The Patellar Tendon Reflex, commonly known as the knee jerk test, is a routine procedure performed during a neurological examination. This quick test involves tapping the tendon below the kneecap to observe the involuntary leg extension that follows. The response provides immediate, objective information about the function of the peripheral and central nervous systems. By evaluating the reflex, doctors gain insight into whether the nerves, spinal cord, or muscles responsible for this action are working correctly.
The Anatomy of the Patellar Reflex Arc
The knee jerk is a classic example of a stretch reflex, operating through a simple circuit called a reflex arc. This arc is monosynaptic, meaning the sensory signal transfers directly to the motor signal with only one connection (synapse) between the neurons within the spinal cord. The reflex begins when tapping the patellar tendon causes a sudden, slight stretch of the quadriceps muscle.
Specialized sensory receptors, called muscle spindles, detect this stretch and send an impulse along a sensory neuron. This neuron travels through the femoral nerve and enters the spinal cord at the L2, L3, and L4 segments. It then connects directly with a motor neuron, which sends the signal back out to the quadriceps muscle, causing it to contract and the leg to kick forward. This entire pathway bypasses the brain, allowing for an extremely rapid response, often occurring in less than 20 milliseconds.
Technique for Eliciting the Reflex
To perform the test, the patient is typically seated with their legs hanging freely over the side of the examination table, ensuring the muscles are relaxed. This positioning allows the quadriceps to be stretched when the tendon is struck, and the lower leg can swing unimpeded. The doctor locates the patellar tendon, the tough cord just below the kneecap, and strikes it briskly with a reflex hammer.
The clinician observes the speed, force, and amplitude of the resulting leg movement. If the reflex is weak or cannot be elicited easily, a reinforcement technique known as the Jendrassik maneuver may be used. This involves asking the patient to interlock their fingers and pull them apart just as the tendon is tapped. The distraction generated by this action temporarily reduces the brain’s natural inhibitory input, making a subtle reflex more apparent.
Clinicians use a standard grading scale to quantify the response, typically ranging from 0 to 4.
Reflex Grading Scale
- Grade 0: The reflex is absent.
- Grade 1+: A diminished response.
- Grade 2+: An average, normal reflex.
- Grade 3+: A brisk response that may be normal or slightly exaggerated.
- Grade 4+: A very brisk response often accompanied by clonus (a rhythmic, involuntary muscle contraction).
Understanding Abnormal Reflex Responses
Hyporeflexia (Diminished or Absent Response)
A diminished (hyporeflexia) or absent (areflexia) response (Grade 1+ or 0) suggests a failure somewhere along the reflex arc itself. This finding often localizes the issue to the lower motor neuron (LMN) pathway.
Damage to the LMN pathway includes problems with the motor neurons in the spinal cord, the peripheral nerve (such as the femoral nerve), or the muscle itself. Conditions like peripheral neuropathy (common in diabetes) or compression of the spinal nerve roots at the L2-L4 level can reduce the reflex. Muscle diseases, such as muscular dystrophy, can also result in hyporeflexia because the muscle tissue is unable to contract effectively.
Hyperreflexia (Exaggerated Response)
In contrast, an exaggerated or hyperactive reflex (Grade 3+ or 4+) indicates a problem in the upper motor neuron (UMN) pathway. UMNs originate in the brain and descend to the spinal cord, normally exerting an inhibitory influence on the reflex arc.
When the UMN pathway is damaged, this inhibition is lost, causing the spinal cord reflexes to become overly sensitive. Hyperreflexia can be a sign of conditions affecting the brain or spinal cord above the reflex’s level, such as a stroke, multiple sclerosis, or a spinal cord injury. An asymmetrical response, where one leg is significantly different from the other, helps the doctor distinguish between a central nervous system issue and a peripheral nerve issue, guiding further diagnostic testing.