Secondary interstitial lung disease specific to adulthood associated with a systemic disease

Secondary interstitial lung disease (ILD) specific to adulthood associated with a systemic disease is a group classification used by Orphanet (ORPHA:264745) to categorize forms of interstitial lung disease that arise as a secondary manifestation of an underlying systemic condition in adult patients. Systemic diseases that can lead to secondary ILD include connective tissue disorders (such as rheumatoid arthritis, systemic sclerosis, systemic lupus erythematosus, and dermatomyositis/polymyositis), sarcoidosis, vasculitides, and other autoimmune or inflammatory conditions. In these cases, the lung parenchyma becomes affected by inflammation and/or fibrosis as part of the broader systemic disease process, rather than as a primary isolated lung disorder. The primary body system affected is the respiratory system, with progressive damage to the lung interstitium — the tissue and space surrounding the air sacs (alveoli). Key symptoms typically include progressive dyspnea (shortness of breath), chronic dry cough, reduced exercise tolerance, and fatigue. Physical examination may reveal bibasilar inspiratory crackles ("velcro crackles") and, in advanced cases, digital clubbing and signs of pulmonary hypertension. Pulmonary function testing commonly shows a restrictive pattern with reduced diffusing capacity for carbon monoxide (DLCO). High-resolution computed tomography (HRCT) of the chest is essential for characterizing the pattern of lung involvement, which may include usual interstitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP), organizing pneumonia, or lymphocytic interstitial pneumonia patterns, depending on the underlying systemic disease. Treatment is directed at the underlying systemic disease and typically involves immunosuppressive or immunomodulatory therapies such as corticosteroids, mycophenolate mofetil, azathioprine, cyclophosphamide, or rituximab. Antifibrotic agents (nintedanib, pirfenidone) have shown benefit in progressive fibrosing ILD regardless of the underlying cause and are increasingly used. Supportive care includes supplemental oxygen, pulmonary rehabilitation, and management of comorbidities. In severe, refractory cases, lung transplantation may be considered. Prognosis varies widely depending on the specific systemic disease, the pattern of lung involvement, and the response to treatment.

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