Large Animal Neurology. Joe Mayhew

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СКАЧАТЬ These arise segmentally from the vertebral nerve that follows the vertebral artery up the neck after the vertebral nerve has left the stellate ganglion near the thoracic inlet (see Figure 2.10).

When the skin of the lateral neck of a horse just above the jugular groove from the level of the atlas to the shoulder is prodded firmly with a blunt probe, the cutaneous coli muscle contracts, which causes skin flicking. With stronger stimuli, the sternocephalicus and brachiocephalicus muscles often contract, causing the shoulder to be pulled cranially and even the head to be flexed laterally. This response tends to be less obvious in other species. In horses, there is also flicking of the ear rostrally, blinking of the eyelids, and lip retraction when the test is performed. Originally introduced by Rooney,³⁰ these are termed the local cervical and cervicofacial responses, respectively. The precise anatomic pathways are not entirely clear, although they must involve several cervical segments and the facial nucleus in the medulla. At least in the cranial cervical region, even the local cervical reflex may well be a cervical/trigeminal–facial reflex associated with the anastomoses between the cervical branch of the facial nerve with cutaneous branches of the C₂–C₆ segmental nerves.³¹ Cervical lesions that involve gray and white matter can cause depressed or absent cervical responses; however interpretation of abnormal responses may need to be expressed as imprecisely as for example “consistent with a right‐side caudal cervical lesion” or “consistent with a cranial cervical lesion.” In contrast, the segmental absence of the cutaneous trunci reflex can be very useful in delineating the precise cranial extent of a thoracic spinal cord lesion particularly when such a lesion is asymmetrical.

Sensory perception from the neck and forelimbs must be assessed. This can be difficult to evaluate accurately in stoic and in excited animals. Perception of a noxious stimulus is noted by observing the animal’s behavioral response while observing the local cervical responses and continuing the skin prodding over the shoulders and down the limbs to include testing the autonomous zones for the thoracic limbs (Figure 2.15). As with any cutaneous sensory test, a two‐step technique is recommended.³² This is accomplished by initially tenting and grasping a fold of skin between the jaws of heavy duty hemostats or needle holders. After pausing to allow the patient to settle, a second, sharp skin pinch is applied to determine superficial sensation. There may be reflex withdrawal of the part, with or without a cerebral response such as vocalization or moving the whole body away from the stimulus; the latter events being taken as representing the conscious perception of the noxious stimulus.

Figure 2.15 Autonomous zones are areas of desensitivity that can be detected when individual peripheral spinal nerves to the limbs are not functioning.

      Source: Adapted from Blythe³³; Blythe and Engel³⁴; Blythe and Kitchell³⁵.

      If a large, adult animal has significant gait abnormality and it is feasible to cast it to lateral recumbency, then this should be done to assess the spinal reflexes. If the animal is ambulating well, it may be assumed that the flexor and extensor spinal reflexes are at least intact. These reflexes can be studied in all smaller patients.

When evaluating wobblers with evidence of a neurologic abnormality in both thoracic and pelvic limbs and no evidence of brain disease, one should allow for a lesion to be present anywhere from C₁ through T₂. Conversely, when there is evidence of a mild neurologic gait abnormality in the pelvic limbs but not the thoracic limbs, then the possibility of a lesion anywhere from C₁ through S₂ must be considered. If the signs of pelvic limb ataxia and/or paraparesis in a large adult patient are moderate or even marked, then a lesion can be considered anywhere in these segments. The reason to include lesions sites at C₆–T₂ is because such lesions, when intramedullary, can be very selective and spare white matter tracts and gray matter involving the thoracic limbs resulting in no definitive thoracic limb signs. Such has been the case in adult horses suffering from S. neurona myelitis, fibrocartilagenous thromboembolism, granulomatous meningoencephalomyelitis, and migrating helminth parasites affecting C₆–T₂ spinal segments.

       Trunk and hindlimbs

If the examination of the head, gait and posture, and neck and thoracic limbs reveals evidence of a nervous system lesion(s), then an attempt should be made to associate such lesions with any further signs found during examination of the trunk and hindlimbs. If there are only signs in the trunk and hindlimbs (Figure 2.16), then the lesion(s) must be between C₁ and S₂, but most probably between T₃ and S₂, or in the trunk and pelvic limb nerves or muscles. This part of the examination helps localize such lesions more precisely. However, the examiner must remember that with a subtle grade neurologic gait abnormality in the pelvic limbs, the lesion may be anywhere between the rostral brainstem and the mid‐sacral spinal cord.

Figure 2.16 This Holstein calf (A) suffered from vertebral trauma during an assisted delivery that resulted in paraplegia due to an epiphyseal separation at caudal L4 vertebra. With nursing care, the calf recovered quite well to be able to stand and walk but with a tendency to knuckle over on its fetlocks as shown. The residual gross lesion shown below (B), particularly the focal necrosis of dorsal gray and white matter at L_4‐5, explains the clinical syndrome that related mostly to bilateral deficits of the sciatic/peroneal nerve distribution with very poor flexor reflexes present in the pelvic limbs.

The trunk and hindlimbs must be observed and palpated for malformation and asymmetry (Figure 2.17). Lesions affecting thoracolumbar gray matter cause muscle atrophy, which is a helpful localizing finding. With asymmetric myelopathies, scoliosis of the thoracolumbar vertebral column often occurs, initially with the concave side opposite the lesion, but changing with subsequent fibrous contracture setting in. Once again, evidence of muscle atrophy, especially common over the gluteal region (Figure 2.18), should be taken as evidence of an underlying lameness until there is additional evidence that it is neurogenic.

Figure 2.17 Very occasionally, portions of muscles, whole muscles, or muscle groups are found to be absent and often there are no definitive neurologic signs present. Such was the case with this horse in which a large portion of the left medial thigh muscles, at least including the medial quadriceps group, was missing with no evidence of weakness or ataxia in either hind limb. Whether individual cases are congenital or acquired (from say S. neurona) is usually very difficult to determine when present for such a long time.

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