Imaging Pneumonia in Immunocompetent and Immunocompromised Individuals

John H. Reynolds and Arpan K. Banerjee

Disclosures

Curr Opin Pulm Med. 2012;18(3):194-201. 

In This Article

Immunocompromised Patients

The immunocompromised group comprises a wide range of patients with disordered immune systems. These include primary immune deficiency, AIDS, and patients who are immunocompromised from medical treatment such as chemotherapy or following organ or bone marrow transplantation. The largest group of immunocompromised patients comprises those with AIDS, a disease which was first recognized in 1981 and has been responsible for the death of over 25 million patients since the beginning of the epidemic. There are currently around 35 million people who are living with the HIV virus.[9–11]

Bacterial Pneumonia

It has been recognized since the Pulmonary Complications of HIV Infection Study that patients with HIV infection had an increased incidence of community-acquired bacterial pneumonia, with 5.5 episodes of pneumonia per hundred person years of the HIV-infected cohort compared with 0.9 episodes per hundred in the non-HIV group.[12] Since the dawn of antiretroviral treatment, the incidence of community-acquired bacterial pneumonia in immunocompromised patients has declined. The commonest pathogens include S. pneumoniae and H. pneumoniae. The majority will present with segmental lobar or multilobar consolidation on a chest radiograph[13] (Fig. 6). There are no significant differences in the findings of pneumococcal pneumonia in patients who are receiving antiretroviral treatment as compared with those who are not.[14] Other manifestations such as diffuse alveolar patterns and interstitial infiltrates may occasionally be seen.

Figure 6.

Bacterial pneumonia in a case of HIV infection. Right upper lobe consolidation due to a bacterial pneumonia in an HIV-infected patient.

Other bacterial infections which may be seen in the immunocompromised host include P. aeruginosa,[15]L. pneumophila,[16] and Rhodococcus equi,[17,18] all tending to produce consolidation on the chest radiograph, sometimes with progression to cavitation. Radiographic appearances of Nocardia asteroides infection can mimic tuberculosis. Features on a chest radiograph include multilobar infiltration as well as reticular nodular infiltration, solitary masses, and effusions.[19]

Fungal Infection

Originally known as Pneumocystis carinii, but now called Pneumocystis jiroveci, this organism was first classified as a protozoan, but has since been shown to be a fungus.[20,21] With lung involvement, chest radiographs classically reveal bilateral symmetrical perihilar alveolar shadowing. Unusual presentations include unilateral disease, focal consolidation, and linear densities. CT findings (Fig. 7) include ground glass opacity and interlobular septal thickening – sometimes combining to form a 'crazy paving pattern'.[22] In addition, there may be thin-walled cysts or pneumatoceles in up to 10% of the cases. Cysts may develop during treatment for pneumocystis as well as at presentation of the initial disease. Pleural effusions are very uncommon as is lymphadenopathy adenopathy.[23]

Figure 7.

Pneumocystis jiroveci infection. Computed tomography image showing ground glass opacity, consolidation, and interlobular septal thickening in an HIV-infected patient with pulmonary P. jiroveci infection.

Infection with Candida albicans typically shows nodules less than 10mm in diameter which may be associated with consolidation[24] (Fig. 8).

Figure 8.

Candida albicans infection. Computed tomography image revealing consolidation and nodular opacities in a patient with an underlying haematological malignancy and pulmonary infection with C. albicans.

Cryptococcus neoformans can infect both the immunocompetent as well as the immunocompromised patient. Chest radiographic abnormalities seen include diffuse interstitial infiltration, focal consolidation, nodules and hilar nodes, and pleural effusions may also be seen, as well as cavitation, although this is not common.[25,26] The chest radiograph may be normal in up to 10% of cases.

The fungus Histoplasma capsulatum may cause infections in the immunocompromised particularly in North America and the Caribbean regions. Disseminated forms of histoplasmosis may present initially with normal chest radiograph, but as the disease progresses, nodular infiltrates can mimic the appearances of tuberculosis[27] (Fig. 9).

Figure 9.

Histoplasmosis. Chest radiograph showing bilateral diffuse nodularity in an immunocompromised patient with histoplasmosis.

Coccidioides immitis infection leads to diffuse reticular nodular shadowing on the chest radiograph again mimicking tuberculosis. Focal nodules may also be seen, but are a rare presentation as are cavities.[28] Pleural effusions and hilar nodes have also been reported.

Immunocompromised patients are at risk of developing aspergillosis – invasive infection due to various Aspergillus species. This not only includes patients with HIV but also patients on steroids, those who have a low white count and those on broad-spectrum antibiotic drugs. Radiographic appearances may include unilateral or bilateral chest infiltration as well as cavitating lesions, nodules, and pleural effusions. CT findings include ill-defined nodules which may or may not cavitate, ground glass opacity, and consolidation.[24] Nodules may have a halo sign on the plain radiography and CT scan[29,28,29–31] (Fig. 10). The reverse halo sign has also been reported in association with invasive pulmonary aspergillosis[32] (Fig. 11).

Figure 10.

Invasive aspergillosis. Computed tomography image in a patient with pulmonary invasive aspergillosis with left-sided consolidation and a right-sided nodule with a surrounding halo of ground glass opacity.

Figure 11.

Invasive aspergillosis. Computed tomography image showing the 'reverse halo' sign of thickening of interlobular septa in a case of pulmonary invasive aspergillosis.

Viruses

Immunocompromised patients are more prone to viral infections which can affect the respiratory system and cause pulmonary disease. The range of radiological signs is essentially the same as for immune competent individuals.[6] The commonest agent, particularly in the HIV-infected patients, is cytomegalovirus (CMV). Chest radiography and CT may reveal a ground glass opacification as well as alveolar infiltrates, and/or reticular nodular shadowing (Fig. 12). CMV pneumonia may also cause pleural effusions. Some of these signs will overlap with infections with Pneumocystis, although pleural effusions are more common in CMV pneumonia.

Figure 12.

Cytomegalovirus infection. Computed tomography image from a lung transplant recipient. The lungs show patchy ground glass opacity and infection with cytomegalovirus was confirmed.

Other viruses which may possibly cause a pneumonitis or a focal pneumonia in immunosuppressed patients include herpes simplex virus[33] and influenza viruses including the H1N1 strain. Changes on the chest radiography may be secondary to coexisting bacterial infection as well as due to the primary effect of the virus itself and include perihilar infiltrates and consolidation[34] (Fig. 13).

Figure 13.

H1N1 infection. Computed tomography image from a patient with a bone marrow transplant who developed pulmonary infection with the novel swine-origin influenza A H1N1 virus.

Parasites

Toxoplasma gondii, a protozoan, can affect the respiratory system. On a chest radiograph, bilateral infiltrates may be identified with reticular nodular shadowing and the pattern of appearance may resemble pneumocystis infection. Pleural effusions have been described in Toxoplasma pneumonia.[24]

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