OVERVIEW OF THE EROSIVE FORMS
As stated previously, two main erosive forms of IMPA exist—idiopathic and a polyarthritis that affects greyhounds.
Idiopathic erosive IMHA occurs in all dog breeds but is most frequent in smaller breeds, with an average age of onset of 2
to 6 years and no sex predilection.6,16,27-30 Early in its course, dogs present with stiffness (especially after rest), intermittent lameness, and swelling of single
or multiple joints.27-30 The carpal, tarsal, and phalangeal joints are most often affected, and bilaterally symmetric joint involvement is common.27-30 Clinical signs wax and wane over time, and lameness and stiffness may be accompanied by fever, lethargy, inappetence, and
lymphadenopathy. The disease is progressive, but the rate of progression varies.27-30 Chronic disease results in connective tissue degeneration, including the joint capsule and intra-articular ligaments, which
leads to further joint instability, causing subluxations and luxations.27-30
The treatment of erosive IMPA involves administering immunosuppressive, disease-modifying, or anti-inflammatory medications.
Overall, lifelong therapy is needed, and response to treatment and long-term prognosis are poor.
Erosive polyarthritis in greyhounds
Erosive polyarthritis in greyhounds is a sporadic disease first reported in Australia in the 1970s.6,16,30,31 It has subsequently been recognized in the United Kingdom and United States. This disease affects young greyhounds between
the ages of 3 to 30 months and has no sex predilection.6,16,30,31 Its clinical presentation is similar to that associated with idiopathic erosive polyarthritis in which distal joints are
affected, but it appears to be a more slowly progressive disease, causes a nonsuppurative synovitis, and has less severe subchondral
erosions than idiopathic erosive polyarthritis.6,16,30,31 Treatment is similar to idiopathic erosive IMPA, and response is variable.
DIAGNOSIS OF IMPA
The key in the diagnosis of IMPA is synovial fluid analysis; however, a comprehensive diagnostic evaluation should be performed
to rule out infectious causes and identify associated disease. Diagnostics should include a complete blood count (CBC), serum
chemistry profile, urinalysis, urine culture, rickettsial titers, thoracic and abdominal radiography, joint radiography, and
synovial fluid analysis with bacterial culture. Other diagnostics to consider on a case-by-case basis include an abdominal
ultrasonographic examination, an antinuclear antibody (ANA) test, a muscle biopsy, a synovial biopsy, or CSF analysis.
1. Placing a drop of synovial fluid between fingers is one way to assess synovial fluid viscosity.
Synovial fluid analysis with bacterial culture
Clinical suspicion of polyarthritis, even in the absence of obvious joint effusion, should prompt you to perform arthrocentesis.1,2,6,16,18,30 Normal synovial fluid is relatively clear and viscous and does not clot on exposure to air (Figures 1 & 2). The volume of fluid collected from normal joints is < 0.1 to 0.25 ml.32 Normal synovial fluid is relatively acellular, with a protein concentration < 2.5 g/dl and a nucleated cell count < 3,000
cells/µl.32 Mononuclear cells predominate.
2. Collect joint fluid in a purple top (EDTA) tube for cytologic analysis and a red top tube for culture and sensitivity.
In cases of erosive or nonerosive IMPA, joint fluid may be thin, turbid, and increased in volume. The protein concentration
is usually > 2.5 g/dl, and nucleated cell counts are > 3,000 cells/µl, ranging from 4,000 to 300,000 cells/µl.32 Nondegenerate neutrophils typically account for most cells, but synovial fluid with greater than 12% nondegenerate neutrophils
is still consistent with IMPA (Figures 3 & 4).5,11-13 Recent studies have shown that pleomorphic inflammation can be present with IMPA in which mononuclear cells account for 50% or more of the synovial cell count.12,13 This appears particularly true if a dog has been receiving immunosuppressive medications or has a waxing and waning form
3. Cytologic examination of synovial fluid from a dog with IMPA demonstrating large numbers of nondegenerate neutrophils.
Greater than 2 cells per high power field is elevated (500X, high power field).
The amount of synovial fluid collected from arthrocentesis is often small. If quantity limits analysis, determining cell counts
and types is most important. This can be accomplished with a microscopic evaluation of one drop of synovial fluid on a slide.
Greater than 2 cells/hpf is abnormal,5,7,8,32 and an estimate of cell count and type is enough to aid in diagnosis.
Varying amounts of synovial fluid and inflammation will be present in different joints, so sample multiple joints. Most commonly,
arthrocentesis of the carpal, tarsal, and stifle joints is recommended.5,7,8,18 One study obtained a diagnosis of IMPA most readily from arthrocentesis of bilaterally symmetrical tarsal joints.8
Bacterial infections should also be ruled out through aerobic and anaerobic bacterial culture of synovial fluid. Polyarticular
joint infections are rare and occur through hematogenous spread of organisms and have been described with omphalophlebitis
in neonates and bacterial endocarditis in mature animals.1,2,7
4. Synovial fluid from an IMPA patient demonstrating nondegenerate neutrophils (1000X, oil immersion).
Degenerate neutrophils and bacteria in synovial fluid are diagnostic for a bacterial infection. However, synovial fluid from
infected joints can also have a predominance of nondegenerate neutrophils and no obvious bacteria.32 A critical assessment of bacterial culture results with other clinical, physical, and diagnostic findings is essential,
as only 50% to 70% of bacterial joint infections will have positive bacterial culture results.32