing the examination.

Table of Contents

1. Introduction
2. What is Clonus?
3. Sites Where Clonus Can Be Elicited
4. Sustained vs. Ill-Sustained Clonus
5. Conditions Associated with Clonus
6. How to Perform the Ankle Clonus Test
7. Conclusion
8. Key Takeaways

Introduction

Ankle clonus is a rhythmic, involuntary alternation of muscle contraction and relaxation, usually seen when the ankle is dorsiflexed. Its presence indicates upper motor neuron lesions and is commonly associated with hyperreflexia in other parts of the body.

What is Clonus?

• Clonus is a series of involuntary, rhythmic muscle contractions and relaxations.
• The frequency is usually around 5–8 Hz.
• It is most commonly seen in the ankle but can also be elicited at other joints.
• Clonus is a hallmark of upper motor neuron lesions.

Sites Where Clonus Can Be Elicited

Clonus is not limited to the ankle. It can also be demonstrated at:

1. Ankle (most common)
2. Knee (patella)
3. Wrist
4. Triceps
5. Biceps

Sustained vs. Ill-Sustained Clonus

• Ill-sustained clonus: Fewer than 5 beats of rhythmic jerks.
• Sustained clonus: More than 10 beats, continuing as long as the joint is held in position.

Conditions Associated with Clonus

Clonus is usually associated with conditions involving upper motor neuron dysfunction or systemic disease. Common causes include:

1. Neurological Disorders
   • Cerebrovascular accidents (ischemic or hemorrhagic stroke)
   • Multiple sclerosis
   • Cerebral palsy
   • Encephalopathy
2. Systemic Conditions
   • Hepatic encephalopathy (in advanced liver failure)
   • Serotonin syndrome (important clinical clue)
   • Pre-eclampsia (in pregnancy, presence of clonus is a risk factor for convulsions)

How to Perform the Ankle Clonus Test

1. Position the patient supine with the knee relaxed and slightly flexed (using a pillow beneath the knee or by supporting it with your arm).
2. With one hand, stabilize the patient’s leg.
3. With the other hand, grasp the foot and suddenly dorsiflex the ankle.
4. Observe for rhythmic contraction and relaxation of the foot.
5. Assess whether the clonus is sustained or ill-sustained.

Conclusion

Ankle clonus is a simple but powerful neurological sign. Its presence often points toward an upper motor neuron lesion or systemic pathology such as hepatic encephalopathy, serotonin syndrome, or pre-eclampsia. Performing the test correctly ensures reliable results, making it an essential bedside skill for all clinicians.

Key Takeaways

• Clonus is a rhythmic, involuntary contraction-relaxation cycle of muscle groups, commonly seen at the ankle.
• It signifies upper motor neuron lesions and is often associated with hyperreflexia.
• Ill-sustained clonus is less than 5 beats; sustained clonus is more than 10 beats.
• Causes include stroke, multiple sclerosis, cerebral palsy, encephalopathy, hepatic failure, serotonin syndrome, and pre-eclampsia.
• The test is performed by suddenly dorsiflexing the ankle while the patient’s knee is slightly flexed and relaxed.