Page 89 - Canine Lameness

4.3 The Neurologic Examination 61

spinal nerve in the region. Like cutaneous areas of peripheral nerves, dermatomes are areas of skin
innervated by one spinal nerve. These sensory nerves synapse on both sides of the cord but pre-
dominantly on the contralateral side, before coursing cranially. Thus, this is a bilateral reflex. The
efferent portion is mediated by the C8–T1 spinal cord segment, the lateral thoracic nerve, and the
cutaneous trunci muscle. As such, this reflex is useful in detecting C8 and T1 spinal cord lesions,
the lateral thoracic nerve, and locating the level of a transverse thoracolumbar spinal cord lesion.
To test this reflex, the dog should be either standing squarely or lying straight in sternal recum-
bency. This reflex is present in the thoracolumbar region but is absent in the cervical and sacral
regions. Testing should begin at the level of the fourth or fifth lumbar vertebra. The skin just lateral
to midline of one side is gently grasped and pinched. Hemostats positioned perpendicular to the
vertebral column work best, especially those with curved tips directed downward. As with the
withdrawal reflex, only enough pressure should be applied to elicit the reflex. The normal response
is bilateral contraction of the skin overlying the cutaneous trunci muscle along the dorsal and lat-
eral trunk. If a normal reflex is elicited caudally, then there is no need to continue cranially; since
the nerves course in a cranial direction, the entire pathway must be intact. If an abnormal reflex
response is encountered, testing up to the level of the first thoracic vertebra is performed and the
location where the reflex becomes normal (cutoff point) is noted. The opposite side is then tested
in the same manner.
Similar to the flexor reflexes, two responses can be observed in the normal patient, a twitch of
the skin and/or a behavioral response. Contraction of the cutaneous trunci muscle indicates the
reflex arc is intact; a behavioral response indicates perception of discomfort. A lateral thoracic
nerve lesion will result in a diminished or absent reflex on the affected side (i.e. LMN dysfunction)
and a transverse spinal cord lesion in the thoracolumbar region will result in a bilateral cutoff
whereby the muscle contraction caudal to the level of a spinal cord lesion (approximately 1–4
spinal segments) will be diminished or absent. Similarly, a lesion affecting the brachial plexus
causes an LMN lesion to the ipsilateral lateral thoracic nerve and results in loss of the ipsilateral
cutaneous trunci reflex. For example, in a patient with a right brachial plexus injury, the muscle
contraction will be elicited only on the left side of the trunk, regardless of where the skin is tested
(Video 16.1).
In some normal animals, a cutaneous trunci reflex will not be elicited. In these circumstances, it
should be interpreted as equivocal since no localizing information can be inferred if there is no
cutoff.

4.3.8 Sensory Testing and Palpation
At this point in the neurologic examination, some information has already been gathered about the
patient’s sensory system, for example during testing of cranial nerves, proprioceptive positioning,
and spinal reflexes. The last portion of the neurologic examination is further evaluation of the
sensory component. This includes nociception (perception of pain), spinal and limb palpation, and
cutaneous sensory testing. To maintain accuracy in localizing the source of pain or abnormal sen-
sation, sensory testing should be completed prior to administering sedation. The age, breed, and
temperament can also influence a patient’s level of response.

4.3.8.1 Nociception
Many publications describe testing for superficial and deep pain by varying the degree of compres-
sion applied to the skin, implying there are different pathways carrying information about each.