Practice Essentials
Pneumonia is predominantly a clinical syndrome. The classic etiologic agents of atypical pneumonia are Legionella species, Mycoplasma pneumoniae, and Chlamydia pneumoniae. Many other diseases, caused by various pathogens, should be considered in the differential diagnosis. Such etiologic agents include fungi, mycobacteria, parasites, and viruses (eg, influenza virus, adenovirus, respiratory syncytial virus, human parainfluenza virus, measles, varicella zoster, Hantavirus). [1, 2, 3, 4, 5, 6]
In immunosuppressed patients, outbreaks of isolated cases of respiratory virus infections with atypical presentations have been reported. These infections can be severe and may have concomitant bacterial etiologies. In endemic areas, certain zoonotic infections should be considered when patients present with atypical pneumonia. Noninfectious etiologies must be considered in atypical and nonresolving pneumonias.
Lung imaging with chest radiography has been the standard method of diagnosing pneumonia; however, lung ultrasonography has the potential to more accurately and efficiently diagnose pneumonia, as well as pleural effusions, pneumothorax, pulmonary embolism, and pulmonary contusions. Because lung ultrasonography can be performed quickly at the bedside and lacks ionizing radiation, it can be a diagnostic option in primary care and emergency department settings. [7, 8, 9] Robbins and Kumar described Legionella infection in an HIV patient who was found on MRI to have a lesion of the splenium of the corpus callosum. [10]
(Scans of atypical bacterial pneumonia are depicted below.)
A 53-year-old patient with severe Legionellapneumonia. Chest radiograph shows dense consolidation in both lower lobes.
A 40-year-old patient with Chlamydia pneumonia. Chest radiograph shows multifocal, patchy consolidation in the right upper, middle, and lower lobes.
A 38-year-old patient with Mycoplasma pneumonia. Chest radiograph shows a vague, ill-defined opacity in the left lower lobe.
Chest computed tomography scan shows ill-defined, airspace infiltrate in the left lower lobe.
Radiologic phases
A phase of active hyperemia occurs, lasting approximately 24 hours before radiologic consolidation of the alveoli appears. This phase is characterized by engorgement of the arterial blood vessels. Edematous fluid, which may be seen in the alveolus, contains few exudative cells.
The next stage is referred to as red hepatization. Neutrophils and fibrin material fill the alveoli, and massive extravasation of red blood cells produces a homogeneous opacity.
The red hepatization phase is then followed by gray hepatization. Fibrin and exudative cells accumulate, appearing on radiographs as a clear zone adjoining the alveolar and acinar cells.
If the process extends to the pleural space, associated empyema may be present.
Radiography
Radiologic findings alone are not reliable in differentiating pneumonia into typical or atypical forms. Therefore, the radiographic findings should be used along with clinical and laboratory data to narrow the possibilities. Structural lung disease with abnormal lung parenchyma affects the pattern of infiltrates. In cases of severe emphysematous lung disease, clinicians may tend to underestimate the presence of infiltrates on chest radiographs.The radiographic characteristics of Legionella, Mycoplasma, and Chlamydia pneumonias are discussed below. [11, 7, 1]
Legionella pneumonia
Legionella species are implicated in 2-15% of community-acquired pneumonia (CAP) cases. Because many manifestations of Legionella are similar to other typical and atypical pneumonias, clinical symptoms or radiologic evidence is of little value for diagnostic purposes. [2, 12, 13, 10, 5]
These organisms usually cause a patchy, localized infiltrate in the lower lobes. Associated hilar adenopathy may be present. Pleural effusion is seen in up to 30% of cases. In rare instances, Legionella infection is associated with cavitation and a masslike appearance. Radiologic resolution of Legionella pneumonia may take 6-12 months. Permanent residual fibrosis is observed in as many as 25% of patients. An early progression of infiltrates can occur despite clinical improvement. [12, 5]
(Legionella pneumonia is shown in the image below.)
A 53-year-old patient with severe Legionellapneumonia. Chest radiograph shows dense consolidation in both lower lobes.
Mycoplasma pneumonia
M pneumoniae is implicated in 2-30% of all cases of CAP. Mycoplasma pneumonia is usually mild and results in a rapid resolution of any radiologic findings. However, it tends to be more severe in patients with sickle cell anemia. Radiographic resolution in 40% of patients occurs in 4 weeks, and 80% of cases resolve by 8 weeks. Residual radiographic abnormalities are uncommon. [1, 2, 3, 9]
The infiltrates in Mycoplasma pneumonia can be unilateral, multilobar, or bilateral. [3] In about 20% of patients, pleural effusion or hilar adenopathy may be present.
(Mycoplasma pneumonia is shown in the image below.)
A 38-year-old patient with Mycoplasma pneumonia. Chest radiograph shows a vague, ill-defined opacity in the left lower lobe.
