Clinical usefulness of MRI and myelography in the diagnosis of intervertebral disc extrusion in dogs
The objectives of the first study were: (1) to determine the accuracy of the ventrodorsal (VD) myelographic projection for predicting the circumferential location of thoracolumbar intervertebral disc (IVD) extrusion in small dogs, and (2) to describe paradoxical contrast obstruction (PCO). 93/104 (89%) of dogs had VD myelographic lateralization and 89% matched the surgical findings. In 83% dogs with bilateral contrast column gaps of unequal length, disc material was found surgically at the side of the shorter contrast gap. This was termed PCO. The objective of the second study was to compare the diagnostic and descriptive value of magnetic resonance (MR) imaging and myelography for the diagnosis of naturally-occurring thoracolumbar IVD extrusion in small breed dogs. Agreement between MR imaging, myelography and surgery for identifying the site of extrusion was substantial (kappa (k)=0.78 [CI=0.67, 0.84]). Agreement between MR imaging, myelography and surgery for identifying the side was moderate (k=0.45 [CI=0.32-0.59]). Signal intensity changes on T1- and T2-weighted, gradient echo (GRE) and gadolinium-enhanced T1-weighted images were reviewed and compared with myelographic and surgical findings. MR imaging was more useful than myelography for imaging IVD extrusion in small dogs, including identification of site and side and determination of the nature of the extradural material. Myelography should be reserved for cases when MR imaging is not available or follow MR imaging when multi-level extrusions are noted. The objective of the third study was to determine whether MR imaging and histological findings correlated in dogs with naturally-occurring thoracolumbar spinal cord trauma. All dogs had severe neurological dysfunction. MR signal intensity changes suggest abnormal parenchymal histology, however, absence of change does not rule out parenchymal damage. In conclusion, MR imaging is a superior imaging modality to myelography for the circumferential localization of naturally-occurring thoracolumbar IVD extrusion in small dogs. Myelography was useful to determine the site of extrusion but was inconsistent in determining the side and did not provide visualization of the spinal cord. MR imaging provided images of the spinal cord but did not reliably provide detailed images of acute lesions. Clinical findings and histopathology are more consistent in the assessment of spinal cord injury.