Chapter 8 - Reflex evaluation
Reflexes are the most objective part of the neurologic examination and they are very helpful in helping to determine the level of damage to the nervous system. We will first discuss the various reflexes used in clinical practice and will conclude the chapter with a discussion of the significance of the findings. In some situations, reflexes may be the major part of the examination (e.g., the comatose patient). They have the value of requiring minimal cooperation on the part of the patient and of producing a response that can be objectively evaluated by the examiner. A list of all possible reflexes would be almost endless and a tangle of eponymic jargon for those with an historical bent. It is necessary to know the most commonly elicited reflexes and this knowledge is not terribly difficult to acquire. However, the interpretation of the reflex response requires some discussion. Table 8-1 is a list of many reflexes, some of them in common clinical use (and some less common). As a group, these reflexes can aid in evaluation of most of the segmental levels of the nervous system from the cerebral hemisphere through the spinal cord.
In this chapter we will discuss the evaluation of commonly tested reflexes of the spinal cord. We have previously considered reflexes involving the cranial nerves such as the pupillary light reflex, the jaw-jerk reflex, the baroreceptor reflex and gag. We have also discussed reflex eye movements and many of the autonomic reflexes (such as the oculocardiac and the pupillary light reflex). Here we will consider muscle stretch reflexes and superficial reflexes that are used to evaluate sensorimotor function of the body.
All reflexes, when reduced to their simplest level, are sensorimotor arcs. At the minimum, reflexes require some type of sensory (afferent) signal, and some motor response. While the simplest of reflexes involve direct synapse between the sensory fiber and the motor neuron (monosynaptic), many reflexes have several neurons interposed (polysynaptic reflexes).
It is important to note that, even with the simplest of reflexes, there are multiple inhibitory and facilitatory influences that can affect the excitability of the motor neuron and thus amplify or suppress the response. These influences can arise from various levels of the nervous system. There are intrasegmental and intersegmental connections in the spinal cord, as well as descending influences from the brain stem, cerebellum, basal ganglia and cerebral cortices. All of these can influence the excitability of motor neurons, thereby altering reflex response.
Lesions that damage the sensory or motor limb of a reflex arc will diminish that reflex. This can occur at any level of the sensory or motor pathway (in the case of the muscle stretch reflex, for example, this can include: the peripheral nerve and receptors; the dorsal root or dorsal root ganglion; the spinal cord gray matter; the ventral root; the peripheral nerve; the neuromuscular junction; or the muscle).
Most of the pathways that descend the spinal cord have a tonic inhibitory effect on spinal reflexes. For this reason, the net result of lesions that damage the descending tracts is facilitation of reflexes that are mediated at only the level of the spinal cord (a classic example being the muscle stretch reflex). With few exceptions, this means that these spinally mediated reflexes become hyperactive. After acute lesions, spinal reflexes often pass through an initial stage of hypoactivity. This stage has been called "spinal shock" or diaschisis and is more severe and long lasting in proportion to the degree of damage. For example, transection of the spinal cord removes the greatest amount of higher influence and may be associated with weeks of hypoactivity. Small lesions may have little effect on reflexes. When reflexes return after spinal transection, they become extremely hyperactive.
Some reflexes, such as the muscle stretch reflex, are semi-quantitatively graded. This is also true for responses such as the pupillary light reflex, where the speed of reaction may indicate a "sluggish" response. On the other hand, many reflexes are simply noted as present or absent. This is true of the superficial reflexes (see Table 8-1) and the "primitive reflexes" that are associated with diffuse bilateral hemispheric dysfunction. In this latter case, the reflexes are often designated as "dysinhibited" because these are infantile responses that are suppressed in the normal adult nervous system.
The muscle stretch (myotatic) reflex is a simple reflex, with the receptor neuron having direct connections to the muscle spindle apparatus in the muscle and with the alpha motor neurons in the central nervous system that send axons back to that muscle (Figure 8-1). Normal muscle stretch reflexes result in contraction only of the muscle whose tendon is stretched and the agonist muscles (i.e., muscles that have the same action). There is also inhibition of antagonist muscles.
Reflexes are graded at the bedside in a semi-quantitative manner. The response levels of deep tendon reflexes are grade 0-4+, with 2+ being normal. The designation "0" signifies no response at all, even after reinforcement. Reinforcement requires a maximal isometric contraction of muscles of a remote part of the body, such as clenching the jaw, pushing the hands or feet together (depending on whether an upper or lower limb reflex is being tested), or locking the fingers of the two hands and pulling (termed the Jendrassik maneuver). This kind of maneuver probably amplifies reflexes by two mechanisms: by distracting the patient from voluntarily suppressing the reflex and by decreasing the amount of descending inhibition.
