Immune mediated polyarthritis is an auto-immune condition where the body starts attacking its joints. There are two forms- erosive or non-erosive.
Immune-mediated, nonerosive polyarthritis (IMNPA) is characterized by neutrophilic inflammation of the synovium. Antigen-antibody complexes are deposited in the synovial membrane of affected joints with resultant synovitis. This inflammation leads to weakening of intra-articular and peri-articular structures. Erosion of subchondral bone and periosteal proliferation does not occur with non-erosive polyarthritis. Ligament laxity, particularly in the carpal and tarsal joints occurs with secondary arthritis formation.
Affected dogs and cats often exhibit joint pain, joint swelling, and lameness involving more than one joint.
Intermittent stilted gait, stiffness, difficulty in rising, reluctance to walk, lameness, swelling, warmth, painful joints, lethargy, depression, anorexia, lymphadenopathy, polydipsia (increased drinking), and fever.
Predominantly, it is found in dogs, rarely in cats.
Smaller, distal joints are most commonly affected, for example the carpi (wrists), tarsi (ankles), and interphalangeal joints.
IMPA can be divided into 3 different types:
Immune-mediated, erosive polyarthritis (IMEPA) can be compared to rheumatoid arthritis in humans. This is when erosion of subchondral bone and periosteal proliferation occurs due to an antibody directed against type II collagen. This anti-body has been found along the joint surface as well as rheumatoid factors are often found within the joint fluid. The erosive forms of immune-mediated polyarthritis are uncommon compared to the non-erosive forms, and have been reported to represent only about 1 to 2% of all canine polyarthritis cases, although a recent paper reported an incidence of 16%, probably because if followed up for long enough (over a year or two) some non-erosive IMPA cases eventually become erosive.
Erosive forms of IMPA are characterized by the presence of radiographic changes consistent with subchondral bone destruction. Radiographic changes may include irregular joint surfaces, a narrowing or widening of the joint space, and punched out lesions along the joint surface. It is important to note that radiographic changes can take up to 6 months, or sometimes, even longer to appear. Therefore, dogs with apparently non-erosive forms of polyarthritis in which clinical signs persist should be periodically re-evaluated for erosive changes.
Occasionally, bloodwork can demonstrate abnormalities including hypoalbuminemia, hyperproteinemia, mild elevations of ALP, and increased canine C-reactive protein. Serum albumin was significantly lower as the number of affected joints increased.
Culture of the affected joints may be positive, but negative joint fluid cultures do not reliably rule-out septic arthritis because of the high incidence of false-negative results.
Radiographs are often normal in the early stages of disease, but in the later stages, radiographic findings can include collapse of joint spaces and osteophytosis similar to erosive polyarthritis. Thoracic and abdominal radiographs may be helpful to rule out reactive polyarthritis associated with neoplasia.
Diagnosis of IMNPA is based primarily on joint fluid cytology from multiple joints that reveals large numbers of nondegenerate neutrophils in the presence of compatible clinical signs.
Screening for heartworm disease, tick-borne diseases, and systemic fungal infections is important in geographic areas where those diseases are common.
Response to immunosuppressive therapy can be used to help with symptoms and help confirm a diagnosis.
Rehabilitation techniques for this condition focuses on reducing swelling in the joints initially. Physical modalities such as laser, ultrasound, shockwave, PEMF, and TENS to help control pain and inflammation will be utilized. Gentle prescriptive exercise programs will be helpful to maintain and promote active range of motion and strengthening to maintain or increase muscle mass. Manual therapies will also be used to improve passive and active range of motion of the affected joint and relieve compensatory taut muscles.
