Figure 3.15 A, B & C are transverse, dorsal, and median plane views, respectively, of CT myelogram centered on the C3‐4 intervertebral site of a 16‐month‐old ataxic Thoroughbred colt with Type‐I CVM. The dashed blue line in A traces the outer circumference of the C3‐4 intervertebral disc. Transverse (A) and dorsal (B) projections were reconstructed in the planes shown as a and b in C. Although dorsal flaring of the caudal vertebral body of C3 (arrow) and a kyphotic angle between C3 and C4 are obvious in C, spinal cord compression is not clearly revealed in A. Conventional radiographic myelography was required to demonstrate dynamic dorsoventral spinal cord compression at this site. Early osteochondral degenerative changes are evident in the articular process joints. There are osteophytes at the dorsal joint margins, sclerosis of the articular aspects of the C3 processes and cavitation of articular surfaces of C4 with possible bony fragmentation (orange circles).

Magnetic resonance imaging

Undoubtedly, magnetic resonance (MR) imaging with and without gadolinium contrast is superior for identifying lesions primarily involving nervous tissues, and studies of the normal variability of brain and spinal cord MR morphology of large animals are available.^164–166 Identifying pathologic changes has assisted in diagnosing a wide range of diseases, including brain abscess, brain injury, pituitary adenoma, nigropallidal encephalomalacia, otitis media, hydrocephalus, cerebellar disorders, aplasia of corpus callosum, inflammatory foci, and coenurosis.^{144,150, 151,164–173} An example of the clinical application of MR imaging to equine brain disorders is given in Figure 3.17.

      In making judgements from any test procedure such as cervical radiographs, it is axiomatic that, depending on cutoff criteria taken from reference values, there will be variable proportions of false‐positive and false‐negative diagnoses. As well as considering normal variance, test error, and clinician bias, any test is only a snapshot in time so that results may well have changed since the onset of the problem.

Figure 3.16 CT myelograms of an 18‐month‐old ataxic Thoroughbred horse that underwent ventral stabilization surgery at C6‐7 for Type‐II CVM. A, B & C are cranial views (left is to the right side) of transverse plane images at C5‐6, C3‐4, and C6‐7, respectively. The stainless‐steel basket implant can be seen ventrally in C. In A, there are prominent osteophytes on the dorsal margins of the APJs and the left intervertebral foramen at that site is markedly narrow with adjacent sclerotic changes suggesting contact across the foramen (orange circle). In B there is marked, asymmetric osteochondral modeling of APJs that is much worse on the left, narrowing of the joint spaces bilaterally, a region of osteolysis (arrow) and patchy areas of bony attenuation in the left C6 articular process. There is extensive modeling of both sets of APJs and joint spaces and intervertebral foramina are markedly narrow at C6‐7 in C, although as in B, such narrowing seen on one slice does not constitute reason to diagnose foraminal stenosis with intervertebral neurovascular bundle compression. Spinal cord compression is not evident in any of these views.