eMedicine Specialties > Radiology > Chest

Pneumonia, Pneumocystis Carinii

Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, Consultant Radiologist, North Manchester General Hospital, The Pennine Acute NHS Trust, Manchester UK
Coauthor(s): Klaus L Irion, MD, PhD, Consulting Staff, The Cardiothoracic Centre Liverpool NHS Trust, The Royal Liverpool University Hospital, UK; Sumaira MacDonald, MBChB, PhD, MRCP, FRCR, Lecturer, Sheffield University Medical School; Endovascular Fellow, Sheffield Vascular Institute; Carolyn M Allen, MB, BCh, MRCP, FRCR, CCST, Consultant Radiologist, Department of Clinical Radiology, North Manchester General Hospital, UK
Contributor Information and Disclosures

Updated: Jul 3, 2008

Introduction

Background

Pneumocystis carinii pneumonia (PCP) is caused by the ubiquitous unicellular eukaryote, P carinii. This organism is a rare cause of infection in the general population, but it is a frequent cause of morbidity and mortality in persons who are immunocompromised, especially patients with acquired immunodeficiency syndrome (AIDS).^1,2,3,4

Patients who do not have AIDS but are immunocompromised and at risk for PCP include individuals with hematologic malignancies⁴ ; organ transplant recipients^5,6 ; and those receiving long-term steroid or cytotoxic therapy, including patients with systemic vasculitis or other autoimmune deficiency. Other patients with immune deficiency disorders who are at particular risk for PCP include those with thymic dysplasia, those with severe combined immunodeficiency, and those with hypogammaglobulinemia. Severe malnutrition may predispose patients to PCP.

For excellent patient education resources, visit eMedicine's Lung and Airway Center, Pneumonia Center, Immune System Center, and Sexually Transmitted Diseases Center. Also, see eMedicine's patient education articles Viral Pneumonia, Acute Respiratory Distress Syndrome, HIV/AIDS, and HIV Testing.

Pathophysiology

P carinii has a controversial taxonomy. However, on the basis of findings from ultrastructural and molecular analysis of its mitochondrial DNA and its affinity for fungal staining, the organism is currently recognized as a fungus. P carinii organisms consist of small cysts, each of which produces as many as 8 intracystic sporozoites. Upon maturity, the cysts rupture and release the sporozoites, which in turn differentiate into trophozoites. The trophozoites subsequently develop into cysts and repeat the cycle. The mode of transmission is believed to be via the inhalation of air-borne cysts.

Almost all children acquire antibodies to P carinii by the age of 2 years. Disease develops only in individuals who are immunocompromised, probably as a result of reactivation of a latent infection. Trophozoites attach to the cell membrane of type 1 alveolar pneumocytes, with subsequent cell death and leakage of proteinaceous fluid into the alveolar spaces. Histologically, the alveolar spaces are filled with an amphophilic, foamy, amorphous material composed of the parasites and cell debris; this material resembles proteinaceous edema fluid. Trophozoites are visible on electron microscopy and on smears or sections stained with toluidine blue or polychrome, but they are not visible by using routine histologic techniques.

No reliable antigen detection methods are available yet. The organism cannot be grown in culture; therefore, the diagnosis of PCP relies on morphologic identification of the organism. The standard method of diagnosis is via cytologic examination of induced sputum specimens or bronchoalveolar lavage (BAL) washings. BAL has a sensitivity of as high as 90% and a specificity of 82%. In a minority of patients, transbronchial, percutaneous, or open lung biopsy may be necessary. Alternatively, patients who present with typical clinical and radiographic manifestations may be treated empirically, with invasive procedures reserved for patients who have atypical presentations or in whom empiric therapy fails to elicit a satisfactory response.

Frequency

United States

Pneumocystis carinii pneumonia (PCP) was exceptionally rare before the advent of the AIDS epidemic, with a reported incidence of 0.3 cases per 1,000,000 persons per year. The AIDS epidemic led to an initial dramatic increase in the incidence of PCP, although this subsequently declined significantly following the introduction of trimethoprim-sulfamethoxazole (TMP-SMZ) PCP prophylaxis. PCP is no longer the most common pulmonary complication or infection in patients with AIDS, having been supplanted by bacterial infection. However, PCP remains the most common pulmonary opportunistic infection, and it still affects 60% of patients during the course of the illness.

PCP remains a common AIDS-defining illness, although its incidence as an index disease has declined. The reduction is due partly to a true reduction in incidence, attributed mainly to the widespread use of effective PCP prophylaxis, and partly to a broadening of the diagnostic criteria for AIDS. PCP accounts for approximately 40% of index diagnoses, as compared with approximately 60% at the start of the AIDS epidemic.

