Nontraumatic, Nonvascular Chest Emergencies

Anatomic Considerations

Anatomic clues facilitate a pattern-based approach to image interpretation. Distinguishing whether opacity lies in the alveolar space, airway, or pleural space seems basic but is requisite for accurate diagnosis. Basic anatomic landmarks such as the bronchi, fissures, vascular and nonvascular mediastinal structures, pleura, and soft tissues are important when navigating conventional radiography, especially if only a single view is obtained.

Not all opacities represent disease. Common anatomic variants such as pectus excavatum and benign masses can mimic acute disease. Recognition of these entities can help resolve diagnostic dilemmas. Displacement of fissures or mediastinal structures can be the first clue in recognizing lobar collapse or even pneumothorax. Left upper lobe collapse can be confounding and is often misinterpreted if secondary signs are not identified ( Fig. 9-1 ). Secondary signs include tenting of the diaphragm, hilar retraction toward the collapsed lung, and the Luftsichel sign (due to compensatory hyperinflation of the superior segment of the left lower lobe). When opacities abut and silhouette the pleura, the presence of obtuse or acute interfaces can help localize the lesion to the pleural or subpleural compartments, respectively. Accurate localization is vital in the setting of aspirated foreign bodies or penetrating trauma for surgical planning. Infiltrative diseases such as a necrotizing pneumonia or neoplasm can be multispacial. Thin-section computed tomography (CT), through the use of 1- to 2-mm collimation, provides exquisite anatomic detail as compared to conventional radiography but requires a greater understanding of segmental and subsegmental pulmonary anatomy to interpret the multitude of patterns that can be encountered.

Beyond lobar and segmental anatomy ( Fig. 9-2 ), the emergency radiologist should be familiar with the smallest unit visible on CT, which is the secondary pulmonary lobule. The walls of the secondary pulmonary lobule are composed of the interlobular septa. The interlobular septa are composed of connective tissue and house the pulmonary veins and lymphatic drainage to each lobule. Additional venolymphatic drainage is provided via the subpleural interstitium along the fissures and the periphery of the lung. At the center of each lobule is a bronchovascular bundle composed of a terminal bronchiole and pulmonary artery that supply oxygenated air and deoxygenated blood to the alveoli, where gas exchange occurs. They can be seen in disease states such as pulmonary interstitial edema when the septa become thickened due to vascular engorgement and increased hydrostatic pressure; radiographically these are manifested as Kerley B lines.

Pattern-Based Approach

Common patterns can be seen across varying disease states. For example, hematogenous spread of disease will manifest as randomly distributed nodules throughout the lungs regardless of cause. Therefore metastatic disease can mimic septic emboli. Cavitation can be seen in both metastatic disease and infection. Recognizing these blood-borne disease patterns helps to guide clinical treatment. Nodules in a perilymphatic distribution with septal involvement are more commonly associated with insidious disease processes such as sarcoidosis or lymphangitic carcinomatosis. Similarly, alveolar opacities, often characterized as ground-glass opacities, can represent a variety of disease states, including infection, hemorrhage, edema, neoplasm, or inflammation.

The commonly described tree-in-bud pattern seen on thin-section CT was first attributed to endobronchial spread of Mycobacterium tuberculosis (TB; Fig. 9-3 ). It described linear branching centrilobular opacities corresponding to impaction of distal airways. This finding is associated with other disease states where fluid, debris, pus, and mucus fill the distal bronchioles or if significant thickening of the distal bronchiolar walls is present. This pattern can indicate endobronchial spread of other bacterial, viral, or fungal infections such as cytomegalovirus and allergic bronchopulmonary aspergillosis (ABPA). The tree-in-bud pattern can also be seen in the setting of chronic airway disease states such as cystic fibrosis, after toxic inhalation or aspiration. Correlation with clinical and laboratory data is necessary when there is severe underlying chronic airways disease to exclude superimposed acute infection.

Several other common patterns of pulmonary disease can be seen in the acute setting. The “crazy-paving” pattern ( Fig. 9-4 ) seen on thin-section CT is characterized by scattered or diffuse ground-glass attenuation with superimposed interlobular septal thickening. This was initially described in acute pulmonary alveolar proteinosis but subsequently has been reported in a wide range of infectious and inflammatory diseases such as Pneumocystis jiroveci pneumonia ( Fig. 9-5 ), acute respiratory distress syndrome (ARDS), organizing pneumonia, and even pulmonary edema. The presence of this pattern should prompt the radiologist to alert the clinician about possible underlying immunodeficiency disorders, most commonly acquired immunodeficiency syndrome (AIDS). Another commonly encountered pattern is “mosaic” perfusion of the lungs, which can be seen in chronic thromboembolic disease, small airways disease with air trapping, and hypersensitivity pneumonitis (HP).

Cavitary masses are commonly associated with fungal and tuberculous infections. Community-acquired pneumonias can also be complicated by necrosis and cavitation. In adults cavitary lesions typically contain mixed flora, including anaerobes such as Klebsiella . In children staphylococcal pneumonia is the most common cause of cavitary pneumonia. Primary lung neoplasm and metastatic disease may cavitate and are usually included as differential considerations particularly in smokers or patients with known primary malignancy. Computed tomography can be important for additional characterization to exclude bronchopulmonary abscess and empyema.

Clinical Considerations

For the emergency radiologist, acquiring accurate history can be a challenge because patients are often triaged quickly before relevant clinical information is obtained or available. Vague clinical indications such as “cough,” “chest pain,” or “shortness of breath” are commonplace. Direct communication with clinical providers can yield important data that can augment radiologic interpretation. The added history of chronic disease states such as chronic renal failure, sickle cell disease, or malignancy sheds light on imaging findings. Clinical data regarding acute exposure to inhalational toxins, thermal injury, or patient’s immune status allow the radiologist to refine his or her interpretation. Reviewing basic laboratory data and patient medical records is frequently a worthwhile endeavor that can yield information unbeknownst even to the clinician.

