Joe Mayhew - Large Animal Neurology

Regarding the assessment of posture and postural abnormalities, flexing the foot to attempt to make the animal stand on the dorsum of the pastern and determine how long the animal leaves the foot in this state before returning it to a normal position is said to be a test for conscious proprioception in dogs and cats. Almost certainly, this involves somatic afferent (tactile) pathways as well, and a very weak patient may not be able to move the foot from many abnormal positions. This test can be attempted in large animals, but in our hands has not been helpful in adding accurate information to lesion location. Inactive and somnolent patients, especially calves, often allow the foot to rest on the dorsum for prolonged periods. Horses need to have almost total paralysis of the limb or to have a nociceptive sensory deficit in the limb before they allow such postural anomalies to be maintained. Other tests, such as manually crossing the limbs or placing one limb on a sack and slowly sliding the sack to the side, have been tried to test conscious proprioception, but again in our hands have proven to be noncontributory to the examination process. Rather than manually placing limbs in abnormal positions, it appears more reliable to maneuver the horse rapidly, say in a circle, and stop the maneuver abruptly ( Figure 2.14). This often results in an initial awkward placement of the limbs, and then the examiner can determine how long the horse leaves the limbs in such an abnormal posture to determine the presence or not of conscious proprioceptive deficits. This procedure probably does test for deficiencies in conscious proprioception. Examination of horses walking across kerbs has not proven to be a useful test of proprioceptive dysfunction. Normal horses, particularly if distracted, will often trip and those that are moving cautiously, even if quite weak and ataxic, can often maneuver such obstacles.

Figure 2.14 Stopping a patient abruptly after maneuvering it may result in abnormal limb postures being adopted and maintained. This is evident here in a pony (A) and a young ram (B), both of which have cervical spinal cord compression, and this may be taken as evidence for abnormal conscious proprioceptive input from the limbs to the forebrain. On the other hand, an obtunded patient or one with prominent weakness may not correct such abnormal limb positioning without having any specific conscious proprioceptive pathway lesion.

Gait alterations can occur in all four limbs with lesions affecting the white matter in the caudal brainstem when head signs such as cranial nerve deficits are used to help define the site of the lesion. Subacute to chronic lesions affecting the forebrain usually cause no substantial change in gait. However, postural reactions, such as hopping, are abnormal and sometimes the gait is slowly initiated on the thoracic limb contralateral to the side of a forebrain lesion.

In smaller patients, other postural reactions can be performed. These primarily help detect signs of subtle proprioceptive and motor system lesions when the straight‐line gait is normal. Wheelbarrowing the patient to make it walk on just the thoracic limbs, hopping it laterally on each individual thoracic and each individual pelvic limb, and hemistanding 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 C 6–T 2, 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 C 6–T 2or involving thoracic limb peripheral nerves or muscle ( Chapter 26).

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 C 1–T 2spinal 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. 27,28However, 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 C 1to 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.