NSIP
(Non-Specific Interstitial Pneumonia)

a spectrum of non-fibrotic and fibrotic disease, often due to SARD or drug toxicity, characterized histologically by homogenous, uniform thickening of alveolar walls by interstitial inflammation and/or fibrosis

What does it look like?

HRCT Findings

  • Non-fibrotic NSIP: Lower lobe predominant, bilateral, symmetric ground glass, with subpleural sparing in up to 50% of cases, but may be absent.

  • Fibrotic NSIP: Lower lobe predominant, bilateral and irregular reticulation and traction bronchiectasis; ground glass may be present and can represent a superimposed cellular component. Honeycombing is rare and should be minimal if present. Subpleural sparing is helpful but may be absent.

Histopathological Findings

  • Cellular NSIP: lymphocytic infiltrate of alveolar walls without fibrosis or granulomas

  • Fibrotic NSIP: diffuse thickening of alveolar walls by fibrosis with traction bronchiolectasis; bronchiolocentricity, honeycombing, and granulomas should be absent

What diseases can produce it?

  • Systemic and autoimmune rheumatic disease

  • Drug toxicity

  • Hypersensitivity pneumonitis

  • Alveolar macrophage pneumonia

  • Cryptogenic or secondary organizing pneumonia

  • Idiopathic NSIP (rare)

How confidently can we recognize it?

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  • confident if CTD or high-risk culprut drug

  • In other cases, due to difficulty distinguishing from UIP, BIP, AMP —> consider tissue sampling

How does it inform clinical reasoning?

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  • Fibrotic NSIP is the most common ILD pattern in SARD-ILD

  • When NSIP occurs alongside radiographic or histologic OP, SARD is especially likely

  • behavior heterogenous - cellular + IS can improve; fibrotic NSIP may progress anyway

finecrackles

  • extensive without honeycombing;

  • The chronic, bilateral ground glass of NSIP is typically quite symmetrical. Significant asymmetry should prompt consideration of other etiologies, especially invasive mucinous adenocarcinoma, which can mimic the appearance of inflammation or pneumonia on CT Chest.

  • ***

References

  • Ryerson CJ, Adegunsoye A, Piciucchi S, Hariri LP, Khor YH, Wijsenbeek MS, Wells AU, Sharma A, Cooper WA, Antoniou K, Borie R. Update of the international multidisciplinary classification of the interstitial pneumonias: an ERS/ATS statement. European Respiratory Journal. 2025 Dec;66(6):2500158.

  • Elicker BM, Webb WR. Fundamentals of High-Resolution Lung CT: Common Findings, Common Patterns, Common Diseases, and Differential Diagnosis: Common Findings, Common Patters, Common Diseases and Differential Diagnosis. Lippincott Williams & Wilkins; 2012 Dec 29.