occasional bi-lateral response to patellar tendon reflex

[evphysio]

Hi there,

when assessing a patient and testing patellar tendon stretch reflexes an occasional bilateral response is elicited (other leg bounces with tested leg).

What is this response called/indicative of?

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[gcoe]

Hi Evphysio

That is a really good question and I don't know the answer. The occasional bilateral response is documented but as far as I know never really explained. It generally occurs in those with more exaggerated responses - whether for pathological reasons or because they have a natural tendency to a more brisk response. so upper motor neurone syndrome due to CNS pathology or a more "nervy" patient without pathology. For example the anxious patient is much more likely to have exaggerated or brisk reflexes.

The monosynaptic stretch reflex is the accepted explanation for the DTR. As far as I know this holds that the afferent neuron from the muscle spindle excites the alpha motor neurons of the homologous muscle and that there are branches crossing over to the other side of the spinal cord to the contralateral alpha motor neurons that are inhibitory. So the monosynaptic stretch reflex doesn't explain the phenomenon. However that doesn't mean there may not be some excitatory neurons - just may not have been identified to date - or maybe they have been and I just don't know. Perhaps when the gamma motor system is primed or the descending inhibitory inputs are damaged such a pathway could be more easily facilitated.

Keen to know if someone else has a better explanation.