Why is it when a doctor gives you a physical examination, he taps your knee with a rubber hammer? My knee always jerks when he does this, and the same goes for everyone I have ever spoken to. Which makes me wonder if anyone has ever failed it, and what became of them. Does the medical community just go on looking, looking, hoping to find a person who fails the test? Or is there actually some hideous disease which has as one of its early symptoms that your knees do not jerk when struck?
Illustration by Slug Signorino
When I consulted the Straight Dope Science Advisory Board about this, one member informed me that if no knee-jerk reflex can be elicited, “this is one of the diagnostic signs that the patient is dead.” Ho ho! But of course there’s more to it than that. The actual purpose of knee tapping is to test for pathological conditions that, while not common, are far from nonexistent. These conditions fall into two categories.
(1) Hyperactive deep tendon reflex (knee jerks too much): amyotrophic lateral sclerosis, brain tumor, cerebrovascular accident (stroke), hepatic encephalopathy (associated with liver disease), hypocalcemia (low calcium), hypomagnesemia (low magnesium), hypothermia, multiple sclerosis, preeclampsia, spinal cord lesion (e.g., tumor), and tetanus.
(2) Hypoactive deep tendon reflex (knee doesn’t jerk enough): botulism, Eaton-Lambert syndrome, Guillain-Barre syndrome (nerve inflammation), peripheral neuropathy, polymyositis, syringomyelia, tabes dorsalis, and other ailments too scary to pronounce much less have.
When the doctor tests your reflexes she’s tapping the tendon that connects the muscle to the bone, which causes the muscle to stretch slightly. This sends a nerve impulse to your spinal cord where it triggers a motor impulse that returns via a parallel nerve and causes the muscle to twitch.
A faulty reflex in itself is not conclusive evidence that you have one of the problems above. For example, in the case of preeclampsia, a form of hypertension, you also have to be pregnant. But a bad reflex does tell the doctor to investigate further. One way to do this is by testing other reflexes. The doctor usually starts with your knee-jerk response, also known as the patellar reflex, because it’s quick and easy. But she can also whale away on your elbow (triceps reflex), crook of your arm (biceps reflex), wrist (brachioradialis reflex), or back of your ankle (Achilles tendon reflex).
If you’ve got feeble reflexes all over plus muscle weakness and blurred and double vision, maybe you’ve got botulism. If you’ve got hyperactive reflexes on one side of the body only, that’s a sign of brain tumor or stroke. If your patellar reflexes bite but your triceps reflexes are OK, that may mean you’ve got a lesion (injury) between your second lumbar vertebra and your — ah, never mind, only your doctor needs to know the details. And don’t despair. If the little hammer won’t make you jump, just wait till you see the bill.
Send questions to Cecil via firstname.lastname@example.org.