Case 18

Section 1

What are the abnormalities? Answer

Find 2 thickened interlobular septa in the left lung.

Section 2

Histologic Findings Alveolar walls are slightly thickened by congested capillaries and a few type II cells. Alveoli are filled with a foamy exudate. A special stain was positive for organisms. What stain was it and what was the organism? Answer Find 2 examples of foamy alveolar exudate.

Differential diagnosis of ground-glass opacity/consolidation and thickened septa on HRCT: Viral, pneumocystis, or mycoplasmal pneumonia, alveolar proteinosis, pulmonary hemorrhage, pulmonary edema, acute respiratory distress syndrome, acute interstitial pneumonia, and bronchioloalveolar carcinoma.

Histologic differential diagnosis: Pneumocystis pneumonia, alveolar proteinosis, edema, mucinous bronchioloalveolar carcinoma

Diagnosis: Pneumocystis carinii pneumonia

Diagnostic features of Pneumocystis carinii pneumonia on HRCT

• Ground-glass opacity: diffuse or perihilar; confluent, patchy, or nodular
• Consolidation
• Interlobular septal thickening
• Cavitation/cyst formation (usually chronic)

Comment: When pneumocystis pneumonia is considered clinically, HRCT is useful because it has a very high sensitivity to detect subtle ground-glass opacity in the presence of a normal chest film. Also, HRCT can distinguish changes typical of pneumocystis pneumonia from changes seen in some other HIV-related conditions, such as

• tuberculosis or fungal disease, which tend to appear as nodular or consolidative processes
• acute bacterial bronchopneumonia, which appears as patchy consolidation with centrilobular nodules
• Kaposi's sarcoma, which appears as consolidation in a bronchovascular distribution associated with contiguous interlobular septal thickening.

See Case Study 26 for another example of pneumocystis pneumonia.

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a) Patchy ground-glass opacity
b) Patchy consolidation
c) Thickened interlobular septa

Note: This combination of findings can be seen with the "crazy-paving" characteristic of alveolar proteinosis. However, this term should be reserved for the coexistence of patchy septal thickening and ground-glass opacity in a lobular pattern, often in sharp contrast to regions of adjacent normal lung (see Case 15) (use the "back" button on the menu bar to return). The jumbled mixture of ground-glass, consolidation, and thickened septa seen in the present case is not characteristic of "crazy-paving."

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Thickened interlobular septum

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Foamy alveolar exudate containing organisms

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A Gomori methenamine silver stain would stain the pneumocystis spore cases (cysts) present in this foamy exudate.

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