Immune Mediated Polyarthritis in Dogs (IMPA)

IMPA is defined as an accumulation of immune complexes that attract white blood cells into the synovial (inner joint space) space causing pain and inflammation. 

Dogs with IMPA often present with an abnormal gait or lameness on one or more limbs. 

To diagnose this the veterinarian must determine if the underlying problem involves the muscles, the nervous system, the long bones of the limbs, soft tissue or joints. This will include a thorough patient history and examination, x-rays of the affected limbs and arthrocentesis with the joint fluid sent to an outside laboratory for analysis. One of the challenges in diagnosing IMPA is the variety of other possible causes of limping. The most common signs noted are reluctance to walk, a change in gait, lameness and multiple painful swollen joints.  Dogs with IMPA are often described as “walking on eggshells”.  Lameness may vary from joint to joint and even spinal pain may be noticed. Other signs may include, fever, lethargy, inappetance, vomiting or diarrhea.

Canine IMPA has been classified into 4 types; idiopathic (of unknown cause or origin), secondary to non- joint infections, secondary to gastrointestinal disease or secondary to cancers. These can be simplified into primary IMPA and secondary (also known as reactive) IMPA.  Idiopathic IMPA  would be considered a primary cause whereas the others are considered secondary as the immune complexes that infiltrate the joint are from outside of the joint itself. Due to these causes outside of the joint IMPA can be difficult to diagnose. 

Most cases of IMPA are treated with immunosuppressive medications, however, these medications may be contraindicated for other diseases that present with similar symptoms or even unnecessary for some underlying causes. One of the main focuses of treatment of IMPA is pain control, this may be achieved with either non-steroidal antiinflammatories (NSAIDS), gabapentin, tramadol, or steroids. It is important to note that steroids and NSAIDS cannot be used together due to the incidence of gastric ulceration.  In cases where pain is severe and not controlled by other measures a narcotic pain patch may be used to provide analgesia. Antibiotics may be used in cases suspected of being caused by bacterial infections. If primary IMPA has been ruled out and a secondary cause identified, treating that condition may cause the IMPA to resolve.