Page 409 - Canine Lameness

21.3 ­Neurorogico giNciNi AANicgio ctNe Nolgi gim 381

and form the lumbosacral plexus. The lumbar portion of this plexus innervates the cranial and
medial muscles and skin of the thigh while the sacral plexus supplies the caudal muscles and skin
of the thigh, tarsus, and foot. Because the series of sacrocaudal nerve roots are forming the S1‐
caudal spinal cord segments and their respective nerves resemble a horse’s tail, this portion of the
spinal cord and spinal roots is called the cauda equina. Thus, the cauda equina is part of the PNS.
Injury or disease affecting only the cauda equina will not cause paresis or lameness because of no
contribution to the femoral and sciatic nerves.
The major spinal nerve contributions to the lumbosacral plexus are summarized in Table 21.2,
though individual variations exist on the actual contributions to the named nerves. The two nerves
that would result in a monoparesis are the femoral nerve and the sciatic nerve with its tibial and
fibular branches. Clinically significant deficits in the obturator and genitofemoral nerves are rarely
reported in dogs. Sensory loss to the skin innervated by the genitofemoral nerve may be appreciated
during testing and can further aid in mapping of deficits.
The femoral nerve arises predominantly from the fifth lumbar spinal nerves, along with substan-
tial portions from L4 and L6. It is formed within the psoas major muscle and continues caudally,
protected within this muscle. Proximally it supplies the psoas major and iliopsoas muscles as well
as sending the saphenous nerve before diving between the rectus femoris and vastus medialis. It
supplies all four heads of the quadriceps (rectus femoris, vastus medialis, vastus intermedius, and
vastus lateralis) and therefore plays the major role in extending the stifle. Consequently, injury to
this nerve will result in inability to support body weight in the affected limb(s); when the dog
attempts to bear weight on the limb, the stifle will passively flex. The saphenous branch of the
femoral nerve innervates the skin on the medial surface of the foot, stifle, and thigh.
The sciatic nerve (also known as the ischiatic nerve) is a mixed sensory and motor nerve that arises
from the spinal cord segments L6, L7, S1, and occasionally S2 and is the extrapelvic continuation of
the lumbosacral trunk (the largest part of the lumbosacral plexus). As the nerve exits the plexus, it
continues as the sciatic nerve and exits the pelvis caudomedial to the coxofemoral joint, deep to and
in between the tuber ischii and the greater trochanter of the femur. It courses distally along the thigh
between the semimembranosus and biceps femoris muscles. Branches of the sciatic nerve supply
the muscles that extend the hip (biceps femoris, semimembranosus, and semitendinosus), flex the
stifle (biceps femoris, semimembranosus, and semitendinosus), and extend the tarsus (biceps femo-
ris). The sciatic nerve divides at the level of the distal femur into fibular (previously called peroneal)
and tibial nerves. The fibular nerve innervates the muscles that flex the hock ( cranial tibial and long
digital extensor muscles) and extend the digits (long digital extensor muscle). Therefore, injury to
this nerve will result in unopposed extension of the tarsus and knuckling of the digits. The tibial
nerve supplies the tarsal extensors (gastrocnemius, semitendinosus, and superficial digital flexor
muscles) and digital flexors (superficial digital flexor and deep digital flexor muscles). Therefore,
injury to this nerve will result in hyperflexion of the hock and plantigrade posture.

21.3 Neurological Diseases Affecting the Pelvic Limb

21.3.1 Myelopathies and Radiculopathies
Unilateral spinal cord lesions caudal to the T2 spinal cord segment (i.e. T3–L3 or L4–S3) can cause
pelvic limb monoparesis; however, the quality of paresis will depend on the location of the injury.
A lesion at the lumbosacral intumescence affecting the L4–S3 spinal cord segments will result in a
lower motor neuron (LMN) paresis and coinciding deficits, while a lesion in the T3–L3 spinal cord