126: Overview of Benign Pleural Conditions: Anatomy and Physiology of Pleura

The pleural space represents one of the body's potential spaces and can be affected pathologically by liquid, gas, or solid components, all of which can alter respiratory function. These processes may result from benign or malignant conditions, including infectious, inflammatory, or traumatic benign etiologies, as well as primary and secondary malignancies. This chapter provides a brief overview of the clinical presentation, diagnosis, etiology, and treatment of pleural conditions.

The pleural space lies within the layer of pleura that covers the lung and the pleural layer that lines the chest wall, diaphragm, and mediastinum. The pleural layers are composed of a monolayer of mesothelial cells supported by a thin membrane of collagen and elastin connective tissue.¹ The pleural surfaces are categorized as the visceral and parietal pleurae. The interior surface, termed the visceral pleura, covers the lung. The exterior surface, denoted by the parietal pleura, lines the chest wall, mediastinum, and diaphragm (Fig. 126-1). An analogous representation of this configuration can be created by invaginating an inflated balloon with one's fist (Fig. 126-2). The portion of the balloon that covers the hand is analogous to the visceral pleural surface, and the exterior surface of the balloon represents the parietal pleural surface. The transition from visceral to parietal pleura occurs at the hilum of the lung. Inferior to the hilum, the anterior and posterior leaves of the visceral pleura fuse together at the inferior pulmonary ligament and anchor the medial aspect of the lower lobe to the mediastinum. The sulci, or sinuses, of the pleural space are defined by various structures: the upward bowing of the diaphragm into the hemithorax, the costophrenic sinus, the costomediastinal sinuses anteriorly and posteriorly, and the mediastinophrenic sinus medially. As the diaphragm descends with inspiration, these sinuses are occupied with inflated lung.

The pleural space develops between the fourth and seventh weeks of gestation. The lateral plate of the embryologic mesoderm differentiates into the splanchnopleure, which give rise to the parietal pleura and the somatopleure, which become the visceral pleura. At the end of the seventh week of gestation, the diaphragm has separated the pleural and pericardial compartments from the peritoneal compartment, and over the next month of embryologic development, the pleural space expands cranially and caudally to surround the developing pericardial sac.

As a potential space, the pleural space is capable of transferring the mechanical forces of the expanding hemithorax to the lung, permitting unimpeded inflation. Coupling of chest wall and diaphragmatic forces to the lung across the pleural space is facilitated by the presence of about 1 mL of pleural fluid. Composed primarily of protein, with a smattering of cells (e.g., mesothelial cells, monocytes, and lymphocytes), this physiologic pleural fluid may also facilitate smooth sliding of visceral and parietal pleural surfaces against one another as the lung volume changes.^2–5 Pressure within the pleural space ranges from -2 to -40 cm H₂O as the lung expands from its functional residual capacity to maximum inspiration.²

The blood supply of the pleura is divided between the systemic and pulmonary circulations. The parietal pleura is supplied largely by the intercostal arteries, whereas the visceral pleura is supplied by both bronchial and pulmonary arteries (Fig. 126-3).^2,3,6 Similarly, the parietal pleural is innervated by the intercostal nerves.

The lymphatic flux through the pleural space occurs through lymphatic channels within the visceral and parietal pleurae, as well as through the thoracic duct, which conveys the chyle generated by the intestinal lymphatics through the thoracic cavity to drain into the junction of the left jugular and left subclavian veins. The thoracic duct exhibits some variability in its course through the thorax, existing as duplicate ducts or foregoing the usual route between the aorta and azygos vein in the lower right hemithorax before it crosses to the left at the T4 level (see Chapter 133). The lymphatic capillaries of the pleura are found within the submesothelial connective tissue. The parietal pleura also has 1- to 6-μm stomata that facilitate drainage into the submesothelial lymphatic lacunae and from there through intercostal, mammary, and mediastinal lymphatics into the thoracic duct (Fig. 126-4).^7,8

