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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 in the left ear as shown by there being no brainstem electrical activity resulting when the left ear is stimulated (top two traces) compared to the normal responses recorded from the left (third trace down) and right (bottom trace) sides of the brain when the right ear is stimulated.

Notable though usually inconsequential radiographic findings on equine cervical radiographs include the following:

 Variations and asymmetry in the shape of the intervertebral notch or orifice at the cranial border of C2.

 Irregularities to the dorsal aspect of the caudal physes of C2–C7 projecting into the intervertebral space.

 Separate ossification of the caudal projection of the transverse processes of C6.

 One or two transverse processes of C6 sometimes transferred onto C5 or onto C7.

 Serpentine lucent vascular channels on the spine of C2.

 Relative lucencies of the pedicles of the arches of the vertebral canal of C1 and C2.

 Irregular border to the caudal aspect of the spine of C2.

 Circular 3‐20 mm, cyst‐like lucencies occasionally present in the arches or bodies of all cervical vertebrae

 Variable size of, and irregular dorsal border to spines of C3‐6

 large and cranially‐projecting, irregularly‐mineralized, dorsal spinous process on C7 and T1

Figure 3.10 Magnetic motor evoked potential (mMEP) recording can be undertaken in the fully conscious cooperative patient or with the patient sedated with alpha‐2 and synthetic narcotic drugs. A magnetic stimulator (1) creates a very brief magnetic pulse in the attached coil (2) held centrally over the forebrain of the patient. At the onset of the stimulus, the generator triggers an electrodiagnostic unit (not shown—see Figure 3.9) to begin recording from the skin clip electrodes (3) placed over muscles such as cranial tibial (CT) and extensor carpi radialis (ECR) limb muscles. One or a few traces can be summated to produce mMEP waveforms that indicate the time for impulses to be produced in the brain motor centers and propagated along the central motor pathways to the peripheral nerves and ultimately the muscles from which a summated motor unit action potential can be recorded. Inset (4) shows a printout of the mMEP waveforms recorded from the ECR (top) and CT (bottom) muscles. Conduction times from stimulus till muscle response for these recordings were 24 ms to the ECR muscle and 46 ms to the CT muscle.

Figure 3.11 Radiographic evidence of a chronic lesion in the area of the occipital bursa such as the irregular mass of mineralized tissue seen here (arrows) has been suggested to be one cause for headshaking in horses. This horse was not a headshaker, and the radiographs were taken for other reasons.

Obtaining high‐quality radiographs is absolutely paramount to the accurate quantification of any measurements such as minimal sagittal diameter of the vertebral canal (Figure 3.12).^96,109,110 Standardized oblique radiographic views (Figure 3.13) can be useful in lateralizing changes seen on vertebral (and skull) radiographs especially where DV views are difficult or impossible to obtain.⁹⁷ Interpretation of plain radiographs of other large animal species^111,112 is less frequently undertaken and thus more problematic¹¹³ such that control images often need to be consulted.¹⁸

Figure 3.12 Obtaining accurate measurements such as minimal sagittal diameters from radiographs does depend upon obtaining a true lateral projection and good exposure factors—the latter being more readily obtained with digital radiography. A and B are images from the same average quality, standing radiograph of the C3–4 intervertebral articulation of a yearling with CVM using different postprocess exposures. The sites that need to be precisely identified to accurately measure the sagittal ratios are indicated by circles in A and are very indistinct. With image manipulation, reasonable measurements can be obtained for the intravertebral (white bar) and intervertebral (yellow bar) sagittal diameters. Part of the problem with making measurements lies with the superimposition of structures including the pedicles of the articular processes overlying the laminae of the dorsal arch of the vertebral canal. Also blurred and paired lines of the dorsal and especially ventral margins of the vertebral canal due to obliquity, add to the imprecision of measurements.

Figure 3.13 Oblique lateral radiographs can assist in lateralizing alterations to vertebral articular processes in wobblers and horses suffering from neck pain and unusual thoracic limb gait abnormalities. This results in skyline views of the articular surfaces as shown here using a 34° (above lateral) oblique projection for one of the C2–3 articulations (between yellow arrows). One of the C3–4 articulations (between green arrows) is not quite as well defined on this projection as the more caudal cervical articular surfaces are on a greater angle such that at C6–7 the X‐ray beam needs to be directed from at least 45° above horizontal with the horse’s neck on a horizontal axis.

The techniques of positive contrast myelography have been described in large animals, particularly horses.^{103,114–122} These procedures are useful for defining spinal cord compression and swelling of the spinal cord, although experience is required to be able to obtain satisfactory studies (Figure 3.14). Complications do occur, and the procedures can be prolonged and distressing to the patient.^123,124 Consequently, it is suggested that these procedures be performed only by an experienced radiographer who has the correct equipment and only when the clinician is prepared to attempt whatever surgical and medical approach is indicated, including euthanasia, as necessary at the end of the procedure. We do not condone performing positive contrast myelography on nonanesthetized large animal patients.¹²⁵ Notwithstanding these issues, contrast myelography and subjective and objective assessment of measured parameters of compression of spinal cord taken from these studies are useful to define lesions prior to corrective surgical procedures being carried out^{99,121,126–132} and to help focus diagnostic postmortem studies.^{53,113, 133,134}

Figure 3.14 Thinning of ventral (yellow arrows) and dorsal (white arrow heads) myelographic contrast columns as shown here in two cases of EPM with swollen spinal cords at C6–7 can be mistaken for epidural compression and thus possible evidence for spinal cord compression and CVM.