Accumulation of pleural fluid is a common clinical problem associated with malignancy. Malignant pleural effusions (MPEs) are the second most common cause of a pleural exudate, with more than 150,000 patients diagnosed annually in the United States alone.1,2 MPEs represent advanced disease and are generally a poor prognostic indicator. Median survival for patients with MPE ranges from 3 to 12 months and depends on the tumor origin.3 In addition, MPEs are a frequent cause of dyspnea and discomfort, which adversely affect a patient’s quality of life. This group of patients requires substantial medical support to manage the burden of their disease, and providing effective therapeutic management remains a challenge. In the United States, there are approximately 126,000 admissions for MPE annually, with a median length of stay of 5.5 days.4 Thirty-day readmission rates are almost 40%, which is approximately 1.5 times higher than for acute myocardial infarction and 2 times higher than for congestive heart failure.5 In addition, palliative measures for patients with MPE are probably underutilized.6
This review is the first of 2 articles focusing on the management of MPE. Here, we discuss the pathophysiology of this disease process and provide an overview of the evaluation and diagnosis of MPE; available therapeutic options for the management of MPE are reviewed in a separate article.
Pathogenesis and Etiology
Normally, the thoracic cavity contains less than 15 mL of pleural fluid. Therefore, the visceral and parietal pleura are usually in close proximity to each other and the space between them is a potential space. Negative intrapleural pressures generated during regular breathing create a gradient for fluid movement into the pleural space from the parietal pleura dictated by Starling forces. Pleural fluid normally has low protein content and is primarily drained back into lymphatics through stomata lining the parietal pleura.7 This system’s ability to remove pleural fluid exceeds normal fluid production by 20- to 30-fold, suggesting that accumulation of excess pleural fluid requires a combination of increased fluid production and/or impaired fluid removal.8
Several mechanisms have been associated with the development of MPE. Pleural involvement by malignancy may occur from direct invasion of the pleural cavity by tumor (eg, lung cancer, breast cancer, chest wall neoplasms) or hematogenous spread of tumor to the pleura (eg, metastasis, non-Hodgkin lymphoma).9,10 Pleural malignancies can produce cytokine and inflammatory mediators, which may directly increase fluid production or indirectly alter vascular permeability.11,12 Tumor cells can also disrupt lymphatic drainage by occluding either pleural stomata or downstream lymphatic drainage. However, tumor involvement of the pleura does not always result in the development of an effusion and is only associated with fluid accumulation in approximately 60% of cases.13,14 MPE have also been strongly associated with mediastinal metastases, likely resulting from obstruction of mediastinal lymphatics.13,15,16
Pleural effusions with negative fluid cytology and pleural biopsies may result from secondary effects of tumor burden without direct pleural involvement and are referred to as paramalignant effusions. Common causes include thoracic duct obstruction (eg, Hodgkin lymphoma), bronchial obstruction, pneumonia, atelectasis, pulmonary embolism, trapped lung, and effects related to radiation or chemotherapy.15
Lung cancer is the most frequent cause of MPE and accounts for approximately one-third of cases. Other common primary tumor sites include breast, lymphoma, ovary, and gastrointestinal. Combined, these etiologies comprise about 75% of cases (Table 1).4,5 Females comprise a greater percentage of patients with MPE mainly due to the prevalence of ovarian and breast cancer. Mesothelioma-related effusions may be more prevalent in certain parts of the world due to associated exposure to asbestos.17 The primary tumor origin remains unknown in approximately 10% of cases.4
Clinical Presentation and Response to Therapeutic Drainage
More than 75% of patients with MPE are symptomatic. Dyspnea is the most common symptom and is present in more than half of patients.15 The mechanism of dyspnea caused by large effusions may not be solely due to impaired lung volumes or gas exchange. Other associated factors include decreased chest wall compliance, mediastinal shift causing decreased volume of the contralateral lung, paradoxical motion of the diaphragm, inefficient muscle length-tension relationships resulting from the stretch of respiratory muscles, and reflex stimulation from the lungs and chest wall.18-20 Other common presenting symptoms include cough, orthopnea, and chest pain. Hemoptysis suggests endobronchial involvement of the large airways. And, given the advanced nature of most MPEs, patients may also present with weight loss and cachexia.