Fibrotic hypersensitivity pneumonitis

Fibrotic hypersensitivity pneumonitis (fibrotic HP), also known as chronic hypersensitivity pneumonitis, is a form of interstitial lung disease that results from a sustained or repeated immune-mediated inflammatory response to inhaled environmental antigens (such as mold spores, bird proteins, or certain chemicals) in susceptible individuals. Over time, this chronic inflammation leads to progressive and irreversible fibrosis (scarring) of the lung tissue. The fibrotic changes distinguish this condition from non-fibrotic (inflammatory) hypersensitivity pneumonitis, which may be partially or fully reversible. The disease primarily affects the lungs and respiratory system. Key clinical features include progressive exertional dyspnea (shortness of breath with activity), chronic dry cough, fatigue, and reduced exercise tolerance. As fibrosis advances, patients may develop clubbing of the fingers, crackles on lung auscultation, and progressive decline in pulmonary function. High-resolution computed tomography (HRCT) of the chest typically shows a combination of fibrotic changes (reticulation, traction bronchiectasis, honeycombing) along with features suggestive of hypersensitivity pneumonitis such as mosaic attenuation and air trapping. Lung biopsy, when performed, may show a pattern of fibrosis with features of chronic inflammation and poorly formed granulomas. Management of fibrotic hypersensitivity pneumonitis centers on identification and strict avoidance of the causative antigen, which is critical to slowing disease progression, though fibrosis is generally irreversible. Immunosuppressive therapies such as corticosteroids and mycophenolate mofetil have been used, particularly when there is an active inflammatory component. More recently, antifibrotic medications (nintedanib) have been studied and shown to slow the rate of lung function decline in progressive fibrosing interstitial lung diseases, including fibrotic HP. Lung transplantation may be considered for patients with advanced disease and respiratory failure. Prognosis is variable but generally worse than non-fibrotic HP, with some patients experiencing a progressive decline similar to idiopathic pulmonary fibrosis.

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