Pleural Effusion: When Should You Be Concerned?

Dr. Rūta Nutautienė

 Introduction

Pleural effusion is diagnosed in approximately 1 million patients worldwide each year. The normal amount of fluid in the pleural cavity is approximately 1.0–15 ml, regulated by the balance between hydrostatic and oncotic pressure in the visceral and parietal pleural blood vessels and by lymphatic drainage. When this balance is disrupted, fluid accumulates in the pleural cavity.

When the fluid volume reaches 200–300 ml, it can be detected on a posteroanterior chest X-ray. On a lateral decubitus chest X-ray, pleural fluid can be observed when as little as 100–300 ml is present.

In any case, when a larger amount of fluid is present, it is necessary to investigate and identify the etiological factors. The most common causes are heart failure, pneumonia, oncological processes, pulmonary embolism, and, in our country, tuberculosis.

Classification and Diagnosis of Pleural Fluid

The main diagnostic procedure for determining the origin of pleural fluid is pleural cavity puncture. It is performed in all patients with fluid in the pleural cavity, except when the amount of fluid is very small and puncture cannot be performed safely.

After aspiration of the pleural cavity, the color, transparency, and odor of the pleural fluid are evaluated first. The nature of the fluid is particularly important: it may be bloody (hemorrhagic pleurisy or hemothorax), purulent, or chylous. A foul odor is characteristic of pleural empyema caused by anaerobic bacteria. Other colors of pleural fluid are not significant for differential diagnosis.

Traditionally, fluid in the pleural cavity is classified as either transudate or exudate according to diagnostic criteria published in 1972 by R. W. Light and colleagues. These criteria are still used in many clinics worldwide (Table 1). Light's criteria have a sensitivity of 98% and a specificity of 83%.

Transudate is believed to develop when the balance between oncotic and hydrostatic pressure is disrupted. However, the possibility of iatrogenic entry of fluid into the pleural cavity due to improper positioning of a central venous catheter or nasogastric tube should not be overlooked.

Exudate is more commonly caused by inflammatory conditions that require interventional and therapeutic procedures. Inflammatory processes increase protein secretion into the pleural cavity, followed by fluid accumulation. As the fluid accumulates, it stimulates further progression of the process. Possible causes of transudate and exudate are presented in Table 2.

It has been observed that in patients taking diuretics who have an increased protein concentration in pleural fluid, Light's criteria may incorrectly classify the fluid as an exudate. Therefore, it is recommended that albumin concentrations in both pleural fluid and blood be measured in these patients. If the albumin gradient (blood albumin concentration minus pleural fluid albumin concentration) is greater than 12 g/l, the pleural fluid is classified as a transudate; if it is less than 12 g/l, it is classified as an exudate.

If the fluid is determined to be a transudate, further investigation is not necessary. It is important to determine which of the four most common causes—heart failure, liver cirrhosis, nephrotic syndrome, or pulmonary artery thrombotic embolism—is predominant. If the fluid is an exudate, further investigation is required, including cytological, biochemical, and bacteriological examinations.

As with other diseases, it is important to evaluate the previously mentioned causes of pleural fluid accumulation and systematically obtain the patient's medical history, including previous or current illnesses involving the heart, kidneys, liver, and other organs; occupational or living conditions, including contact with asbestos; harmful habits such as smoking and alcohol or drug use; chest trauma or medical interventions; medications currently or previously taken; and possible neoplastic processes.

Other, rarer diseases can be excluded when obtaining the medical history and reconsidered if none of the main conditions mentioned above are confirmed. It is also important to establish when the clinical signs of pleurisy began, for example, following pneumonia, trauma, medical interventions, or other causes.

The patient may complain of cough, shortness of breath, and pleuritic pain. These symptoms may occur individually or in combination, but they are nonspecific and rarely help determine the cause of fluid accumulation in the pleural cavity.

Spiral computed tomography of the chest helps differentiate pleural disease from pulmonary parenchymal disease and provides a better assessment of mediastinal lymph node involvement, pulmonary parenchyma, pleura, chest wall, bony structures, mediastinal changes, the localization and extent of the pathological process, and the nature of the fluid in the pleural cavity.

