Corpus: Patellar reflex

The patellar reflex is a deep tendon reflex that causes a contraction of the quadriceps muscle in response to a tap on the patellar tendon, leading to knee extension. It is mediated by the femoral nerve and involves the motor neurons of spinal segment L3, as well as the adjacent segments L2 and L4.

The reflex is best elicited with the patient in a seated position.

• The patient sits on a chair or examination couch with one leg loosely crossed over the other. Alternatively, the examiner can slightly lift the leg from behind the knee.
• The reflex is triggered by a firm but not excessive tap on the patellar tendon just below the kneecap using a reflex hammer. If the response is particularly brisk, the examiner can also elicit the reflex by placing a finger on the upper edge of the patella and striking the finger with the hammer.
• The test should be repeated several times at approximately two-second intervals.
• The opposite side should also be examined for comparison.
• If the reflex response is weak, the patient may be asked to perform the Jendrassik maneuver to facilitate the reflex.

In bedridden patients, the leg is flexed at the knee by gently lifting it from behind. The reflex is then elicited in the same manner as described above. Since the muscle response is often less pronounced in this position, the examiner should focus on detecting visible contraction of the quadriceps rather than leg movement.

• Absent reflex: Possible causes include lumbar disc herniation at L3 or peripheral nerve injury.
• Diminished reflex: Suggests neuropathy.
• Exaggerated reflex or expanded reflex zone: Indicates a pyramidal tract lesion