Recognizing parenchymal lung disease patterns of susceptible populations can alert the radiologist to assess for associated potential complications. For example, sickle cell anemia is a frequently encountered disease in the acute care setting. Patients are predisposed to infection, as well as bouts of acute vasoocclusive crises known as acute chest syndrome . Even without history, the recognizable radiographic stigmata of sickle cell disease such as osteonecrosis and cardiomegaly can alert the radiologist to assess for changes suggestive of acute chest syndrome—which is defined as the presence of a new consolidation on chest radiography ( Fig. 9-6 ). Sickle cell patients are 100 times more susceptible to pneumonia than the general population, with a 30% recurrence rate. Pulmonary edema related to left ventricular failure can also be seen. Chronic findings of pulmonary hypertension related to microvascular occlusion and chronic hypoxia are manifested as scarring, architectural distortion, and enlargement of the pulmonary arteries.

Patients with obstructive lung physiological findings, most commonly long-time smokers, appear to have hyperinflated lungs because of pulmonary emphysema and are predisposed to infection and neoplasm. Parenchymal damage due to cystic or bullous changes can predispose this population to secondary spontaneous pneumothorax. Another commonly encountered entity is cystic fibrosis. Patients present with frequent respiratory tract complaints, including cough, dyspnea, and fever. The radiographic findings of cystic fibrosis include bronchiectasis and parenchymal scarring. Computed tomography findings include bronchial wall thickening, progressive bronchiectasis, and mucoid plugging within the bronchi with upper lobe predominance. Hyperinflation and secondary spontaneous pneumothorax can also be seen. Consideration should also be given to the wide range of disease states that result in immunosuppression. In addition to human immunodeficiency virus (HIV)/AIDS, patients undergoing chemotherapy, long-term corticosteroid use, or recent organ transplantation are susceptible to opportunistic infections.

Infection

Infectious disease processes of the thoracic cavity are diverse in clinical and radiologic presentation depending on the pathophysiological characteristics of the offending organism. Lobar and interstitial pneumonias, cavitary pneumonias, endobronchial infection, pleural-based infections, and hematogenous spread of infection are all encountered in the acute care setting, and the emergency radiologist should be alert to the radiographic findings and common associations of these disease states.

Fungal Infections

Fungi are uncommon pathogens in otherwise healthy populations. Fungal infections occur more frequently in susceptible populations. Fungal infections can manifest with unique radiologic findings. Pulmonary aspergillosis is an opportunistic infection that has four distinct forms: ABPA, aspergilloma, and invasive and semi-invasive aspergillosis. ABPA can be seen in long-standing asthmatic patients typically under the age of 40 years. Eosinophilia and elevated serum immunoglobulin E levels are features of ABPA. During the initial stages of ABPA, chest radiographs may appear normal or depict asthmatic changes. If mucoid impaction within the dilated central bronchi is present, the classic “finger-in-glove” appearance may be seen ( Fig. 9-7 ). Computed tomography findings include fleeting alveolar opacities, central upper lobe bronchiectasis, bronchial wall thickening, and mucoid impaction. Lung distal to the impacted region may become hyperinflated or atelectatic.

Aspergilloma or fungus ball can appear as an air crescent sign, a round focal intracavitary mass surrounded by crescent of air ( Fig. 9-8 ). Early signs include increasing wall thickness of a preexisting cavity or cyst, most commonly prior cavitary TB. The mass may shift with patient position. Invasive aspergillosis appears mainly in severely immunocompromised patients, and it may demonstrate patchy nodular opacities or lobar type of air-space disease in cases with vascular invasion. Computed tomography may disclose a halo or ground-glass attenuation and is more accurate in the detection of early disease. Cavitation frequently develops with time and typically results in the air crescent sign. Semi-invasive aspergillosis has an indolent course of onset. Radiologic features of semi-invasive pulmonary aspergillosis include areas of focal consolidation or masslike lesions with upper lobe predilection and pleural thickening.

Pneumocystis pneumonia is an opportunistic infection caused by the yeastlike fungus P. jiroveci . It is seen most commonly in HIV patients with a CD4 count of less than 200 cells/mm. Patients present with worsening dyspnea and a nonproductive cough. Plain radiographic features vary and may include a fine reticular interstitial pattern or patchy ground-glass peribronchial opacities (see Fig. 9-5 ). Computed tomography is more sensitive than radiography and may identify diffuse ground-glass opacities, septal thickening, and pneumatoceles (30% of cases). The crazy-paving pattern seen in pneumocystis pneumonia is characterized by scattered or diffuse ground-glass attenuation with superimposed interlobular septal thickening. Atypical radiologic features in those treated prophylactically include cavitating nodules, lymphadenopathy, consolidation, and pleural effusion. The upper lobe–predominant, cystic form poses an increased risk for pneumothorax.

Blastomycosis is an airborne fungal disease endemic in the southern and central United States. Imaging manifestations are nonspecific but most commonly include air-space disease, nodular masses, interstitial disease, and cavitation. Histoplasmosis is a systemic fungal infection caused by breathing the spores of Histoplasma capsulatum , mostly found in the Mississippi and Ohio River valley regions. Although immunocompetent hosts may be asymptomatic with no abnormal radiographic findings, disseminated histoplasmosis mostly occurs among the immunocompromised and those with chronic, debilitating diseases. Nearly 50% of these patients will initially have normal radiographic findings. In others radiographic findings may include diffuse, interstitial, reticulonodular or miliary opacities. Smaller nodules may coalesce and produce air-space disease and ARDS.