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Emergency And Acute Medicine – Pneumocystis Pneumonia
Pneumocystis pneumonia, commonly referred to as PCP, is caused by Pneumocystis jirovecii and remains the most common opportunistic infection in patients with HIV, particularly when the CD4 count is less than 200 cells/mm³. Although previously known as Pneumocystis carinii pneumonia, the organism is now classified as a fungus. Transmission is believed to occur via the respiratory aerosol route. Organisms colonize the respiratory tract, cysts rupture, and trophozoites proliferate within the alveoli, producing a characteristic foamy exudate that interferes with gas exchange. Most cases likely represent reactivation of latent infection, although person-to-person transmission has been suggested. PCP also occurs in patients with impaired cellular immunity due to cancer, chronic corticosteroid therapy, organ transplantation, or malnutrition. In children, infection tends to be more severe.
The clinical presentation is typically subacute. Up to 7% of patients may initially be asymptomatic. Patients often report fever, nonproductive or minimally productive cough, and progressive dyspnea. In HIV-positive patients, symptoms may develop gradually over weeks to months, whereas in non–HIV immunocompromised hosts the course may progress over days. Dyspnea on exertion with exercise-induced oxygen desaturation is common. Associated symptoms include chills, fatigue, weight loss, and chest discomfort. Patients receiving inhaled pentamidine prophylaxis may present with milder pulmonary symptoms but have a higher incidence of pneumothorax and extrapulmonary disease. On examination, tachypnea and tachycardia are common, and lung auscultation may reveal crackles or rhonchi, although findings can be minimal relative to the degree of hypoxemia.
Essential evaluation includes complete blood count, electrolytes, arterial blood gas, lactate dehydrogenase (LDH), blood cultures, and chest imaging. Arterial blood gas analysis should be obtained in all suspected cases to calculate the alveolar–arterial gradient, which is usually elevated. Adjunctive corticosteroids are indicated when the A–a gradient exceeds 35 mm Hg or the PaO₂ is less than 70 mm Hg. LDH is often elevated in HIV-associated PCP and higher levels may correlate with worse prognosis, though it is nonspecific. Chest radiography classically demonstrates bilateral interstitial or diffuse alveolar infiltrates. However, up to 25% of patients may have a normal radiograph early in the disease. Atypical findings include lobar infiltrates, cysts, pneumothoraces, pleural effusions, or nodular infiltrates. High-resolution CT is highly sensitive and typically shows patchy ground-glass opacities.
Definitive diagnosis requires identification of Pneumocystis organisms in respiratory specimens. Induced sputum examination has high specificity but variable sensitivity depending on specimen quality and laboratory expertise, and it is less sensitive in non–HIV patients or those on pentamidine prophylaxis. Bronchoalveolar lavage is recommended when induced sputum is nondiagnostic and clinical suspicion remains high, with sensitivity approaching 80–100%.
Initial management follows airway, breathing, and circulation principles. Supplemental oxygen should be administered, escalating from nasal cannula to nonrebreather mask as needed. Endotracheal intubation is required for refractory hypoxemia or hypercapnic respiratory failure. Intravenous fluids should be given for hypotension or dehydration. Empiric antimicrobial therapy should be initiated promptly when PCP is suspected. Intravenous trimethoprim–sulfamethoxazole is the first-line therapy and should be continued for 21 days. Intravenous pentamidine is an alternative for patients intolerant of first-line therapy. Oral therapy may be appropriate in mild cases. Alternative regimens include trimethoprim–dapsone, clindamycin–primaquine, or atovaquone. Adjunctive corticosteroids must be started within the first 72 hours in patients with significant hypoxemia to reduce mortality and the risk of respiratory failure. Suspected PCP patients should be isolated from other immunocompromised individuals.
Admission is indicated for moderate to severe disease, defined by hypoxemia or elevated A–a gradient, inability to tolerate oral medications, or unreliable follow-up. Intensive monitoring is required for patients with respiratory compromise. Selected patients with mild disease, stable oxygenation, and reliable follow-up may be managed as outpatients. Close follow-up with an infectious disease specialist is essential.
PCP should always be considered in immunocompromised patients presenting with dyspnea and diffuse infiltrates, particularly those with HIV or suspected undiagnosed HIV infection. Well-appearing patients with unexpectedly low oxygen saturation are at higher risk for rapid deterioration. Clinicians must also consider coexisting infections such as tuberculosis or atypical bacterial pneumonia in this population.
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