Large Animal Neurology. Joe Mayhew

listeriosis and equine neuroborreliosis. Fungal and protozoal diseases usually cause mixed cell responses. Protozoal, and particularly helminth parasite infestations, may produce an eosinophilic and neutrophilic response in the CSF, as well as hemorrhage. In contrast to bacterial meningitis where the neutrophils are degenerate and show toxic changes, with parasitic invasions and specific viral diseases the polymorphonuclear cells are nondegenerate and multilobulated due to their age. In most chronic inflammatory states and in diseases in which there is much CNS tissue necrosis, the CSF can contain many large mononuclear cells or macrophages.

      Although immune‐associated CNS disorders such as canine steroid‐responsive meningitis have not yet been reported in large animals, there would be an expected modest pleocytosis, usually mononuclear, and, extrapolating from canine practice, it may well be worth sampling CSF from both the cervical and lumbar regions to maximize the likelihood of identifying the major CSF cytologic response.⁵¹ This recommendation also likely holds true for many of the infectious spinal disorders.

      With traumatic injury to the CNS, there is often some hemorrhage into the CSF with resulting yellow discoloration that can persist for days after the insult. This xanthochromia remains after red cells have been centrifuged off. Neutrophils, not showing toxic changes, followed by macrophages, will usually appear in the CSF in response to hemorrhage.

      In most toxic, nutritional, and metabolic neurologic diseases, the results of routine CSF analyses are generally normal. However, for those diseases in which there can be considerable tissue destruction, such as lead poisoning, sodium salt/water intoxication and polioencephalomalacia in ruminants, and moldy corn‐associated leukoencephalomalacia in horses, there may be some protein leakage into, and a mononuclear cell response within, the CSF.

      Typically, there is leakage of protein and some xanthochromia without any significant pleocytosis in many vascular diseases. If the hemorrhage is large, then neutrophils and macrophages may also be seen.

Primary genetic (degenerative) diseases do not typically cause any changes in CSF constituents, and most frequently, the CSF analysis is normal. Early in the course of cell degeneration and especially in very young patients, some protein and even cellular response may be expected. Those resulting in the accumulation of products such as occurs in the storage diseases may result in macrophages containing waste material in the CNS and thence in CSF.

      Neoplasms can act like other space‐occupying lesions, such as abscesses, granulomas (such as cholesterinic granulomas in horses) and hematomas, and can increase intracranial pressure. The most frequent CSF change in patients with neoplasia is a slight elevation in protein content. Sometimes there is evidence of mild injury, xanthochromia, and a few macrophages. Rarely, there have been atypical lymphocytes detected in CSF from cattle with CNS lymphosarcoma. Atypical cells such as melanoblasts have been detected in CSF samples, but it is worth considering whether such cells may have been disrupted from meningeal sites during the course of CSF collection.

      Because of the lymphatic‐like drainage system of the CNS from perivascular and Virchow–Robin spaces ultimately to the subarachnoid space, any process that is contained within the parenchyma of the CNS may ultimately cause the leakage of pigments or protein, or the exfoliation of cells into the CSF.

Electrodiagnostic testing

Electroencephalography (EEG), needle electromyography (EMG) (Figure 3.2), quantitative EMG, and nerve stimulation and conduction testing contribute to a complete neurological evaluation. The techniques are used in large animals^52–63 and are the same as those used in small animal neurology.^64–66 An ambulatory EEG system has been described that allows telemetric recording from freely moving horses.⁶⁷ Considerable experience in using these ancillary aids is required to be able to interpret electrophysiologic studies in large animals because findings in normal animals are not well defined.^6068–70 This fact, and the expense of the equipment, makes these procedures rather out of reach for most large animal clinicians. However, visual evoked, brainstem auditory evoked, and other evoked potential testing (Figure 3.9) definitely have become more widely used in large animal neurology over the last decade or so, and references can be consulted for techniques and initial methods of interpretation.^71–84

Certainly, the reasonably innocuous testing for transcranial magnetic and electrical motor evoked potentials (Figure 3.10) now has received attention in large animal neurology,^85,86 and it appears to be a very sensitive and quite specific electrophysiologic test for the disruption of somatic motor pathways in disease states.^87–89 This and the additional use of more elaborate but error‐prone quantitative EMG investigations^58,60,62,70 should allow more accurate identification of neuromuscular disorders.

      Finally, measuring conduction velocity across the cauda equina can be simply performed by using one stimulating electrode placed over the dura mater at the lumbosacral space and another stimulating electrode in the sacrocaudal cauda equina, with anode in adjacent skin. Induced motor action potentials are recorded using electrodes in a ventral coccygeal muscle. No conduction from the cranial stimulating site, with some potentials recorded from the caudal site early in a case of a fractured sacrum, can reasonably be taken as the cauda equina being totally severed. In this context, measuring the maximal bladder contraction pressure and maximal urethral closure pressure is a technique⁹⁰ that should assist in better defining the site of a lesion and should particularly assist in monitoring the use of drugs that can be used to treat patients suffering from urinary incontinence. Of note is the fact that there appears to be discrepancies in reported normal values.⁹⁰ However, the measurement of intravesicular and urethral pressure profiles could well be useful in monitoring horses with urinary incontinence.^91,92

Neuroimaging

       Plain and contrast radiography

Radiography of the calvaria and vertebral column (Figure 3.11) is indispensable for identifying normal variations, bony malformations, fractures and osteomyelitis,^93–104 and even evidence of reactivity to parasites.¹⁰⁵ Undoubtedly, the use of digital radiography will continue to be used more widely allowing better routine radiographic imaging and decreased radiation exposure.

Almost certainly, the cervical region is the area most often radiographed in neurologic large animal patients. However, even though the radiographic anatomy of equine cervical vertebrae is well described, many normal variations and inconsequential findings must still be realized in interpreting such radiographs.^{99,104,106,107} The frequent finding in <5% of Thoroughbred and Warmblood horses of transposition of ventral processes of C₆ onto C₅ and more often onto C₇ on one or both sides must be considered when vertebrae are being identified on radiographs.^104,108 This can also occur occasionally in other breeds including ponies.

Figure 3.9 Brainstem auditory evoked potential (BAEP) recording is a minimally invasive procedure as being performed here in a patient suspected as having vestibulocochlear (CN VIII) nerve disease. Broad band clicks are generated (1) and passed to insert earphones. The electroencephalogram is recorded with subcutaneous electrodes and passed through a preamplifier (2) to be recorded in an electrodiagnostic unit (3). By signal averaging many individual electrical responses, a waveform is produced that represents electric activity in the brainstem because of the clicks, which can be printed out as a BAEP trace (shown below). This horse indeed is deaf