Osteoarthritis in cats (Proceedings)
The incidence of chronic pain in cats is not well documented but is associated with many conditions including osteoarthritis, cancer, interstitial cystitis, dental and gum disease and long-standing dermatitis and wounds. It is only recently that we have begun to appreciate what the true incidence of osteoarthritis or degenerative joint disease might be in cats and it appears to be much more common than previously thought and could be a major cause of discomfort especially in senior (> 10 years of age) cats.
What is the current state of knowledge?
The terms degenerative joint disease (DJD) and osteoarthritis (OA) are often used interchangeably but Clarke and others (2005) who have described the radiographic findings in cats in detail pointed out that enthesiophytes and soft tissue mineralization may not represent osteoarthritis whereas osteophytes and subchondral bone sclerosis are features of OA. Osteoarthritis involves a pathologic change of a diarthrodial synovial articulation including deterioration of articular cartilage, osteophyte formation, bone remodeling, soft tissue changes and low grade non-purulent inflammation.In one of the first studies designed to determine the prevalence of degenerative joint disease (DJD) in cats, 100 radiographs (taken as part of a diagnostic workup for multiple reasons) only of cats over 12 years of age were retrospectively reviewed and 90% of them showed radiographic evidence of the disease. When medical records of these cats were examined, only 4 contained any mention of DJD but severe DJD of the vertebral column was associated with neurological disease. The key question from this study was "did the failure to observe clinical signs truly represent a lack of clinical signs in the presence of radiographic findings, or a failure to recognize signs?" Certainly in dogs, the radiographic findings of osteoarthritis do not correlate well with clinical function. In another retrospective radiological study but this time involving cats of all ages, 22% showed evidence of radiographic OA and when patient records were consulted, 33% of these cats also had clinical signs. Affected cats were significantly older than the control population. These authors also suggested that there may be little correlation between radiographic and clinical findings or that clinical sings of OA in cats are not easily recognized. In a recent study at a University referral hospital, the prevalence of radiographic signs of DJD was 33.9% and the prevalence of clinical signs was 16.5% with most affected cats being 10 years of age or older. These authors further classified their findings into DJD and OA, the second being less common.
Which joints are most commonly affected?
The elbow joint was the most commonly affected joint in some published studies but when OA (rather than DJD) was specifically described, the hip joint was the most frequently affected. Many cats have multiple affected joints and bilateral involvement is common.
Causes of DJD or OA in cats
Osteoarthritis may be termed primary or secondary. In primary OA there is no clear underlying cause whereas with secondary OA an underlying cause can be implied, such as hip dysplasia or previous bone fracture. In the study by Clarke and others (2005) half of the cats with OA had an identifiable or historical cause, including hip dysplasia, in another study by the same group, 71% of cats had no obvious cause and these were classified as having primary or idiopathic OA.