Large Animal Neurology. Joe Mayhew

Figure 4.1). The spinal cord can be harvested by preparing cervical, thoracic, and thoracolumbosacral vertebral column sections. Then, a dorsal laminectomy is performed using bone cutters for smaller samples allowing for removal of the spinal cord sections using traction applied to the dura mater and careful sharp sectioning of neural and vascular attachments. Alternatively, the spinal cord sections can be exposed by making a sagittal saw cut along the vertebrae exposing the vertebral canal and lateral spinal cord and removing and labeling each section of cord for orientation. A spinal cord removal instrument⁹ can be useful in removing long sections of spinal cord from trimmed blocks of vertebral column in large animals (Figure 4.2). The dura mater should be carefully split along the dorsal surface and almost complete transverse sections made with a new razor blade through the cord at least at every segment. Alternatively, in wobblers, the cervical vertebrae may be disarticulated and each segment of spinal cord labeled and oriented as to cranial versus caudal. If there is good evidence from clinical and imaging studies of the site of a focal spinal cord lesion, then the preparation of the site of interest can be more focused and carefully performed. As a rule, there will be no epidural fat evident on opening each dorsal intervertebral space at sites of spinal cord compression. This then becomes a signal of a likely compressive lesion, warranting care when that segment of cord is harvested. If in large wobbler cadavers the site of a suspected compressive lesion(s) is not known, the cervical vertebral column can be sectioned transversely through the middle of each vertebral body so that each segment of spinal cord can be removed after cutting the spinal nerve roots. After spinal cord removal, each block of vertebral sections or adjacent halves of two vertebrae can be cut on the median plane to expose all intervertebral sites where most compressions occur in the neck of large animals.

Figure 4.1 To remove the brain intact, a craniectomy can be performed as depicted here. Two saw cuts through the bones of the cranium are made (between arrows 1 and between arrows 2) on each side and then a transverse cut is made over the rostral frontal region just caudal to the level of the caudal extent of the bony orbits (arrows at 3). With further careful sawing and leverage (arrowhead) with a hand wedge (4), the cranial roof can be removed. The brain is then removed by tilting the nasal region up and over while cutting the exposed cranial nerves as they exit the calvarium.

      Because the removal of the entire spinal cord from an adult large animal is particularly difficult, the procedure of sectioning the entire or appropriate parts of the vertebral column with enclosed spinal cord, trimming off excess soft tissues, and immersing the sections of vertebral column in large volumes of 10% formalin is an appropriate alternative means of preservation for shipment to a pathology laboratory. A band saw may not be available to perform a laminectomy or sagittal vertebral cut to remove the spinal cord, and it may be unpractical to preserve large sections of vertebral column for the pathologist. A hatchet may then be used to remove the vertebral bodies from the vertebral arches using a ventral approach. The cuts are made in a slightly medial direction, from the angle between the vertebral bodies and the transverse processes in the cervical and lumbar regions, or from the rib remnants in the thoracic region. Alternatively, a spinal cord removal tool⁹ may be used (Figure 4.2).

       Gross artifacts

      Vertebral and cranial fractures and separations sometimes occur at the time of euthanasia and during any manipulation of the cadaver of both immature and heavy large animals. This may be the result of falling, electrocution, dragging, and slinging. Surprisingly, considerable hemorrhage may be present around such fracture sites, and the histologic appearance can be difficult to interpret. Irrespective of this, large, mainly subarachnoid, spinal and cranial hemorrhages also occur, especially if decapitation is performed immediately after death.

      Gross estimates of an excessive (or deficient) volume of CSF are almost impossible to make, and except for noting any other major malformation, the evaluation of an apparent slight increase in the size of ventricles should await measurements made after fixation.

Areas of postmortem damage to nervous tissue can appear as a softening (malacia) and discoloration. Saw cuts and sites of tension may be apparent, but the spinal cord in particular must be removed carefully to avoid an effect akin to squirting toothpaste from a tube; tension or compression on the spinal cord within its dura mater easily causes constrictions, areas of softening, discoloration, and dislocation of parts of spinal cord tissue.

Figure 4.2 With the vertebral column cut into cervical, cranial thoracic, and thoracolumbosacral sections, a laminectomy may be performed to remove the spinal cord sections or a spinal cord removal instrument as shown can be used to gently cut the spinal nerves in the epidural space allowing withdrawal of the spinal cord segments. Such an instrument as shown at top is ~60–80 cm long and has a blunt‐ended, curved blade with indented cutting surface at one end (two close‐up views are shown beneath).

      A common error in preserving large masses of neural tissue is underestimating the volume of aldehyde fixative to use resulting in autolysis of tissues and even bacterial growth in the cramped samples sitting in a small volume of fixative! A ratio of one‐part nervous tissue to ten parts fixative is ideal, and changing the fixative after 24 h is good practice and helps reduce any blood staining of the preserved specimens. Partial and differential fixation can produce localized changes in texture and color, so comment is best reserved until fixation is complete. This does not preclude initial sectioning of the brain and spinal cord, as this also aids in fixation.

       Gross lesions

Malformations of the CNS usually are apparent on careful gross inspection. However, associated structures such as eyeballs, peripheral nerves, bones, and muscles as well as midline structures such as heart and great vessels should be scrutinized for accompanying defects. The outside of the brain and spinal cord should be studied for abnormalities in color, configuration, regional size and texture, and the brain can then be cut into four to six transverse sections to assist in fixation. Examining these freshly cut surfaces, especially for symmetry, can help locate areas of malacia, hemorrhage, atrophy, and swelling. Small (and even large) lesions may not be clinically significant in several regions of the forebrain (Figure 4.3). If cerebellar atrophy or hypoplasia is suspected, the cerebellum to total brain weight ratio should be calculated (normal >7%). Parasite tracts and inflammatory lesions can be inconspicuous. A culture taken from the meninges and saving a small portion of brain tissue for viral and bacterial isolation, especially from areas of discoloration, is advisable. In animals suspected of having rabies, appropriate samples need to be prepared for submission to the local animal health authorities. This may entail preparing the whole head or all/part of the brain for shipment and requires that appropriate precautions be followed during harvesting and shipping.

Figure 4.3 Small volumes of necrosis of CNS tissue result in astrocytic scars. Such lesions of many millimeters and greater usually result in cystic cavities lined by astrocytic fibers