Ultrasound is the simplest and most accessible method for detecting fluid in the pleural cavity, estimating its volume, and even assessing how long the fluid has been present. In terms of specificity, it is equivalent to computed tomography. According to the literature, the distance between the lung and the parietal pleura can be used to estimate the amount of fluid. Although the measured distance may vary, it is sufficient to provide an approximate estimate of fluid volume.

On a frontal chest X-ray, pleural effusion becomes visible when more than 150–170 ml of fluid is present. It most commonly appears as a shadow in the costophrenic angles.

Magnetic resonance imaging (MRI) allows good visualization of pleural changes, differentiation of solid structures from fluid in the pleural cavity, and assessment of changes in the diaphragmatic pleura and chest wall. However, there are no comparative studies between contrast-enhanced computed tomography and MRI, making it difficult to determine which examination is superior.

Bronchoscopy is important in the differential diagnosis of pleural fluid accumulation, including neoplasms, tuberculosis, and foreign bodies, in the following cases:

● lung infiltration observed on chest X-ray or computed tomography;
● presence of bloody fluid;
● a large amount of fluid in the pleural cavity, occupying more than 3/4 of the pleural cavity;
● displacement of the mediastinum toward the pleural fluid.

Treatment of Pleural Fluid

Fluid accumulation in the pleural cavity is a consequence of other diseases, so treatment depends on the underlying cause.

When a large amount of fluid causes discomfort and respiratory distress, it should be removed by puncture or drainage.

Drug-induced fluid accumulation is usually not significant. Discontinuing the medication suspected of causing fluid accumulation, such as procainamide, hydralazine, quinidine, nitrofurantoin, amiodarone, procarbazine, or methotrexate, is often effective, and additional interventions are usually unnecessary.

In cases of purulent fluid with a pH below 7.2, encapsulated fluid, or bacterial infection, urgent drainage with a large-bore drain is necessary. In patients undergoing drainage and receiving appropriate antibiotic treatment, the condition usually improves within a week. If a large amount of fluid remains or increases after one week, as determined by ultrasound or other methods, repeat drainage of the pleural cavity should be considered, and other causes of fluid accumulation should be reassessed.

Pneumonia may coexist with a neoplastic process or tuberculosis. In such cases, computed tomography and FBS are recommended.

Pleural fluid in oncology patients is usually a sign of disease progression. The average survival of these patients is approximately 1 year. Fluid accumulation is usually first diagnosed when slowly progressive dyspnea develops. Because the process is gradual, patients may attribute their symptoms to the underlying disease and seek medical attention only after a large amount of fluid has accumulated.

Patients feel considerably better after the fluid is removed. However, the improvement is temporary, and fluid recurrence occurs within 2–3 weeks. Repeat pleural punctures are performed to remove the fluid. It has been observed that fluid is removed more effectively when pleural drainage is used. In some cases, spontaneous fusion of the pleural cavity occurs as a result of long-term drainage. This represents an alternative to pleurodesis surgery.

Tuberculous pleural fluid usually resolves spontaneously with adequate antituberculous treatment. Surgical intervention is used to clarify the causes of fluid accumulation when no other diagnostic options are available or solely for therapeutic purposes.

During visually controlled thoracoscopy, local or general anesthesia is used to examine the pleural cavity, obtain biopsy material, and remove accumulated fluid. Pleural abrasion may be performed when necessary. The procedure is classified as minimally invasive.

When the pleural cavity becomes organized for certain reasons, decortication is performed—the thickened pleura is removed and the lung is released. Failure to remove the damaged pleura results in lung entrapment, leading to shortness of breath because the lung cannot expand properly during breathing. This is a major surgical procedure requiring thoracotomy.

Drug treatment is directed at the underlying disease. If pleural fluid accumulation is caused by heart failure, the most effective treatments include diuretics, vasodilators, and other cardiovascular medications. Diuretics will not be effective if the fluid is caused by lung inflammation, a tumor, or tuberculosis.

In cases of pleural empyema, antibiotics are essential in addition to pleural cavity drainage. Combinations of cephalosporins and antianaerobic agents may be used. In uncomplicated parapneumonic effusions, a single antibacterial medication directed against the suspected or confirmed pathogen is sufficient.

Publication "Internist" (152)

References in the editorial office