John P. Graham, MVB, MSc, DVR, MRCVS, DACVR, DECVDI
The objectives of this presentation are to:
Review normal radiographic anatomy of the spine;
Review radiological signs of degenerative disc disease (DDD); and
Review radiological signs of non-disc lesions which may be detected from survey radiographs.
Clinical Diagnostic Points
DDD is the most common spinal disorder of dogs.
Survey radiographs provide limited data about DDD.
Surgical treatment of DDD requires specific lesion localization by myelography, CT or MRI.
Survey radiographs may be more helpful to confirm or exclude other causes of spinal pain, paresis or paralysis.
'Pre-radiographic' diskospondylitis occurs, but rarely.
Aetiologic and Pathophysiologic Points
Older chondrodystrophic dogs may have multiple degenerate discs.
Spinal neoplasia may involve either the skeletal structures of the spine or the soft tissues.
Survey radiographic changes in spinal neoplasia may be subtle and visible on one view only.
Survey radiographs do not provide sufficient data for surgical intervention in DDD.
Diskospondylitis lesions combined with sternebral or long bone lesions consistent with osteomyelitis usually indicate systemic aspergillosis.
Progression of diskospondylitis lesions or development of new lesions in the face of therapy may be a poor prognostic sign.
Intervertebral Disc Disease
Degenerative disease of the intervertebral discs is common in middle aged and older dogs, especially chondrodystrophic breeds. Spondylosis deformans is new bone formation around the margins of or bridging an intervertebral disc space. This new bone forms in an attempt to stabilize a degenerative or unstable disc and is commonly associated with degenerative disc disease. However, spondylosis deformans is very common in middle-aged and older dogs and is almost invariably clinically insignificant. The clinical signs associated with intervertebral disc disease are variable, ranging from pain or gait abnormality to paralysis with loss of deep pain sensation. Degeneration of the disc nucleus results in loss of concussive capacity due to dehydration. In chondrodystrophic dogs, mineralization of the disc nucleus is a common event so the detection of mineralized disc nuclei is not, in itself, a significant finding. Degeneration of the fibres of the annulus fibrosus results in protrusion of the disc into the vertebral canal or prolapse of nuclear material into the canal if the annulus ruptures. When evaluating the spine for evidence of a disc prolapse or protrusion one should keep in mind that some older animals may have many such lesions and that the discs with obvious radiographic changes may not be clinically significant at the time of presentation. The cardinal sign of disc prolapse is narrowing of the intervertebral space. This should be assessed by comparison of the suspect disc with the disc spaces cranial and caudal. The film must be well positioned and only the discs in the centre portion of the image should be assessed as the x-ray beam diverges and parallax distortion will cause discs at the periphery of the image to appear narrowed. If a disc is suspected to be abnormal, a film should be taken centered over it to confirm or refute this suspicion. In addition, to evaluating the disc space, the intervertebral foramina and facet joint spaces should also be assessed for narrowing. A mineralized disc nucleus contained within an intact annulus appear as a well defined slightly ovoid object. If the annulus is weakened, the dorsal aspect of the nucleus may appear pointed as it bulges into and through the annulus. A thin line of mineralized disc material may be seen dissecting through the annulus in some cases of disc prolapse, the so-called toothpaste sign. A mineralized disc nucleus within the vertebral canal may cause a faint or hazy increased opacity in the intervertebral foramen or appear as a well defined, well mineralized structure within the vertebral canal. Although survey films are helpful, definitive localization of a cord compressive lesion requires myelography, CT or MRI. With this in mind, one should consider survey radiographs as primarily excluding other lesions rather than confirming DDD.
While degenerative disc disease is the most common acquired lesion of the vertebral column, it is important to evaluate the entire structure and to not focus on the disc spaces alone. Each vertebra should be assessed in two views for size, shape, opacity and margination. Tumours may selectively destroy only a small part of the bone such as a single pedicle or transverse process. One should examine all the components of each vertebra in turn--endplates, ventral cortex, floor of the vertebral canal, pedicles, lamina and facets. Pathological compression fractures may be evident as a subtle foreshortening of the vertebral body. In such fractures it is important to assess the opacity of the vertebra and to look for evidence of new bone formation. Traumatic compression fractures result in increased opacity as the bone mineral is compacted into a smaller volume. Pathologic compression fractures usually occur as a result of an osteolytic process and the opacity of the fractured vertebra is equal to or less than the adjacent normal vertebrae. New bone will form within 7-10 days of a traumatic facture so the presence of new bone in a more recent fracture usually indicates it is pathological.
Cervical spondylomyelopathy, which is commonly called 'Wobbler syndrome', is a disease of large and giant breed dogs, especially the Doberman Pinscher and the Great Dane. The disease is characterized by malformation of the canal of cervical vertebrae which is funnel shaped (usually C5, C6 and C7) causing a narrowing of the vertebral canal at the cranial orifice. There may also be deformity of the vertebral bodies, which are more triangular in shape than normal and instability of the discs causing malalignment and subluxation. The soft tissues of the disc annulus and the vertebral ligaments hypertrophy in an attempt to reduce the instability. This proliferative tissue is a significant component of the spinal cord compression. In middle-aged and older dogs, there is often coexisting degenerative disc disease. The cord compression is usually dynamic, i.e., exacerbated by movement of the spine, and diagnosis and evaluation require myelography.
Discospondylitis or intradiscal osteomyelitis is defined as an infection originating within the intervertebral disc space. The infection first destroys the soft tissues of the disc and the earliest visible change is narrowing or collapse of the disc space. Lysis of the vertebral endplates occurs resulting in an apparent widening of the disc space. The margins of the vertebral bodies are quite ragged and irregular, and the vertebrae may appear mildly to moderately foreshortened. In chronic cases, exuberant spondylosis deformans develops in an attempt to contain the infection and stabilize the disc. Staphylococcus spp., Brucella canis and Aspergillus spp. are common etiologic agents. The organisms can usually be isolated from blood or urine. If a discospondylitis lesion is detected, it is recommended that survey radiographs of the entire spine be obtained as some cases have multiple lesions. Radiographic progression of healing is slow. Spondylosis deformans develops and initially the new bone appears active and disorganized. Complete fusion of the vertebral bodies is the desired end point.