Pleural space diseases can be grouped into abnormalities of liquid, solid, and gas, although there may be some overlap between physical states. Consideration of extrathoracic disease also must be given, since the upper intra-abdominal contents may be located above the level of the costal margin, while still below the dome of the diaphragm, especially after full exhalation. Subphrenic infections, gallbladder inflammation, and splenic rupture all can lead to pain in the lower chest on inspiration. Occasionally, infradiaphragmatic pathology can lead to a pleural process. There have been case reports of gallstones migrating into the pleural space.⁹ With the so-called hepatic hydrothorax, a transudate accumulates in the pleural space of cirrhotic patients as ascitic fluid fluxes across the diaphragm, possibly through blebs or fenestrations.¹⁰ Other examples include recurrent pancreatitis that may fistulize to the pleural space.¹¹

The workup of a pleural abnormality begins with a complete history and physical examination. A thorough review of systems should be taken including specific queries about local symptoms such as pain, dyspnea, or cough, as well as more systemic findings such as fevers, weight loss, or neurologic symptoms.

A complaint of chest pain should be explored further—is the pain pleuritic? Was there antecedent trauma? Does the pain radiate to the top of the shoulder or scapula or back? The sensation of pain by the afferent nerve fibers of the parietal pleura secondary to inflammation or infection may be observed with hemothorax, parapneumonic effusion, or empyema (i.e., purulent fluid in the pleural space). On the other hand, changes in intrapleural pressures and stretching of parietal pleural surfaces on the chest wall or diaphragm may account for sensations of discomfort in cases of pneumothorax. The time course of the pain also might yield insight into the pathologic process. For example, a history of retching followed by acute epigastric or substernal pain and then progression to left-sided chest pain would suggest an esophageal rupture with initial mediastinitis followed by contamination of the left chest.

Dyspnea may reflect a true decrement in the lung function if the pathologic process usurps a significant volume within the hemithorax. This can be seen in patients in whom fluid or solid volume prevents full lung expansion. The respiratory mechanics also can be disrupted, such as may occur in cases of pneumothorax, where there is an uncoupling of the mechanical forces of the chest wall and diaphragm from the lung, with a resulting diminution of lung volume caused by the lung's intrinsic elastic recoil. However, dyspnea also may result without a significant loss of lung function. The sensation of incomplete chest expansion that accompanies some pleural processes may lead to dyspnea in the absence of hypoxia or hypoventilation.

A history of cough should be elicited to determine the characteristics of the sputum produced: color, odor, viscosity, amount, and frequency. A period of coughing that is productive of purulent sputum, in conjunction with a fever and chest pain, followed later by a persistent dry cough, may be an indication that a chronic empyema cavity has formed (see Chapter 107). A history of hemoptysis, weight loss, and heavy tobacco use in a patient with a pleural effusion would be suspicious for a malignant effusion or pleural malignancy.

A comprehensive physical examination often, but not always, corroborates the suspicions raised by an abnormal history. Auscultation, percussion, palpation, and testing for tactile fremitus or egophony all represent important aspects of the respiratory examination. It is equally important to detect pathology in other systems because the pleural space may represent only one manifestation of many in a particular disease. For example, a patient complaining of shortness of breath with stair climbing also may relate a long-standing history of joint pain and swelling. On examination, in addition to a dullness to percussion that shifts to the dependent aspects of the chest when the patient is moved from the sitting to the decubitus positions, one also might observe the severely gnarled joints and ulnar deviation of rheumatoid arthritis.

In practice, few diagnoses are made solely by physical examination. Radiologic examination of the pleural space is accomplished with plain chest radiographs, ultrasound, CT scan, and MRI. Plain films are usually the initial investigation of benign conditions from any of the three categories described earlier (Fig. 126-5). The scant volume of pleural fluid present in the healthy state is not visible on chest x-ray, but a pleural effusion of 50 mL should be detectable on a lateral chest film, evidenced as blunting of the posterior costophrenic sinus. A 200-mL effusion is evidenced by blunting of the lateral sulcus.¹² On anteroposterior (portable) films, often the meniscus of the pleural effusion is indistinct, and atelectatic or consolidated lung may contribute to the basilar opacification. An upright posteroanterior and lateral film is preferable because it should be technically superior, but even that technique still may not distinguish the fluid component of such a finding. In these cases, a lateral decubitus chest x-ray may be performed to assess the effusion, which should layer dependently if it is free flowing and not loculated.

