Thoracic Coccidioidomycosis Imaging 

Infection with Coccidioides immitis, a soil-inhabiting fungus, causes an illness in humans called coccidioidomycosis.

C immitis thrives in soil, and its growth occurs in either of 2 phases: the mycelial arthrospore phase in the soil and the spherule endospore phase in infected tissues. The mycelia are the least infectious, but the hyphae develop into arthrospores that become airborne and are highly infectious.^[1]

After the organism is inhaled into the lungs, the arthrospore develops into a thick-walled spherule that is filled with endospores (as shown in the image below). Once it is released, each endospore can start the development of a new spherule, and the infection in the host progresses. Coccidioidomycosis is not known to transmit from person to person.

The risk of infection is the highest in the dry summer months; a secondary period of high risk usually occurs in the late fall, terminating with winter rains. Dust exposure is a critical route for C immitis infection; individuals who dig in the soil or who are exposed to the disrupted earth are at the greatest risk.

Radiographic and computed tomography (CT) images of coccidioidomycosis nodules are seen below. In addition, C immitis can disseminate from the lungs and thoracic cavity to infect other organs, such as bone, joints, skin, and meninges. Dissemination usually occurs within weeks or months of the primary pneumonia. However, in some patients with disseminated disease, radiographs may not show evidence of previous pulmonary disease, and the patients will have no history of a preceding respiratory illness.

Preferred examination

Chest radiography is readily available and is usually the first imaging study performed. It assists in clinical staging of the disease and is useful in following up the progression or resolution of the disease. However, chest radiographic findings are nonspecific and variable.

Asymptomatic patients may have normal chest radiographic findings, and a normal result generally excludes significant clinical disease. The chest radiographic findings may progress from single or multiple areas of airspace consolidation to the formation of nodules or cavities, which may further progress to diffuse reticulonodular lung disease and upper-lobe scarring.

In a patient who is either living in or has visited an endemic area, the chest radiographic findings described in the following sections are highly suggestive of thoracic coccidioidomycosis. The diagnosis must be established by means of cultures, histopathologic examination, or serologic tests.

Limitations of techniques

Chest radiographic findings alone are not diagnostic of thoracic coccidioidomycosis, because other infectious diseases and neoplastic processes may mimic the disorder.

Asymptomatic patients may have normal chest radiographic findings. Images may depict a small, calcified granuloma and, rarely, areas of lung scarring or pleural thickening.

Primary pulmonary coccidioidomycosis

The thoracic manifestations of primary infection include parenchymal disease, intrathoracic adenopathy, and pleural effusion.

Parenchymal consolidation is the most common manifestation, being seen in 75% of patients. The segmental or subsegmental consolidation is single or multiple; it is usually unilateral and in perihilar or basal distribution. It may resolve spontaneously within 1-2 weeks. (See the images below.)

In 20% of patients, nodular lung disease (as demonstrated in the image below) is seen. The nodules frequently are well defined, simulating metastasis, or they may have ill-defined margins. They have a parahilar and lower-lobe distribution and are 5-25 mm.

In approximately 20% of patients, hilar adenopathy is present. This is usually unilateral and concomitant with parenchymal lesions. Mediastinal adenopathy is seen with severe and prolonged infection and is associated with a higher risk of dissemination.

Pleural effusion is seen in less than 20% of patients, although pleuritic chest pain occurs more frequently (in 50-75% of cases), as determined clinically. The effusion usually is small, although a massive effusion in children may suggest more severe disease, and it may even represent evidence of acute dissemination.

Scattered patchy infiltrates called persistent coccidioidal pneumonia are a less common presentation and may require as long as 1-2 months to resolve. Hilar lymphadenopathy is present in about 20% of primary infections. Pleural thickening or a small pleural effusion may be present in 20% of patients with coccidioidomycosis. A pleural effusion may occur without parenchymal or lymph node involvement.

Persistent or chronic pulmonary coccidioidomycosis

Approximately 5% of patients may develop a persistent pulmonary disease when the primary disease is present for longer than 6 weeks. This disease may include persistent pneumonia with or without adenopathy, nodules and cavities, pleural disease, bronchiectasis, empyema, or calcifications. Only 25% of patients with chronic changes have a history that is suggestive of an antecedent acute primary pulmonary coccidioidal illness.

Persistent coccidioidal pneumonia generally occurs in a severely ill patient with dense, extensive consolidation. Depending on the size of the consolidation, 3-21 months may be required for its resolution. Despite the slow resolution, fibrosis is less common. In immunocompromised patients with persistent pneumonia, the clinical symptoms become severe or even fatal, as in two thirds of cases.

Chronic, progressive coccidioidal pneumonia occurs in less than 1% of patients; it clinically and radiographically mimics chronic pulmonary tuberculosis or histoplasmosis. However, patients with this finding do have the chronic presence of C immitis on sputum cultures. Apical fibronodular lesions with cavities and volume loss are seen on radiographs, which generally show dramatic resolution with amphotericin B treatment.

