See also separate article Rheumatoid Arthritis.

Lung disease is a well-recognised and important extra-articular manifestation of rheumatoid arthritis. A study in New Zealand of patients recently diagnosed with rheumatoid arthritis found that 30% of patients reported respiratory symptoms (dyspnoea, cough, wheeze), 20% of patients had physiological evidence of airflow obstruction and 40% had reduced gas transfer. The prevalence of abnormalities found with high-resolution CT was decreased attenuation in 67%, bronchiectasis in 35%, bronchial wall thickening in 50%, ground glass opacification in 18%, and reticular changes in 12% of patients.^[1] The range of pulmonary problems associated with rheumatoid arthritis includes:

• Rheumatoid nodules:
  • The only pulmonary manifestation specific to rheumatoid arthritis.
  • They are typically benign but can lead to pleural effusion, pneumothorax, haemoptysis, secondary infection, and bronchopulmonary fistula.
• Caplan's syndrome:^[2] 
  • See also separate article Caplan's syndrome.
  • In 1953, Caplan described a characteristic radiographic pattern in coal miners with rheumatoid arthritis that was distinct from the typical progressive massive fibrosis pattern of coalworkers' pneumoconiosis.
  • CXRs show multiple well-defined rounded nodules, which are about 0.5-2 cm in diameter, distributed throughout the lungs but predominantly at the lung periphery.
• Interstitial lung disease (ILD):^[3] 
  • ILD most often occurs in middle-aged men. It is usually associated with severe arthritis and high serum levels of rheumatoid factor.
  • Presents with insidious onset of dyspneoa with occasional dry cough.
  • Associated pulmonary vasculitis may cause pulmonary hypertension.
• Bronchiolitis:
  • Bronchiolitis obliterans with organising pneumonia: clinical features include cough, shortness of breath and bilateral crackles. The vital capacity is slightly decreased, and the diffusing capacity is moderately to severely decreased. High-resolution chest CT scan shows bilateral ground-glass opacities with air bronchograms and triangular, pleura-based opacities. Steroids are the best treatment option. The prognosis is generally good with resolution and cure for most patients.^[4] 
  • Obliterative bronchiolitis: a rare, usually fatal condition. Associated with penicillamine, gold, and sulfasalazine treatment. Presents with rapid-onset dyspnoea and dry cough. Fever is uncommon.
• Bronchiectasis:^[5] 
  • See also separate article Bronchiectasis.
  • Bronchiectasis co-existing with rheumatoid arthritis differs from the other types of bronchiectasis.
  • Patients with rheumatoid arthritis and bronchiectasis have worse obstructive airways disease, increased susceptibility to recurrent lower respiratory tract infections, faster lung function decline and higher mortality compared with subjects with bronchiectasis alone.
• Arteritis:
  • Arteritis of the pulmonary artery and lung is rare; signs of systemic vasculitis are usually present.
• Infection:
  • Respiratory infections account for 15-20% of deaths in rheumatoid patients.
• Drug toxicity:
  • Acute interstitial pneumonitis may occur in 1-5% of patients treated with methotrexate (see 'Methotrexate-associated lung disease in rheumatoid arthritis', below).
  • Penicillamine and gold may also cause pulmonary complications.^[6] 
• Pleural effusions:
  • See separate article Pleural effusion.
  • Pleural effusions in rheumatoid arthritis are usually small, unilateral, and asymptomatic.^[3] 
  • Occasionally, an empyema may develop
• Lung cancer: is more common in patients with rheumatoid arthritis than in normal control subjects.
• Other diseases:
  • Rheumatoid arthritis patients can get apical fibro-bullous disease (apical fibrotic cavity lesions similar to ankylosing spondylitis).
  • Thoracic cage immobility causing restrictive lung disease.
  • Primary pulmonary hypertension (rare); secondary pulmonary hypertension (due to ILD) is more common.