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[1] 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[4]. 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.

Clinical signs

Compared to the radiographic features of feline OA the clinical signs are not well documented. Unlike dogs, lameness is not a common clinical sign of DJD or OA in cats. Because of the small size of cats and their innate agility they can cope with severe orthopedic disease. In addition, bilateral involvement, for example, both elbow joints is common and this makes lameness difficult to detect. It is also notoriously difficult to elicit pain on clinical examination of some cats.

There is only one published clinical study reporting a series of cats affected with OA. Twenty-eight cats that had historical evidence and / or clinical evidence of OA together with radiographic evidence of OA were recruited for a prospective study; their median age was 11 years. In addition to a clinical examination by one veterinarian the owners filled out a questionnaire on their cat's general demeanor, food intake and lameness. On physical examination, peri-articular thickening was a common finding, but decreased range of motion was not. Crepitus was not detected and synovial effusions were seldom present. Cats were treated with meloxicam for one month and reevaluated. The majority of owners felt their cats made an improvement and the most common clinical sign that improved was the unwillingness to jump and the height of the jump. Improvement of the stiff gait seen in many cats at the beginning of the study was also significant.

Clinical experience suggests that the behavioral changes that accompany osteoarthritis may be insidious and easily missed, or assumed inevitable with advancing age therefore the owner does not seek veterinary advice. Because of a pet cat's lifestyle, lameness or exercise intolerance are not common owner complaints. Changes in behavior including decreased grooming, reluctance to jump up on favorite places, inability to jump as high as before and soiling outside the litter box should prompt the veterinarian to look for sources of chronic pain. Other changes that owners report are altered sleeping habits (an increase or decrease), withdrawing from human interaction, hiding and dislike of being stroked or brushed. Inactivity, which may result from chronic joint pain, is much more difficult to determine in cats since they naturally sleep a lot and are often solitary. In many cases the owners are not home all day to monitor their cat's activity level and may not know if it has changed. Activity monitors attached to cat collars have been used to monitor daily movement in cats diagnosed with OA during treatment with meloxicam or placebo. In that study the activity counts increased with NSAID treatment suggesting alleviation of musculoskeletal discomfort.

It is common for owners and veterinarians not to realize how affected the cat was until they see dramatic improvements following treatment, emphasizing that analgesic trials may sometimes be the only way to confirm that pain was present.

Contributing factors

As in dogs, obesity may contribute to osteoarthritis in cats. In a prospective study of almost 1,500 cats where body scores were assigned and their health status followed for 4.5 years, an association between excess body weight and lameness was substantiated. Compared with optimal weight cats, heavy cats (number 5 on a scale of 1 to 6) were 2.9 times as likely to become lame and in obese cats (6 out of 6) this rose to 4.9 times more likely. In this study the cause of lameness was not confirmed but assumed to be related to degenerative joint disease. Excess weight was not found to be correlated with DJD or OA in a recent survey of 218 cats.

Treatment

Treatment can be considered under different categories such as life-style changes, pharmacological and non-pharmacological intervention.

Life-style changes

Much of the supportive management recommended for canine osteoarthritis, such as weight loss and controlled exercise, is applicable to cats but often more difficult to implement. Overweight cats should be placed on a carefully monitored weight loss program and there are several good commercial diets available to achieve set goals. With creative use of toys and owner participation, exercise can be encouraged in cats.

Chronic pain may prevent some cats form grooming and owners must perform this task, although the cat may resent this until pain is reduced to a tolerable level. Environmental modifications can simplify the cat's daily routine; for example placing food bowls and litter boxes in positions that do not require leaping or jumping for access. If cats cannot leap onto favorite resting places it impacts on their quality of life; therefore owners can construct ramps and steps so that cats may still reach these spots, for example window ledges and cat towers.

Drugs

Any drugs that are prescribed for the treatment of chronic pain must be in a formulation that is simple to administer and well accepted by cats otherwise owner compliance will be less than optimal.

Opioids

Because of their side-effects (including euphoria, inappetence and constipation) after long-term use (days / weeks) opioids are best reserved for terminal / hospice care in cats or for "break-through" pain while the cat is receiving other analgesics or undergoing other treatment modalities.

