Pneumocystis jirovecii Pneumonia

Pathology and Pathogenesis

The alveoli are Þlled by a foamy, pale, and eosinophilic exudate (Fig. 20.6). The fungus is unstained in Hematoxylin and Eosin preparations but with silver stain the alveoli are seen to contain numerous round cysts that measure about 5 μm across. Crescent-shaped forms represent collapsed cysts. Other helpful feature is a dot, generally seen on the edge of the cyst. In bronchoalveolar lavage ßuids, frothy exudate is frequently seen and it may be a diagnostic tool if a biopsy cannot be studied [9].

Symptoms and Signs

Pneumocystis jirovecii pneumonia can occur in immunocompromised individuals, especially hematopoietic stem and solid organ transplants, those receiving high-dose corticosteroid therapy, and persons with advanced HIV infection [10]. A slow indolent time course with symptoms of pneumonia progressing over weeks to months is characteristic in HIV-infected patients. Fulminant respiratory failure associated with fever and dry cough is typical in non-HIV-infected patients.

CT Findings

Thin-section CT (TSCT) typically shows bilateral areas of GGO [11] (Fig. 20.6). The areas of GGO may be patchy, have a geographic appearance, be diffuse, or involve mainly the perihilar regions or upper lobes. Reticulation or interlobular septal thickening is seen in approximately 20 % of patients forming Òcrazy-paving appearanceÓ (Fig. 20.6). Cyst formation is seen in about 30 % of patients. The cysts are variable in appearance, but most measure 5Ð30 mm in diameter, are thin walled, and are located in the upper lobes. Unusual manifestations include nodules and a miliary pattern [12].

CT–Pathology Comparisons

The GGO reßects thickening of alveolar wall by edema or interstitial pneumonitis and foamy nature of alveolar exudates, whereas the reticulation reßects the organization of intra-alveolar materials or Þbrosis [8].

Patient Prognosis

The Þrst-line drug for treatment and prevention is trimethoprimÐsulfamethoxazole. To reduce the incidence of