The Ankle Jerk Reflex(S1, S2) also known as the Achilles reflex, is performed by gently bending the knee and holding the foot in dorsiflexion. The tendon hammer then drops onto the Achilles tendon.
The response is plantar flexion of the foot with contraction of the gastrocnemius.
The absence of an ankle jerk in a patient with back and or sciatica pain may indicate nerve root compression. We must beware as this reflex may be normally absent in a number of patients.
One study found that patients < 60 years of age, < 5% had one absent ankle jerk reflex, but in those > 60 years of age up to 30% had both reflexes naturally absent(1)
Some of the more common causes of Absent ankle jerk reflex:
- Normal with age
- Sacral radiculopathy
- Cauda Equina
- Peripheral neuropathy secondary to
- Charcot marie Tooth
- Bowditch MG et al. The significance of an absent ankle reflex. J Bone Joint Surgery Br 1996 March;78(2):276-279