Mixed connective tissue disease
Summary
Mixed connective tissue disease (MCTD) is an overlap autoimmune disorder featuring clinical features of SLE, systemic sclerosis, and polymyositis, characterized serologically by high-titer anti-U1 ribonucleoprotein (anti-RNP) antibodies. It predominantly affects women and commonly presents with Raynaud phenomenon, puffy/edematous hands, arthralgias/arthritis, and myositis. Pulmonary involvement (interstitial lung disease, pulmonary hypertension) is a major cause of morbidity and mortality.
Detail
MCTD is considered a distinct overlap syndrome combining features of systemic lupus erythematosus, systemic sclerosis (scleroderma), and polymyositis/dermatomyositis, unified by the presence of high titers of antibodies against U1-ribonucleoprotein (anti-U1RNP), typically in the absence of anti-Smith or anti-dsDNA antibodies (which helps distinguish it from classic SLE). Pathophysiology involves immune dysregulation with autoantibody production, immune complex deposition, and vasculopathy, though the exact mechanism remains incompletely understood.
Clinical features: Raynaud phenomenon is often the earliest and most common manifestation, frequently preceding other symptoms by years. Patients develop 'puffy hands' (diffuse edema progressing to sclerodactyly), polyarthralgia/arthritis (often non-erosive), myositis with proximal muscle weakness and elevated CK, and esophageal dysmotility similar to scleroderma. Pulmonary manifestations are critical to monitor—interstitial lung disease and pulmonary arterial hypertension are leading causes of death in MCTD and require regular screening with PFTs and echocardiography. Renal disease is less common and typically milder than in SLE, though can occur. Serositis, lymphadenopathy, and mild cytopenias may be seen.
Diagnosis requires clinical overlap features plus high-titer anti-U1RNP antibodies (per Alarcon-Segovia or Kahn criteria), with speckled pattern ANA. Some patients may evolve over time into a more defined connective tissue disease (usually systemic sclerosis or SLE).
Management is tailored to the predominant organ involvement: NSAIDs/hydroxychloroquine for mild arthritis, corticosteroids and immunosuppressants (e.g., methotrexate, azathioprine, mycophenolate) for myositis or more severe disease, calcium channel blockers for Raynaud phenomenon, and pulmonary vasodilators (e.g., endothelin receptor antagonists, PDE-5 inhibitors) for pulmonary hypertension. Regular cardiopulmonary monitoring is essential given the high morbidity from pulmonary complications.
High-yield boards point: Anti-U1RNP antibody is the serologic hallmark; think MCTD when a patient has overlapping features of lupus, scleroderma, and myositis.
Sources
- First Aid for the USMLE Step 1
- Kasper DL et al., Harrison's Principles of Internal Medicine
- Kumar V et al., Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Mixed connective tissue disease
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