Disorders of the Mediastinum¶

Chapter 306 | Part 7: Disorders of the Respiratory System · Part 7 – Respiratory Disorders · Chapter 306

Key Clinical Points¶

The mediastinum is defined as the region between the pleural sacs bound by the thoracic inlet superiorly and the diaphragm inferiorly.
Tension pneumothorax is a medical emergency requiring immediate needle decompression via the second anterior intercostal space.
Thymomas are the most common neoplasm in the anterior mediastinum and are closely tied to paraneoplastic syndromes, most notably myasthenia gravis (supported by serum anti-acetylcholine receptor antibodies).
Elevated tumor markers, α-fetoprotein and β-human chorionic gonadotropin, distinguish between seminomatous and even nonseminomatous germ cell tumors.
Seminomas are generally responsive to radiation therapy, whereas nonseminomatous germ cell tumors are treated with standard chemotherapy.
The incidence of mediastinitis after median sternotomy is 0.4–5.0%.
Fibrosing mediastinitis usually follows a history of granulomatous infections, commonly due to histoplasmosis or tuberculosis.
SVC obstruction often occurs; diagnosis can be established with mediastinal needle aspiration.
Neurogenic tumors are the most common cause of posterior mediastinal tumors and are often benign.
Surgical resection is the mainstay of treatment for most anterior mediastinal masses, such as localized early stages of thymic carcinomas, symptomatic teratomas, and mediastinal goiters.

DEFINITION & OVERVIEW¶

• General Definition: Disorders of the mediastinum encompass a broad range of diseases including, but not limited to, neoplastic and nonneoplastic masses, congenital or acquired malformations of anatomical structures, infections, and chronic fibrosing mediastinitis. • Anatomical Definition (Harrison's 22e):

Definition: The mediastinum is the region between the pleural sacs bound by the thoracic inlet superiorly and the diaphragm inferiorly.

Table 306-1: The Three Compartments of the Mediastinum | Anatomical boundaries | ANTERIOR COMPARTMENT | MIDDLE COMPARTMENT | POSTERIOR COMPARTMENT | | --- | --- | --- | --- | | Anatomical boundaries | Manubrium and sternum anteriorly; pericardium, aorta, and brachiocephalic vessels posteriorly | Anterior mediastinum anteriorly; posterior mediastinum posteriorly | Pericardium and trachea anteriorly; vertebral column posteriorly | | Contents | Thymus gland, anterior mediastinal lymph nodes, internal mammary arteries, and veins | Pericardium, heart, ascending and transverse arch of aorta, superior and inferior vena cavae, brachiocephalic arteries and veins, phrenic nerves, trachea, and mainstem bronchi and their contiguous lymph nodes, pulmonary arteries, and veins | | | Common abnormalities | Thymoma, lymphomas, teratomatous neoplasms, thyroid masses, parathyroid masses, mesenchymal tumors, giant lymph node hyperplasia, hernia through foramen of Morgagni | Metastatic lymph node enlargement, granulomatous lymph node enlargement, pleuropericardial cysts, bronchogenic cysts, masses of vascular origin | Neurogenic tumors, meningocele, meningomyelocele, gastroenteric cysts, esophageal diverticula, hernia through foramen of Bochdalek, extramedullary hematopoiesis |

EPIDEMIOLOGY¶

• Mediastinitis Incidence: ◦ Rate: 0.4–5.0% following median sternotomy. • Tumor Markers: ◦ Markers: α-fetoprotein and β-human chorionic gonadotropin. ◦ Clinical Use: Distinguish between seminomatous and nonseminomatous germ cell tumors.

ETIOLOGY & PATHOPHYSIOLOGY¶

• Pathology Localization: The first step in evaluating a mediastinal mass is to place it in one of the three mediastinal compartments (Anterior, Middle, or Posterior), as each has distinct characteristic lesions. • Fibrosing Mediastinitis: ◦ Etiology: Usually follows a history of granulomatous infections. ◦ Common causative agents: Histoplasmosis or tuberculosis.

CLINICAL FEATURES¶

• General Presentation: Symptoms vary widely; assessment depends on clinical symptoms, patient age, and location of the mediastinal tumor to determine likelihood of malignancy and need for intervention. • Pneumothorax Symptoms: ◦ Acute illness with chest pain and dyspnea (specifically in cases of esophageal rupture due to mediastinal infection). • Mediastinal Mass Symptoms: ◦ Common manifestations: Dyspnea, chest pain, SVC syndrome, hemoptysis, dysphagia, and Horner's syndrome. • Mediastinitis Symptoms: ◦ Primary presentation: Wound drainage. ◦ Other presentations: Sepsis and a widened mediastinum.

