Immune-mediated polyarthritis (IMPA) represents a group of diseases that cause marked joint pathology and systemic illness.
IMPA is defined as an inflammatory process that affects the synovium of two or more joints, has no identifiable infectious
component, and is responsive to immunosuppressive therapy.1-3 Early recognition, diagnosis, and treatment are essential in reducing morbidity and mortality associated with the disease.
Classifying IMPA will help you determine treatment and prognosis and is based on clinical, radiographic, pathologic, and serologic
findings (Table 1).1,4,5
IMPA is categorized as erosive or nonerosive (Table 1). Nonerosive IMPA demonstrates no radiographic signs of bone or cartilage destruction.1,4,5 Erosive IMPA demonstrates radiographic evidence of cartilage and subchondral bone destruction in one or more joints.1,4,5 Erosive forms are rare and account for less than 1% of all reported IMPA cases.5-7 (See boxed text "The pathophysiology of IMPA".)
The pathophysiology of IMPA
OVERVIEW OF THE NONEROSIVE FORMS
Although several retrospective studies have suggested the susceptibility of certain breeds and sizes of dogs to nonerosive
IMPA, inconsistencies among reports exist.7-9 Thus, nonerosive IMPA may occur in any breed or size of dog. It occurs at a mean age of 4 to 6 years with no definite sex
Most dogs present with a stiff, stilted, or "walking on eggs" gait; lameness; reluctance or inability to stand; and joint
pain and effusion.1,2,4,5,7-9 Commonly affected joints include the carpus, tarsus, stifle, and elbow; bilaterally symmetric joint involvement is frequent.5,7-9 Spinal pain may also occur because of intervertebral joint inflammation.10
Systemic signs include anorexia, weight loss, fever, lethargy, and lymphadenopathy.1,2,4,5,7-9 Up to 25% of dogs present with subtle or no signs of gait abnormality, lameness, joint effusion, or joint pain but have
systemic signs of illness.5,11-13 IMPA should be considered as an underlying cause of fever of unknown origin; it accounted for 20% of dogs presenting with
a fever of unknown origin in one study population.14