Long term use of NSAIDs in cats

The mainstay of drug therapy for osteoarthritis in most species are non-steroidal anti-inflammatory drugs (NSAIDs). This poses a challenge in cats due to their susceptibility to NSAID toxicity, especially with long term dosing. Pharmacokinetic data is only readily available for single doses of NSAIDs in cats. For a comprehensive review of NSAIDs in cats see Lascelles et al 2007. Most NSAIDs have a relatively long half-life in cats and repeated dosing must be done carefully to avoid toxicity. There have been reports of gastrointestinal toxicity with carprofen usage, generally associated with concurrent disease and prolonged administration of the oral formulation. Problems with repeated dosing are likely a result of individual variation in pharmacokinetics because the half-life of carprofen in cats can vary from 9 to over 40 hours[10], [11]. Meloxicam is a COX-2 selective NSAID which is available as an injectable and oral formulation. The injectable formulation is licensed for use in cats in the USA, but only as a single dose, at 0.3 mg/kg. The honey flavored oral liquid marketed for dogs is widely used (off label) in cats because it is palatable and small doses are easily measured. In several European countries an oral formulation for cats is now labeled for long term use in cats with musculoskeletal disease.

One published study evaluated the use of short term NSAIDs use for musculoskeletal pain in cats. Sixty-nine cats with acute or chronic locomotor disorders were randomly assigned to receive meloxicam (liquid formulation 0.3 mg/kg orally on day one then 0.1 mg/kg for 4 more days), or ketoprofen (tablet formulation, 1 mg/kg orally for 5 days)[12]. Based on general attitude, appetite, weight bearing, lameness and pain on manipulation, both drugs were equally effective but meloxicam was more palatable and easier to administer. In a prospective study[5], many cats with OA improved with NSAID intervention (61% showed a marked improvement, 14% a moderate improvement and 25% a slight improvement). With care, meloxicam can be used long-term in cats and doses as low as 0.025 mg/kg 3 to 4 times a week can markedly improve the comfort of cats with pain related to osteoarthritis. Most owners are able to titrate the dose down to that which keeps their cat comfortable; the key to successful chronic NSAID administration in cats is to use of the lowest effective dose.

Monitoring cats on long term NSAID therapy

As with dogs, cats receiving NSAIDs for chronic pain should be monitored for side-effects related to renal and hepatic function and gastrointestinal erosions. There are no accepted monitoring guidelines but a biochemistry panel, packed cell volume, and total protein measurement before initiating treatment and repeated at 1 and 4 weeks. After this, time of follow-up blood work is arbitrary and will vary in each patient. As with dogs, it is important to discuss the possible side-effects of these drugs and if vomiting or anorexia occurs, the client should stop administration and call for veterinary advice. It is also imperative to inform the owner that under no circumstances should any "over the counter" anti-inflammatory drugs such as aspirin be given in conjunction with an NSAID. Continual reassessment of the patient both by the veterinarian and owner is important so that the dose can be tapered to the smallest effective amount.

Other analgesic drugs

The role of other drugs in alleviating chronic OA related pain has not been well studied in cats. Several drugs have however been suggested. Tricyclic antidepressants including amitriptyline, clomipramine and imipramine can provide relief in humans with chronic neuropathic pain and are thought to act by altering the actions of serotonin and nor-adrenaline both centrally and peripherally. Amitriptyline (2.5 –12.5 mg/kg orally, SID) may be effective for chronic pain syndromes including osteoarthritis. The anticonvulsant gabapentin is clinically effective in some types of chronic pain in humans although the mechanism of action is not clear[14]. Based on individual case reports this drug shows promise in cats, and suggested doses starting at 10 mg/kg PO BID have been published.

Although not classified as an opioid, tramadol has weak binding affinity at mu-receptors and is thought to activate monoaminergic spinal inhibition of pain although this may not apply to non-primate species. It can be administered by multiple routes including orally and is effective for moderately severe acute and chronic pain in humans and seems remarkable devoid of the usual undesirable side-effects of opioids such as respiratory depression and tolerance. The pharmacokinetics of tramadol and its major metabolite has recently been reported for the cat[18] and it would seem that doses for cats will be less than those for dogs (on a mg/kg basis) and the dosing interval may be longer. As of yet there are no published studies reporting the efficacy of tramadol for OA pain in cats.

Non-pharmacologic treatments

Rehabilitation therapy, nutraceuticals and acupuncture are also used to treat OA in cats with varying success and many cats are more tolerant of these interventions than many people thought they would be!

References

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