Clonus is an abnormal, repetitive, rhythmic contraction of a muscle or group of muscles in response to sudden stretching. Normally, a stretched muscle responds with a single contraction followed by relaxation. In clonus, stretching triggers a series of involuntary, oscillatory contractions. Clonus may also be observed in seizure activity, such as in grand mal epilepsy, reflecting hyperexcitable motor circuits.

Clonus is a neurological sign indicative of upper motor neuron (UMN) lesions or hyperactive reflex pathways.

Pathophysiology

• Clonus arises from disruption of inhibitory signals from the motor cortex to the spinal cord, resulting in hyperactive stretch reflexes.

• Normally, the corticospinal (pyramidal) tract inhibits excessive reflex activity. Damage to this tract causes:

  • Loss of supraspinal inhibition

  • Enhanced excitability of alpha motor neurons

  • Oscillatory, repetitive contractions of the affected muscle

• Common mechanisms include:

  • Stroke, traumatic brain injury, or spinal cord lesions

  • Demyelinating diseases (e.g., multiple sclerosis)

  • Severe metabolic or infectious encephalopathies

Examination Technique

Patient Positioning

• Place the patient supine or relaxed in a comfortable position.

• Ensure the muscle to be tested is relaxed.

Eliciting Clonus

1. Ankle Clonus (most commonly tested):

   • Rapidly dorsiflex the foot while stabilizing the leg.

   • Observe for rhythmic, involuntary plantar flexion contractions.

2. Knee or Wrist Clonus:

   • Rapidly extend or flex the joint.

   • Observe for repetitive oscillatory movements.

Assessment

• Count the number of contractions or beats; sustained clonus (>5–10 beats) suggests significant UMN pathology.

• Compare bilateral responses for asymmetry.

Clinical utility

• Indicator of upper motor neuron lesions: Clonus is a hallmark of corticospinal tract dysfunction.

• Adjunct to neurological examination: Often assessed alongside spasticity, hyperreflexia, and Babinski-like signs.

• Monitoring disease progression: Useful in stroke recovery, multiple sclerosis, or spinal cord injury.

Differential Diagnosis

Pediatric considerations

• Mild clonus may be physiological in infants and young children due to immature corticospinal inhibition.

• Persistent or asymmetrical clonus beyond infancy warrants neurological evaluation.

Geriatric considerations

• Clonus in older adults commonly reflects stroke, degenerative spinal cord disease, or other UMN lesions.

• May be accompanied by spasticity, hyperreflexia, and gait abnormalities.

Limitations

• Clonus can be elicited in normal individuals under extreme conditions (anxiety, voluntary movement).

• Interpretation requires correlation with other neurological findings.

Patient Counseling

• Explain that clonus is a sign of abnormal nerve function, not a disease itself.

• Reassure that assessment guides further evaluation and management.

• Highlight the importance of reporting new or worsening symptoms such as weakness, numbness, or spasticity.

Conclusion

Clonus is a repetitive, rhythmic muscle contraction in response to stretch, serving as a reliable marker of upper motor neuron dysfunction. Proper assessment, including localization, duration, and symmetry, enhances diagnostic accuracy and guides management in conditions such as stroke, spinal cord injury, multiple sclerosis, and seizures.

References

1. Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science. 5th ed. New York: McGraw-Hill; 2013.

2. Ropper AH, Samuels MA, Klein JP. Adams and Victor’s Principles of Neurology. 11th ed. New York: McGraw-Hill; 2021.

3. Pandya DN, et al. Clonus and Hyperreflexia: Clinical Assessment and Neurophysiology. Neurol Clin. 2000;18(3):611–626.

4. American Academy of Neurology. Neurological Examination Guidelines. 2022.

5. Frowein RA, Mumenthaler M. Clinical Examination of Upper Motor Neuron Signs. Neurol Clin. 2000;18:595–610.