Björn P. Meij, DVM, PhD, DECVS
In veterinary literature, spondylosis deformans (hereafter called spondylosis) is defined as a non-inflammatory, degenerative disease of the region peripheral to the end-plate associated with new bone formation originating several millimetres from the intervertebral junction. The osteophytes vary from small spurs to bony bridges (Figure 1) across the intervertebral disc space, leaving most of the ventral surface of the vertebral body unaffected. As in humans, the aetiology of spondylosis in companion animals is not clear.
Spondylosis is diagnosed on radiographic (Figure 1) or histopathological examination. Using radiographs, a distinction has been made between end-plate osteophytes (type 1) described to be the result of spondylosis, and a group of three other types (types 2, 3 and 4) of new bone formation. The spurs of type 2 and 3 have, compared to type 1, a broader base of origin at the vertebral body and grow out to be type 4, which consists of a contiguous ventral band of new bone (Figure 2). These last three types were described to be comparable with those seen in human diffuse (or disseminated) idiopathic skeletal hyperostosis (DISH), formerly called ankylosing hyperostosis. Most other studies of canine vertebral hyperostosis did not specifically distinguish between spondylosis and DISH. In these studies, all bridging ossifications were thought to be severe spondylosis. Dogs diagnosed with spondylosis have been subdivided into three subclasses according to the degree of osteophytes development. In grade 1, the bony spur does not protrude beyond the caudal/cranial edge of the vertebral border, in grade 2 it does protrude beyond the caudal/cranial edge of the vertebral border, and in grade 3, a bony bridge is formed from the corner of one vertebra to the next (Figure 1). It has been proposed that in the past DISH in dogs was possibly described as a severe variant of grade 3 spondylosis. Although radiographic differentiation between DISH and severe spondylosis may be challenging, the two disorders do differ in radiographic appearance.
|Figure 1. Spondylosis deformans of L5–L6 in a cat. |
|Figure 2. Radiograph (A) and computed tomography (B) of thoracolumbar diffuse idiopathic skeletal hyperostosis (DISH) in two Boxers. |
The prevalence of canine spondylosis increases with age, with a described breed predilection for Boxers. In Norwegian Boxers, a prevalence of 26% (104/402) of spondylosis was found. In Italian Boxers, an even higher prevalence (50%) of grade 3 spondylosis was reported. The prevalence and the degree, or grade, of spondylosis were described to increase with age.
In cats a high prevalence of spondylosis is also reported. In the 1960s prevalence ranging from 34–68% was reported. In 55 out of 100 cats more than one site of the axial column was affected. The caudal thoracic region, cranial lumbar region and the lumbosacral region were reported to be most often affected by spondylosis. Spondylosis in the cervical spine is described less often. In cats, spinal segments T4–T10 are described to be most frequently affected by spondylosis. More extensive new bone formation was found more frequently in the caudal part of the vertebral column.
Clinical signs reported to be related to severe canine spondylosis are stiffness in the back, lameness, changes in gait and pain. In working dogs the diminished spinal flexibility limits activity. Osteophyte formations extending dorsolaterally can incidentally compress spinal nerve roots at the level of intervertebral foraminae. Spondylosis was also detected on computed tomography (CT) images in 62% of dogs with degenerative lumbosacral stenosis (DLS). Although the presence of spondylosis has been suggested to be correlated with Hansen's type 2 disc protrusion, spondylosis is also found in combination with healthy intervertebral discs. Generally, spondylosis has been described not to be of great clinical relevance in dogs. In cats, the spondylosis-associated changes of the intervertebral junction, a cartilaginous joint, are dissimilar to those associated with osteoarthritis of synovial joints. Nevertheless, both osteoarthritis and spondylosis are considered as part of degenerative joint disease and are suggested to be a possible overlooked cause of clinical disease in cats. Cats are known to mask signs of skeletal pain and often adjust their behaviour as a coping mechanism.
