Etiology and epidemiology
P. carinii pneumonitis is the initial AIDS-defining disease in approximately 50 per cent of patients in developed countries ( Table...2), although this proportion is falling with the wider use of co-trimoxazole as primary prophylaxis. As a result P. carinii pneumonitis often presents for the first time at lower CD4 counts and as a second or subsequent complication of AIDS.
The most common symptoms are dyspnea, fatigue, fever, and cough. The patient frequently appears to be clinically less ill than the chest radiograph and arterial blood gases suggest, and the only physical signs may be fine inspiratory crackles on auscultation of the chest. Deterioration on treatment may be due to super-added bacterial infection, fluid overload, or pneumothorax.
The chest radiograph may be normal in mild or early cases but more commonly shows bilateral interstitial shadowing, most prominently at the midzones. Atypical radiographic changes occasionally seen include apical and unilateral shadowing. The majority (over 90 per cent) of patients will have hypoxemia at rest or after exercise. The diagnosis is confirmed in most cases (over 90 per cent) by Grocott's methenamine silver staining of alveolar lavage fluid obtained at bronchoscopy. Microscopy of sputum induced by the inhalation of nebulized saline has a sensitivity of approximately 40 per cent compared with bronchoscopy, but the diagnostic yield is variable as it is operator dependent.
Treatment is outlined in Table..3. The majority of patients (over 85 per cent) improve on co-trimoxazole therapy. Those who require mechanical ventilation have a poor prognosis, with 25 per cent or less surviving their hospital stay.
Table 3 Management of P. carinii pneumonitis
At CD4 counts over 200 * 106/l HIV-infected patients are susceptible to common pathogens such as Streptococcus pneumoniae, Staphylococcus aureus, and Hemophilus influenzae. At lower CD4 counts a wider range of pathogens will be found including Pseudomonas species and organisms that are uncommon in the immunocompetent (e.g. Serratia marcescens).
At higher CD4 counts the illness is similar to community-acquired bacterial pneumonia seen in the immunocompetent, usually presenting as a lobar pneumonia. At lower CD4 counts the clinical features are similar to P. carinii penumonitis, although the cough is more likely to be productive. Approximately 20 per cent of P. carinii pneumonitis will be complicated by bacterial infection.
The chest radiograph will normally show lobar or segmental consolidation at higher CD4 counts, but at lower levels (below 300 * 10 6/l) patchy bilateral consolidation and interstitial shadowing indistinguishable from changes due to P. carinii pneumonitis or other infections may be seen. Blood and sputum cultures should be performed and brochoscopic alveolar lavage performed if the etiology is in doubt.
A second- or third-generation cephalosporin such as cefuroxime 750 mg intravenously four times daily, with additional cover for atypical organisms such as Mycoplasma pneumoniae (e.g. erythromycin 500 mg four times daily) is appropriate in patients with higher CD4 counts. In the more immunocompromised (CD4 < 200 * 106/l) a broad-spectrum antibiotic with anti-pseudomonal activity is required (e.g. ceftazidime 1-2 g intravenously twice daily). Supplemental oxygen by mask may be required to maintain Pao2 above 8 kPa (60 mmHg).
Management is the same as for P. carinii pneumonitis.
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