Peripheral nerves and reflexes

Testing of reflexes.

The peripheral nervous system (PNS) is composed of cranial nerves, spinal nerves, the  distal branches of these nerves, and  ganglia. Any  disorder of nerve  function is  called a neuropathy.  Your textbook describes methods of testing for neuropathies of the cranial nerves. The spinal cord and spinal nerves are tested by  assessing  various somatic reflexes. Testing a reflex helps a clinician evaluate not only the  individual  components of a reflex arc (receptors, neurons,  and muscles) but also the overall state of the nervous  system. Abnormalities of reflex function,  coupled  with other information  gathered  during  the physical examination, provide valuable clues to diagnosis. One advantage of reflex testing is  that it is easy to do; it requires only simple tools and  good powers of observation. Although just a few reflexes are routinely tested, many others can be tested if necessary. Reflexes are usually graded on a scale of 0 to 4+:

0 Areflexia, absence of response

1+ Hyporeflexia, a somewhat diminished response

2+ An average or normal response

3+ Hyperreflexia, a stronger than normal response, possibly indicating disease

4+ Intense hyperreflexia with sustained clonus, indicating disease

Areflexia or hyporeflexia typically indicates a segmental  lesion of the spinal cord segment or nerve root that innervates the  muscle.

Hyperreflexia typically indicates a suprasegmental  lesion of higher levels of the CNS that normally inhibit the reflex. The following discussion describes a few of the deep and superficial reflexes most  commonly tested in adults and some of  the reflexes tested in infants.

Deep Reflexes

The testing of  deep reflexes  usually  involves striking the skin with a  reflex hammer to stretch specific tendons and stimulate the tendon organs and muscle spindles. Clinicians  commonly test the biceps, knee, and ankle reflexes and may  test for clonus.

Biceps Reflex     

The patient  lies supine with the elbow  flexed about 30°. The clinician  presses on the  cubital fossa to  stretch the  biceps  tendon and strikes his or her own fingers with  the  reflex hammer. In a normal response, the  biceps should contract slightly, but not enough to flex the  elbow. If there is a lesion of the  musculocutaneous  nerve or segment C6 of the spinal cord, the  biceps does not contract  but  the finger flexors may  contract slightly. If there is a suprasegmental lesion, the biceps may  contract more forcefully than  normal and the brachioradialis or finger flexors may contract.

Knee  Reflex         

The patient either  sits or  lies supine with the knee flexed 90°. The  examiner strikes the patellar ligament with the  reflex hammer. Normal responses range from a slight twitch of the  quadriceps  femoris muscle t o extension of the knee, lifting the  leg. The  absence of a response indicates a  disease of lumbar  nerve roots L3 and L4 or the femoral nerve.

Ankle Reflex       

There are several  ways to  test this reflex. One is to have the patient kneel on the examining table with the  foot extending beyond the end of the table. The examiner  presses slightly against the foot to dorsiflex it, thus stretching the gastrocnemius muscle, and  then strikes the calcaneal tendon with the reflex hammer. The gastrocnemius should contract and  plantar flex the foot. Other positions  and methods can be tried if this one fails, but if no response is obtained by any method, a disease of  the first sacral nerve  root or the tibial nerve is indicated.

Clonus     

Clonus was  described in  the preceding chapter in connection  with epilepsy  (clonic seizures), but it can also be elicited in  normal persons by the proper test. The patient  should lie supine with the hip and knee flexed at 30° to 45° angles. The examiner then  produces a  sudden and sustained  contraction of the  gastrocnemius and soleus muscles by passively dorsiflexing  the  foot. In normal people, the calf muscles contract, relax, and  contract  again for about two or  three beats. This occurs because the  contraction of  one  muscle stimulates the stretch receptors in the  antagonistic muscle.  When the antagonist contracts, it stimulates the stretch receptors in the original muscle and triggers a reflex  contraction. In people with suprasegmental lesions, the clonus  continues for as long as the examiner dorsiflexes the patient’s foot.

Superficial Reflexes

Superficial reflexes are tested by stimulating the skin. Following are three examples of these tests.

