Page 285 - Canine Lameness

16.3 ­Neurorogico giNciNi AANicgio ctNe trucigi gim 257

a pre- or post-fixed plexus, respectively. Nevertheless, two or three spinal nerve branches combine
to form the major named nerves of the brachial plexus, specifically the suprascapular, subscapular,
axillary, musculocutaneous, radial, median, ulnar, and lateral thoracic nerves. Table 16.2 provides
a summary of each nerve and the spinal cord segments from which they arise. Wide variations
exist among individuals but as a general guideline, nerves exiting cranially in a plexus will inner-
vate more cranial and proximal muscle groups of the limb, while more caudally exiting nerves will
innervate caudal and distal muscle groups of the limb.
The flexors of the shoulder joint (infraspinatus, teres major, deltoideus, and teres minor) are
innervated by the suprascapular and axillary nerves with small contributions from the radial nerve
(Hermanson 2013). Elbow flexion is facilitated through activation of the biceps brachii and bra-
chialis muscles innervated by the musculocutaneous nerve. Lastly, carpal and digit flexion are
mediated by median and ulnar nerve innervation to the palmar antebrachial muscles (flexor carpi
radialis and flexor carpi ulnaris muscles, and superficial digital flexor and interosseous muscles).
Muscles and their respective nerves responsible for extension of the thoracic limb include the
shoulder extensors (supraspinatus, biceps brachii, and coracobrachialis) supplied by suprascapu-
lar, axillary, and musculocutaneous nerves; the elbow extensors in the caudal brachial muscle
group (triceps brachii, anconeus, tensor fasciae antebrachia, and extensor carpi radialis) all inner-
vated by the radial nerve; and the carpal and digital extensors belonging to the craniolateral ante-
brachial muscles (extensor carpi radialis, ulnaris lateralis, common digital extensor, lateral digital
extensor, extensor digiti, and pollicis longus) supplied by the radial nerve (Hermanson 2013).
Compromise to these muscles and/or nerves, particularly the radial nerve responsible for elbow
extension, will affect the weight-bearing abilities of the limb. Injury to the other nerves may cause
gait abnormalities but the animal will be able to support its own weight.

16.3 Neurological Diseases Affecting the Thoracic Limb

Signalment and clinical progression are key factors in considering likely causes of monoparesis or
lameness affecting the thoracic limb. Commonly encountered conditions include Type I interver-
tebral disc (IVD) extrusion, neoplasia, fibrocartilaginous embolism (FCE), myelitis/meningomy-
elitis, and brachial plexus injury (Table 16.1).

16.3.1 Myelopathies and Radiculopathies
Unilateral injury or damage to the C6–T2 spinal cord segments forming the cervical intumescence
(i.e. a myelopathy) or to the associated nerve roots (i.e. a radiculopathy) can cause gait abnormali-
ties that are either restricted to only one thoracic limb, such as a lameness or monoparesis, or that
are more pronounced in both thoracic limbs (e.g. short-strided gait).
Damage to the motor neuron cell bodies in the ventral gray matter of the cervical intumescence
causes a lower motor neuron (LMN) paresis in the thoracic limbs characterized by a stiff, short-
strided gait with symmetric lesions, or a monoparesis if the lesion is unilateral. Cervical myelopa-
thies often also involve the white matter, containing the caudally directed general proprioceptive/
upper motor neuron (GP/UMN) pathways to the ipsilateral pelvic limb causing a hemiparesis, or
to both pelvic limbs causing a paraparesis (Figure 4.1); a GP ataxia may also be noted in either
scenario. Deficits to GP/UMN tracts manifest as a long-strided gait in the pelvic limbs and, when
present with a short choppy LMN thoracic limb stride, this combination is referred to as a