Osteoarthritis (OA) is a progressive, degenerative disease of the joints. Although dramatically under-recognized, it is one of the most common chronic diseases of cats. It is estimated that more than 90% of cats over ten years of age have radiographic evidence of OA.
What causes osteoarthritis?
The contributing causes of OA are many and varied. Some genetic abnormalities can prevent joints from developing normally. OA can also result from a traumatic injury to a joint (e.g., in a cat that has fallen out of a tree or been hit by a car).
Cats who are overweight or obese have an increased risk of OA. The joints of an overweight or obese cat are subjected to repetitive overloading, which over time, contributes to the initiation and progression of OA and its degenerative, painful processes.
"Cats who are overweight or obese have an increased risk of OA."
What can I do to help my cat with OA?
Your veterinarian is the best source for a comprehensive OA management plan. If your cat is overweight or obese, that is where to start, but there is much more that can be done to help support the joints of a cat with OA. In addition to diet modifications, exercise, weight loss, nonsteroidal anti-inflammatory drugs (NSAIDs), and anti-nerve growth factor antibody treatment (Solensia®), management strategies for OA may include a disease-modifying osteoarthritis drug (DMOAD).
What is a DMOAD?
A DMOAD is a medication that can slow or alter the progression of OA. One of the DMOADs commonly used in cats with OA is an injectable medication containing the active ingredient polysulfated glycosaminoglycan, or PSGAG (Adequan® and Cartrophen®).
PSGAG has several effects that help manage OA. Once injected, the PSGAG is distributed into joint fluid and cartilage. Although the exact mechanism of action is not entirely understood, PSGAG inhibits the enzymes contributing to cartilage destruction, thus slowing cartilage breakdown in OA joints. By blocking cartilage destruction, PSGAG helps decrease inflammation, a significant source of pain in an OA patient. PSGAG also contributes to cartilage healing by providing the body with the building blocks of cartilage. Finally, this medication improves the consistency of joint fluid, providing better joint lubrication, improving joint mobility, and increasing comfort in cats with OA.
"A DMOAD is a medication that can slow or alter the progression of OA."
Is PSGAG safe for cats, and if so, how is it given?
The U.S. Food and Drug Administration (FDA) approval for PSGAG specifies 12 injections given into a muscle over eight weeks in dogs with OA; however, PSGAG has also been shown to be safe for cats. In addition, studies have shown that injecting under the skin results in the same distribution of PSGAG to the joints as injections into a muscle. These subcutaneous injections are much less painful for cats and much easier to give. Most veterinarians teach their clients to give PSGAG injections under the skin at home (just like giving insulin but much less frequently).
Although the original FDA label for PSGAG in dogs specifies an eight-week treatment period, OA is unfortunately never cured; it is only managed. For this reason, most veterinarians recognize the need for ongoing joint support in cats with OA. The use of PSGAG for ongoing support in cats is off-label. Many drugs are commonly prescribed for off-label use in veterinary medicine. Your veterinarian will carefully weigh the pros and cons of ongoing treatment and will discuss the treatment strategy for your cat with you.
Are there any side effects of PSGAG to watch for?
Mild, short-lived diarrhea has been reported with PSGAG use at the recommended dose. PSGAG is not recommended for use in cats with bleeding disorders. Vomiting, lethargy, and loss of appetite have also been reported but are not common.
How long after starting PSGAG will I notice an improvement in my cat?
Because PSGAGs are but one part of a comprehensive approach to OA, most cats will improve quickly. That said, the effects of PSGAG appear to be cumulative, so optimal results can be expected by the eighth week, when the frequency of injections drops to monthly.
"...the effects of PSGAG appear to be cumulative, so optimal results can be expected by the eighth week..."
How long will my cat need PSGAG injections?
OA is a progressive lifetime disease. Fortunately, PSGAG is generally considered beneficial over the long term. Because the risk of adverse side effects is relatively low, long-term use of PSGAG is generally well-tolerated.
Cats with OA present a unique challenge to veterinarians because of the small number of medications available for them. In many cases, a cat's OA management can be fine-tuned so that ongoing PSGAG dosing allows for a decreased dose or even withdrawal of pain medications (e.g., NSAIDs) that have a higher risk of side effects.