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Degenerative spinal diseases of German boxers - spondylosis deformans and diffuse idiopathic skeletal hyperostosis

MVDr. Renata Kvapilová, MVDr. Roman Kvapil

            A common radiological finding on the spine of dogs is growths. In a large number of cases, they are a secondary finding on chest and abdominal radiological examination.¹². These are diseases that are characterized by the formation of new bone in the spine. These include spondylosis deformans (SD), diffuse idiopathic hyperostosis (DISH) and osteoarthritis of the intervertebral joints⁵. In most studies looking at the prevalence of SD and DISH in dogs, the German Boxer is the breed with the highest prevalence of these diseases.  

            SD and DISH are both non-infectious, non-inflammatory and degenerative diseases of the spine¹⁰. Both occur spontaneously and can occur in the same animal. They are characterized by the formation of bony processes on the vertebrae. Histological examination has shown that they are different diseases. The main criteria are the location and extent of new bone formation⁸. The fact that SD and DISH are two different diseases is also shown by the findings on MRI.¹⁵. The nomenclature of these diseases is often unclear for the following reasons:

1. The terms used to describe these conditions are often used quite haphazardly rather than adhering to precise diagnostic criteria. For example, in the past the term severe SD has been used to describe what is more accurately called DISH. Another example is the confusion between the term 'spondylitis', which indicates an inflammatory condition, and the term 'spondylosis', which is non-inflammatory.
2. Another source of confusion is the tendency to incorrectly attribute a patient's clinical symptoms to one of these conditions, when in fact the patient is suffering from a condition unrelated to spinal disease. The reason for this confusion is that new bone formation is often so obvious and sometimes so dramatic on the radiograph. On the other hand, more clinically significant lesions such as intervertebral disc herniation result in only subtle changes on X-ray and are not obvious without more advanced imaging techniques such as magnetic resonance imaging⁵.

Spondylosis deformans (SD)

            Bony growths on vertebrae arise at the point where the anulus fibrosus of the intervertebral disc is attached to the cortical surface of the adjacent vertebral body by Sharpey fibers. These bony processes are often described as osteophytes, but true osteophytes arise at the osteochondral junction of the synovial joints. The precise term for these bony proliferations of SD is enthesophytes. These enthesophytes are localized between the intervertebral disc and the vertebra. They typically grow ventrally and laterally but not dorsally. Enthesophytes are small protrusions to bony bridges over the disc space and do not extend over part of the ventral surface of the vertebral body.

            The exact pathogenesis of SD is unclear, but changes in the peripheral fibers of the annulus fibrosus appear to be the most important inciting cause. In most cases, there is a breakdown of the peripheral annulus fibers (Sharpey fibers) with increasing age, leading to disruption and weakening of the disc connection to the vertebra. The consequence is stress on the ventral longitudinal ligamentum, which is attached to the vertebral body. Enthesophytes develop in the area of stress and grow by a process of enchondral ossification. Bone growth continues ventrally and then laterally but rarely dorsally. This may be due to differences in the attachment of the ventral and dorsal longitudinal ligament and the ventral and dorsal attachment of the annulus fibrosus and vertebral body. SD can also develop as a result of other causes of disc damage such as spinal trauma, ventral slot surgery or discospondylitis⁵.

            The clinical significance of SD is not clear. SD is usually a secondary finding, so there is no clear correlation between the presence of spondylosis and clinical symptoms of spinal disease. Bony growths typically do not extend into the vertebral canal and therefore do not compress the spinal cord to cause neurological deficits. Because new bone formation may extend dorsolaterally, it is possible that nerve roots or spinal nerves may be compressed. However, this compression has a slow progression and the clinical signs associated with SD are difficult to confirm.

Diffuse idiopathic skeletal hyperostosis (DISH)

            Known as ankylosing hyperostosis or Forestier's disease in humans, DISH is characterized by calcification and ossification of the entheses that form where ligaments, tendons or joint capsules enter the bone. Wright in 1982 was the first to identify a condition identical to human DISH in dogs. To date, several studies have been published describing DISH in dogs. Different authors have used different diagnostic criteria. When considering the risk of error inherent in extrapolating the clinical features of human disease to veterinary medicine, it is still unclear what the best diagnostic criteria are for dogs.