A plain upright chest x-ray is also the mainstay of radiologic evaluation of pneumothorax. Small amounts of air in the pleural space may be detected by observing the visceral pleural line (Fig. 126-5, inset) or noting the absence of lung markings. In an otherwise normal pleural space, the first location of detectable pneumothorax is usually the apex. Thus, in most instances, an upright plain film is the appropriate study to order, although there is experimental evidence from cadaver studies that a lateral decubitus film may be even more sensitive.^13,14 The progressive collapse of the lung with increasing pneumothorax will continue to shorten the radial distances between the visceral pleural surfaces of the pulmonary lobes and the hilum. In a supine patient, the distribution of air within the pleural space is altered, and the pneumothorax may be noted as the “deep sulcus sign,” where the lateral sulcus is sharper and more lucent on the affected side or as lucency over the right or left upper quadrants.^15,16

Chest CT scanning increases the sensitivity of detection of solid, liquid, and gas within the pleural space. Fluid collections can be further characterized by measuring Hounsfield units to distinguish between simple effusions and hemothorax or evolving empyemas.¹⁷ CT scanning is also more sensitive for the detection of pneumothoraces.^18,19 It is worth stating that tension pneumothorax is not a diagnosis that should be made with CT or chest x-ray, but rather clinical assessment. Distinguishing between exudative and transudative pleural effusions is not reliably accomplished with CT scanning alone.²⁰ In terms of surgical planning, a chest CT scan can be extremely useful in planning video-assisted thoracic surgery (VATS) port sites or even thoracotomy to avoid lung adhesions and obtain optimal access to the intrapleural pathology.

MRI of the chest may be useful in the workup of malignant disease because it demonstrates tumor invasiveness and thus resectability. It has a sensitivity of up to 100% and a specificity of up to 93% in the detection of malignant disease during a workup of pleural masses.²¹ Its role in the workup of benign disease is less clear. Although it is not accurate in distinguishing between exudate and transudate, MRI can detect hemothorax reasonably well.²² In practice, however, it is unlikely to be a necessary adjunct to clinical evaluation.

Ultrasound is less sensitive than CT scanning for the detection of pleural fluid and pneumothorax,^18,19,23 although it may have increased specificity for distinguishing pleural thickening from pleural fluid.²⁴ Ultrasound is also a portable tool, which has implications in the rapid assessment of critically ill patients in the trauma or intensive care settings. Compared with plain radiography, ultrasound is more sensitive for detecting and quantifying pleural fluid.^25,26 For reasons of safety and efficacy, it is now recommended that procedures for evaluation of pleural effusion (e.g., thoracentesis) be performed with ultrasound guidance.²⁷

Thoracentesis represents the first invasive modality in the workup of a pleural effusion. It bears mentioning that thoracentesis typically is not necessary in patients with bilateral pleural effusions and clinical scenarios that point to transudative causes.²⁸ In one analysis it was found that the clinical judgment of the physician was correct in predicting a transudative effusion in 16 of 16 cases and in 15 of 17 exudative effusions.²⁹

