Large Animal Neurology. Joe Mayhew
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Название: Large Animal Neurology
Автор: Joe Mayhew
Издательство: John Wiley & Sons Limited
Жанр: Биология
isbn: 9781119477198
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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 out99,121,126–132 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.
Computed tomography
Computed tomography (CT) and CT contrast myelography have some advantages over unenhanced radiography but at an expense. More detailed imaging of bony structures such as vertebrae and petrosal, basilar and hyoid bones, being able to view structures in three planes and perform anatomical reconstructions can be definite advantages for diagnostic purposes.102,135–141 CT also images some soft tissue lesions well.113,142–163 The utility of CT for imaging of large animals has been enhanced at a few high‐end referral centers by the development of large‐bore mobile scanners and robotic‐controlled, gantry‐less digital systems. Both of these methods enable high‐resolution CT imaging of the heads and cervical vertebrae of standing, awake horses. Examples of the detail in image definition and some aspects of the diagnostic utility of CT myelography are shown in Figures 3.15 and 3.16.
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.