Chlamydia pneumonia
On initial chest radiographs, the predominant finding is a unilateral pattern of alveolar infiltrates or bronchopneumonia. Findings are often limited to a single lobe, with lower lobe involvement occurring more frequently than involvement of the middle lobe or upper lobe . Interstitial pneumonia is rare. Up to 25% of patients may have a small to moderate-sized pleural effusion. Hilar or mediastinal lymphadenopathy is uncommon. Chest radiographs show 50% resolution in 4 weeks. In 20% of cases, resolution takes longer than 9 weeks. [1, 2]
(Chlamydia pneumonia is shown in the image below.)
A 40-year-old patient with Chlamydia pneumonia. Chest radiograph shows multifocal, patchy consolidation in the right upper, middle, and lower lobes.
Computed Tomography
Computed tomography (CT) scans are increasingly being used in clinical practice. Some authors have questioned CT scanning's usefulness in evaluating consolidations, suggesting that the value of CT in the diagnosis of pneumonia is limited to specific cases involving the following [14, 15] :
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An indistinct, abnormal opacity on chest radiographs
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Patchy, ground-glass, linear, or reticular opacities on chest radiographs
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Possible pleural effusion
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Neutropenia and fever of unknown origin (for which ultra–thin-section CT scanning may be helpful)
Coinfection with several organisms is not uncommon. Underlying parenchymal lung abnormalities usually predispose patients to pneumonia. Therefore, in patients with pneumonia, the overall clinical and radiologic picture must be considered in place of an independent, dichotomous view.
(See the image below.)
Although this patient smokes, this lesion most likely has an inflammatory etiology, given the clinical symptoms and a recent, normal CT scan. Appropriate management includes repeat CT scanning in 3 months if the lesion persists or enlarges despite clinical improvement.
Chlamydia pneumonia
A significant increase in airway dilatation or pulmonary emphysema, or both, are common CT findings in patients with C pneumoniae, but such findings do not reliably distinguish C pneumoniae from other pneumonia pathogens. On CT, findings of C pneumoniae are widely variable. In a study by Sharma et al, predominant findings included involvement of more than one lobe, with consolidation and bronchovascular bundle thickening. Bilateral lung involvement was seen in 50% of patients. [2]
(A CT scan of chlamydial pneumonia is shown in the image below.)
Chest computed tomography scan in a 45-year-old patient with Chlamydia pneumonia shows a right upper-lobe infiltrate.
Legionella pneumonia
Mild Legionella pneumonia may manifest with bilateral involvement of the lung parenchyma. Multiple segments are affected, and peripheral lung consolidation with ground-glass opacity and pleural effusion may be seen. With more severe infection, lung cavitation and bulging of the fissure have been reported. Residual lung parenchymal scarring can be found, even after the acute infection resolves. [13]
(A CT scan of Legionella pneumonia is shown in the image below.)
Image in a 66-year-old patient with Legionella pneumonia. Chest computed tomography scan shows dense alveolar consolidations in both lower lobes.
Mycoplasma pneumonia
The infiltrates in Mycoplasma pneumonia can be unilateral, multilobar, or bilateral. In about 20% of patients, pleural effusion or hilar adenopathy may be present. [3] In high-resolution chest CT of community-acquired M. pneumoniae infection, findings include lobule centricity nodules and bronchial wall thickening. [4]
Ultrasonography
Ultrasonography can help differentiate between consolidation and effusion. Consolidated lung tissue may appear as hypoechoic areas with blurred margins. The texture varies with the amount of aeration, being more heterogeneous with aeration and more homogeneous with dense consolidation. Ultrasonography may also aid in the diagnosis of empyema and abscesses. However, in clinical practice, ultrasonography's usefulness may be limited to the identification and quantification of parapneumonic effusions. Once found, the area where an effusion occurs can be marked for subsequent diagnostic or therapeutic thoracentesis.
Long et al found that lung ultrasound can help diagnose adult pneumonia with a high accuracy. rate Their meta-analysis of 12 studies with 1515 patients found the sensitivity and specificity to be 0.88 (95% CI: 0.86–0.90) and 0.86 (95% CI: 0.83–0.88), respectively. [8]
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A 53-year-old patient with severe Legionellapneumonia. Chest radiograph shows dense consolidation in both lower lobes.
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A 40-year-old patient with Chlamydia pneumonia. Chest radiograph shows multifocal, patchy consolidation in the right upper, middle, and lower lobes.
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A 38-year-old patient with Mycoplasma pneumonia. Chest radiograph shows a vague, ill-defined opacity in the left lower lobe.
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Chest computed tomography scan shows ill-defined, airspace infiltrate in the left lower lobe.
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Chest computed tomography scan in a 45-year-old patient with Chlamydia pneumonia shows a right upper-lobe infiltrate.
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Image in a 66-year-old patient with Legionella pneumonia. Chest computed tomography scan shows dense alveolar consolidations in both lower lobes.
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Although this patient smokes, this lesion most likely has an inflammatory etiology, given the clinical symptoms and a recent, normal CT scan. Appropriate management includes repeat CT scanning in 3 months if the lesion persists or enlarges despite clinical improvement.
