In a normal person, when a muscle tendon is tapped briskly, the muscle immediately contracts due to a two-neuron reflex arc involving the spinal or brainstem segment that innervates the muscle. The afferent neuron whose cell body lies in a dorsal root ganglion innervates the muscle or Golgi tendon organ associated with the muscles; the efferent neuron is an alpha motoneuron in the anterior horn of the cord. The cerebral cortex and a number of brainstem nuclei exert influence over the sensory input of the muscle spindles by means of the gamma motoneurons that are located in the anterior horn; these neurons supply a set of muscle fibers that control the length of the muscle spindle itself.
Hyporeflexia is an absent or diminished response to tapping. It usually indicates a disease that involves one or more of the components of the two-neuron reflex arc itself.
Hyperreflexia refers to hyperactive or repeating (clonic) reflexes. These usually indicate an interruption of corticospinal and other descending pathways that influence the reflex arc due to a suprasegmental lesion, that is, a lesion above the level of the spinal reflex pathways.
By convention the deep tendon reflexes are graded as follows:
- 0 = no response; always abnormal
- 1+ = a slight but definitely present response; may or may not be normal
- 2+ = a brisk response; normal
- 3+ = a very brisk response; may or may not be normal
- 4+ = a tap elicits a repeating reflex (clonus); always abnormal
Whether the 1+ and 3+ responses are normal depends on what they were previously, that is, the patient's reflex history; what the other reflexes are; and analysis of associated findings such as muscle tone, muscle strength, or other evidence of disease. Asymmetry of reflexes suggests abnormality.
Technique
Valid test results are best obtained when the patient is relaxed and not thinking about what you are doing. After a general explanation, mingle the specific instructions with questions or comments designed to get the patient to speak at some length about some other topic. If you cannot get any response with a specific reflex—ankle jerks are usually the most difficult—then try the following:
- Several different positions of the limb.
- Get the patient to put slight tension on the muscle being tested. One method of achieving this is to have the patient strongly contract a muscle not being tested.
- In the upper extremity, have the patient make a fist with one hand while the opposite extremity is being tested.
- In general, any way to distract the patient from what you are doing will enhance the chances of obtaining the reflex. Having the patient count or give the names of children are examples.
Biceps Reflex (C5)
The forearm should be supported, either resting on the patient's thighs or resting on the forearm of the examiner. The arm is midway between flexion and extension. Place your thumb firmly over the biceps tendon, with your fingers curling around the elbow, and tap briskly. The forearm will flex at the elbow.
Triceps Reflex (C7)
Support the patient's forearm by cradling it with yours or by placing it on the thigh, with the arm midway between flexion and extension. Identify the triceps tendon at its insertion on the olecranon, and tap just above the insertion. There is extension of the forearm.
Brachioradialis Reflex (C6)
The patient's arm should be supported. Identify the brachioradialis tendon at the wrist. It inserts at the base of the styloid process of the radius, usually about 1cm lateral to the radial artery. If in doubt, ask the patient to hold the arm as if in a sling—flexed at the elbow and halfway between pronation and supination—and then flex the forearm at the elbow against resistance from you. The brachioradialis and its tendon will then stand out.
Place the thumb of the hand supporting the patient's elbow on the biceps tendon while tapping the brachioradialis tendon with the other hand. Observe three potential reflexes as you tap.
- Brachioradialis reflex: flexion and supination of the forearm.
- Biceps reflex: flexion of the forearm. You will feel the biceps tendon contract if the biceps reflex is stimulated by the tap on the brachioradialis tendon.
- Finger jerk: flexion of the fingers.
The usual pattern is for only the brachioradialis reflex to be stimulated. But in the presence of a hyperactive biceps or finger jerk reflex, these reflexes may be stimulated also.
