A 62-year-old man presents with 6 months of progressive dyspnea and dry cough. He has bilateral submandibular gland enlargement, mild jaundice from autoimmune pancreatitis, and retroperitoneal fibrosis on imaging. Serum IgG4 is markedly elevated at 850 mg/dL. HRCT chest shows interlobular septal thickening, peribronchovascular nodules, and ground-glass opacities. Lung biopsy confirms IgG4-related disease with dense lymphoplasmacytic infiltrate, storiform fibrosis, and obliterative phlebitis. Pulmonary function testing reveals the pattern marked **B** in the diagram. Which of the following best explains the physiologic basis of this PFT pattern in IgG4-related interstitial lung disease?
A. Increased FEV1/FVC ratio with normal TLC and reduced DLCO due to small airway obstruction
B. Normal FVC and TLC with isolated reduction in DLCO due to alveolar-capillary membrane thickening alone
C. Reduced FVC and TLC with preserved FEV1/FVC ratio and reduced DLCO due to parenchymal infiltration and fibrosis
D. Reduced FVC with elevated FEV1/FVC ratio and normal DLCO due to chest wall restriction
Explanation
Why option 1 is correct
The restrictive pattern marked B (reduced FVC 68%, reduced TLC 70%, preserved FEV1/FVC ratio 82%, reduced DLCO 55%) is the hallmark pulmonary function signature of IgG4-related interstitial lung disease. The dense lymphoplasmacytic infiltrate, storiform fibrosis, and obliterative phlebitis documented on lung biopsy cause parenchymal involvement that reduces lung volumes (FVC and TLC) while preserving the FEV1/FVC ratio (because both FEV1 and FVC fall proportionally). The reduced DLCO reflects the interstitial inflammation and fibrosis affecting the alveolar-capillary interface. This restrictive pattern with reduced DLCO is typical of IgG4-RD pulmonary involvement (Inoue classification, alveolar-interstitial pattern). [Stone et al., NEJM 2012; Inoue 2009]
Why each distractor is wrong
Option 2: An increased FEV1/FVC ratio is characteristic of obstructive disease (pattern A), not the restrictive pattern marked B. IgG4-RD causes parenchymal infiltration and fibrosis, not airway obstruction. Small airway disease would not produce the preserved FEV1/FVC ratio seen here.
Option 3: Pattern D (isolated DLCO reduction with normal lung volumes) does not match this patient's PFT results. The patient has reduced FVC and TLC, not normal lung volumes. Isolated DLCO reduction without volume restriction is not typical of IgG4-RD interstitial disease.
Option 4: Chest wall restriction would not produce the pathologic findings of IgG4-RD (lymphoplasmacytic infiltrate, storiform fibrosis, obliterative phlebitis in lung tissue). Furthermore, chest wall disease typically does not reduce DLCO; the reduced DLCO here reflects parenchymal pathology.
High-YieldNEET PG
IgG4-related interstitial lung disease presents with a restrictive PFT pattern (reduced FVC, TLC, preserved FEV1/FVC) and reduced DLCO due to parenchymal lymphoplasmacytic infiltration and fibrosis.
[Stone et al., NEJM 2012, IgG4-Related Disease; Inoue 2009]
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