Pneumocystis pneumonia (PCP) is an atypical pulmonary infection and the most common opportunistic infection in patients with acquired immunodeficiency syndrome (AIDS).
Classically, PCP was the acronym for pneumocystis carinii pneumonia but, the causative organism was reclassified as pneumocystis jirovecii. Strictly speaking, pneumocystis carinii refers to a species found in rats, while pneumocystis jirovecii refers to the human isolate. However, physicians have been slow to adopt the updated terminology, and there continues to be widespread use of the term PCP; the post-hoc justification for its use is that it stands for pneumocystis pneumonia.
Pneumocystis pneumonia is essentially not seen in individuals with an intact immune system. It is particularly seen in patients with AIDS and therefore the demographics match those of the AIDS population.
In order to be susceptible to PCP, the patients CD4 count usually has to fall below 200 cells/mm3. 8
Presentation is usually non-specific and insidious, with the most common symptoms being dyspnoea and/or a non-productive cough. In patients who are profoundly immunocompromised, onset may be more dramatic and resemble other pulmonary infections 7,9.
The diagnosis can often be confirmed with bronchoalveolar lavage which is 85-90% sensitive 8.
Pneumocystis jirovecii is a yeast-like fungus of the genus pneumocystis, which had previously thought to be a protozoan.
Histology of infected lung demonstrate intralveolar eosinophylic masses with a foamy appearance, due to small cysts within which the pneumocystis jirovecii organism is found 9.
Although up to 90% of chest x-rays in patients with PCP are abnormal, appearances are often non-specific. Between 10-15% of patients with PCP have normal chest radiographs and close to 30% have non-specific or inconclusive findings 2-4,6-7 .
Features which are highly suggestive of PCP in patients with CD4 counts below 200/mm3 include 5:
- small pneumatocoeles, subpleural blebs
- a fine reticular interstitial pulmonary pattern
- there is often a perihilar distribution
Pleural effusions are normally not a feature being seen in less than 5% of cases 9.
High resolution computed tomography (HRCT) is more sensitive and may be used to exclude PCP in patients with clinical suspicion for PCP but normal or inconclusive chest radiographs 3.
Features include 2-3,7:
- predominantly involving perihilar or mid zones
- there may be a mid, upper or lower zone predilection depending on whether the patient is on prophylactic aerosolised medication
- if they are, then the poorly ventilated upper zones are prone to infection 9, whereas in those who are not the lower zones are more frequently involved
- there may be relative preservation of previously irradiated areas
- predominantly involving perihilar or mid zones
- reticular opacities or septal thickening may also be present
- a crazy paving pattern may therefore be seen when both ground-glass opacities and septal thickening are superimposed on one another
- pneumatoceles are seen in up to 30% of cases 7
- pleural effusions are rare 2,3
- lymphadenopathy is uncommon (10%)
Especially in patients treated prophylactically, features are more frequently atypical including 7:
- nodules (granulomas) which may cavitate
- pleural effusions are also more frequently encountered in this group of patients
A cystic form of PCP is also recognised, again more frequently in patients receiving aerosolised prophylaxis. In this pattern, features include 7:
- thin walled cysts: in most cases these are pneumatoceles
- upper lobes
- maybe bilateral
- increased risk of pneumothorax
Gallium67 lung scintigraphy is highly sensitive for PCP, and normal gallium scan makes the diagnosis of PCP very unlikely. The gallium scan in patients with PCP demonstrates diffuse pulmonary uptake, which may be heterogeneous or homogeneous.
Despite this, the specificity of the gallium scan is low, making it most useful in patients in whom bronchoalveolar lavage may be less diagnostic (e.g. in suspected relapse).
Treatment and prognosis
Most patients with an acute infection are treated with trimethoprim-sulfamethoxazole, combined with corticosteroids in patients with moderate to severe infections 8. The same agent may be used as prophylaxis. A number of alternative agents may also be employed, both for acute treatment and for prophylaxis, although these are beyond the scope of this article.
Overall, with prompt treatment, survival is good (50-95%), although relapses are common 9.
The differential diagnosis on HRCT is of course strongly influenced by knowledge of HIV status and CD4 count. In these patients the differential includes:
- viral pneumonitis: similar but cysts are absent
- mycobacterium tuberculosis: when upper zone cysts are prominent
- angioinvasive aspergillosis
- immune reconstitution inflammatory syndrome (IRIS): in patients receiving HAART
Other conditions to be considered, which also occur in non-AIDS patients include:
- for prominent cysts consider
- for a reticulonodular pattern consider
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- 7. Müller NL, Franquet T, Lee KS et-al. Imaging of pulmonary infections. Lippincott Williams & Wilkins. (2007) ISBN:078177232X. Read it at Google Books - Find it at Amazon
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