Histopathology of idiopathic diffuse interstitial lung diseases

The diagnosis of interstitial disease requires a multidisciplinary approach and interaction between clinicians, radiologists, and pathologists. Pathologists must identify a model or pattern of histologic lesions that suggests a given interstitial disease, without prejudice to its idiopathic nature. Usual interstitial pneumonia (UIP), the designation used by pathologists for the clinical equivalent, idiopathic pulmonary fibrosis, is characterized histologically by fibrosis with a temporal and spatial heterogeneity, by the presence of fibroblastic foci, and by patches of subpleural and paraseptal honeycomb remodeling. Nonspecific interstitial pneumonia (NSIP) is the principal differential diagnosis for UIP, but contrary to UIP, has a good prognosis. The histologic appearance of NSIP is non-specific, and collagenoses, drug toxicity, immunologic lung diseases, and some infectious processes can present in this form. The diagnosis of NSIP is therefore a provisional diagnosis proposed by pathologists, while awaiting additional clinical and laboratory investigations to identify the cause; it is considered idiopathic only by default.