Theankle jerk reflex, also known as theAchilles reflex, occurs when theAchilles tendon is tapped while the foot isdorsiflexed. It is a type of stretch reflex that tests the function of thegastrocnemius muscle and the nerve that supplies it.A positive result would be the jerking of the foot towards itsplantar surface. Being a deep tendon reflex, it ismonosynaptic. It is also a stretch reflex. These are monosynapticspinal segmental reflexes. When they are intact, integrity of the following is confirmed:cutaneous innervation, motor supply, andcortical input to the corresponding spinal segment.
This reflex is mediated by the S1 spinal segment of the spinal cord.[1]
Ankle of the patient is relaxed. It is helpful to support the ball of the foot at least somewhat to put some tension in the Achilles tendon, but don’t completelydorsiflex the ankle. A small strike is given on the Achilles tendon using a rubber hammer to elicit the response. If the practitioner is not able to elicit a response, aJendrassik maneuver can be tried by having the patient cup their fingers on each hand and try to pull the hands apart. A positive response is marked by a briskplantarflexion of the foot. The response is also graded into Grade 0-4 according to thereflex grading system.[2]
The Achillesreflex checks if the S1 and S2[3]nerve roots are intact and could be indicative ofsciatic nervepathology. It is classically delayed inhypothyroidism. This reflex is usually absent indisk herniations at the L5—S1 level. A reduction in the ankle jerk reflex may also be indicative ofperipheral neuropathy.
Grade 4 ankle hyperreflexia is called ankle clonus. 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and plantarflexion on passive dorsiflexion of the foot by the examiner till the force applied by the examiner is withdrawn.
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 hyperreflexia and clonus. Some other causes of clonus are