The designation 1+ means a sluggish, depressed or suppressed reflex, while the term trace means that a barely detectible response is elicited. Reflexes that are noticeably more brisk than usual are designated 3+, while 4+ means that the reflex is hyperactive and that there is clonus present. Clonus is a repetitive, usually rhythmic, and variably sustained reflex response elicited by manually stretching the tendon. This clonus may be sustained as long as the tendon is manually stretched or may stop after up to a few beats despite continued stretch of the tendon. In this case it is useful to note how many beats are present.
One sign of reflex hyperactivity is contraction of muscles that have different actions while eliciting a muscle stretch reflex (for example, contraction of thigh adductors when testing the patellar reflex or contraction of finger flexor muscles when testing the brachioradialis reflex). This has been termed "pathological spread of reflexes."
Practice observing normal reflexes in patients and initially among students is an excellent way to determine the range of normalcy. Almost any grade of reflex (outside of sustained clonus) can be normal. Asymmetry of reflexes is a key for determining normalcy when extremes of response do not make the designation obvious. The patient's symptoms may facilitate the determination of which side is normal, i.e., the more active or the less active side. If this is a problem, the remainder of the neurologic examination and findings usually clarify the issue.
Decreased reflexes should lead to suspicion that the reflex arc has been affected. This could be the sensory nerve fiber but may also be the spinal cord gray matter or the motor fiber. This motor fiber (the anterior horn cell and its motor axon coursing through the ventral root and peripheral nerve) is termed the "lower motor neuron" (LMN). LMN lesions result in decreased reflexes. The descending motor tracts from the cerebral cortex and brain stem are termed the "upper motor neurons" (UMN). Lesions of the UMNs result in increased reflexes at the spinal cord by decreasing tonic inhibition of the spinal segment.
Lesions of the cerebellum and basal ganglia in humans are not associated with consistent changes in the muscle stretch reflex. Classically, destruction of the major portion of the cerebellar hemispheres in humans is associated with pendular deep-tendon reflexes. The reflexes are poorly checked so that when testing the patellar reflex, for example, the leg may swing to-and-fro (like a pendulum). In normal individuals, the antagonist muscles (in this example, the hamstrings) would be expected to dampen the reflex response almost immediately. However, this is not a common sign of cerebellar disease and many other signs of cerebellar involvement are more reliable and diagnostic (see Chapter 10). Basal ganglia disease (e.g., parkinsonism) usually is not associated with any predictable reflex change; most often the reflexes are normal.
Superficial reflexes are motor responses to scraping of the skin. They are graded simply as present or absent, although markedly asymmetrical responses should be considered abnormal as well. These reflexes are quite different from the muscle stretch reflexes in that the sensory signal has to not only reach the spinal cord, but also must ascend the cord to reach the brain. The motor limb then has to descend the spinal cord to reach the motor neurons. As can be seen from the description, this is a polysynaptic reflex. This can be abolished by severe lower motor neuron damage or destruction of the sensory pathways from the skin that is stimulated. However, the utility of superficial reflexes is that they are decreased or abolished by conditions that interrupt the pathways between the brain and spinal cord (such as with spinal cord damage).
Classic examples of superficial reflexes include the abdominal reflex, the cremaster reflex and the normal plantar response. The abdominal reflex includes contraction of abdominal muscles in the quadrant of the abdomen that is stimulated by scraping the skin tangential to or toward the umbilicus. This contraction can often be seen as a brisk motion of the umbilicus toward the quadrant that is stimulated. The cremaster reflex is produced by scratching the skin of the medial thigh, which should produce a brisk and brief elevation of the testis on that side. Both the cremaster reflex and the abdominal reflex can be affected by surgical procedures (in the inguinal region and the abdomen, respectively). The normal planter response occurs when scratching the sole of the foot from the heel along the lateral aspect of the sole and then across the ball of the foot to the base of the great toe. This normally results in flexion of the great toe (a "down-going toe") and, indeed, all of the toes. The evaluation of the plantar response can be complicated by voluntary withdrawal responses to plantar stimulation.
The "anal wink" is a contraction of the external anal sphincter when the skin near the anal opening is scratched. This is often abolished in spinal cord damage (along with other superficial reflexes).
