Patellar Clonus Test
The medical term for the test where the knee is positioned at 45-degree flexion and dorsiflexion of the ankle elicits clonus is called the patellar clonus test or knee clonus test. However, based on your description involving ankle dorsiflexion causing clonus, this appears to be ankle clonus testing performed with the knee flexed at 45 degrees, which is the standard position for eliciting ankle clonus 1, 2.
Test Methodology
Position the patient supine with the knee flexed to approximately 45 degrees to relax the gastrocnemius-soleus complex and optimize stretch reflex testing 1.
Rapidly dorsiflex the ankle and maintain gentle upward pressure on the ball of the foot to sustain the stretch 1, 2.
Positive test: Rhythmic, involuntary oscillations of the ankle between plantar flexion and dorsiflexion at a frequency of 5-8 Hz (approximately 160-200 ms per cycle) 1.
Clinical Significance
Clonus indicates upper motor neuron pathology such as spinal cord injury, stroke, multiple sclerosis, or severe metabolic derangements like hepatic failure or serotonin syndrome 1, 2, 3.
Sustained clonus (>5 beats) is pathological, while brief unsustained clonus (2-3 beats) may be normal, particularly in anxious patients or those with hyperreflexia 4.
The mechanism involves both central spinal generators and peripheral stretch reflexes, not solely recurrent muscle-tendon stretch as previously thought 5, 6.
Key Examination Features
Plantar flexion comprises 45% of each oscillation cycle, dorsiflexion 55%, with the first beat always longer than subsequent beats, which stabilize by the 4th-5th cycle 1.
Synchronous EMG activity occurs in soleus, gastrocnemius, and tibialis anterior during clonus, indicating central pattern generation rather than simple reflex arc activation 5.
Clonus frequency remains constant (5-8 Hz) regardless of the force applied, though amplitude varies with dorsiflexion force strength 1, 6.
Common Pitfall
Do not confuse ankle clonus with patellar clonus, which is tested by sharply displacing the patella downward with the knee extended and observing rhythmic patellar oscillations—a much less common finding 1.