ANNA-LUISE A. KATZENSTEIN and JEFFREY L. MYERS
Idiopathic pulmonary fibrosis (IPF), also referred to as cryptogenic fibrosing alveolitis, is a progressive interstitial lung disease of unknown etiology (1, 2, 3). Although the pathologic features of inflammation and fibrosis are widely appreciated, other diagnostically relevant findings are generally poorly understood. There is a belief in some circles, in fact, that the pathologic features of IPF are nonspecific, and that the diagnosis is made only by excluding other histologically specific entities (4). The clinical features of IPF are variable. Although the disease occurs most often in middle-aged adults, there is a wide age range, with cases reported even in young children and infants. In most patients the presentation is insidious, with a progressive course spanning several years, but cases with acute onset and fulminant course have also been described. A favorable response to corticosteroid or cytotoxic therapy occurs in one-third or less of patients. To date there has been no satisfactory explanation for the differences in age at onset, presentation, clinical course, and response to therapy of some patients with IPF. This review provides evidence that four distinct forms of interstitial pneumonia are included in the category of IPF. Failure to recognize these different entities may explain the clinical diversity observed among patients with this condition. This pathologic classification of IPF should also aid in assessing prognosis in and choosing therapy for IPF, as well as in enhancing our understanding of its pathogenesis.