How to Assess for Ankle Clonus
To assess for ankle clonus in a patient with hyperactive lower extremity reflexes, sharply dorsiflex the foot and maintain this position while supporting the knee.
Correct Technique
The proper method for testing ankle clonus involves:
- Sharply dorsiflexing the foot (bringing the toes toward the shin)
- Maintaining this dorsiflexed position while holding the foot
- Supporting the knee in a flexed position to relax the gastrocnemius muscle
- Observing for rhythmic, involuntary oscillations (beats) of the foot between plantarflexion and dorsiflexion
This technique stretches the Achilles tendon and soleus muscle, which can trigger the hyperexcitable stretch reflex characteristic of upper motor neuron lesions 1.
Why This is the Correct Answer
Ankle clonus represents a pathological manifestation of hyperactive stretch reflexes. When you rapidly dorsiflex and hold the foot in this position, you're applying sustained stretch to the plantarflexor muscles. In patients with upper motor neuron dysfunction, this sustained stretch triggers repetitive, rhythmic muscle contractions that you can both see and feel as the foot "beats" back and forth 2, 3.
The research demonstrates that the plantarflexion phase of clonus beats occurs at approximately 71-73 milliseconds, corresponding to the medium-latency reflex response in the soleus muscle 2. This timing confirms that clonus is a reflex-mediated phenomenon triggered by the dorsiflexion maneuver.
Why the Other Options are Incorrect
- Tuning fork to lateral malleolus: This tests vibration sense (large fiber sensory function), not reflexes or clonus 4
- Flexing knee, rotating externally, striking Achilles tendon: This describes testing the Achilles (ankle jerk) reflex, not clonus 5
- Stroking lateral sole from heel to ball: This is the Babinski test for plantar reflex, not clonus assessment
Clinical Context
When assessing hyperactive reflexes, you're looking for signs of upper motor neuron pathology. The presence of sustained clonus (typically ≥5 beats) is pathological and indicates significant hyperreflexia. Transient clonus (1-3 beats) may be normal, especially in anxious patients 6.
The neurological examination should include assessment of deep tendon reflexes at multiple levels, but when specifically testing for clonus, the dorsiflexion-and-hold technique is the standard method 7, 6.