            It is unclear how often DISH results in clinical symptoms and most patients are asymptomatic. However, spinal pain and stiffness have been described. Kornmayer et al. described Weimaraners with DISH who had two separate spinal fractures associated with minor trauma over a 2-year period. Impaired biomechanics due to fusion of multiple intervertebral spaces can increase degeneration at adjacent, unconnected intervertebral spaces resulting in intervertebral disc disease at these sites⁵.

Clinical significance of spondylosis deformans and diffuse idiopathic skeletal hyperostosis

            As already mentioned, neither SD nor DISH are the cause of clinical symptoms. However, most veterinarians consider these diseases to be the cause of spinal and back pain in dogs. A proper history and clinical examination will then help localize the painful area. We can assume a spinal problem if the patient has difficulty moving up and down stairs, if the patient whines when we take him in our arms. Given the large number of differential diagnoses, in most cases we should not dispense with further diagnostic steps, usually using imaging methods (radiological examination, CT, MRI). Differentially, we must consider discospondylitis, intervertebral disc herniation, neoplasia, spinal exostoses, lumbosacral or cervical stenosis, fractures and luxations³.

Diagnosis of spondylosis deformans and diffuse idiopathic skeletal hyperostosis

            The basis of diagnosis of SD and DISH is radiology. A lateral view of the spine is taken. Radiologically, enthesophytes associated with spondylosis deformans begin as small triangular growths localized a few millimeters from the edge of the vertebral endplate. As the disease progresses the enthesophytes may look like bridges of the intervertebral space, although true ankylosis is rare. SD can also be detected on computed tomography (CT) and magnetic resonance imaging (MRI) scans.

            New bone formation in DISH represents enthesophytes affecting the ventral longitudinal ligament continuing on the ventral side of at least four adjacent vertebral bodies. This is in contrast to spondylosis deformans, in which bone formation begins in the area attached to the ventral endplate of the vertebra, and is usually confined to the region of the intervertebral disc⁵.

            The basic radiological finding in SD and DISH is the formation of new bone on the vertebrae, especially on their ventrolateral aspect. There are many classification schemes for both SD and DISH. In the evaluation of SD in purebred boxers in the Boxing Club of the Czech Republic, the classification scheme most similar to that proposed by Eichelberg and Wurster in 1982 is used, see Table 1¹².

Table 1: Classification scheme of spondylosis deformans used by the Boxing Club of the Czech Republic

                                                                       http://www.decker.cz/rtg_hodnoceni_SA

Authors' note: this classification scheme still uses the name for osteophyte growths and the description of very severe grade is similar to DISH

Seven criteria have been proposed to distinguish DISH from SD:

1. Bridging ossification on the ventral and lateral aspect of three adjacent vertebral bodies
2. Relative protection of the width of the intervertebral space in the affected part and absence of changes of degenerative disc disease (sclerosis of the end disc, calcification of the nucleus pulposus, or localized spondylosis deformans)
3. Osteoarthritis of the dorsal intervertebral processes of the intervertebral joints
4. Pseudoarthrosis processus spinosus
5. Enthesopathy of attached soft tissues in axial and appendicular skeleton
6. Osteophytes, sclerosis and ankylosis of the sacroiliac joint
7. Bony ankylosis of the symphysis pubis.

At least four criteria must be met to confirm DISH in a dog¹⁴.

Therapy of spondylosis deformans and diffuse idiopathic skeletal hyperostosis

            In patients with obvious spinal pain and stiffness, conservative therapy is appropriate and should be comprehensive. Comprehensive therapeutic options for musculoskeletal pain include: administration of anti-inflammatory and centrally acting analgesics, acupuncture, transcutaneous electrical nerve stimulation, improved biomechanics of movement, movement therapy, massage, thermotherapy, physical therapy (ultrasound, laser, neuromuscular electrical stimulation, magnet therapy), chiropractic care, weight loss and nutritional supplements⁴.

In patients with neurological deficits or severe persistent spinal pain, further diagnostic investigations such as CT, MRI and cerebrospinal fluid examination are indicated in an attempt to detect the cause of the clinical symptoms, rather than to assess that the spondylosis seen on the radiograph is the cause of the clinical symptoms.

Therapy of spondylosis deformans and spondylarthritis through rehabilitation

            Patients with spondylosis deformans may also benefit from rehabilitation and physical therapy. On clinical examination, we usually find tenderness in the spine, stiffening of the long back muscles, signs of inflammation (heat, pain) at the sites of the growth, varying degrees of muscle atrophy (especially in the pelvic limbs) and kyphosis¹. These symptoms can be categorised according to severity into severe, moderate and mild, which determines the intensity and frequency of physical therapy and rehabilitation.