Effective PCP prophylaxis not only reduces the incidence of PCP but also lengthens disease-free intervals between episodes.

In patients without AIDS who are immunocompromised, the incidence of PCP before the administration of prophylactic therapy is 10-20%; with prophylactic therapy, it decreases to 0-10%.

International

In the African population, the incidence of Pneumocystis carinii pneumonia (PCP) in patients with AIDS is lower than that of other groups, and only 9% of patients are affected.

Mortality/Morbidity

Although Pneumocystis carinii pneumonia (PCP) is less common than bacterial respiratory tract infection in patients with AIDS, this condition accounts for more deaths. Initially, 50% of deaths due to respiratory failure in patients with AIDS were attributed to P carinii; the organism was identified in two thirds of cases. However, with a declining incidence of PCP, wider recognition, and more effective treatment, the mortality rate has decreased to approximately 5% overall.

• Slowly progressive forms, atypical features, and complications such as pneumothorax are associated with significantly higher mortality rates. Mortality rates increase to 75-100% in patients whose condition fails to respond to therapy during the first 5-10 days.
• The mortality rate in patients without AIDS but who are immunocompromised is 40%.

Race

Pneumocystis carinii pneumonia (PCP) is 3 times more common in white patients than black patients, despite a higher use of prophylactic therapy in the white population.

Sex

Rates of infection of Pneumocystis carinii pneumonia (PCP) in homosexual and bisexual males are more than twice that of those in females.

Age

Individuals of any age can be affected with Pneumocystis carinii pneumonia (PCP), particularly when their immune system is compromised.

Presentation

Pneumocystis carinii pneumonia (PCP) can occur at any level of immune compromise, but this disease is more common in patients who are not receiving PCP prophylaxis and who have CD4 levels less than 200 cells per cubic millimeter (cells/mm³). PCP is most common in persons with profound levels of immunodeficiency, that is, in those with CD4 levels less than 100 cells/mm³.

PCP usually appears with an insidious onset of malaise, weight loss, and low-grade fever (79-100%) that is associated with a dry cough (59-91%). However, the symptoms may be more severe, and dyspnea (29-95%), cyanosis, and respiratory failure (5-30%) may be present. Immunocompromised patients without AIDS but with PCP tend to present more acutely and have more fulminant disease. Chest pain (14-23%) and productive cough (23-30%) are reported. Patients classically demonstrate marked desaturation with exercise. An elevated serum lactate dehydrogenase level is sensitive for PCP, but this finding is nonspecific and has limited diagnostic value.

Complications of PCP include spontaneous pneumothorax and hypoxemia secondary to an adult respiratory distress –like syndrome. Intubation may be needed, and this complication has a significantly worse prognosis.

Extrapulmonary disease occurs in 1% of patients due to hematogenous spread of the organism. In particular, this is associated with nebulized pentamidine, which was once used as a prophylactic agent; however, this therapy has been superseded by oral antibiotics. With nebulized drug administration, less than 10% of the dose enters the systemic circulation, potentially allowing the organism to cause systemic disease or lie dormant in extrapulmonary sites, acting as a source of recurrent infection. The bone marrow, spleen, liver, lymph nodes, small bowel, and eyes can be affected. Less commonly involved areas include the adrenal glands, kidneys, pituitary gland, skin, thyroid gland, and gastrointestinal tract.

A subset of patients presents with atypical clinical and radiographic features termed chronic PCP. These patients have a prolonged clinical course over months or years, with persistent stable symptoms and radiographic abnormalities corresponding to pathologic findings of interstitial fibrosis, traction bronchiectasis, and honeycombing.

Preferred Examination

Chest radiographs should be included in the initial evaluation for Pneumocystis carinii pneumonia (PCP). Frequently, these are the only images required. High-resolution computed tomography (HRCT) scanning^7,8,9,10,11 and, occasionally, gallium-67 (⁶⁷ Ga) scanning^12,13,14,15 are useful in symptomatic patients in whom chest x-ray findings are normal or equivocal.

Limitations of Techniques

Chest radiographic findings may be normal in 10-39% of patients with Pneumocystis carinii pneumonia (PCP). With both CT and⁶⁷ Ga scanning, the appearance of PCP is nonspecific.

Differential Diagnoses

Bronchiolitis Obliterans Organizing Pneumonia
Extrinsic Allergic Alveolitis
Kaposi Sarcoma, Thoracic
Lung, Drug-Induced Disease

Other Problems to Be Considered

AIDS-related lymphoma
Kaposi sarcoma
Other opportunistic infections
Pulmonary edema
Pulmonary hemorrhage

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