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The diagnosis of exogenous lipoid pneumonia is suggested by a history of oil aspiration or inhalation, along with radiographic findings.
Radiographic findings
There is no characteristic radiographic picture of lipoid pneumonia. Initial chest radiographs may show an alveolar mottling as a result of accumulation of lipid in the alveolar spaces. Later, interstitial images develop due to the migration of macrophages and thickening of alveolar septae with fibrosis. Chest radiographs may also show a localized nodule (paraffinoma), which can easily be confused with several entities—neoplasm or tuberculosis; bilateral cavitary lesions; cavitary lesions that are quite similar to Wegener's granulomatosis; or asynchronous bilateral infiltrates. In a retrospective study of 44 patients with exogenous lipoid pneumonia, the most common radiographic findings were alveolar consolidation, ground-glass opacities, and alveolar nodules.3 These features were bilateral, predominant in the posterior and lower zones of the lobes, and hypodense, and the subpleural zones were spared.
Another report identified bilateral air space consolidation, irregular masslike lesions, and a reticulonodular pattern as the most prominent radiographic findings. In a review of chest x-rays of 24 children who developed lipoid pneumonia after the aspiration of animal fat, had bilateral multilobar consolidation, 5 had bilateral perihilar infiltrates with or without associated lobar consolidation, 5 had right perihilar infiltrates, and 2 had unilateral multilobar consolidation.16
Imaging studies
Since chest x-rays have a low diagnostic accuracy, computed tomography (CT) scanning is considered the modality of choice for establishing the diagnosis of endogenous lipoid pneumonia. Chest CT typically shows low-density consolidation with negative attenuation values, indicating the presence of fat, Computed tomography scans may also demonstrate diffuse parenchymal consolidation and fat with localized areas of consolidation and subpleural pulmonary fibrosis. In scans with diffuse consolidation, the attenuation is decreased, but it is greater than that of subcutaneous fat.
High-resolution CT scanning has demonstrated pulmonary consolidation with fatty changes, unspecific but low attenuation values, areas of ground-glass opacities, septal lines, and centrilobular interstitial thickening. The CT appearance of ground-glass attenuation can be seen. In 5 of 7 patients with exogenous lipoid pneumonia, a "crazy-paving pattern" (ground-glass attenuation with superimposed septal thickening;) of various spread was seen on high-resolution CT, either isolated or surrounding the pulmonary consolidation. In 2 cases, traction bronchiectasis and cystic changes consistent with fibrosis were seen.
Magnetic resonance imaging (MRI) has demonstrated pulmonary consolidation of high intensity on T1-weighted images consistent with lipid content. Findings on lipid-sensitive (chemical-shift) MRI have included a loss of signal intensity in an area of airspace disease that was considered specific for the presence of lipid.19
Definitive Diagnosis
Definitive diagnosis can be made by examining bronchoalveolar lavage (BAL) fluid20 or specimens obtained via transbronchial biopsy. The BAL fluid may appear normal, milky, or hemorrhagic; examination may reveal lipid-laden macrophages,21 many eosinophils, and few activated lymphocytes. In 1 retrospective study, BAL examination of specimens from 39 patients with exogenous lipoid pneumonia showed 23% had a lymphocytic alveolitis, 14% had a neutrophilic alveolitis, and 31% had mixed alveolitis (lymphocytic and neutrophilic).
The cytologic demonstration of lipid-laden macrophages in the BAL specimen is consistent with, but not specific for, lipoid pneumonia. These macrophages can be seen in many other disorders. The lipid-laden macrophage index may be helpful for diagnosis. This is a quantification technique in which the amount of lipids per cell is graded from 0 to 4 in 100 consecutive alveolar macrophages. The grades are then summed. A score of 100 or more suggests aspiration lipoid pneumonia. This index cannot, however, differentiate between the endogenous and exogenous forms of lipoid pneumonia in adults.
Subsequent research invalidated the use of the lipid-laden macrophage index as proof of aspiration pneumonia in infants because there were many index values greater than 100 in the samples of tracheal aspirates from infants who were not receiving intravenous lipids, as well as from those who were receiving the infusions.
Transbronchial biopsy or CT-guided needle biopsy or thoracotomy may be necessary to confirm the diagnosis of lipoid pneumonia and to rule out primary or metastatic tumors of the lung. Other conditions in the differential diagnosis are pulmonary alveolar proteinosis, eosinophilic pneumonia, pulmonary hemosiderosis, bronchoalveolar pneumonia, bronchial carcinoid, and hamartomas.
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