Page 292 - Canine Lameness

264 16 ­Neurorogico giNciN rAectNe trucigi gim

16.3.2.1 Neoplasia
Tumors affecting the nervous system of the thoracic limb, particularly neoplasia of spinal nerves
(Video 4.1), occur commonly and are an important differential diagnosis for thoracic limb lame-
ness. These are discussed in Chapters 11 and 17.

16.3.2.2 Traumatic Neuropathies

16.3.2.2.1 Brachial Plexus Injuries Brachial plexus injuries are the most common traumatic
neuropathies in veterinary medicine (Gemmill and Mckee 2012). The term brachial plexus avulsion
is often used to describe these injuries, but the term brachial plexus injury is more appropriate as
there is not always a true avulsion (i.e. physical separation). They are most often due to severe
traction of the thoracic limb or abduction of the scapula as the result of severe trauma, for example
vehicular, falls from a height, gunshot, and bite wounds. These injuries may result from
compression, stretching, tearing, crushing, laceration (from fractures), or complete transection of
the nerves or nerve roots. Most commonly the damage occurs at the level of the spinal nerve roots
where there is less resistance to stretch than in more peripheral regions of nerves owing to the lack
of epineurium. The ventral (motor) roots appear to be more susceptible to injury than the dorsal
(sensory) root (Steinberg 1988; Platt and Da Costa 2012).
A clinical diagnosis is often made based on history of a traumatic injury, clinical presentation,
and neurologic examination. Depending on the extent of the trauma, all or only part of the bra-
chial plexus may be injured. Clinical signs will vary depending on which nerves are involved and
the severity of the injury. The major motor and sensory distribution of the brachial plexus is out-
lined in Table 16.2.
Partial injuries most commonly involve the caudal plexus, originating from the ventral branches
of C8, T1, and sometimes T2 spinal nerves. The muscles innervated by these nerves are involved in
elbow extension which is essential for weight-bearing and locomotion, as well as the cutaneous
trunci muscle. Clinically the animal cannot bear weight and stands with the elbow and shoulder
flexed and dropped; the carpus is knuckled (Video 16.1). Cutaneous sensation may be lost distal to
the elbow and over the caudolateral aspect of the antebrachium. If the avulsion is severe enough, it
may damage spinal cord pathways causing ipsilateral pelvic limb deficits. A partial Horner syn-
drome, characterized by anisocoria due to ipsilateral sympathetic dysfunction and resulting miosis,
can develop if the T1 nerve root is involved. Complete Horner syndrome is not commonly seen in
brachial plexus injuries in dogs. If the C8 and T1 nerve roots supplying the lateral thoracic nerve are
affected, there will be ipsilateral loss of the cutaneous trunci (panniculus) muscle reflex (Chapter 4).
Injury to the cranial plexus roots of C6 and C7 causes loss of elbow flexion and shoulder move-
ment; however, the animal can still support weight since the elbow extensors are spared. Loss of
cutaneous sensation may be appreciated cranially and medially. Complete injury to all brachial
plexus roots (C6–T2 nerve roots) causes flaccid LMN paresis or paralysis, inability to support
weight, and loss of cutaneous sensation over the entire limb.
Orthopedic injuries, particularly humeral fractures can cause nerve damage, and therefore radi-
ographs are indicated to rule out osseous pathology. Electrodiagnostics may assist in identifying
the affected nerves, although spontaneous electrical activity (indicating denervation) cannot be
detected until five to seven days after the injury. An MRI may demonstrate the abnormalities in the
nerves, intumescence, or surrounding soft tissues that are consistent with focal inflammation,
edema, and/or hemorrhage, but this is not often performed when there is definite history of trauma
since the treatment is unlikely to change. Nevertheless, MRI is recommended if there is no wit-
nessed trauma nor any apparent cause to explain the clinical signs.