To establish the frequency of APE, we retrospectively analyzed our experience over a one-year period. The frequency with which PE was asymptomatic was 16 percent. Based upon this, we have substantiated the clinical impression of Light1 that APE are not uncommon.
Spectrum of Causes
Asymptomatic PE had a similar spectrum of causes compared to SPE. While our study and the literature suggest that any disease that can cause SPE can also
cause APE, malignancy, congestive heart failure, pneumonia and abdominal and thoracic surgery accounted for approximately 75 percent of the effusions in both groups. Although postpartum and benign asbestos effusions were not diagnosed in any of our patients, the literature suggests that they commonly present asymptomatically. Therefore, they deserve further discussion.
Childbirth is a common, yet infrequently considered cause of APE. Hughson et al radiographically examined the prevalence of PE within 24 hours of vaginal delivery. Up to 67 percent of patients had small-to-moderate size effusions which were often bilateral. Almost all of these patients were asymptomatic. They did not have an increased incidence of postpartum complications despite the lack of a specific diagnosis. Although this study requires further confirmation, it seems that asymptomatic effusions in the immediate postpartum period are common and in the absence of other findings, these effusions do not require evaluation.
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Benign asbestos effusion (BAE) is the most common manifestation of asbestos-related pulmonary disease within 20 years of first exposure. Epler at al identified PE in 4.8 percent of 1,135 asbestos-exposed workers undergoing a multi-year surveillance, a prevalence five times that of the non-exposed control population. Thirty-four of 54 (63 percent) were determined to be BAE, and of these, two thirds were asymptomatic. Characteristics of BAE include: 1) usually unilateral location; 2) small-to-moderate size; 3) negative cultures; 4) negative cytology; and 5) serous or bloody exudative fluid. The prognosis of these patients is excellent despite frequent recurrence of the effusion. Management of BAE involves exclusion of other causes of PE and close observation.
Why and When Are Pleural Effusions Asymptomatic?
The mechanisms by which symptoms develop in PE are not known. Dyspnea has been attributed to an alteration in chest wall mechanics and to hypoxemia resulting from atelectasis. Lack of improvement in these parameters following removal of600 to 1,800 ml of pleural fluid despite subjective improvement in dyspnea suggests otherwise. Coexistent parenchymal disease and splinting from pleuritic pain may also contribute to dyspnea.
The mechanism of cough in PE is also unclear. Possible causes include stimulation of pleural or airway cough receptors by the presence of pleural inflammation or atelectasis of the lung causing airway collapse. Also, cough is a common complication of thoracentesis and may be related to stimulation of pleural or airway cough receptors by removal of a large volume of fluid.
Chest pain in PE is a manifestation of pleural 194 inflammation. Pain can be either sharp or dull and aching. Its presence implies involvement of the parietal pleura since the visceral pleura is without pain fibers. Chest pain can be localized to the involved area of the pleura or referred. Important sites of referred pain are the abdomen and the ipsilateral shoulder. Abdominal pain occurs because of the presence of intercostal innervation above and below the diaphragm. Shoulder pain indicates involvement of the diaphragmatic pleura in the phrenic nerve distribution.
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We can only speculate why some PEs are asymptomatic. The absence of a primary inflammatory process involving the pleura may be one reason (postpartum or left ventricular failure effusions). Alternatively, small effusions, even bilateral ones, may not compress airways and lead to symptoms in the sedentary patient. Perhaps symptoms associated with the primary process distant from the chest (eg, colon cancer) absorb most of the patients attention, thus minimizing respiratory symptoms, or cases without pulmonary symptoms may be discovered inadvertently before development of symptoms. Finally, altered mental status associated with underlying disease can result in masking thoracic symptoms. Hepatic encephalopathy as seen in our own series or uremia could lead to this result.