A 38-year-old woman with a 3-year history of rheumatoid arthritis on methotrexate 15 mg weekly presents with new-onset dyspnea on exertion, dry cough, and bilateral basal crackles on auscultation. Chest X-ray shows bilateral interstitial infiltrates. High-resolution CT chest confirms usual interstitial pneumonia (UIP) pattern. Pulmonary function tests show FVC 62% predicted and DLCO 48% predicted. She denies fever or hemoptysis. What is the most likely diagnosis and what is the most appropriate next step?

Explanation

## Clinical Diagnosis and Differential **Key Point:** This patient has RA-associated interstitial lung disease (RA-ILD), not methotrexate-induced pulmonary toxicity. The UIP pattern on HRCT is the classic histological signature of RA-ILD, which occurs in 5–10% of RA patients and is a major cause of morbidity and mortality in RA [cite:Harrison 21e Ch 297]. ## Distinguishing RA-ILD from Methotrexate Toxicity | Feature | RA-ILD | Methotrexate Toxicity | |---------|--------|----------------------| | **Onset** | Insidious; may precede RA or occur years into disease | Usually within 6–12 weeks of initiation or dose increase | | **HRCT pattern** | UIP (usual interstitial pneumonia), NSIP, organizing pneumonia | Hypersensitivity pneumonitis, organizing pneumonia | | **Symptoms** | Progressive dyspnea, dry cough, no fever | Acute dyspnea, fever, malaise | | **Methotrexate status** | Often present in seronegative or seropositive RA | Temporal relationship to MTX dose | | **Prognosis** | Poor; median survival 5–10 years if UIP | Better if MTX is stopped early | **High-Yield:** UIP pattern on HRCT is pathognomonic for RA-ILD, not methotrexate toxicity. The patient's 3-year disease duration and insidious presentation favor RA-ILD over drug toxicity. ## Management of RA-ILD **Clinical Pearl:** The mainstay of treatment is immunosuppression combined with antifibrotic therapy: 1. **Continue methotrexate** — paradoxically, MTX may slow RA-ILD progression and should not be automatically stopped unless there is acute hypersensitivity reaction (fever, acute dyspnea within days of dose increase). 2. **Start prednisolone** — typically 0.5–1 mg/kg daily, then taper based on response. 3. **Add antifibrotic agent** — pirfenidone or nintedanib (based on recent trials showing slowing of FVC decline in RA-ILD) [cite:Harrison 21e Ch 297]. 4. **Consider mycophenolate mofetil (MMF)** — alternative or adjunctive immunosuppressant if corticosteroid-sparing is needed. **Mnemonic:** **RAPID-ILD** — Recognize RA-ILD, Add antifibrotic + prednisolone, Persist with methotrexate, Immunosuppress aggressively, Delay progression. ## Why Discontinuing Methotrexate Is Wrong Methotrexate-induced pulmonary toxicity is rare (0.3–1% of RA patients) and typically presents acutely (within weeks of dose increase) with fever and hypersensitivity features. This patient's insidious 3-year disease course and UIP pattern are inconsistent with MTX toxicity. Stopping MTX would deprive the patient of its disease-modifying benefit in RA and may worsen systemic inflammation.