|
Article Excerpt
Byline: SACHIN R. PENDHARKAR, MD, FRCPC and ALAIN TREMBLAY, MDCM, FRCPC, FCCP
Abstract: Pleural effusion is a common condition with a long list of potential causes. A multitude of diagnostic tests are useful in the elucidation of these causes, which is a key step in determining appropriate therapy for the patient. Ultrasonography detects pleural effusions with greater accuracy than chest radiography, whereas CT scans are valuable for differentiating pleural disease from lung abscess and other thoracic diseases and for diagnosing pleural malignancy. Positron emission tomography (PET) and CT-PET are useful adjuncts in the diagnosis and staging of malignant mesothelioma. Thoracentesis should be performed early because pleural fluid analysis is a critical component of the workup. Numerous criteria have been proposed to differentiate exudates from transudates; the most commonly used are Light criteria; an alternative criterion is the serum-effusion albumin gradient. (J Respir Dis. 2007;28(12):565-582)
Key Words: Pleural effusion, Malignant mesothelioma, Pleural fluid analysis
---
Pleural effusions are a common problem seen in isolation or in association with a number of pulmonary and extrapulmonary diseases. The approach to the diagnosis of pleural effusion is multifaceted, beginning with a careful clinical assessment followed by radiological, laboratory, and cytopathological studies and recognition of the common clinical patterns.1
In this review, we will discuss the clinical presentation and differential diagnosis of pleural effusion; imaging modalities that are useful in evaluating pleural diseases; the important components of pleural fluid analysis; and invasive pleural procedures, such as thoracentesis, closed pleural biopsy, and thoracoscopy. Finally, a general clinical approach will be proposed.
PLEURAL ANATOMY AND PHYSIOLOGY
The pleural space is bound by a visceral and parietal layer that meet at the hilum. Blood supply is from the intercostal arteries for the parietal pleura and via the bronchial circulation for the visceral pleura. As with other structures supplied by the bronchial arteries, drainage of the visceral pleura is through the pulmonary veins. Lymphatic drainage occurs primarily through stomata in the parietal pleura to infracostal lymphatics and into parasternal and periaortic lymph nodes. Sensory innervation provided by the intercostal and phrenic nerves is only found in the parietal pleura.2
Pleural fluid is generated from pleural vessels as a result of negative intrapleural pressure and exits via the parietal pleural lymphatics. The entry of pleural fluid occurs at approximately 0.5 mL/h in an adult male; the exit of fluid varies with the pressure generated by excess pleural fluid and can exceed the normal entry rate 30-fold.2 The accumulation of pleural fluid implies an increase in fluid production exceeding the capacity of lymphatic removal and/or an obstruction of drainage of the pleural space (Figure 1).
CLINICAL FEATURES
Pleural effusion should be suspected in any patient with dyspnea; other features include pleuritic or nonpleuritic chest pain and nonproductive cough. Symptoms of the underlying disease causing the effusion may also be present.
Physical signs of pleural effusion include asymmetric chest expansion, dullness to percussion, decreased breath sounds, reduced vocal fremitus, and the presence of a pleural rub. A comparison study of physical signs in a cohort of patients with respiratory symptoms revealed that asymmetric chest expansion and dullness to percussion had adjusted odds ratios of 5.22 and 12.80, respectively.1 Negative predictive values for asymmetric chest expansion, reduced vocal fremitus, dullness, decreased breath sounds, and reduced vocal resonance were all above 90%.1
IMAGING STUDIES
Radiography
As fluid accumulates in the pleural space, it fills the costophrenic sulci, first posteriorly and subsequently on the lateral aspect. The result is a meniscus seen on a chest radiograph.3 Pleural fluid can be consistently identified on lateral and frontal radiographs when the volume of fluid is greater than 50 and 200 mL, respectively. Complete obscuration of the hemidiaphragm usually occurs at a fluid volume greater than 500 mL.4 These volumes may vary with the size of the patient; larger patients may accumulate more fluid before the radiographic findings appear abnormal.5
It has been recommended that a lateral decubitus view be routinely obtained to identify pleural effusion, assess for loculation, and determine whether the volume is sufficiently large to warrant drainage.5 Light and associates6 determined that a pleural fluid level 10 mm above the lateral chest on a decubitus radiograph predicted failure of antibiotics in patients with parapneumonic effusion; this value has since been correlated with a 5-cm pleural fluid column height on a lateral projection, possibly obviating the need for a decubitus radiograph.5 Supine radiographs are notoriously poor for the diagnosis of pleural effusion, with sensitivity and specificity of only 65% to 70%.7
Other radiographic challenges include a "pseudotumor" appearance of fluid in an interlobar fissure (Figure 2)8 and a subpulmonic effusion (Figure 3), which may be suggested by a steep lateral drop from a slightly peaked hemidiaphragm.9 A massive effusion may shift the mediastinum to the contralateral side, raising a suspicion of lobar collapse or mediastinal fixation if this shift is not seen.3
Ultrasonography
Pleural fluid appears as an echo-free space between the pleural layers. Its morphology may change with respiration, as evidenced by a change in color signal on Doppler images.10 Lung movement with respiration may be impaired if an inflammatory pleural process results in adhesions or septations.11
Pleural effusions are described as anechoic, complex and nonseptated, complex and septated, or echogenic depending on the amount of cellular and fibrinous debris within the effusion. While transudates are typically anechoic,12 this schema does not correlate with clinical or biochemical parameters in the differentiation of nonmalignant exudates.9,13,14 Pleural irregularity or nodular thickening in association with pleural fluid should raise suspicion of malignancy in the absence of previous asbestos exposure.14
Ultrasonography detects pleural effusions with greater accuracy than chest radiography,10,15 particularly when patients are supine.11 Other advantages include lack of radiation exposure, low cost, and real-time guidance for thoracentesis or drain placement for smaller or loculated effusions.2,16
We recommend liberal use of thoracic ultrasonography for the bedside identification of pleural fluid in patients in whom plain radiography is indeterminate and as a guide to thoracentesis or chest tube placement, particularly in patients with small or loculated effusions.
CT
Thoracic CT scans can help to differentiate parenchymal from pleural abnormalities based on higher-resolution images of tissue planes and variable densities of opacity.2 Specifically, uniform thickening of visceral and parietal pleura, obtuse angulation of the abnormality as it abuts the chest wall, lung compression, and pleural separation ("split pleura" sign) differentiate empyema from lung abscess (Figure 4).17 Bolus intravenous contrast highlights these and other findings in the differentiation of pleural disease from other thoracic disease.18
CT scans also have excellent diagnostic value in identifying pleural malignancy. In addition to demonstrating suspicious parenchymal nodules or masses, the finding of parietal pleural thickening greater than 1 cm, the presence of a pleural rind, nodularity, and involvement of mediastinal pleura each have specificities of greater than 88% for malignancy, albeit lower sensitivity (36% to 56%).19
In one study, mesothelioma was differentiated from pleural metastases because of a lack of isolated pleural effusion on a CT scan, although the number of cases was small for both.19 Pleural plaques and bilateral pleural involvement on CT scans helped identify benign or malignant asbestos-related pleural disease with specificities of 94% and 74%, respectively. Other CT findings seen in mesothelioma include volume loss, preferential mediastinal pleural thickening, and involvement of the interlobar fissures.20
PET...
|
