Abstract
The cat is a small agile obligate carnivore. It has precise design specifications. A high proportion of fast-twitch muscle fibres ensure it is capable of ballistic activity — to sprint and pounce on prey, climb a near-vertical surface to avoid a predator, or to jump and land safely from a height (Fig 1). Its central nervous system and musculoskeletal machinery have been engineered through evolution. As Da Vinci said, ‘The smallest feline is a masterpiece.’ However, akin to a sports car, the stresses associated with this design configuration and the associated lifestyle come at a price. It is therefore no surprise that ‘wear and tear’ on articular cartilage and the ligaments and tendons that support the joints is considerable.
Two high-performance design models: the cat (this is Miss Binks, who belongs to RM); and the Aston Martin
There are two further considerations. In the wild, surfaces are ‘natural’, so the environment is more forgiving. The soft earth absorbs some of the kinetic energy after landing, and trees do not arise as sheer vertical surfaces, but have points of purchase and often some angulation. Furthermore, in nature, cats are lean, fit and conditioned to athletic activity, compared with the domestic situation in which cats are often obese (greatly increasing the kinetic energy to be dissipated after landing) and exposed to hard, unforgiving surfaces such as concrete. Also, certain pedigree cats have poor vision and obvious design flaws — the Siamese and Persian breeds immediately come to mind! Certain inbred pedigree lines of Maine Coon and Devon Rex cats can have specific genetic problems, such as hip dysplasia and luxating patellae (more ‘lemons’, to continue the automotive analogy).
For these reasons, it is no surprise that cats have a very high prevalence of osteoarthritis, especially as their lifespan is prolonged by domestication, good nutrition and modern veterinary interventions. And it's no surprise that the elbow joint is typically affected most, as it bears the brunt of the impact when cats land after jumping. Finally, as an obligate carnivore, cats have lost many of their enzymatic biochemical pathways through reductive evolution, and so have difficulty metabolising many of the drugs used to treat osteoarthritis in other species.
The knowledge base concerning feline degenerative joint disease (DJD) is limited, as little evidence-based medicine was available before the seminal paper by Hardie et al in 2002, 1 and the series of papers from Bennett's group in Glasgow.2–4 Certainly, the problem is common — Hardie et al found that 90% of cats older than 12 years had radiographic evidence of DJD, 1 Godfrey found X-ray evidence of DJD in 22% of a convenience sample of cats of all ages, 5 while Clarke and Bennett found 34% of 218 cats were affected. 2 The elbow, shoulder, hips and spine were most commonly affected, although all joints were at risk.
There are some definitional issues, with the term DJD encompassing changes affecting the vertebral column in addition to osteoarthritis affecting the appendicular joints. Critically, clinical signs and physical findings associated with feline DJD are somewhat cryptic, requiring the clinician to have a high index of suspicion to diagnose this condition. Telltale signs include: alterations in the ability to climb or jump, or in the height of the jump; a stiff or lumbering gait; reduced activity and mobility; reduced grooming, resulting in a poor, unkempt coat; lameness; lumbosacral pain; and discomfort when posturing to urinate or defecate, sometimes resulting in inappropriate voiding.
Some of these signs may be evident on physical examination, in addition to other more specific signs such as crepitus, pain on flexion and extension, and reduced range of motion of joints. In some cases, however, despite a careful physical examination, definitive physical findings may be difficult to elicit. A presumptive diagnosis may be confirmed using imaging studies — currently radiography (ideally digital), although in the future computed tomography 6 and scintigraphy may become more important. In addition, arthrocentesis with cytological examination and culture of joint fluid can be helpful. 7 In many instances, however, limitations of practice will mean that a tentative diagnosis is most conveniently confirmed using a trial of a safe non-steroidal anti-inflammatory agent, such as meloxicam. 4
In this issue of JFMS, Tan et al discuss an illustrative case study of a lame cat with a synovial osteochondroma of the elbow. 8 Although this is an extreme example, less severe variations of this occur commonly in a variety of joints in cats afflicted by osteoarthritis. This condition is thus an uncommon manifestation of a very common disease. Likewise synovial cysts (Fig 2) can occur in cats with DJD,6,7 and again the elbow joint is most commonly affected, reflecting another unusual manifestation of elbow osteoarthritis. Feline clinicians should be alert to the diagnosis and management of both these clinical variants — we reckon they are more common than you think!
Cat with a synovial cyst secondary to osteoarthritis of the elbow (a), and the corresponding lateral radiograph (b). From White et al 7