Pendular knee jerk is an abnormal response to the patellar tendon reflex (knee-jerk reflex) where, after the initial extension of the leg, the leg swings back and forth more than four times before coming to rest.
Understanding the Pendular Knee Jerk
The knee-jerk reflex, or patellar tendon reflex, is a monosynaptic reflex that tests the integrity of the spinal cord at the L2-L4 levels. Normally, when the patellar tendon is tapped, the leg extends once and then quickly comes to rest. A pendular knee jerk signifies a disruption in the normal damping mechanisms within the nervous system.
Characteristics of Pendular Knee Jerk
- Exaggerated Swinging: The leg swings excessively (more than 4 times) after the initial knee jerk response.
- Loss of Damping: The normal dampening effect of the nervous system, which controls the extent and duration of the reflex, is impaired.
Possible Causes
Pendular knee jerk is often associated with cerebellar lesions or other conditions that affect the cerebellum's ability to coordinate and control movement. Specifically, it can be caused by:
- Cerebellar Disease: Damage or dysfunction of the cerebellum.
- Hypothyroidism: In some cases, severe hypothyroidism may lead to pendular reflexes.
- Other Neurological Conditions: Conditions affecting the pathways involved in motor control.
Clinical Significance
The presence of a pendular knee jerk is a significant neurological sign. It suggests that further investigation is needed to determine the underlying cause, which might include neuroimaging (like MRI) to examine the cerebellum.
Normal vs. Abnormal
| Feature | Normal Knee Jerk | Pendular Knee Jerk |
|---|---|---|
| Leg Movement | Single extension followed by quick stop | Initial extension followed by >4 back-and-forth swings |
| Damping Mechanism | Intact and effective | Impaired |
| Significance | Indicates normal neurological function | Suggests cerebellar or other neurological issue |