The British Thoracic Society guidelines for workup of a unilateral pleural effusion are depicted in Figure 126-6. The first goal of pleural fluid analysis is to distinguish between exudative effusions (e.g., cloudy serum signifying disease of the pleura itself) and transudative effusions (e.g., clear serum but abnormally high concentration). The different diagnoses associated with each type of effusion are listed in Table 126-1. As recommended, the fluid is then sent in three separate tubes for pleural chemistries (e.g., lactate dehydrogenase [LDH], pH, and protein), microbiologic analysis (e.g., aerobic and anaerobic culture, Gram stain, fungal culture, and AFB stain and culture), as well as cytology. Light's criteria are used to determine whether the fluid is transudative or exudative (pleural:serum LDH >0.6, pleural:serum protein >0.5, or pleural LDH >two-thirds of upper normal serum value).^29,30 Whereas other assays have been described to distinguish exudates from transudates, Light's criteria have been validated and in clinical practice remain a mainstay of the workup.³¹ It is important to remember that the data may not return as classic profiles of malignant or benign disease. Certain data will “trump” others. For example, the finding of malignant cells on cytology yields a definitive diagnosis of malignant pleural effusion regardless of the protein and LDH levels. Similarly, the finding of pleural food particles suggests viscus (esophageal) perforation, independent of other findings. Other specific analyses can be run on the fluid, depending on the clinical situation. For example, the presence of milky fluid, suggestive of chylous effusion, would prompt the addition of a triglyceride level to the fluid analysis. Amylase levels of the pleural fluid specimen may be determined in suspected cases of pancreatic disease, keeping in mind that malignancy, esophageal rupture, tuberculosis, abdominal trauma, uremia, and radiation pleuritis also have been found to be associated with increased levels of amylase.³²

In cases of exudative effusion, when the diagnosis is still suspect, a pleural biopsy is warranted.³³ The preferred route at our institution is by means of VATS. This is done under general anesthesia, with single-lung ventilation on the nonaffected side. The exploration and biopsy usually can be accomplished through a single 12-mm port. It is important to bear in mind that the pleural tissue obtained should be sent for both pathologic and microbiologic analyses.

Empyema

Empyema is defined as a collection of pus in the pleural space. In cases associated with pneumonia, there is a continuum from simple parapneumonic effusion to complex parapneumonic effusion to empyema. Parapneumonic effusion is defined as any effusion found in association with pneumonia.³⁴ The stages of evolution of these effusions have been described: exudative, fibrinopurulent, and organizing.³⁵ A simple parapneumonic effusion may resolve with proper antibiotic therapy of the underlying pneumonia. It is worth noting, however, that antibiotic penetration into empyemas is variable and less likely to sterilize anything more advanced than the earliest stages of a developing empyema.³⁶

A complex parapneumonic effusion is one that demonstrates a pH greater than 7.20, LDH greater than three times the normal serum LDH value, glucose less than 60 mg/dL, and positive bacterial cultures. These should be drained, but since loculations form inevitably, it is increasingly more difficult to achieve complete drainage with a tube alone. Progression to the thick, fibrous “rind” of the third stage necessitates a decortication procedure to free the lung of the rind and allow full reexpansion (Fig. 126-7). An algorithm to assess and treat patients with pneumonia who are discovered to have a pleural effusion is depicted in Figure 126-8. Thoracentesis should be performed, and pleural chemistries, microbiology, and a postthoracentesis plain radiograph should be assessed to determine whether the effusion demonstrates poor prognostic factors. There exists debate regarding the use of fibrinolytic therapy to optimize drainage of infected pleural fluid. The largest prospective trial (MIST1) was reported in 2005 and randomized 430 patients to receive placebo or intrapleural streptokinase in conjunction with chest tube drainage of infected pleural fluid.³⁷ There did not appear to be an advantage over placebo in terms of need for surgery, mortality, or length of stay. The MIST2 trial utilized alteplase (a direct plasminogen activator) instead of streptokinase (indirect) but again did not show an improvement over placebo with regard to the rate of surgical intervention, duration of hospitalization, radiographic improvement, or mortality.³⁸ The MIST2 trial, however, did show an improvement when the enzyme DNase was used in conjunction with alteplase in outcome measures of radiographic improvement, need for surgery, and length of stay. A recent meta-analysis of seven randomized controlled trials on fibrinolytic therapy concluded that despite the recent negative MIST1 and MIST2 trials, there was a benefit of fibrinolytic therapy for empyema when looking at the composite endpoint of mortality and freedom from surgical intervention, as well as the single endpoint of surgical intervention alone.³⁹

