Large Animal Neurology. Joe Mayhew
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and hemiwalking the animal by making it stand and then walk sideways on both left and then both right limbs are three useful postural reactions to perform. Even in large, adult animals, particularly horses, it is possible to perform a modified hopping response test with the thoracic limbs. This is performed by lifting each thoracic limb in turn while using the shoulder to make the horse hop laterally on the other thoracic limb. This test can help the clinician decide if there are subtle neurologic abnormalities involving the control of a thoracic limb. Brainstem and spinal cord lesions appear to result in postural reaction deficits on the same side as the lesion, whereas cerebral lesions produce contralateral abnormalities.
At the conclusion of the examination, a most likely site of any acute nervous system lesion frequently can be defined accurately by determining the precise characteristics and severity of any gait and posture abnormalities present. The degree of weakness, ataxia, hypometria, hypermetria, and conscious postural deficits should be graded for each limb (Table 2.4).
With peracute lesions, particularly those of an inflammatory nature and those with soft tissue compression of the spinal cord such as with caudal cervical arthritis and synovial cyst formation, resulting signs can wax and wane quite dramatically over periods of hours to days. Such signs usually stabilize with subacute to chronic lesions. For example, a horse that has suffered a single insult of cervical spinal cord compression a year prior to the examination may have an unusual, perhaps hypermetric, mild ataxia in the pelvic limbs with no evidence of pelvic limb weakness and no signs in the thoracic limbs other than a questionably poor response to hopping. The anatomic diagnosis in such cases may be a lesion in the thoracolumbar or cervical spinal cord, or diffuse or multifocal spinal cord disease. A moderate or severe abnormality in the pelvic limbs, and none in the thoracic limbs, is however far more consistent with a thoracolumbar spinal cord lesion. With a very mild and a very severe neurologic abnormality in the thoracic and the pelvic limb gaits respectively, one must also consider a severe thoracolumbar lesion plus a mild cervical lesion or a diffuse spinal cord disease. Lesions involving the brachial intumescence at C6–T2, with involvement of the gray matter supplying the thoracic limbs, and diffuse spinal cord lesions may both result in a severe gait abnormality in the thoracic limbs and the pelvic limbs. A severely abnormal gait in the thoracic limbs, with normal pelvic limbs, indicates final motor neuron involvement of the thoracic limbs; a lesion is most likely present in the ventral gray columns at C6–T2 or involving thoracic limb peripheral nerves or muscle (Chapter 26).
Localizing findings in spinal cord disease
Neck and forelimbs
If a gait alteration was detected in the thoracic limbs and there were no signs of brain involvement, then this part of the examination can confirm involvement of the C1–T2 spinal cord or thoracic limb peripheral nerves or muscles; it should also help localize the lesion within these regions.
Results of the thoracolaryngeal adductor response or the slap test can be a useful part of the complete neurologic evaluation of horses suspected to be suffering from lesions of the vagal or recurrent laryngeal nerves, caudal medulla oblongata, or cervical and cranial thoracic spinal cord. As most emphasis is placed on its utility in diagnosing cervical spinal cord disease in wobbler horses, some aspects of testing will be reiterated here. The test can be performed in cooperative horses by palpating the dorsal and lateral laryngeal musculature while simultaneously slapping the contralateral dorsolateral thoracic region from the cranial withers to near the last rib, while the horse is exhaling. If there is difficulty in interpretation of this test, observing the larynx via an endoscope while performing the test may be necessary. It should be emphasized that the thoracolaryngeal response is not consistently absent in horses with cervical spinal cord disease or caudal brainstem disease and can be absent in horses with no evidence of CNS disease.27,28 However, a reduced or absent slap reflex on the left side of the larynx must be taken as strong evidence for the presence of idiopathic recurrent laryngeal neuropathy or prior laryngeal surgery, although exercise and endoscopic examination of the horse will be necessary to confirm any clinical problem of reduced laryngeal function and of roaring. Bilateral absence of the response without other signs of severe laryngeal or cervicomedullary disease must be interpreted cautiously, particularly in an excitable horse. A normal response on the left side of the larynx and absent response on the right side will indicate a neurologic disease, most likely right‐sided brainstem or spinal cord C1 to cranial‐thoracic, other than classical idiopathic recurrent laryngeal neuropathy. Thus to repeat, the only useful result from the right to left test is a positive test, whereas both unequivocal positive and negative test results from left to right have utility.
The commonest cause of acute, acquired, severe bilateral laryngeal paralysis is hepatoencephalopathy.
Observation and palpation of the neck and forelimbs will detect gross skeletal defects, asymmetry in the neck and muscle atrophy. These signs may be associated with neurologic disease and thus be localizing findings. The neck should be manipulated to assess a normal range of movement. Interpretation of what appears to be reluctance to move the neck passively or actively in any direction as indicating neck pain is fraught with difficulties. On the other hand, if a horse will not lower or bend its head to eat, drink, or reach for a treat, or the head and neck gets “locked” in an abnormal position/posture, this usually indicates a mechanical or painful disruption to movement of the cervical vertebrae, particularly involving intervertebral disk damage in the caudal neck and cranial thoracic region.29 Cervical vertebral arthrosis, involvement of cervical nerve roots, and marked cervical spinal cord disease can cause scoliosis and even torticollis.
Importantly, as musculoskeletal diseases are far more common than neurologic disease and as disuse atrophy can occur within at least weeks of the onset of lameness, muscle atrophy, especially common over the scapula, should be taken as evidence of an underlying lameness until there is additional evidence that it is neurologic in origin.
Clearly delineated regions of cervical and thoracic sweating can be useful indicators of localized spinal cord or peripheral nerve disease in that they can represent sympathetic denervation or decentralization of the vasculature in the skin, resulting in increased circulating adrenalin stimulating sweat glands to secrete. Care must be taken in interpreting patchy sweating that is not well delineated. Very asymmetric patchy sweating can occur in horses that are excited or distressed, particularly when in a draughty area, without a specific sympathetic lesion being present. Involvement of the peripheral pre‐ and postganglionic sympathetic neurons in the horse results in localized sweating; this can be an extremely helpful localizing sign. Horner syndrome will result if the cervical sympathetic trunk is damaged. In the horse, dermatomal patterns of sweating on the neck and cranial shoulder occur with involvement of the C3–C8 branches of the sympathetic fibers. 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