Page 1075 - Small Animal Internal Medicine, 6th Edition

CHAPTER 58 Lesion Localization and the Neurologic Examination 1047

reaction testing is used to determine whether animals can SPINAL REFLEXES
recognize the position of their limbs in space (propriocep- Spinal reflex evaluation helps classify a neurologic disorder
VetBooks.ir tion). Sensory receptors for proprioception originate in the as being UMN or LMN. Spinal reflexes and muscle tone will
be diminished to absent in limbs affected by LMN disorders
muscles, tendons, and joints, and spinal cord propriocep-
tive tracts relay this sensory information to the cerebral
reflexes are best assessed in a relaxed animal restrained in
cortex. Most proprioceptive tracts ascend the ipsilateral and normal to increased in UMN disease. Spinal limb
spinal cord and cross midline in the rostral brainstem (see lateral recumbency. Each reflex is judged to be absent (0),
Fig. 58.5). Abnormalities detected during the manipulations decreased (+1), normal (+2), or increased (+3 or +4). LMN
performed to test postural reactions do not provide precise lesions severe enough to cause weakness and abnormal gait
localizing information but are sensitive indicators that will reliably cause an absent or decreased reflex. UMN lesions
suggest the presence of neurologic dysfunction somewhere cause an increased reflex that will not always be distinguish-
along the neurologic pathway. A careful and systematic able from normal. In the absence of other neurologic deficits,
evaluation of postural reactions may permit the examiner an exaggerated reflex means little and can be observed in an
to detect subtle deficits not observed during routine gait excited or nervous animal. The most useful limb reflexes in
examination and to determine whether each limb is neu- dogs and cats include the patellar reflex, sciatic reflex, pelvic
rologically normal or abnormal. Postural reaction testing limb withdrawal (flexor) reflex, and thoracic limb with-
should include knuckling, hopping, wheelbarrowing, and drawal (flexor) reflex. Because other reflexes are found
hemiwalking (Fig. 58.12) (Video 58.4). When performed inconsistently in normal animals, they are not routinely
by an experienced clinician comparing the right and left evaluated. The spinal reflexes and the spinal cord segments
limbs in an animal that has voluntary movement, hopping responsible for mediating each reflex are listed in Table 58.4.
is the most sensitive and reliable postural reaction test. The
animal is supported bearing weight on one limb, and its Patellar Reflex
body is leaned and hopped laterally so that its center of With the animal restrained in lateral recumbency, the exam-
gravity is no longer adequately supported by that limb. The iner evaluates the reflex in the upper (nonrecumbent) limb
normal response is to immediately lift the limb and replace by holding the stifle in partial flexion and striking the patel-
it directly under its center of gravity. Any delay in this lar ligament with the flat surface of the reflex hammer (plex-
response is abnormal. In animals with significant weakness, imeter), stretching the fibers of the quadriceps muscle (Fig.
it is important to support most of the body weight during 58.13). The normal response is a reflex contraction of the
postural reaction testing. Animals with neuromuscular dis- quadriceps muscle. This is a monosynaptic myotactic
orders that still have normal sensation and the ability to (stretch) reflex, with both sensory and motor components
voluntarily move their limbs will hop quickly (normal) as contained in the femoral nerve and the L4, L5, and L6 spinal
long as their weight is supported because their proprio- nerves, nerve roots, and spinal cord segments. A weak or
ception is normal. For the purpose of lesion localization, absent patellar reflex indicates a lesion of the femoral nerve
abnormalities of postural reaction testing are usually inter- or the L4-6 spinal cord segments or nerve roots. A lesion
preted as UMN signs, which must then be confirmed with cranial to the L4 spinal cord segment will typically cause an
testing of muscle tone and spinal reflexes (see Box 58.4 and exaggerated reflex. Although this is the most reliable tendon
Table 58.1). reflex for evaluation, it is sometimes difficult to interpret the
response. Occasionally a lesion of the sciatic nerve or the
MUSCLE SIZE/TONE L6-S2 spinal cord segments will cause the patellar reflex to
Muscle atrophy and muscle tone should be assessed by appear increased by decreasing tone in the muscles opposing
careful palpation and movement of each limb through a stifle extension (pseudohyperreflexia). The patellar reflex is
range of motion. Muscle atrophy can occur slowly as sometimes difficult to elicit in animals with significant
a result of disuse or rapidly as a result of a lesion of the orthopedic disease of the stifle. It is occasionally decreased
LMN supplying a muscle (neurogenic atrophy). If focal or absent in normal dogs (especially large-breed puppies)
muscle atrophy is detected in a limb, this can be used to and may be absent in some geriatric dogs, presumably due
precisely localize lesions of the peripheral nerve, nerve to an age-related neuropathy affecting the sensory side of the
roots, or spinal cord gray matter because the spinal cord reflex arc. In tense patients, the reflex is sometimes decreased
segments and peripheral nerves responsible for innervat- or absent in the upper limb but normal in the relaxed recum-
ing each of the individual limb muscles are well known. bent limb, making it important to check this reflex in both
Muscle swelling or enlargement is a feature of some limbs in both recumbencies.
myopathies. Muscle tone is generally decreased in animals
with significant lesions of the LMN, whereas extensor Pelvic Limb Withdrawal (Flexor) Reflex
muscle tone is usually increased with UMN lesions (see The examiner squeezes a digit with enough pressure to elicit
Table 58.1). Extreme alterations in muscle tone can be flexion of the hip, stifle, hock, and digits (Fig. 58.14, A and
seen in animals with Schiff-Sherrington syndrome and B). If manual pressure is inadequate, the examiner squeezes
with decerebrate and decerebellar rigidity (see Figs. 58.8 the base of a toenail with a pair of forceps. The pelvic limb
and 58.9). withdrawal reflex is complex. Sensory input is through the