The pleura are anatomically and physiologically distinct structures. The pleural cavities are formed early during embryonic development from the intraembryonic coelom. Anatomically, the pleura are divided into two structures: the visceral and the parietal pleura. The visceral pleura lines the lungs and is intimately invested with the lung parenchyma. The parietal pleura lines the chest wall, diaphragm, mediastinum, and the superior portion of the lungs.
Pathophysiology
The pleura and pleural fluid production are tightly regulated anatomical and physiologic entities. The pleura function as serous membranes that line the lungs and chest wall by creating a sealed space between the lungs and atmosphere, allowing for the development of distinct intrathoracic pressures necessary for respiration and the production of pleural fluid. Pleural fluid serves as a lubricant as well as a mechanical coupler between the chest wall and the lungs during respiration. The pleural cavity is a potential space that exists until various pathologic processes interrupt the normal anatomy or physiology of the space, exposing the cavity. Air, excess pleural fluid, chyle, infections, and fibrous tissue are all potential pathologic entities that can disrupt the pleura and pleural space.
Clinical features
Patients presenting with pleural disorders often complain of a myriad of symptoms; however, most commonly patients will present with one or more of the following: dyspnea, chest pain, and cough. The history and physical examination help to differentiate the etiology of these symptoms. Questions that investigate the onset and duration of these symptoms, their character, and past medical and surgical history can all be illuminating. The physical examination is indispensable in investigating pleural diseases. Dullness or hyperresonance on percussion, decreased or increased tactile fremitus, breath sounds, and visual inspection of the chest wall and respiratory pattern can further refine the diagnosis.
Diagnostics
Diagnosing pleural disorders often starts with a standard posteroanterior and lateral chest x-ray. The lungs, chest wall, and mediastinum are visualized, and if a pathologic process occurs, the pleural cavity is also visualized. More advanced visualization techniques such as a computed tomography (CT) scan and ultrasound may be necessary to further define the process. In some cases, invasive techniques (i.e., thoracentesis in pleural effusions) are required to finalize a diagnosis.
Management
Benign pleural disorders are a diverse group of diseases that require complex management and often a combination of both medical and surgical approaches. For example, for pneumothoraces, conservative management involves evacuating the pleural space. If conservative management fails or there is worsening of the disease or recurrence, the patient proceeds to surgical therapy. Conservative management attempts to decrease the disruption within the pleural space and treat the underlying cause. Surgery is often reserved for refractory cases or patients with contraindications to conservative management.
Outcomes and prognosis
The term “benign” is misleading in describing nonmalignant pleural disorders. If left untreated or not properly treated, these diseases can lead to significant morbidity and even mortality. Some disease processes do recur and ultimately require more extensive surgical management or chronic conservative treatment. However, with prompt and appropriate treatment, patients can be managed effectively, and morbidity and mortality rates reduced significantly.
The pleura and lung work in concert to perform the mechanics of respiratory function. The potential space between the pleura and lung provides negative pressure, serves as a lubricant, and is a mechanical coupler between the lung and chest wall during respiration. This delicate balance can be interrupted by benign processes that alter the normal anatomy of the pleura and pleural space. With an estimated incidence of 1.5 million pleural effusions in the United States annually, an understanding of the diagnosis and treatment of this and other benign pleural diseases is valuable in one’s clinical armatorium.
The pleural cavities are formed during the fourth week of embryonic development. Initially, the septum transversum evaginates into the intraembryonic coelom separating the coelom into a primitive pericardial cavity and peritoneal cavity. The septum transversum migrates to its final position inferior to the lung buds, where it eventually forms part of the diaphragm. The primitive pericardial cavity is separated into the pleural and pericardial cavities by pleuroperitoneal folds that grow from the lateral walls of the cavity medially. The medial fusion of these projections forms three distinct cavities: the two lateral pleural cavities and the medial pericardial cavity. Inferiorly, the pleural cavities communicate with the peritoneal cavity but are eventually closed by dorsoventral growth from the pleuroperitoneal membranes (failure to close in the fetus results in congenital diaphragmatic hernia).1,2
The lung forms from a respiratory bud that evaginates from the embryonic foregut and grows ventrally into the newly forming pleural cavities. The respiratory epithelium of the lungs is derived from the endodermal cells of the foregut; however, the supportive cartilage, muscle, and the visceral pleura are formed from the mesoderm that the respiratory bud is surrounded by as it evaginates. The mesoderm, which composes both the visceral and the parietal pleura, differentiates into a single layer of mesothelial cells and a supportive tissue forming a serous membrane that allows frictionless movement in the pleural cavity as the lungs expand and retract.1,2
In adults, the pleura is divided into the visceral and parietal pleura. The visceral pleura is a thin serous layer that completely invests the lungs, including the fissures. There is no separation between the visceral pleura and the lung parenchyma. The blood supply to the visceral pleura is through the bronchial arteries, and venous drainage through the pulmonary veins. Lymphatic drainage of the visceral pleura occurs primarily through deep pulmonary plexuses within the lung parenchyma at the interlobar and peribronchial spaces. The visceral pleura is not innervated by the somatic nervous system.
