What is a clonus? | What is Ankle & Patellar Clonus?

What is a clonus?
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  • It is a set of involuntary and rhythmic muscular contractions and relaxations.
  • It is a sign of certain neurological conditions, particularly involving upper motor neuron lesions.
  • In many cases it is accompanied by spasticity (another form of hyperexcitability). 
  • Term infants can be hyperreflexic, and a few beats of clonus can be a normal finding in this population
  • In adults, it is generally pathological

What is the mechanism?

  • 2 Theories
    • It is due to hyperactive stretch reflex which causes a self-perpetuating reactivation of peripheral muscle stretch circuits, with each beat producing the next
    • When the central oscillator is turned on by a peripheral event, it will continue to rhythmically excite motor neurons

Clinical Significance

  • As a marker of hyperreflexia, which is part of an upper motor neuron syndrome.
  • It is also often evaluated in suspected serotonin syndrome.
  • Drugs including psychiatric (monoamine oxidase inhibitor, selective serotonin reuptake inhibitor, serotonin-norepinephrine reuptake inhibitor, tricyclic antidepressant) or street drugs (cocaine, ecstasy, amphetamines) can cause clonus
  • It has been identified as an effective way to predict seizures in patients who have overdosed on tramadol

Sites where it can be seen

  • Ankle
  • Patella
  • Triceps surae
  • Wrist
  • Jaw
  • Biceps brachii

What is its frequency?

5-8 Hz 

How to demonstrate an Ankle Clonus?

  • At the ankle it is tested by rapidly dorsiflexing the foot
  • This induces a stretch to the gastrocnemius muscle.
  • The subsequent beating of the foot will result in a clonus.

When is is called as Sustained?

It is called as sustained if there are greater than 10 beats.

Case Review

A 3-year-old baby boy who is a known case of post meningitis hydrocephalus came with a complaint of an Exposed VP shunt. The NCCT Brain demonstrated that the shunt was also non-functional. The scan also showed gross hydrocephalus with multiple loculations and septations. On clinical examination, the child had significant mental retardation, ophthalmoplegia, and spastic quadriparesis. All deep tendon reflexes were exaggerated and he also had patellar and ankle clonus.

Video of Ankle Clonus in a 3 year old baby

Video Of Knee Clonus in a 3 year old Baby

Video demonstration of Patellar Clonus

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