Pulmonary nodules from coccidioidomycosis are the most common radiographic findings in persistent pulmonary infection. Nodular lesions (coccidioidomas) represent localized foci of incompletely resolved consolidation. Nodules may also form from filling in of a cavity. Nodules usually are well circumscribed and round, averaging 1.5-2 cm. They usually are single, and they tend to occur in the periphery of middle and upper lung zones. In contrast to the nodules in tuberculosis, these nodules may develop in the anterior segment of an upper lobe. These nodules may remain stable for months and eventually regress; only rarely is slow growth observed.

Calcification in coccidioidomas is much less frequent than it is in tuberculosis and histoplasmosis. In the evaluation of these nodules, malignancy is a primary concern for the clinician. In a review of 200 solitary pulmonary nodules that were surgically resected in patients from endemic areas, 33.5% of the nodules were malignant.^[2]

Pulmonary nodules may be better defined on CT scans of the lungs than on standard chest radiographs, and they may show marked enhancement after the intravenous administration of contrast material.

Cavities

Cavities may develop as a result of necrosis in an area of pneumonia or may be produced by excavation of a nodule. Those cavities formed by means of excavation have been reported in 10-15% of patients. Usually, the cavities appear singly and are located in the upper lobes. They may have thin or thick walls; thin-walled cavities have a tendency to change in size, which possibly reflects check-valve communication with the bronchial tree.

A rapid change in the size of a cavity suggests coccidioidal infection rather than any other granulomatous infection. A single, asymptomatic, thin-walled cavity is probably more common than other types. Most of the cavities close spontaneously in 2 years, although some may remain stable in size; they are not known to produce disseminated disease. Rarely, these cavities may be colonized by Aspergillus organisms, resulting in a mycetoma. The cavities may wax and wane over the years. A subpleural cavity may break down into the pleural cavity, causing a pneumothorax, pyopneumothorax, or bronchopleural fistula.

Bronchiectasis may occur in 1-2% of patients with chronic disease. Endobronchial coccidioidal infection can be present in rare cases, or bronchial stenosis from scarring may be evident. Calcified residual lesions in the lungs and lymph nodes contain viable organisms and are a potential source of dissemination.

Disseminated pulmonary coccidioidomycosis

Disseminated coccidioidomycosis may occur as a complication of primary illness, a late complication of chronic coccidioidomycosis, or the reactivation of latent disease in susceptible individuals. Dissemination of infection occurs hematogenously to the lungs and extrathoracic organs. Although the disease may affect any organ of the body, the principal sites of involvement are the skin, bones, joints, kidneys, and meninges. In white patients, meningitis is commonly present, and in black patients, lymph node or subcutaneous abscess is commonly associated with disseminated disease.

The radiographic manifestations of dissemination include a miliary pattern that resembles miliary tuberculosis, although the nodules in coccidioidomycosis are less well defined. Hilar and mediastinal adenopathy are almost always associated with disseminated disease. Lung biopsy is usually required for diagnosis. The differential diagnoses for the miliary pattern include other mycotic infections, tuberculosis, silicosis, sarcoidosis, and metastatic disease. Pericardial involvement may lead to pericardial effusion, cardiac tamponade, or constrictive pericarditis.

Degree of confidence

All chest radiographic patterns for thoracic coccidioidomycosis can occur with several other disease processes.

False positives/negatives

Although the sensitivity of a chest radiographic finding is high in a patient from an area with endemic disease, the specificity is low. The chest radiograph may suggest coccidioidomycosis; however, confirmation with another diagnostic test is mandatory. The chest radiographic patterns of coccidioidomycosis have a broad range of differential diagnoses; careful clinical evaluation and workup are recommended to exclude other disorders.

In select cases of thoracic coccidioidomycosis, CT scanning is more useful than are other modalities in defining the morphology of the lesions that are seen on chest radiographs^[3] . Additional lesions are frequently noted, and intrathoracic adenopathy is better seen on CT scans than it is on other images. The coccidioidomas often show marked enhancement after administration of contrast material (see the images below).

In a retrospective study of 19 immunocompetent patients with chronic pulmonary coccidioidomycosis, several CT scan abnormalities were reported. Solitary 1- to 2-cm nodules were seen in 17 patients, a focal area of ground-glass attenuation was seen in 1 patient, and focal consolidation was seen in 1 patient. Ten nodules had homogeneous attenuation on the CT scan, 2 showed cavitation, another 2 had foci of calcifications, and 1 had a central lucency. Most nodules (those in 14 patients) were peripheral, and some (those in 3 patients) were central. Ground-glass attenuation, representing granulomatous inflammation, surrounded 3 nodules.

Degree of confidence

The CT scan findings may be caused by other infectious or neoplastic or inflammatory disorders.

False positives/negatives

CT scanning has improved sensitivity in depicting small nodules and small thin-walled cavities that may not be apparent on chest radiographs. However, the specificity for the diagnosis continues to be low. CT scanning may provide the added advantage of depicting an occult calcification that may be consistent with a benign process. Nevertheless, other differential diagnoses should be considered and excluded, and additional diagnostic studies should be performed to confirm the diagnosis of thoracic coccidioidomycosis. A calcified coccidioidomycosis nodule is demonstrated in the image below.