Update on Rare Idiopathic Interstitial Pneumonias and Rare Histologic Patterns

Context.-In 2013, the revised American Thoracic Society and European Respiratory Society classification of idiopathic interstitial pneumonias (IIPs) described 2 rare IIPs and 2 rare histologic patterns. Because of the rarity of the disease, there is limited evidence related to the histology. Because the rare histologic patterns are provisional criteria, no unanimous consensus on histologic diagnostic criteria has yet been reached. Objective.-To review the histologic features for rare IIPs and rare histologic patterns, and to provide diagnostic aids and discuss the differential diagnosis. Data Sources.-Published peer-reviewed literature and the authors' personal experience. Conclusions.-Following the publication of the international consensus classification, evidence regarding rare IIPs and rare histologic patterns has accumulated to some extent, although to date the amount remains insufficient and further evidence is required. Because the diagnosis is sometimes challenging, a multidisciplinary approach represents the gold standard in reaching an accurate diagnosis for these rare disorders.