The parietal pleura lines the chest wall, diaphragm, portions of the pericardium and mediastinum. Thus, the parietal pleura has distinct anatomical names; the costal pleura, diaphragmatic pleura, pericardial pleura, mediastinal pleura, the cupula of the pleura, or cervical pleura, where the pleura reflects over the superior portion of the lungs. The cupula protrudes into the anterior inferior neck, posterior to the sternocleidomastoid muscle, extending approximately 2 to 3 cm cephalad from the superior border of the first rib. The parietal pleura tightly adheres to both the diaphragm and the pericardium, but is dissectible from the ribs, cupula, and mediastinum. Arterial blood is supplied to the parietal pleura through branches of arteries supplying the ribs, diaphragm, and mediastinum. Venous drainage is through the veins running parallel with the arterial supply, and direct drainage into the superior vena cava. Lymphatic drainage of the parietal pleura occurs anteriorly through the internal mammary nodes, posteriorly into the intercostal nodes, and the diaphragmatic parietal pleura drains into the retrosternal and mediastinal nodes. The parietal pleura is innervated by the somatic, sympathetic, and parasympathetic nervous system through the intercostal nerves. Similarly, the diaphragmatic parietal pleura is innervated through the phrenic nerve.
The visceral and parietal pleura are continuous with each other at the lung hilum. At the inferior lung root, the reflections of anterior and posterior pleura come together along the mediastinum and extend inferiorly to the diaphragm, forming the pulmonary ligament. The pulmonary ligament functions to anchor the inferior lung to the diaphragm.
In healthy adults, only a thin layer of serous fluid separates the parietal and visceral pleura. Normally, the pleural cavity is a potential space that exists between the visceral and parietal pleura. This cavity becomes apparent when large amount of fluid or air is present during pathologic processes. The lung fills the pleural cavity almost entirely, except for the inferior recesses of the diaphragm and the anteriomedial mediastinum posterior to the sternum. These spaces are known as the phrenicocostal sinus and costomediastinal sinus. These sinuses are the earliest sites of excess fluid accumulation during pathologic processes.
The pleura consist of five layers: (1) an innermost mesothelial cell layer, (2) a submesothelial interstitial connective tissue layer, (3) an inner thin elastic fiber layer, (4) an outer interstitial connective tissue layer, and (5) a thick elastic fiber layer. The mesothelial cells are lined with microvilli.3 The microvilli function to increase surface area of the mesothelium thereby serving two functions: (1) cellular transport and metabolic activities related to pleural fluid reabsorption and (2) enmesh glycoproteins that aid in lubricating the pleura, decreasing friction during lung movement.4 These functions are supported by the geographic distribution of the mesothelium with microvilli. In general, the visceral pleura contains more cells with microvilli than the parietal pleura, and the density of microvilli increases caudally. This is due to the role the visceral pleura plays in reabsorption of the pleural fluid, and the increased friction at the base of the lung, respectively.
The pleura serves three primary functions. First, the pleura forms a serous layer over the lung and chest wall, preventing adhesions of the lung to the chest wall and allowing for easy deformation of the lungs when in contact with other structures (i.e., the pericardium or diaphragm). Second, the pleura produces pleural fluid, whose function will be described later. Finally, the pleura acts as a barrier between the atmosphere and the alveolar space allowing for the development of distinct intrathoracic pressures necessary for normal respiration.4–6
Pleural fluid is produced primarily by the parietal pleura through the capillaries feeding the parietal pleura. The negative intrathoracic pressure produces a hydrostatic pressure that counteracts the oncotic pressure within the capillaries, thus favoring fluid movement into the pleural cavity. Pleural fluid is absorbed by the visceral pleura through passive and active uptake by the mesothelial cells as well as lymphatic stomas within the parietal pleura. Roughly 3.4 mL/kg of fluid is produced per day by the pleura; however, only 0.1 to 0.3 mL/kg of fluid is present in the pleural cavity at any given time.5,6
The fluid serves two important functions. The first is converting the shearing perpendicular forces generated by the chest wall and diaphragm that are exerted onto the lung into sliding forces, thereby, coupling the movement of chest wall and lung expansion.5 Second, it provides lubrication for the lung and chest wall, which decreases friction of movement during respiration, thus decreasing the amount of work required for respiration.