Abdominal Reflex    

The patient  must be supine and relaxed. The examiner strokes the skin of the  abdomen with a pointed object such as a pencil or the handle of a reflex hammer, moving from the lateral margins of the abdomen toward the midsagittal plane along a given dermatome. Normally, the  underlying muscle contracts and pulls the umbilicus toward the stimulus. An absence of response in a  given dermatome may indicate lesions to spinal nerves or roots T7 to T11. The response is often absent, however, in elderly  patients  and people with lax abdominal muscles.

Cremasteric Reflex        

In males, stroking the inner, upper aspect of the thigh with a pin or pencil point  causes the ipsilateral testicle (but not the scrotum) to rise, owing to  contraction of the cremaster  muscle. Lesions in spinal cord segments or nerve roots L1 to L2 or in the corticospinal tracts abolish this reflex.

Plantar Reflex    

To test this reflex, the  patient must be supine with the  lower limbs extended. The examiner strokes the sole of the  foot firmly with the handle point of the reflex hammer, progressing from the heel toward the toes. Normal  subjects show a  flexor plantar response in which  they quickly flex  the hip and knee, dorsiflex the  ankle, and adduct and  plantar flex (curl) the toes. An abnormal extensor plantar (Babinski) response is a reliable, early warning sign of  corticospinal  tract disease;  the patient  extends  and dorsiflexes the great toe and  abducts  (fans) the other toes. The extensor plantar response also sometimes occurs in persons unconscious from  drug or  alcohol intoxication.

Reflexes of Infants

Because  the nervous system is not  completely developed at birth, neurological  examination of infants differs somewhat from the techniques  used for adults.  Normally, a  neonatal examination is performed between 36 and 60 hours after birth. In addition to reflexes, the infant’s motor pattern and body posture are observed. A normal  infant has flexed limbs, and its head may be turned to one side. The  lower limbs  may be moving or kicking, and the  infant is expected to become more  active and to begin crying during the examination. On the other hand, certain  responses are  considered abnormal. For instance, an infant extending its limbs may have suffered  intracranial  hemorrhage. Asymmetric behavior of the upper  limbs suggests brachial plexus palsy.  Lack of increased  activity during the examination  suggests  anoxia or intracerebral hemorrhage.   Infants are tested for the same reflexes asadults as  well as  some additional ones  described here.

The  trunk incurvation (Galant)  reflex is tested by stroking the back from the  shoulder to the buttocks or vice versa, about 1 cm from the midline. This  stimulus should elicit  contraction of  the ipsilateral back  muscles, causing  the  infant’s shoulders and pelvis to curve toward  the  stimulus while  the trunk curves  away. This  response normally disappears at 2 months of  age. Its  earlier absence may indicate a transverse spinal  cord lesion.

The  grasp  reflex is evaluated by  determining the infant’s ability to forcefully grasp the examiner’s hand when the ulnar palmar surface is stimulated. This reflex normally disappears at 3 t o 4 months of age. Persistence of  the reflex beyond 4 months may indicate cerebral dysfunction.

The  rooting  reflex is a response to  tactile stimulation of the lips. When the corner of the baby’s mouth is stroked, the  baby opens its mouth and turns its head toward  that  side.  When the midline of the upper lip is stroked, the  baby extends its head, and when the midline of the lower lip is stroked, the jaw drops. This reflex disappears at 3 to 4 months of  age, although sleeping infants exhibit it at slightly older ages. Absence of  this reflex before 3 to 4 months indicates severe CNS disease.

The  startle  (Moro)  reflex is a response to a sudden stimulus such as a jolt, a loud noise, or  being dropped a short distance (supporting the  baby in a supine  position  and suddenly lowering it about 2 feet). The normal response is for the infant t o extend and abduct all four limbs and extend and fan the  digits,  then flex  and adduct  the  limbs. Neurologic disease is suspected if this reflex persists beyond 4 months and is almost certain if it persists beyond 6  months. An asymmetric response may indicate hemiparesis, brachial plexus injury, or fracture of the clavicle or humerus. The absence of a startle reflex may indicate  kernicterus—damage to the  basal nuclei or  other  areas of the  CNS by accumulated bilirubin, a  hemoglobin breakdown product seen in  hemolytic  disease of  the newborn and some other conditions.