Large Animal Neurology. Joe Mayhew

reflex in the pelvic limbs (B). All other reflex testing can be problematic in interpretation, and results of such tests rarely change an anatomic diagnosis. Appropriate hyperactive reflexes and crossed extensor reflexes were present as expected in this normal neonatal calf. Reflexes should be tested in both pairs of limbs while uppermost, and while dependent, the most prominent result being taken as real, with prominently asymmetric reflexes helping to lateralize a lesion. Rarely, a reflex cannot be elicited in a normal patient, usually bilaterally.

      Interpreting results of testing the tendon reflexes—triceps, biceps, extensor carpi radialis, etc.—in the thoracic limbs is problematic and does not usually assist in defining the site of neurologic lesion, perhaps except for neonatal animals. Also, patients with profound diffuse neuromuscular paresis can have reflexes that are at least present. However, a general description of two of these reflexes follows—descriptions for the remainder being superfluous. To perform the triceps reflex, the relaxed limb is held slightly flexed, and the distal portion of the long head of the triceps and its tendon of insertion is balloted with a rubber neurology hammer in smaller patients or a heavy metal plexor in larger patients while observing and palpating for contraction of the triceps muscle, which causes extension of the elbow. The triceps reflex involves the radial nerve for its afferent and efferent pathways and spinal cord segments C₇ to T₁. The triceps reflexes, although present, can be extremely difficult to demonstrate in heavy, adult, recumbent patients. The musculotendinous portion of the extensor carpi radialis muscle can be balloted to produce an extension of the knee when the relaxed limb is held in a partially flexed position. This extensor carpi radialis reflex involves afferent and efferent fibers also in the radial nerve, but the reflex may not always be present in normal adult animals.

      All these reflexes are usually active in normal neonates in which there is a prominent crossed extensor reflex present that subsides through the first weeks of life.

      The pelvic limb spinal reflexes may also be evaluated in all animals that can be restrained in lateral recumbency and in all recumbent patients. In addition, the amount of voluntary effort and muscle tone present in the pelvic limbs is assessed in recumbent patients. As described for the thoracic limbs, this can be performed while watching the animal attempt to get up or by observing its struggle in response to stimuli while lying in lateral recumbency. Consideration must be given to possibly exacerbating a fracture.

      The patellar reflex and the flexor reflex are the two most clinically important spinal cord reflexes involving the pelvic limbs. The patellar reflex is performed by supporting the limb in a partly flexed position, tapping the middle patellar ligament with a neurologic hammer or a heavy metal plexor, and observing a reflex contraction of the quadriceps muscle resulting in extension of the stifle (Figure 2.21). The sensory and motor fibers for this reflex are in the femoral nerve, and the spinal cord segments involved are primarily L₄ and L₅. The flexor reflex is performed by pinching the skin of the distal limb with needle holders and observing for flexion of the limb. The afferent and efferent pathways for this reflex are in the sciatic nerve and involve spinal cord segments L₅ to S₃ (Figure 2.16).

      Although two other reflexes can be elicited in most neonatal animals, they are frequently not clearly reproducible in adult patients, and thus results of testing them do not usually contribute to defining the site of neurologic lesions. The gastrocnemius reflex is performed by balloting the gastrocnemius tendon and observing and palpating the contraction of the gastrocnemius muscle, which is accompanied by an extension of the hock. This reflex involves the tibial branch of the sciatic nerve and spinal cord segments L₅ to S₃. Second, the cranial tibial reflex results in contraction of the cranial tibial muscle with hock flexion occurring when the muscle is balloted and the relaxed limb is held partially extended. Variable limb movement and direct impact muscle contraction in response to mechanical stimulation may be interpreted falsely as a positive reflex muscle contraction when both these reflexes are tested.

Distinguishing characteristics of final motor neuron paresis and paralysis and central motor pathway paresis and paralysis are given in Table 2.5. Such distinctions can be straightforward in assisting to anatomically localize the site and extent of spinal cord and peripheral nerve lesions in many patients, but in recumbent heavy patients and those with chronic disease and disuse these classic characteristics can merge such that this distinction becomes problematic.

Table 2.5 Common clinical features with lesions involving central motor pathway and final motor neurons

Clinical sign	Clinical features with lesion in
Central motor pathway	Final motor neuron
Muscle tone	Normal to hypertonic	Hypotonic to flaccid
Muscle atrophy	None or disuse*	Significant
Muscle fasciculations	NOT present	Present
Reflexes	Normal to hyperactive	Hypoactive to absent

      ^* With lameness and disuse in fit horses, this becomes quickly evident in proximal muscles.

      Skin sensation from the pelvic limbs should be assessed independently from reflex activity using the two‐pinch technique focussing on the autonomous zones (Figure 2.15). The femoral nerve is sensory to the skin of the medial thigh region, the peroneal nerve to the dorsal tarsus and metatarsus, and the tibial nerve to the plantar surface of the metatarsus and fetlock. As for the thoracic limbs, lesions of the peripheral nerves to the pelvic limbs, such as the femoral and peroneal nerves, result in specific motor deficits; however, the precise sensory deficits can be difficult to define.

      The patellar reflex is hyperactive in newborn foals, lambs, calves and kids, and probably in all large animal neonates. Also, the cranial tibial and gastrocnemius tendon reflexes are easily performed in healthy, cooperative newborn patients. As with the forelimbs, these patients have normal, strong, crossed extensor reflexes. In addition, an extensor thrust reflex is obtained, in very young patients, by rapidly overextending the toe while the limb is already partially extended. This results in a forceful extension of the limb and possibly represents a Golgi tendon organ reflex that becomes suppressed as the animal matures.

       Rectum, bladder, anus, perineum, and tail

      Tail tone can be assessed prior to testing the perineal reflex, and it is exceedingly variable between patients. A completely flaccid tail with no voluntary movement is indicative of a lesion of sacrocaudal spinal cord segments, nerves, or muscles. Decreased movement of the tail can be detected with lesions cranial to the caudal segments, but usually the spinal cord lesion must be severe for the weakness to be apparent. Some horses with no known neurologic lesion(s) hold their tail to one side, and others are considered natural tail wringers and flick their tails vertically and laterally while moving. A crooked tail carriage appears to be more prevalent in lame horses than non‐lame horses.³⁹

      Fixed torticollis of the caudal vertebrae, or wry tail (caudal vertebral scoliosis), should alert the examiner to the possibility of the presence of other spinal malformations, but it may, particularly in adult cattle, be due to overzealous twisting of the tail.

      The perineal reflex is elicited by briskly prodding the skin of the perineum and anus while observing reflex contraction of the anal sphincter and clamping