Our preference is to perform VATS decortication in patients found to have poor prognostic factors (pus, positive Gram stain/culture, glucose <60 mg/dL, pH < 7.20, LDH >3× serum, loculation) with persistent or recurrent effusion after thoracentesis. Although there is only a single prospective, randomized trial comparing surgery versus fibrinolytic therapy, VATS drainage was found to perform significantly better with measurements of primary treatment success, duration of chest tube drainage, and hospital stay.⁴⁰

The medical management of empyema and other nonmalignant pleural effusions is reviewed in Chapter 127.

Other routes of infection of the pleural space exist, including traumatic, direct extension of infectious processes in the abdomen or neck, and iatrogenic (i.e., surgical or interventional). For additional detail about the surgical management of empyema, see Chapter 107. Aggressive drainage remains the key to treatment for most of these conditions.

The nonspontaneous causes of pneumothorax include trauma, iatrogenic etiologies such as inadvertent laceration of the lung during central venous line placement or intentional biopsy of the lung parenchyma, and postoperative air leak. Primary spontaneous pneumothorax refers to cases that arise in previously healthy lungs, whereas secondary spontaneous pneumothorax occurs in patients with chronic obstructive pulmonary disease, cystic fibrosis, and other lung pathologies (Table 126-2).⁴¹ The pathophysiology of both primary and secondary causes has not been well established, but the rupture of peripheral blebs or bullae is thought to be the underlying cause of primary spontaneous cases.⁴²

Once identified, pneumothorax must be treated with consideration of the patient's underlying condition. An intubated patient who is subject to positive-pressure ventilation will require closed drainage of the pleural space to prevent an unpredictable evolution of a small pneumothorax into a large or tension pneumothorax with respiratory or hemodynamic effects. Similarly, a patient with underlying lung disease and poor pulmonary reserve may tolerate less well a pneumothorax that might be treated conservatively with observation in a healthy patient with normal lungs. Treatment strategies include observation, aspiration, chest tube drainage, chemical pleurodesis, and surgical resection of the underlying pathology (i.e., blebectomy) with or without mechanical or chemical pleurodesis.⁴³

These are discussed in greater detail in Chapter 128.

A pleural fluid triglyceride level greater than 110 mg/dL is diagnostic of chylothorax. The fluid is also rich in lymphocytes, as detected on cell counting. It is suggested by the presence of a milky fluid which does not layer on centrifugation, as might be expected with turbid exudative fluid from an empyema.³³ This visual confirmation can be enhanced by feeding the patient a fat-rich meal before the investigation. Once the chylous effusion is in a collecting system, such as that into which a chest tube drains, there is an immiscible quality to it that allows identification of the milky effluent even after the meal has been digested and the lymphatic fluid begins again to run clear. The etiology of chylothorax includes traumatic/iatrogenic sequelae and medical conditions such as lymphoma, chylous ascites, and lymphatic abnormalities.⁴⁴

Conservative management includes closed drainage of the pleural space with a chest tube, strict nonenteral nutrition or medium-chain triglyceride feedings, careful observation of nutrition status, and surveillance for infections. Unfortunately, conservative management may fail in up to 48% of patients.⁴⁵ Somatostatin and etilefrine have been used as medical adjuncts to conservative management with some success.^46–48 Percutaneous embolization of the thoracic duct also has been used.^49,50 However, the mainstay of treatment involves VATS ligation of the duct. This technique is discussed in Chapter 133.

The spectrum of pleural disease is extensive and must be approached with a careful diagnostic algorithm depending on the clinical situation and the type of abnormality encountered—solid, liquid, or gas.

The pleural space has shaped our surgical approaches to diseases of the chest; a fact that is painfully apparent when the space is obliterated by disease or prior surgery. The function of the pleural space is an intriguing topic for speculation, but a reasonable hypothesis is that this space contributes to the mechanical coupling of respiratory muscle function with alveolar gas exchange.