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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 typical radiographic picture indicative of lipoid pneumonia. Initial chest radiographs may reveal an alveolar mottling resulting from the collection of lipid in the alveolar spaces. Subsequently, interstitial images form because of the migration of macrophages and thickening of alveolar septae with fibrosis. Sometimes a localized nodule(parafinoma) may be visible in chest radiographs which could easily be confused with many implications - neoplasm or tuberculosis; asynchronous bilateral infiltrates or bilateral cavitary lesions; cavitary lesions that are quite similar to Wegener's granulomatosis; In a demonstrative study consisting of 44 patients suffering from exogenous lipoid pneumonia, the most common radiographic observations were alveolar nodules and alveolar consolidation, ground-glass opacities. These features were found to be bilateral, hypodense and mainly prevalent in the lower and posterior zones of the lobes but the sub pleural lobes were spared.
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Another report identified bilateral air space consolidation, consisting of mass like lesions and a reticulonodular pattern as the major radiographic observations. A analysis of chest x-rays of 24 children who developed lipoid pneumonia due to the aspiration of animal fat revealed multilobar consolidation. Five of the children had right perihilar infiltrates, another 5 had bilateral perihilar infiltrates with or without associated lobar consolidation and two of them had bilateral perihilar infiltrates with or without associated lobar consolidation.
Imaging studies
Due to the low diagnostic accuracy of chest x-rays, computed tomography (CT) scanning is considered the optimal method for establishing diagnosis of endogenous lipoid pneumonia. A CT scan of the chest reveals low-density consolidation with negative attenuation values, indicating the presence of fat. The scans may also show diffuse parenchymal consolidation and fat with localized areas of consolidation and subpleural pulmonary fibrosis. In scans showing diffuse consolidation, the attenuation is decreased but is higher than that of subcutaneous fat.
Pulmonary consolidation with fatty changes, areas of ground-glass opacities, unspecific but low attenuation values, septal lines, and centrilobular interstitial thickening have been revealed in high resolution CT scanning. The CT image of ground-glass attenuation is visible. Out of 7 patients with exogenous lipoid pneumonia, a "crazy-paving pattern" (ground-glass attenuation with superimposed septal thickening) was observed on high resolution CT in 5 of them, either in isolation or all around the pulmonary consolidation. In 2 cases cystic and traction bronchiectasis changes consistent with fibrosis were seen.
Magnetic resonance imaging (MRI) has revealed pulmonary consolidation of a high intensity on T1-weighted images consistent with lipid content. Results on lipid sensitive (chemical-shift) MRI include a loss of signal intensity in an area of airspace disease that was considered typical for the presence of lipid.
Definitive Diagnosis
A conclusive diagnosis can be made by analyzing bronchoalveolar lavage (BAL) fluid or samples obtained via transbronchial biopsy. The BAL fluid may appear normal, milky, or hemorrhagic; further examination may reveal lipid-laden macrophages, a few activated lymphocytes and many eosinophils. One demonstrative study of BAL examination of specimens from 39 patients with exogenous lipoid pneumonia revealed that 14% had a neutrophilic alveolitis, 23% had a lymphocytic alveolitis and 31% had mixed alveolitis (lymphocytic and neutrophilic).
The cytologic manifestation of lipid-laden macrophages in the BAL specimen is consistent with, but not specific for, lipoid pneumonia. Such macrophages can be seen in many other disorders. The lipid-laden macrophage index may be useful for diagnosis. This is a technique for quantification in which the amount of lipids in each cell is graded from 0 to 4 in 100 consecutive alveolar macrophages. The grades are then assessed wherein a score of 100 or greater indicates aspiration lipoid pneumonia. However this index cannot differentiate between endogenous and exogenous forms of lipoid pneumonia in adults.
Subsequent research rendered obsolete the use of the lipid-laden macrophage index as confirmation of aspiration pneumonia in infants since there were many index values greater than 100 in the tracheal aspirate specimens from infants who had not received lipids intravenously as well as from those who had received the infusions.
CT-guided needle biopsy or thoracotomy or Transbronchial biopsy may be required to confirm the diagnosis of lipoid pneumonia and to negate the possibility of primary or metastatic tumors in the lung. Some other serious conditions in the differential diagnosis consist of pulmonary alveolar proteinosis, pulmonary hemosiderosis, eosinophilic pneumonia, bronchial carcinoid, pulmonary hemosiderosis and hamartomas.
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