Immune Mediated Disease
When we hear the term arthritis, we usually think about a chronic, degenerative joint condition that results from injury or normal wear and tear of the affected joints. However, immune mediated polyarthritis (IMPA) refers to a very different condition that can be much more dramatic in its presentation.
IMPA refers to an active inflammatory process that usually involves multiple joints. In most patients, a specific underlying cause for the inflammation cannot be identified. Under such circumstances, the condition is presumed to be due to abnormal activation of the immune system directed at tissues within the joints. Nevertheless, it is important to look for possible underlying issues that might need to be treated specifically to achieve a positive outcome.
Initial symptoms can be frustratingly non-specific, with signs like general malaise and reduced physical activity. In many dogs with IMPA, fever is present, but in some, it is not. With time, signs that help to localize the problem to the joints become more apparent. These include difficulty walking or lameness, joint swelling, and joint pain. Many dogs will have other signs of illness not related to their joints, including vomiting and decreased appetite. If joint swelling and pain are present on initial exam, it is easier to know to look to the joints for more information, but in some pets, the joint signs are not as evident, and without some index of suspicion for the disorder, the diagnosis can be delayed.
In dogs with clinical signs and physical exam findings compatible with the possibility of IMPA, it is important to consider a range of diagnostic tests, not just those aimed at assessing the joints. These include a complete blood count, a biochemistry profile, and a urinalysis to screen for evidence of multi-system illnesses that can accompany IMPA. Screening for infectious diseases, including tick- and other vector-borne diseases, is also very important. If signs like vomiting are present, imaging of the abdomen with X-rays and even ultrasound should be considered. Chest X-rays should be taken in older dogs and in dogs with respiratory symptoms, and echocardiography should be considered in dogs with newly identified heart murmurs and evidence of joint disease to look for evidence of endocarditis.
When focusing on the joints, the main diagnostic tools are X-rays and sedation or anesthesia for joint fluid sample collection and analysis. X-rays are taken to look for erosive changes in the joints. If these are present, the outlook for a strong recovery is more guarded. If joints are sampled for collection of joint fluid, at least two joints should be tested, and fluid should be submitted both for microscopic examination and for bacterial culture and susceptibility testing.
If the diagnostic tests support the possibility of IMPA, treatment usually centers on the administration of medications to suppress the inflammation due to abnormal immune system activity. Relatively high doses of steroids like prednisone are usually the first line of treatment. In many dogs, the initial response to treatment is quick and dramatic, and this can help support the diagnosis as much as any test can, since there is no diagnostic test that can prove the diagnosis beyond all doubt. Many dogs will also be treated with a second immunosuppressive medication to try to decrease reliance on prednisone alone, and to try to lessen side effects that are often seen with the high doses of prednisone needed for successful treatment.
Over time, assuming a good response to treatment, medication doses are slowly tapered to the lowest dose that seems to keep signs in check without creating unacceptable side effects. Some dogs are able to come off all treatment without relapsing, but other require long-term and some times indefinite treatment to keep their signs under control. Despite its often-dramatic onset and the severity of the problems it can cause, most dogs with IMPA improve with treatment. Some are not able to return to how they were prior to treatment, and require modification of exercise and activity due to irreversible damage from the disease. A few will experience so much joint damage that surgical procedures to fuse the affected joints may need to be considered for managing joint pain and instability. With earlier diagnosis and aggressive treatment, surgical outcome can usually be avoided.