Check the deep tendon reflexes using impulses from a reflex hammer to stretch the muscle and tendon. The limbs should be in a relaxed and symmetric position, since these factors can influence reflex amplitude. As in muscle strength testing, it is important to compare each reflex immediately with its contralateral counterpart so that any asymmetries can be detected. If you cannot elicit a reflex, you can sometimes bring it out by certain reinforcement procedures. For example, have the patient gently contract the muscle being tested by raising the limb very slightly, or have them concentrate on forcefully contracting a different muscle group just at the moment when the reflex is tested. When reflexes are very brisk, clonus is sometimes seen. This is a repetitive vibratory contraction of the muscle that occurs in response to muscle and tendon stretch. Deep tendon reflexes are often rated according to the following scale:
Deep tendon reflexes are normal if they are 1+, 2+, or 3+ unless they are asymmetric or there is a dramatic difference between the arms and the legs. Reflexes rated as 0, 4+, or 5+ are usually considered abnormal. In addition to clonus, other signs of hyperreflexia include spreading of reflexes to other muscles not directly being tested and crossed adduction of the opposite leg when the medial aspect of the knee is tapped.
Deep tendon reflexes (see Neuroanatomy Through Clinical Cases, Figure 2.21) may be diminished by abnormalities in muscles, sensory neurons, lower motor neurons, and the neuromuscular junction; acute upper motor neuron lesions; and mechanical factors such as joint disease. Abnormally increased reflexes are associated with upper motor neuron lesions. Note that deep tendon reflexes can be influenced by age, metabolic factors such as thyroid dysfunction or electrolyte abnormalities, and anxiety level of the patient. The main spinal nerve roots involved in testing the deep tendon reflexes are summarized in the following table:
|Reflex||Main Spinal Nerve Roots Involved|