Differential Diagnoses
- Pneumococcal pneumonia
- Pneumothorax
- Pulmonary mass
- Mediastinitis
- Aortic dissection
Physical Examination
On physical examination, his vital signs are as follows: temperature, 101.2°F (38.4°C); pulse rate, 108 beats/min; respiration rate, 24 breaths/min; blood pressure, 112/82 mm Hg; and oxygen saturation, 94% on room air. He is alert and oriented, is in no acute distress, and is breathing comfortably but slightly faster than normal. His lungs are clear to auscultation. His heart rate and rhythm are regular, and there is no murmur, rub, or gallop.
Diagnosis
A chest x-ray is obtained (Figure 2) that shows the fairly symmetric bilateral infiltrates of pneumocystis pneumonia (PCP). There is no evidence of a lobar infiltrate, pleural effusion, parapneumonic effusion, or pneumothorax.
Learnings
PCP is caused by a fungus-type organism called Pneumocystis jiroveci, which was previously called Pneumocystis carinii (a parasite). PCP commonly occurs in immunocompromised hosts, generally as a result of acquired immunodeficiency syndrome (AIDS), first recognized in men who have sex with men and in intravenous drug users in 1981. PCP is an AIDS-defining illness and may be the initial presentation of human immunodeficiency virus (HIV) or AIDS in patients who did not know they were HIV-positive. The incidence has decreased with the advent of antiretroviral therapy and use of medications for prophylaxis. For example, between 1994 and 2007, the incidence of opportunistic infections decreased from 89% to 13%.
Pneumocystis jiroveci, originally thought to be a protozoan, is actually a yeast-like fungus, spread through the air. It is ubiquitous; most children have antibodies for it at a young age, with 80% having evidence of exposure by the age of 13. The most common site of infection is pulmonary, but infection may occur in extrapulmonary sites as well, such as the skin, lymph nodes, spleen, and brain. PCP is most likely in patients with AIDS, as defined by a CD4 count of <200 cells/mm3, a CD4 percentage of <14%, or an AIDS-defining illness such as PCP, cerebral toxoplasmosis, esophageal candidiasis, cytomegalovirus retinitis, or mycobacterium avium complex.
Risk factors include immunosuppression, typically in patients with AIDS, though it may also occur in those with malignancies, those who have undergone organ transplantation, and in patients receiving immunosuppressive therapy. If there is concern for undiagnosed HIV or AIDS infection, inquire about risk factors such as these:
- High-risk behaviors such as men engaging in unprotected sex with men
- Intravenous drug use
- Hemophilia with blood transfusions
- Multiple sexual partners
Other factors with undiagnosed AIDS may include the following:
- Lymph node swelling
- Weight loss
- Skin rashes
What to Look For
Pay particular attention to temperature, tachypnea, tachycardia, and hypoxia. Findings on the lung examination may be normal or may reveal rales (crackles), rhonchi, or bronchial breath sounds. Evaluate the state of hydration through such findings as poor skin turgor, dry mucous membranes, and lack of urine output.
The following testing should be done.
- Chest x-rays:
- The typical appearance of PCP is bilateral interstitial infiltrates.
- Infiltrates are often perihilar.
- The appearance is not pathognomonic for PCP, but it may be present in other conditions.
- Mediastinal adenopathy may be present.
- X-ray findings may be normal (25%) early in the course or show mild symptoms.
- Complications of PCP may include pneumothorax.
- Findings may be asymmetric or may present as upper lobe disease. There may be nodules or effusions.
- Computed tomography scanning:
- This modality is more sensitive than chest x-rays.
- The typical appearance is an interstitial pattern and/or diffuse ground-glass opacities.
- There may also be multiple thin-walled cysts.
- Oxygenation:
- Pulse oximetry should be checked in all patients with a diagnosis of pneumonia or suspected hypoxemia because oxygenation will typically be low in patients with PCP.
- Exertional hypoxemia will typically be present; oxygen saturation drops significantly with exertion.
- Laboratory tests:
- HIV testing: Enzyme-linked immunosorbent assays are sensitive for HIV. If findings are positive, they should be confirmed by a western blot test.
- CD4 count testing
- Arterial blood gas is not generally helpful in the outpatient setting.
- Sputum Gram stain and culture is not necessary in the outpatient setting.
- Blood cultures are not generally helpful.
- A complete blood cell count is generally unhelpful, though leukopenia may be suggestive of AIDS or immunosuppression.
- Urine antigens are not recommended, owing to a high number of false-positives.
All patients with a new diagnosis of PCP must be transferred to an emergency department for admission and bronchoscopy to confirm the diagnosis.