The best known (and most important) of the so-called "pathological reflexes" is the Babinski response (upgoing toe; extensor response). The full expression of this reflex includes extension of the great toe and fanning of the other toes. This is actually a superficial reflex that is elicited in the same manner as the plantar response (i.e., scratching along the lateral aspect of the sole of the foot and then across the ball of the foot toward the great toe). This is a primitive withdrawal type response that is normal for the first few months of life and is suppressed by supraspinal activity sometime before 6 months of age. Damage to the descending tracts from the brain (either above the foramen magnum or in the spinal cord) promotes a return of this primitive protective reflex, while at the same time abolishing the normal plantar response. The appearance of this reflex suggests the presence of an upper motor neuron lesion.
We now list the reflex changes associated with dysfunction at various levels of the nervous system.
- Muscle: Stretch reflexes are depressed in parallel to loss of strength.
- Neuromuscular junction: Stretch reflexes are depressed in parallel to loss of strength.
- Peripheral Nerve: Stretch reflexes are depressed, usually out of proportion to weakness (which may be minimal). This is because the afferent arc is involved early in neuropathy.
- Nerve root: Stretch reflexes subserved by the root are depressed in proportion to the contribution that root makes to the reflex. Superficial reflexes are rarely depressed since there is extensive overlap in the distribution of individual nerve roots of the skin and muscle tested in the superficial reflexes. However, extensive nerve root damage can depress superficial reflexes in proportion to the amount of sensory loss in the dermatomes tested or the motor loss in the involved muscles.
- Spinal cord and brain stem: Stretch reflexes are hypoactive at the level of the lesion and hyperactive below the level of the lesion. As noted, during the initial state of spinal shock following acute lesions, the spinal reflexes below the lesion are also hypoactive or absent.
- Superficial reflexes are hypoactive at and below the level of the lesion and normal above. The abdominal superficial reflexes are not reliably present in normal individuals who are excessively obese, who have abdominal scars, or who have had multiple pregnancies, and they are frequently poorly elicited in otherwise normal elderly persons. Therefore, though classically depressed in persons with corticospinal system involvement, one should not place great emphasis on depressed abdominal reflexes if they are the only abnormality found in the examination. The plantar response is an extremely important superficial reflex. Not only does this normal respons disppear when upper motor neurons are damaged, the normal response is replaced by an extensor (Babinski) response.
- Bilateral disease is associated with the same abnormalities bilaterally, and in addition, there may be "primitive reflexes" due to release of these responses from cortical inhibition (see Chapter 2).
- With bilateral damage to the motor cortex (particularly when the corticobulbar system is heavily affected), inhibitory control of the complex emotional expression reflexes becomes defective. These individuals cry or laugh with minimal emotional provocation and the patient usually says that they do not understand why they are crying or laughing. These complex emotional reflexes are subserved by the limbic system and are normally under inhibitory modulation by the neocortex. Bilateral damage may release these responses in a pattern that is termed "pseudobulbar" (see Chpt. 5).
- DeJong, R.N.: The Neurologic Examination, ed. 4. New York, Paul B. Hoeber, Inc., 1958.
- Monrad-Krohn, G.H., Refsum S.: The Clinical Examination of the Nervous System, ed. 12, London, H.K. Lewis & Co., 1964.
- Wartenberg, R.: The Examination of Reflexes: a Simplification. Chicago, Year book Medical Publishers, 1945.
Define the following terms:hyper-reflexia, pathological spread of reflex, clonus, Babinski sign, Hoffmann's sign, myotatic reflex, upper motor neurons, lower motor neurons, reinforcement.
8-1. What is the main effect of descending motor systems on reflexes?
8-2. What are the 7 Deep Tendon Reflex exams (DTRs)? What sensory/motor nerves are they testing?
8-3. What are the superficial reflexes?
8-4. What is the effect of damage to corticospinal fibers on myotatic (deep tendon) reflexes? What is the effect on superficial reflexes?
8-5. What primitive reflexes emerge with diffuse bilateral hemispheric dysfunction?
8-6. What happens to DTRs with lesions in the cerebellum & basal ganglia?
8-7. How are DTRs graded?
8-8. What is the most important consideration in testing reflexes?
8-9. What reflex changes would occur in lesions of muscles?
8-10. What reflex changes would occur in lesions of the neuromuscular junction?
8-11. What reflex changes would occur in lesions of the peripheral nerves?
8-12. What reflex changes would occur in lesions of the nerve root?
8-13. What reflex changes would occur in lesions of the spinal cord and brain stem?
8-14. How can damage to sensory nerve fibers affect reflexes?
8-15. What is the effect of neuropathy on muscle stretch reflexes?
8-16. What are some visceral reflexes that can be tested?