DIFFERENTIAL DIAGNOSIS¶

• Anterior Mediastinum: Thymoma, lymphomas, teratomatous neoplasms, thyroid masses, parathyroid masses, mesenchymal tumors, giant lymph node hyperplasia, hernia through foramen of Morgagni. • Middle Mediastinum: Metastatic lymph node enlargement, granulomatous lymph node enlargement, pleuropericardial cysts, bronchogenic cysts, masses of vascular origin. • Posterior Mediastinum: Neurogenic tumors, meningocele, meningomyelocele, gastroenteric cysts, esophageal diverticula, hernia through foramen of Bochdalek, extramedullary hematopoiesis.

INVESTIGATIONS & DIAGNOSIS¶

Imaging Modalities: • Computed Tomography (CT): Primary tool to evaluate masses; can differentiate fat, water, calcifications, and air. • Magnetic Resonance Imaging (MRI): Diffusion-weighted MRI used for improved tumor characterization and differentiation between cystic and solid structures.
Biopsy Techniques: • Percutaneous fine-needle aspiration (FNA). • Endoscopic transesophageal ultrasound. • Endobronchial ultrasound.
Tension Pneumothorax Diagnosis: • Physical examination findings:
Enlarged hemithorax.
Absence of breath sounds.
Hyperresonance to percussion.
Shift of the mediastinum to the contralateral side.
Hypotension.

MANAGEMENT & TREATMENT¶

Pneumothorax Management: • Initial: Tube drainage for nearly all patients with secondary pneumothorax. • Surgical options: Thoracoscopy or thoracotomy → stapling of blebs and surgical pleurodesis. • Alternative (if not a good candidate or refuses surgery): Chemical pleurodesis.
Mediastinal Mass Management: • Surgery: Mainstay for most anterior mediastinal masses (e.g., localized early stages of thymic carcinomas, symptomatic teratomas, and mediastinal goiters). • Lymphoma: Multimodal therapy (combination of radiation and chemotherapy); surgery is rarely indicated.
Mediastinitis Management: • Procedure: Exploration of the mediastinum. • Actions: Primary repair of the esophageal tear → drainage of the pleural space and the mediastinum.
Chronic Mediastinitis Management: • Granulomatous: Usually asymptomatic. • Fibrosing: Treatment depends on extent and location of invasion of structures such as:
SVC
Large airways
Phrenic or recurrent laryngeal nerve (paralysis)
Pulmonary artery or proximal pulmonary veins (obstruction).

PROGNOSIS & COMPLICATIONS¶

• Mediastinitis Prognosis: ◦ Treatment: Immediate drainage, debridement, and parenteral antibiotic therapy. ◦ Mortality: Exceeds 20% despite treatment. • Fibrosing Mediastinitis Complications: ◦ Severity: Can be devastating in severe cases. ◦ Clinical manifestations: Dyspnea, chest pain, SVC syndrome, hemoptysis, dysphagia, and Horner's syndrome.

KEY PEARLS & CLINICAL TRAPS¶

• Emergency Alert: Tension pneumothorax is a medical emergency. • Germ Cell Tumors: ◦ Seminomas → Responsive to radiation therapy. ◦ Nonseminomatous → Treated with standard chemotherapy. • Thymoma & Myasthenia Gravis: ◦ Thymomas are the most common anterior mediastinal neoplasm. ◦ Associated with paraneoplastic syndromes, specifically myasthenia gravis (supported by serum anti-acetylcholine receptor antibodies). • Diagnostic Pitfall: ◦ Use of clinical symptoms, patient age, and tumor location to determine likelihood of malignancy and need for intervention.

Reference Tables¶

TABLE 306-1 The Three Compartments of the Mediastinum Anatomical boundaries¶

Harrison's 22e, p.2272

	ANTERIOR COMPARTMENT	MIDDLE COMPARTMENT	POSTERIOR COMPARTMENT
Anatomical boundaries	Manubrium and sternum anteriorly; pericardium, aorta, and brachiocephalic vessels posteriorly	Anterior mediastinum anteriorly; posterior mediastinum posteriorly	Pericardium and trachea anteriorly; vertebral column posteriorly
	Thymus gland, anterior mediastinal lymph nodes, internal mammary arteries, and veins	Pericardium, heart, ascending and transverse arch of aorta, superior and inferior vena cavae, brachiocephalic arteries and veins, phrenic nerves, trachea, and mainstem bronchi and their contiguous lymph nodes, pulmonary arteries, and veins
Common abnormalities	Thymoma, lymphomas, teratomatous neoplasms, thyroid masses, parathyroid masses, mesenchymal tumors, giant lymph node hyperplasia, hernia through foramen of Morgagni	Metastatic lymph node enlargement, granulomatous lymph node enlargement, pleuropericardial cysts, bronchogenic cysts, masses of vascular origin	Neurogenic tumors, meningocele, meningomyelocele, gastroenteric cysts, esophageal diverticula, hernia through foramen of Bochdalek, extramedullary hematopoiesis