Large Animal Neurology. Joe Mayhew

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Figure 3.6 Performing CSF collection from the atlantooccipital (AO) (A) and lumbosacral (LS) (B) sites is very straightforward with appropriate restraint, sedation, and/or general anesthesia. In very young patients light sedation is adequate, and in obtunded patients simple restraint suffices to perform an AO collection. Collection of CSF from the LS site is easier with the patient standing as the landmarks are more easily palpable and symmetric.The landmarks are shown for the AO procedure in a calf (A) with the superficial site for skin penetration indicated by . This site is at the intersection of a line joining the cranial borders of the wings of the atlas indicated by linear marks, with the midline, indicated by the spot over the occipital protuberance, while the head is flexed 90° to the neck as shown. A 19–21g, 30–45 mm hypodermic needle with plastic hub is ideal for this procedure, allowing for CSF to appear in the hub instantly the subarachnoid space is penetrated. The basic site for an LS procedure (B) is where a line joining the caudal aspects of the wings of the ilia (yellow dotted line) crosses the midline, as indicated by . This coincides with a palpable depression between the paired tubers sacrale (TS) laterally and the dorsal spines of L6 and S2 on the midline—the spine of S1 normally being too short to palpate. An 8–20 cm, 18 or 20 g spinal needle with fitted stilette is used for all adult animals depending on their size.Penetration of the terminal spinal cord during an LS procedure is essentially inconsequential. Damage to the medulla oblongata, which occurs on pathologic inspection more often than is clinically suspected, can result in additional neurologic signs.

If a patient is in an intensive care facility showing severe focal or diffuse brain injury, then consideration might be given to inserting a separate subarachnoid or subdural cranial catheter at the time of anesthesia for sampling CSF from the AO site to allow the assessment of intracranial pressure and the derivation of an estimate of cerebral perfusion pressure.^27,28 This can be of use in determining the effectiveness of ongoing management of such cases. Patient‐side observations and tests are extremely important at this stage. For example, if initial CSF is pink but clears, iatrogenic hemorrhage is likely the cause. With a sick neonatal patient, cloudy CSF with a low glucose content detected on urine test strips likely indicates suppurative meningitis. Particularly during lumbosacral sampling, small particles can often be seen to swirl in the aspirated sample. These are usually determined to be epidural fat and small aggregates of collagenous tissue introduced into the sample at the time of collection, likely reasons for falsely elevated CSF‐CK activity. When CSF analysis cannot proceed immediately following collection, the addition of a small, quantified volume (~10%) of autologous serum to an aliquot of the sample may help support cells for better cytologic harvesting, at least for 24 h.

      Of note is that estimates of protein content in CSF may be substantially higher, at least when tested by the pyrogallol red method, when CSF is collected into commonly used red vacutainer tubes containing clot activator.⁽²⁹⁾

      The results of CSF analysis can reflect diseases in the brain and spinal cord just as a hemogram detects many systemic disease states. Thus, CSF analysis is one of the most helpful aids in determining the nature of current and progressive CNS lesions. Normal CSF is clear and colorless with a refractive index of 1.3347–1.3350. In addition, it contains no erythrocytes and usually less than 6 × 10⁶/L (=6/μL) small mononuclear cells. Protein content is usually between 0.5 and 1.0 g/L (=50–100 mg/dL) in the horse with an absolute range of 0.1–1.2 g/L depending on the laboratory and can be up to 1.8 g/L in normal neonatal foals. Ruminants and pigs usually have CSF with a protein content of less than 0.75 g/L.¹⁸ Specific electrophoretic protein fractions in CSF can be assayed by numerous methods,¹⁷ but the utility of these results in changing the clinical course of cases is doubtful at present. Neonates of all species may have slightly xanthochromic CSF.

To help account for the blood contamination of CSF samples and to help determine the local versus systemic source of measured CSF immunoglobulin content, the determination of the albumin quotient and the IgG index, respectively, can be of some use.^30,31 Additionally, on the assumption that all red blood cells are from the circulation and not the result of previous subarachnoid bleeding, arithmetical corrections can be made for all measured components of CSF. Such corrections for routinely measured CSF components have not proven to consistently improve the utility of absolute measurements of amounts of CSF constituents.^32–34 Repeat thecal puncture from the lumbosacral or from the atlanto‐axial space 2 weeks apart has been shown not to impact results of CSF analysis.³⁵