When only pain and stiffness are reported, most veterinarians in general practice start out with a conservative treatment similar to that for degenerative osteoarthritis (arthrosis), which consists of bodyweight reduction, controlled exercise or physiotherapy and non-steroidal anti-inflammatory drugs (NSAIDs).
Diffuse Idiopathic Skeletal Hyperostosis
Diffuse (or disseminated) idiopathic skeletal hyperostosis is a systemic disorder of the axial and peripheral skeleton in humans and has incidentally been described in dogs. DISH in dogs is defined as a flowing new bone formation along the ventral longitudinal ligament spanning at least four vertebrae (Figure 2A). Surprisingly, few articles are published on DISH in animals. Until recently, only two case reports of DISH in dogs were published, and there are no studies describing DISH in cats.
Little to nothing has been reported regarding the possible aetiology of canine DISH. In humans, type 2 diabetes mellitus (DM) is often related to DISH. In dogs, DM is, however, most often the result of autoimmune mechanisms affecting the pancreatic β-cell function and therefore more comparable to the human type 1 than type 2 DM. In cats, vitaminosis A is known to give rise to extensive new bone formation throughout the body, including the spine. In humans with vitamin A poisoning or long-term treatment with a vitamin A derivative for dermatological disorders, ligamentous ossification is also reported. In dogs vitamin A toxicity has the opposite effect and causes bone resorption and fractures.
Recently, the overall prevalence of canine DISH in an outpatient population of skeletally mature dogs was reported to be 3.8% (78/2041). In the same dog population, the prevalence of spondylosis was 18.0% (367/2041). Of dogs with DISH, 67.9% (53/78) also had spondylosis, whereas 14.0% (53/367) of dogs with spondylosis also had DISH. The prevalence of DISH increased with age and was more frequent in male dogs, similar to findings in human studies. The prevalence of DISH and spondylosis in Boxer dogs was 40.6% (28/69) and 55.1% (38/69), respectively. Dog breeds represent closed gene pools with a high degree of familial relationship, and the high prevalence in the Boxer may be indicative of a genetic origin of DISH. Therefore, the Boxer breed may serve as an animal model for DISH in humans. Nineteen smaller breeds were not affected by DISH, but did show signs of spondylosis; only Standard Poodles appeared not to be affected by either disorder. It was concluded that DISH and spondylosis can co-occur in dogs.
DISH has probably been previously underdiagnosed and mistaken for severe spondylosis. The diagnosis can be made using radiography, CT (Figure 2B) or magnetic resonance imaging (MRI). On histology, DISH can be distinguished from spondylosis by the location (ventral longitudinal ligament) and extent of new bone formation.
In humans, DISH is often asymptomatic. Symptoms reported are mainly due to altered biomechanics and may consist of pain, stiffness and restriction in range of motion. Reduced flexibility and decreased range of spinal motion can eventually result in fractures of the spine after only minor trauma. These fractures in patients with DISH tend to be unrecognised, unstable and associated with treatment delays and permanent neurological deficits. The clinical relevance of canine DISH is not yet thoroughly described. The dogs in the two case reports showed orthopaedic and neurological abnormalities. Extreme stiffness and pain in the axial and appendicular skeleton, presumably due to DISH, did not seem to respond to treatment and resulted in the owners electing for euthanasia. It was stated that the radiographic and pathological features of canine DISH closely resembled extensive spondylosis but possessed marked radiographic and morphological differences. Vertebral osteophytes associated with spondylosis typically centre ventrally to individually degenerated discs and do not have patterns of flowing bone growth involving contiguous segments or ventral periarticular changes. DISH in dogs may have been described in the past as a variant of spondylosis.
Treatment for DISH is similar to that for spondylosis and the first choice is conservative management. NSAIDs may give some relief and clinical improvement. Especially when the extreme new bone formations either cause pain that is refractory to pain medication or result in isolated neurological deficits, surgical intervention may be considered.
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