A pneumothorax is defined as air in the pleural cavity. Air enters the pleural space by two primary methods7:
Communication between the pleural cavity and the atmosphere.
Communication between alveolar air and the pleural cavity.
Clinically, pneumothoraces can be divided into four major categories: spontaneous, traumatic, tension, or catamenial. Table 20-1 lists the causes of pneumothorax.
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Symptoms of a pneumothorax can be diverse and are often dependent on concomitant disease and methods of presentation. For spontaneous pneumothoraces, sudden onset of dyspnea, unilateral pleuritc chest pain, and cough are the most common symptoms. The chest pain often resolves within 24 h and the dyspnea is dependent on the size of the pneumothorax, but usually it is a minor symptom in an otherwise healthy patient.
The physical examination can be helpful if the pneumothorax is significant. Decreased breath sounds and tactile fremitus in the area of the pneumothorax as well as hyperresonance on percussion can be present. However, it is important to remember that small pneumothoraces will often have normal examinations and clinical suspicion of a pneumothorax should be investigated further.
The radiographic modality of choice is an upright posteroanterior or lateral (with the affected side superior) chest x-ray. Findings on x-ray include a thin pleural line displaced from the chest wall often accompanied by a radiolucent area devoid of lung markings. Small pneumothoraces may be difficult to evaluate on chest x-ray and computed tomography (CT) scans may be necessary. However, CT scans should not be routinely employed to diagnose a pneumothorax.
True tension pneumothorax is a rare, but clinically significant manifestation. It is important to be able to recognize physical signs of a tension pneumothorax, as delay in diagnosis can be catastrophic. Often these patients present with symptoms mentioned earlier, but the dyspnea is out of proportion to the common presentation and is accompanied by diaphoresis, tachycardia, cyanosis, hypotension, and deviation of the trachea away from the affected side. These symptoms are a result of air trapping within the pleural cavity causing a shift of the mediastinum as well as reduced preload and stimulation of the sympathetic nervous system.8 Prompt recognition and resolution is vital, and radiology is often deferred until treatment has been instituted.
Spontaneous pneumathoraces can be subdivided into primary spontaneous pneumothorax (PSP) and secondary spontaneous pneumothorax (SSP). A PSP is a pneumothorax that develops without insult in a patient with no underlying pulmonary disease. Classically, these patients are tall, thin, young adult males, who are found to have ruptured apical subpleural pulmonary blebs. The etiology of pulmonary blebs is often unclear, but it has been postulated to be due to heredity, congenital malformations, or distal airway inflammation. PSPs have a reported incidence of 7.4 to 18 cases/100,000 per year in males and 1.2 to 6 cases/100,000 per year in females. Roughly, 20,000 new cases are reported annually in the United States. Patients are usually between the ages of 10 and 30 years, and cigarette smoking is a significant risk factor.9
SSP occurs without warning in patients with known pulmonary disease. The most common pulmonary disease associated with SSP is chronic obstructive pulmonary disease. Children with underlying pulmonary disease, such as cystic fibrosis, can also present with SSP. Clinically, these patients often present in severe respiratory distress due to the existing respiratory dysfunction. SSPs have a reported incidence of 6.3 cases/100,000 in males and 2 cases/100,000 in females.9
There are iatrogenic and noniatrogenic traumatic pneumothoraces. Noniatrogenic pneumothoraces are commonly encountered with patients presenting with either blunt or penetrating thoracic trauma. Pneumothorax is the second most common sign of thoracic trauma, occurring in up to 50 percent patients.10 In these cases, the diagnosis of pneumothorax may be unclear, but in any patient requiring mechanical ventilation after thoracic trauma, a chest tube should be placed and a CT scan obtained.8
Iatrogenic pneumothoraces are the most common nosocomial pneumothorax. Pneumothorax occurs following transthoracic needle biopsy (24 percent), subclavian vein catheterization (22 percent), thoracentesis (20 percent), transbronchial lung biopsy (10 percent), pleural biopsy (8 percent), and positive-pressure ventilation (7 percent).11 Iatrogenic pneumothorax is often a result of physician’s inexperience with the procedure and therefore diagnosis is often delayed.
