Description
Pneumocystis jiroveci pneumonia (PJP), formerly known as Pneumocystis carinii pneumonia (PCP), is a type of fungal pneumonia caused by the yeast-like fungus Pneumocystis jiroveci. It primarily affects individuals with weakened immune systems, particularly those with HIV/AIDS, but can also occur in those receiving immunosuppressive therapy.
Pathogenesis
Pneumocystis jirovecii is a ubiquitous organism that typically does not cause disease in immunocompetent hosts. In immunocompromised individuals, the fungus colonises the lung alveoli where it proliferates, causing inflammation and damage to the alveolar-capillary barrier. This leads to impaired gas exchange and the clinical manifestations of pneumonia. The organism primarily spreads through airborne transmission, and the disease results from the reactivation of a latent infection or new exposure in susceptible individuals.
Epidemiology, Risk Factors & Associations
PJP is primarily associated with HIV/AIDS, with the disease often occurring when CD4+ T-cell counts fall below 200 cells/μL. Other risk factors include the use of immunosuppressive medications (such as corticosteroids or chemotherapy), organ transplantation, and conditions associated with impaired immunity, such as malignancy or autoimmune diseases. The incidence of PJP is decreasing due to routine antimicrobial prophylaxis.
Clinical Features
- The most common symptoms of PJP are dyspnoea, non-productive cough, and fever.
- The onset of symptoms is usually gradual over several weeks.
- Chest pain, weight loss, and night sweats may also occur.
- In severe cases, respiratory failure can develop.
Pathological Features
The diagnosis of PJP often requires microscopic examination of respiratory specimens, such as bronchoalveolar lavage fluid or lung tissue. The characteristic microscopic finding in PJP is the presence of cysts of Pneumocystis jiroveci, which can be visualised with special stains such as Gomori methenamine silver or toluidine blue.
Pathological Features
Histopathology
- Macroscopic: Diffuse, bilateral lung involvement.
- Microscopic: Alveolar spaces filled with frothy, eosinophilic exudate containing Pneumocystis organisms. Interstitial infiltration with mononuclear cells and occasional plasma cells.
Serology
- Elevated serum lactate dehydrogenase (LDH).
- (1,3)-β-D-glucan assay may be positive.
Biochemistry
- Arterial blood gases showing hypoxia and respiratory alkalosis.
Immunohistochemistry
- Identification of Pneumocystis jirovecii organisms in lung tissue or respiratory samples using specific stains (e.g., Gomori methenamine silver, toluidine blue, immunofluorescence).
Molecular
- PCR for Pneumocystis DNA in respiratory samples.
Radiological Features
General Features
- Bilateral, diffuse, interstitial infiltrates predominantly in the perihilar region and lower lobes.
- Ground-glass opacities, cystic lesions, possible pneumatoceles.
XR
- CXR: Bilateral, symmetrical, interstitial or alveolar infiltrates with perihilar prominence. CXR can be normal in up to 20% of cases, particularly in early or mild disease.
- Evolution of parahilar granular infiltrates to extensive bilateral airspace opacification
CT
CT scan of the chest is more sensitive and specific than chest X-ray in diagnosing PJP.
- Bilateral and diffuse ground-glass opacities with a central or upper lobe predominance
- Cystic lesions in up to 30% of cases and are typically thin-walled and variable in size, may develop to a pneumatoceoele if the underlying disease is untreated, predisposing to pneumothorax or pneumomediastinum.
- Consolidation is less common than ground-glass opacities but can occur, particularly in severe disease. It is often patchy and may be associated with ground-glass opacities.
- Septal thickening is seen in some patients and contributes to the overall crazy paving pattern that can be seen in PJP.
Staging
There is no specific staging system for PJP. The severity of the disease is typically assessed based on the extent of lung involvement and the patient’s clinical status.
Differential Diagnosis
The differential diagnosis for PJP includes other causes of pneumonia (such as bacterial, viral, or other types of fungal pneumonia), as well as non-infectious causes of lung disease (such as interstitial lung disease or pulmonary oedema).
Management
The first-line treatment for PJP is trimethoprim-sulfamethoxazole (TMP-SMX). Alternative treatments, such as pentamidine or atovaquone, may be used in patients who are allergic to or intolerant of TMP-SMX. Supportive care, including supplemental oxygen and mechanical ventilation if needed, is also important. Prophylaxis with TMP-SMX is recommended for individuals at high risk of PJP, such as those with HIV/AIDS and a CD4+ T-cell count below 200 cells/μL.