            The goal of rehabilitation therapy is to control pain, reduce muscle stiffness, strengthen muscles, improve spinal flexibility and improve limb function.

            Physical methods of pain management include acupuncture, transcutaneous electrical nerve stimulation, thermotherapy, therapeutic ultrasound and laser. Using rehabilitation exercises, we can then improve the biomechanics of movement and redistribute the force acting on the musculoskeletal system, leading to a reduction in pain. In addition, we can affect pain with massage, passive and active movement therapy, osteopathic and chiropractic methods. An important part of this therapy is to lose weight if necessary and add nutritional supplements⁴. For severe and moderate pain it is recommended to perform therapy with these methods every day, for mild pain as needed or at least 2 to 3 times a week.

            Muscle stiffness can be removed by massage, transcutaneous electrical nerve stimulation, thermotherapy, stretching and therapeutic ultrasound. We perform this therapy every day.

            We influence muscle atrophy by kinesiotherapy, whether passive or active. A suitable method is to move the dog in a rehabilitation bath with a moving underwater belt. For active movement, we start with slow walking on a leash, walking in tall grass or sand, sit-stand exercise and cavalcade movement, and we can add swimming. For severe muscle atrophy, we exercise every day and when the condition improves and the atrophy is mild, it is enough to do this kinesiotherapy 2 - 3 times a week.¹.

Spondylosis deformans and diffuse idiopathic skeletal hyperostosis in boxers

Prevalence of spondylosis deformans and DISH in boxers

            In his 1973 paper, Mühlenbach found that 92.4 % of all boxers (n=324) had some degree of spondylosis. The most common growths were located in the T12/T13 and L7/S intervertebral spaces. All dogs older than 4 years were affected with SD. Of these, 40 % dogs showed clinical signs of varying degrees of¹¹. A slightly lower prevalence was found in a study by Carnier, 2004, who evaluated X-rays of 849 boxers from Italy. The prevalence of SD was 84 %. Half of the boxers had bony bridging at at least one intervertebral site. The most commonly affected intervertebral spaces were T10/T11 to L2/L3 and intervertebral spaces L6/L7 to L7/S1². Kranenburg's 2011 paper already surveyed the prevalence of both diseases. The prevalence of SD 55.1 % (38/69) and DISH 40.6 % (28/69) in boxers was found. The study found that SD and DISH can co-occur in the same individual. DISH has been misdiagnosed in the past as severe SD⁸.

Etiology of spondylosis deformans and DISH in boxers

            The etiology of both diseases is unknown. Due to the high prevalence in the Boxer breed, a genetic predisposition for SD is assumed. In the Langeland study, the heritability of SD in Boxers was determined by a calculation method for the development of maximum growth grade of 0.42-0.62 and the heritability for the number of affected intervertebral discs of 0.13-0.47. A positive phenotypic correlation between SD and hip dysplasia was also observed. In this study, different heritabilities at different sites of affected intervertebral spaces were also demonstrated⁹. Both of these diseases in boxers may serve as examples to identify the causative genes involved in the (etio)pathogenesis of these diseases or serve as test populations for newly developed therapies⁷.

Clinical significance of spondylosis deformans and DISH in boxers

            Most dogs with SD and DISH are asymptomatic or show only mild clinical signs. Eight dogs with signs of thoracolumbar myelopathy were examined by magnetic resonance imaging for the presence of SD and DISH. However, fusion of two or more adjacent vertebrae by bony bridges correlated with disease of adjacent unconnected intervertebral spaces¹³.

            Togni's 2014 study showed that clinically relevant lesions associated with disseminated idiopathic spinal hyperostosis were rare compared with lesions in SD. Foraminal stenosis and/or intervertebral disc protrusion was considered a clinically relevant lesion (2/15). However, in this study, only 18 dogs were examined radiologically and by magnetic resonance imaging¹⁵.

            Spondylosis deformans and diffuse idiopathic skeletal hyperostosis occur in dogs and with a high prevalence in the boxer breed. These are two distinct entities. They are easily diagnosed on lateral radiographs of the spine. Most dogs with SD and DISH are asymptomatic, so other differential diagnoses must be considered when spinal pain is present. Once other causes have been ruled out, subsequent therapy must be comprehensive, including both pain and inflammation medications and rehabilitation.

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