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Exogenous lipoid pneumonia
Most patients with exogenous lipoid pneumonia are elderly, in the late sixth or seventh decade of life.1 The condition, however, also occurs in infants and has been reported in mentally retarded persons.2 Predisposing factors include an impaired swallowing mechanism secondary to neurologic and esophageal disorders2,3 and the habitual use of different kinds of oils—mineral, animal, or, less common, the vegetable oils found in laxatives, mouth spray, nasal drops, oral lubricants, traditional folk remedies, or insecticides. Lipoid pneumonia is also an occupational hazard of fire eaters.4 The offending agent enters the lungs through inhalation or aspiration or during procedures in which iodized vegetable oils are used as a radiopaque medium, such as bronchography, lymphangiography, myelography, hysterosalpingography, or uterography.
Although vegetable oils are mostly expectorated, residual elements are emulsified and can give rise to exogenous lipoid pneumonia. Mineral oil, which is composed of liquid petroleum or paraffin, is a mixture of long-chain saturated hydrocarbons. It is an irritant to lung tissue and leads to the development of a diffuse parenchymal reaction or localized masses known as paraffinomas.5,6 Animal fats are the most noxious of these substances to lung tissue. They are hydrolyzed by lipase in the pulmonary cells and broken down into free fatty acids, which causes a severe inflammatory reaction and tissue necrosis.5 This process has been described in infants and children with exogenous lipoid pneumonia secondary to the forced feeding of animal fat (Ghee), a cultural practice in Saudi Arabia.
Once oil is introduced into the lung, the pathologic process that leads to the development of exogenous lipoid pneumonia occurs in 3 stages. The early stage consists of a toxic perturbation of capillary endothelium with secondary exudation of plasma into alveoli. In the second stage, alveolar and interstitial macrophages become activated and then phagocytose and degrade the oil. In the late stage, granulomatous and fibrointerstitial reactions develop. These stages vary in severity depending on the quantity and quality of oil present. With animal fat inhalation, for example, the high lipid content suppresses phagocytosis, leaving the pulmonary lymphocytes as the main cells responsible for the removal of the inhaled fat. In such cases, fat-laden macrophages and prominent pleural lymphocytes are seen on histologic examination, which could easily lead to the mistaken diagnosis of lymphatic carcinomatosis.
Endogenous lipoid pneumonia
Endogenous lipoid pneumonia, also called cholesterol pneumonitis, is caused by the accumulation of intrinsic lipids in the lungs. It is usually seen in patients with chronic bronchial obstruction secondary to tumors or foreign bodies. Other predisposing conditions include fat embolism, pulmonary alveolar proteinosis, lipid storage diseases, such as Niemann-Pick disease and Gaucher's disease, and disseminated lipogranulomatosis.
Endogenous lipoid pneumonia has also been found in patients with Hodgkin's disease.8 A combination of endogenous and exogenous lipoid pneumonia and pulmonary alveolar proteinosis reportedly occurred in a 5-year-old child with severe neurodevelopmental disease.2 Endogenous lipoid pneumonia has also been associated with lung cancer. In 1 study, endogenous lipoid pneumonia was diagnosed in the resected lungs of 33 of 147 patients with lung cancer.9 It was found in 16 of 89 (18%) patients with adenocarcinoma and in 17 of 55 (31%) patients with squamous cell carcinoma. In 23 cases, endogenous lipoid pneumonia was localized to lung parenchyma distal to the airway obstructed by the tumor, and 16 of these were in patients with squamous cell carcinoma. In 4 of 5 patients with adenocarcinoma, endogenous lipoid pneumonia was present distal to the tumor and had spread to the isolated segments. These results suggest that transbronchial dissemination of the breakdown products of adenocarcinoma cells, including mucin, may contribute to the spread of the nonobstructive component of endogenous lipoid pneumonia.
Histopathologic examination of the lung tissue of patients with non–small cell lung cancer has shown changes similar to those seen in endogenous lipoid pneumonia, along with pulmonary alveolar proteinosis, in the vicinity of the non–small cell lung cancer.10 One study reported coexisting changes similar to those seen in endogenous lipoid pneumonia in the proximity of primary lung tumors, particularly in tissue samples from patients with squamous cell and large cell carcinoma.
Chemical analysis of normal lung tissue shows a total fat content of 8.63 per 100 g of dry tissue, of which 19% is cholesterol; this percentage is increased in smokers. When these cells break down, lipid gains access to lung parenchyma and is phagocytosed by the pulmonary macrophages. There are no lysosomes in these macrophages, which impairs their ability to defend against bacterial infection, making the patient susceptible to opportunistic infections. If the macrophage nucleus exhibits morphologic evidence of an extreme increase in activity—chromatin clumping, parachromatic clearing, and hyperchromatic prominent nucleoli—a diagnosis of lung carcinoma is possible.
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