Nonspecific Interstitial Pneumonia Pattern as an Atypical Pulmonary Manifestation of Polyarteritis Nodosa.

Polyarteritis nodosa (PAN) is a necrotising vasculitis that primarily affects medium-sized arteries and usually spares the lungs. Pulmonary involvement is rare and may complicate diagnosis when respiratory symptoms mimic infection or interstitial lung disease. We describe a 44-year-old woman with long-standing PAN who presented with several months of persistent dry cough and progressive shortness of breath. High-resolution computed tomography (HRCT) of the chest demonstrated a nonspecific interstitial pneumonia (NSIP) pattern. Given her long-term corticosteroid use, an atypical infection was initially suspected; however, bronchoscopy, sputum cultures, and blood studies were all negative. Despite high-dose corticosteroids, improvement was minimal, and a four-week course of co-trimoxazole led to gradual clinical improvement. Pulmonary function tests (PFTs) showed a markedly reduced diffusing capacity for carbon monoxide (DLCO) of 35% predicted, which improved by approximately 10% after five months. Other pulmonary function parameters were assessed and showed no significant interval change, with the overall pattern remaining restrictive. She had experienced a similar episode two years ago that also improved with combined antibiotic and corticosteroid therapy. Considering the recurrence, negative infectious work-up, and vasculitic background, the pulmonary abnormalities were attributed to PAN. This case highlights that, although rare, pulmonary manifestations of PAN can mimic atypical infection or interstitial lung disease. Clinicians should consider PAN-related lung disease in patients with unexplained respiratory symptoms and interstitial changes, particularly in the context of long-term immunosuppression. Multidisciplinary evaluation and careful exclusion of alternative causes are essential.