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Pulmonary capillaritis is generally secondary to a systemic autoimmune process. Little is known regarding other causes of pulmonary capillaritis, particularly isolated pulmonary capillaritis (IPC). IPC is defined as pulmonary capillaritis in the absence of evidence of a systemic disease. We aim to describe the aetiology of biopsy-proven pulmonary capillaritis to add to the existing knowledge on aetiologies of pulmonary capillaritis and provide a more detailed description of IPC cases.A retrospective cohort of biopsy-proven pulmonary capillaritis was conducted at the Mayo Clinic, Rochester over a 17-year period. Histologic slides were re-reviewed by a lung pathologist. Medical records were reviewed to identify a possible aetiology of the pulmonary capillaritis. A subset analysis of patients identified with IPC was then performed.Thirty-six cases of biopsy-proven pulmonary capillaritis were identified. The majority of cases were secondary to a systemic autoimmune disease, most commonly granulomatosis with polyangiitis. There were four cases of IPC in this cohort. Median follow-up was 116.5 months with no evidence of systemic disease development. No risk factors for IPC were identified. All patients presented sub-acutely with haemoptysis and diffuse alveolar haemorrhage with a delay in diagnosis. After initiation of immunosuppression, most patients obtained remission with a benign clinical course.Pulmonary capillaritis is most commonly secondary to systemic autoimmune disorders, predominantly ANCA-associated vasculitis. IPC is a rare form of pulmonary capillaritis with very few cases described in the literature, the availability of effective treatment makes this rare disease important to recognize.This study aims to describe the aetiology of biopsy-proven pulmonary capillaritis. The results support past studies with granulomatosis with polyangiitis being the most common aetiology. An in-depth examination of isolated pulmonary capillaritis demonstrates that these patients have a good prognosis with therapy, making this rare disease important to recognize.