The etiology of tension pneumothorax is often multifactorial. The underlying pathophysiology is an injury that allows air to enter the pleural space without subsequent removal leading to the development of positive intrathoracic pressures, which cannot be overcome by the respiratory cycle.12 Thoracic trauma is the most easily recognized cause of tension pneumothorax, but other etiologies exist. Spontaneous pneumothoraces can evolve into tension pneumothorax if the parenchymal lesion is not contained with the initial collapse. Iatrogenic pneumothoraces, such as barotraumas, and puncture injures can lead to tension pneumothoraces as well. Treatment of tension pneumothorax should be done promptly and judiciously. The most common primary treatment is decompression with needle thoracostomy. Needle thoracostomy allows for rapid resolution and does not require the procedural skills necessary for tube thoracostomy. However, the definitive therapy for a tension pneumothorax is tube thoracostomy, and therefore placement of a chest tube should not be delayed. Needle thoracostomy is a temporizing agent.
Catamenial pneumothorax is a rare form of spontaneous pneumothorax. It generally occurs in women aged 30 to 40 years. The pneumothorax most often presents between 24 and 72 h after the onset of menstruation. Several theories have been postulated as to the cause, but most commonly, the presence of pelvic or thoracic endometriosis leading to perforation of the diaphragmatic pleura and the introduction of air into the pleural space are the causes. Treatment for catamenial pneumothorax is separate from other types of pneumothoraces, as the first occurrence requires surgical management due to the high probability of recurrence. Mechanical pleurodesis along with closing any openings within the pleura that are visualized should be instituted. Hormonal therapy exists as an option as well; however, the side effects are significant and patients should be counseled on its implementation.8
Management of pneumothoraces is often complex and dependent on multiple variables, such as size of the pneumothorax, hemodynamic stability, associated symptoms, concomitant disease, and primary or recurrent pneumothorax. Management options are observation, aspiration, tube thoracostomy, pleurodesis, and surgery. Observation is generally reserved for small spontaneous pneumothoraces without significant symptoms. Inhaled supplemental oxygen can be instituted in these patients. These patients should be observed in the hospital, as enlargement of the pneumothorax is possible.
Aspiration can be used in patients with a mild-to-moderate pneumothorax. Aspiration allows for evacuation of the pleural cavity and complete lung expansion. Studies have demonstrated that needle thoracostomy carries a risk of minimal complications and patient discomfort as well as success rates comparable with tube thoracostomy.7 When performing an aspiration, the needle can be placed in either the second intercostal space in the midclavicular line or the fifth intercostal space in the midaxillary line, similar to tube thoracostomy. The needle is generally attached to either a stopcock, allowing for manual aspiration, or a one-way valve. Follow-up chest x-ray should be obtained to confirm resolution of the pneumothorax. If the pneumothorax does not resolve, tube thoracostomy should be employed.
Larger pneumothoraces or those causing significant symptoms should be treated with a chest tube. The chest tube should be placed in the fifth intercostal space in the midaxillary line. Creating a subcutaneous track can help prevent air leaks and aid in positioning of the chest tube. The tube should be directed posterior and superior. At our institution, we add additional holes to the chest tube to aid in drainage of pleural effusions at the inferior portion of the pleural cavity (it must be kept in mind to place the final hole through the radiopaque line so that proper placement can be visualized on chest x-ray). On initial placement, the chest tube is placed on suction. Once re-expansion has been demonstrated for 24 h, the chest tube can be placed on water seal.
Tube thoracostomy is 90 percent successful in resolving a first occurrence of a PSP, 50 percent for a first recurrence, and 15 percent for second recurrence. PSPs have been reported to recur in 33 percent of patients, 62 percent of those patients will recur a second time, and 83 percent of those a third time.8,9 Prophylactic treatment to prevent recurrence of PSPs is generally not recommended after the initial pneumothorax, but patients who report recurrence are recommended for treatment.
Pneumothorax recurrence can be treated through two options, chemical pleurodesis or operative management. Chemical pleurodesis is employed in patients who are not good operative candidates or those who refuse surgery. The agents most commonly used are one of the tetracycline antibiotics and sterile talc. The agent is infused through the chest tube and the patient rotated side to side to distribute the chemical. Recurrence rates are high with chemical pleurodesis.
Surgical indications include recurrence as well as bilateral spontaneous pneumothoraces, persistent air leaks, and incomplete lung re-expansion. Patients at high risk of reoccurrence (i.e., aviation personnel, scuba divers) are often treated operatively after a primary occurrence. Operative management consists of irritation of the pleura with or without parenchymal removal. Thorough pleurodesis is the primary goal of the procedure. This is generally accomplished by both mechanical pleurodesis as well as the use of a chemical sclerosant. Visualization of bulla or blebs is a relative indication for removal. Removal without pleurodesis results in high recurrence rates.7 The most common surgical approach is video-assisted thoracoscopic surgery (VATS) but thoracotomy may be necessary depending on the clinical situation.
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