The deep tendon reflexes are used to assess the spinal cord and the peripheral nervous system, testing these reflexes can also be used to determine the presence of neuromuscular disease. The underlying mechanism of the examination is to test muscle contraction in response to stretching of a muscle. The reflex itself is a monosynaptic reflex, which controls skeletal muscle length. Specifically when a muscle lengthens the sensory receptors, or muscle spindle, within the muscle is stretched leading to increased nerve activity. This increase in nerve activity leads to an increase in alpha motor neuron activity, which leads to muscle contraction, thus resisting the stretching of the muscle. The function of this reflex is to maintain the muscle at a constant length.
The reflexes that are tested by tapping on the tendons of specific muscle groups in the upper and lower limbs respectively, these muscles include: the biceps muscle; which tests C5, brachioradialis; which tests C6, triceps; which tests C7 and C8, patellar/quadriceps; which tests L3 and L4, and finally the Achilles; testing S1 and S2. Once tested, the reflexes are graded based on the following scale: 0=absent, 1= present but diminished, 2=normal, 3=exaggerated, and 4= clonus; a grade of 0 or 4 are always abnormal, the others could be considered normal in certain instances. For each reflex tested, the examiner should test one side and then the other sequentially to determine if the responses are equal, rather then testing one side of the body at a time.
Testing deep tendon reflexes can lead a clinician to determine whether the problem is a lower motor neuron lesion, which would lead to decreased reflexes, or and upper motor neuron lesion, which would lead to increased reflexes. I have a distant cousin that has multiple sclerosis and this condition leads to symptoms of an upper motor neuron lesion leading to increased reflexes.
Works Cited
Braunwald E., Fauci, S., Hauser S., Jameson J. Kasper D., Longo D., Loscalso J. (Eds.). (2008). Harrison’s Principles of Internal Medicine (17th edition) New York. McGraw Hill Medical. Pg 2488
