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Key Features

Essentials of Diagnosis

  • Significant immunosuppression

  • Fever, tachypnea, cough, dyspnea

  • Hypoxemia; diffuse interstitial infiltrates

  • Detection of the organism in specimens of pulmonary origin

General Considerations

  • Initial infection is presumed to occur asymptomatically via inhalation, usually in early childhood, and to become a clinical problem upon reactivation during immune suppression

  • Severe signs and symptoms occur chiefly in patients with abnormal T-cell function, such as hematologic malignancies and organ transplantation

  • Pneumocystis also causes severe pneumonia in patients with γ-globulin deficiency and is an AIDS-defining illness for children with advanced HIV infection

Clinical Findings

Symptoms and Signs

  • In most patients, a gradual onset of fever, tachypnea, dyspnea, and mild, nonproductive cough occurs over 1–4 weeks

  • Initially the chest is clear, although retractions and nasal flaring are present

  • Hypoxemia out of proportion to the clinical and radiographic signs is an early finding; however, even minimally decreased arterial oxygen pressure values should suggest this diagnosis in immunosuppressed children

  • In some children with AIDS or severe immunosuppression from chemotherapy or organ transplantation, the onset may be abrupt and progression more rapid

  • Acute dyspnea with pleuritic pain may indicate the complication of pneumothorax

Differential Diagnosis

  • Influenza

  • Respiratory syncytial virus

  • Cytomegalovirus

  • Adenovirus

  • Other viral infections

  • Bacterial and fungal pneumonia

  • Pulmonary emboli or hemorrhage

  • Heart failure

  • Chlamydophila pneumoniae

  • M pneumoniae infections


Laboratory Findings

  • Serum lactate dehydrogenase levels may be elevated markedly as a result of pulmonary damage

  • In moderately severe cases, the arterial oxygen pressure is < 70 mm Hg or the alveolar-arterial gradient is < 35 mm Hg


  • Chest radiographs

    • Normal early in disease course

    • Classic pattern seen on later films: bilateral, interstitial, lower lobe alveolar disease starting in the perihilar regions, without effusion, consolidation, or hilar adenopathy

  • High-resolution CT scanning may reveal extensive ground-glass attenuation or cystic lesions

  • Older HIV-infected patients present with other patterns, including nodular infiltrates, lobar pneumonia, cavities, and upper lobe infiltrates

Diagnostic Procedures

  • Diagnosis requires finding characteristic round (6–8 mm) cysts in a lung biopsy specimen, bronchial brushings, alveolar washings, induced sputum, or tracheal aspirates

  • Tracheal aspirates are less sensitive, but are more rapidly and easily obtained

  • Several rapid stains—as well as the standard methenamine silver stain—are useful

  • Indirect fluorescent antibody method is most sensitive


General Measures

  • Supplemental oxygen and nutritional support may be needed

  • Patient should be in respiratory isolation

Specific Measures

  • Trimethoprim-sulfamethoxazole is drug of choice

    • Dosage: 20 mg/kg/d of trimethoprim and 100 mg/kg/d of sulfamethoxazole in four divided doses intravenously or orally if well ...

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