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Ankle clonus -A complete guide

Clonus is a series of involuntary, rhythmic, muscular contractions and relaxations. Clonus is a sign of upper motor neuron lesions involving descending motor pathways,
Ankle clonus is an involuntary tendon reflex that causes repeated flexion and extension of the foot. . More than four beats of clonus is pathological.It is the grade 4 ankle hyper reflexia.
There is repetitive ankle dorsiflexion and plantarflexion on passive dorsiflexion of the foot by the examiner till the force applied by the examiner is withdrawn.
Method of elicitation of  ankle clonus
  • Patient lies supine. 
  • Support the flexed knee with your left palm (placed in the popliteal fossa) (with both the knee and ankle resting in 90° flexion)
  • Now briskly dorsiflex and partially evert the foot with your right hand sustaining the pressure (do not leave the hand after dorsiflexing). (heel should not touch the bed). 
  • A series of contraction and relaxation of calf muscles are seen if a steady pressure is maintained on the foot by the right hand. Clonus is felt as repeated beats of dorsiflexion/plantar flexion.

Pathogenesis of ankle clonus
Ankle clonus is seen in people with damage to the neuronal pathways in the spinal cord or brain (an upper motor neurone lesion), elicited by a deliberate rapid, stretching of the ACHILLES TENDON, by forcibly flexing the foot. There is a rhythmic contraction of the calf muscles following a sudden passive dorsiflexion of the foot, the leg being semiflexed.The spinal reflex arc is intact but the normal control on it, from above, is abnormal.
Causes of ankle clonus
Any spinal cord lesions, be it traumatic, neoplastic, pyogenic, vascular above the level of S1 can cause clonus.
This is because of the spasticity caused by the UMN type of injury causing hyper reflexia and clonus. Some other causes of clonus are
  • Meningitis
  • Tetanus
  • CJD
  • Cerebral palsy
  • Multiple sclerosis
  • Syringomyelia
  • Pre-eclampsia
  • Ankle clonus: