Biopsy-proven pulmonary capillaritis: A retrospective study of aetiologies including an in-depth look at isolated pulmonary capillaritis

Background and objective 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. Methods 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. Results 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. Conclusion 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.