Purpose of review: There is growing speculation that idiopathic nonspecific interstitial pneumonia (NSIP) is, in reality, a grouping of separate disorders with a common histologic pattern. In this review, distinct clinical, imaging, and serologic features providing support for this premise are detailed and discussed.
Recent findings: The diagnosis of idiopathic NSIP is often uncertain because of its clinical and imaging diversity. In a landmark study of inter-multidisciplinary group diagnostic variation, there were striking discrepancies between seven expert groups (κ = 0.24) in diagnoses of idiopathic NSIP. Recent histologic observations provide support for the concept of an NSIP/organizing pneumonia overlap, distinct from isolated NSIP. An important group of NSIP patients with features of 'undifferentiated connective disease', historically classified as an idiopathic NSIP subgroup, have been shown to have a lower mortality than idiopathic NSIP patients without features of autoimmune disease. The recently proposed entity of 'interstitial pneumonia with autoimmune features' includes many patients with a histologic or imaging pattern of NSIP, shown by Oldham and colleagues to have a similar survival to patients with connective tissue disease-related NSIP.
Summary: The concept of idiopathic NSIP as a grouping of separate disorders with a common histologic pattern provides a template for potentially important pathogenetic insights.