Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (11 page)

   Patients with IBM generally have a more insidious onset compared to PM and DM, and more prominent distal muscle weakness.

   PM and DM can occur at any age, with peak incidence between the ages of 40 and 50, whereas IBM mainly affects individuals >50 years of age.

   Laboratory Findings

Diagnosis of the three conditions is based on clinical manifestations, serum muscle enzymes, autoantibodies, EMG findings, and muscle biopsy. The latter is the definitive test for establishing the diagnosis of IBM, and in PM or DM patients presenting with atypical clinical or laboratory findings.

   Muscle enzymes:

   Levels of the muscle enzyme creatine kinase (CK) are greatly elevated in PM and DM patients (typically >10 folds the upper limit of normal but may be >50 folds), and to a lesser extent in patients with IBM.

   Other muscle enzymes including lactate dehydrogenase (LDH), aldolase, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are also elevated. Similar to CK, elevations are less profound in IBM than in PM or DM.

   ANA test is positive in up to 80% of patients with DM or PM.

   Myositis-specific antibodies are positive in 30% of patients with PM or DM. The most common antibodies are those against histidyl-tRNA synthetase (anti-Jo-1), and titers of these antibodies have been found to correlate with disease activity. Other myositis-specific antibodies include anti-Mi-2 and anti–signal recognition particle (anti-SRP) antibodies. Presence of other connective disease conditions associated with myositis is suggested when another type of autoantibodies is positive (e.g., anti-SSA/Ro, anti-SSB/La, anti-Sm, or anti-RNP).

   ESR is normal or mildly increased.

   Myoglobin is elevated in the serum and urine.

Suggested Reading

Mammen AL. Dermatomyositis and polymyositis: clinical presentation, autoantibodies, and pathogenesis.
Ann N Y Acad Sci.
2010;1184:134–153.

PSORIATIC ARTHRITIS

   Definition

   Psoriatic arthritis (PsA) is a type of arthritic inflammation that occurs in approximately 15% of patients with psoriasis. It can affect any joint in the body causing pain, swelling, and stiffness. CD8
⁺
T cells and T-cell-derived cytokines play a central role in the pathogenesis of PsA.