Peripheral Nerve Deficits in the Hindlimb
Updated: Sep 25, 2019
Sciatic Nerve Paresis:
The sciatic nerve originates from the segments L6-S1 and innervates the semitendinosus, semimembranosus and branches into the tibial and peroneal branches.
Sciatic nerve damage is rare in horses. It usually happens in foals as a result of injections on thigh and rump or occurs transiently due to analgesia caudally to the coxofemoral joint. This happens because the sciatic nerve is located deeper in older horses whereas in foals it is more superficial therefore more susceptible to damage by intramuscular injections.
Horses with sciatic nerve paresis are able to support weight but have loss of tibial and peroneal nerve function. The stifle joint is not involved but the horse will assume a crouched position behind because innervation of gastrocnemius, flexor and extensor muscles cause the hock to “drop” (flex) and fetlock to knuckle forward. Other clinical signs may include loss of sensation in the distal limb.
Femoral Nerve Paresis:
Horses with femoral nerve paresis also assume a "crouched" hind limb posture. However they are unable to bear weight as the stifle substantially drops. Because the reciprocal apparatus is compromised, as the stifle flexes (drops) it causes flexion of the hock and fetlock (knuckling). If bilateral, the horse will most likely be unable to rise for more than a few seconds. Femoral nerve paresis may occur unilaterally or bilaterally after general anesthesia or may result from lower motor neuron disease or injury.
Peroneal (Fibular) Nerve Paresis:
Arises from the sciatic trunk deep to the biceps femoris and is motor to the flexors of the tarsus and the extensors of the digit. Paralysis results in extension of the tarsus and flexion of the distal joints of the pelvic limb. At rest, the limb is held slightly caudally with the distal joints in flexed position and the dorsum of the hoof contacting the ground. During walking, the limb is moved erratically. The toe is dragged along the ground during the weak protraction phase, then pulled caudally as the horse attempts to bear weight. There is atrophy of the anterior tibial and long and lateral digital extensors and immediate cutaneous hypalgesia/anesthesia over the lateral metatarsus.
The tibial nerve is the direct continuation of the sciatic nerve and innervates the gastrocnemius (extensor of the hock) and digital flexors. The limb is held flexed and the foot contacts the ground in normal position, the fetlock often partially collapses into a flexed position (i.e., "knuckles"). The foot is moved in stringhalt-like fashion, with exaggerated flexion of the hock and stifle during protraction followed by sudden extension to the weight-bearing phase of the stride. There is atrophy of the gastrocnemius and cutaneous hypalgesia/anesthesia of the caudal metatarsal region and bulbs of the heels. The flexor reflex is tested by pinching the skin over the dorsal aspect of the fetlock. The reflex is present but weaker than on the normal side.
Diagnosis and Management of Lameness in Horses - Ross & Dyson