Mediastinum

The mediastinum is the central compartment of the thoracic cavity, located between the two pleural sacs. It houses vital structures including the heart, great vessels, trachea, esophagus, and various nerves and lymphatic tissues. Understanding its anatomy is essential for diagnosing thoracic diseases and planning surgical interventions.

Anatomical Boundaries

The mediastinum is defined by specific anatomical boundaries that separate it from other thoracic compartments. Its limits provide a framework for subdividing it into clinically relevant regions.

• Superior boundary: thoracic inlet formed by the first thoracic vertebra, first pair of ribs, and superior border of the manubrium
• Inferior boundary: diaphragm separating the thoracic and abdominal cavities
• Lateral boundaries: mediastinal pleura adjacent to the lungs
• Anterior boundary: posterior surface of the sternum and associated costal cartilages
• Posterior boundary: anterior surface of the thoracic vertebral bodies

Subdivision of the Mediastinum

Traditional Division

The mediastinum is commonly divided into superior and inferior compartments based on anatomical landmarks.

• Superior mediastinum: above the sternal angle extending to the thoracic inlet
• Inferior mediastinum: below the sternal angle extending to the diaphragm

Inferior Mediastinum Subdivisions

The inferior mediastinum is further subdivided into anterior, middle, and posterior compartments to localize structures and pathological processes more accurately.

• Anterior mediastinum: located between the sternum and pericardium
• Middle mediastinum: contains the heart, pericardium, and major vessels
• Posterior mediastinum: situated between the pericardium and vertebral column

Contents of the Mediastinum

Superior Mediastinum

The superior mediastinum contains several vital structures that pass between the neck and thorax. Its contents are essential for circulation, respiration, and neural function.

• Thymus gland, particularly prominent in children
• Great vessels, including the aortic arch, brachiocephalic artery, left common carotid artery, and left subclavian artery
• Trachea and upper portion of the esophagus
• Nerves including the vagus, phrenic, and cardiac plexus
• Lymphatic structures such as the thoracic duct and mediastinal lymph nodes

Anterior Mediastinum

The anterior mediastinum is a small space located between the sternum and the pericardium. It mainly contains connective tissues and remnants of fetal structures.

• Fat and connective tissue
• Remnants of the thymus gland in adults
• Lymph nodes scattered throughout the anterior compartment

Middle Mediastinum

The middle mediastinum houses the heart and the major vessels associated with it, making it central to cardiovascular function.

• Heart and pericardium
• Ascending aorta, pulmonary trunk, and superior vena cava
• Main bronchi
• Phrenic nerves and associated lymph nodes

Posterior Mediastinum

The posterior mediastinum lies between the pericardium and thoracic vertebrae, containing structures primarily related to the digestive and vascular systems.

• Esophagus
• Descending thoracic aorta
• Azygos and hemiazygos veins
• Thoracic duct
• Sympathetic trunks and thoracic splanchnic nerves

Clinical Significance

Mediastinal Masses

Mediastinal masses can arise from any of the compartments and often present with compressive symptoms or incidental findings on imaging.

• Thymoma, thymic cysts, and germ cell tumors commonly in the anterior mediastinum
• Lymphoma affecting superior or middle mediastinum
• Cystic lesions such as pericardial, bronchogenic, or enteric cysts
• Symptoms may include cough, chest pain, dyspnea, or superior vena cava syndrome

Mediastinitis

Mediastinitis is an inflammation of the mediastinal tissues that can be life-threatening if not promptly treated.

• Causes include post-surgical infections, esophageal perforation, or spread from cervical infections
• Clinical features: fever, chest pain, tachycardia, and mediastinal widening on imaging
• Complications: sepsis, abscess formation, and cardiovascular compromise

Trauma and Other Pathologies

The mediastinum is also involved in various traumatic and pathological conditions.

• Pneumomediastinum: presence of air in the mediastinum due to trauma or alveolar rupture
• Aortic injuries and aneurysms, often life-threatening if ruptured
• Esophageal rupture leading to mediastinal contamination and infection

Imaging and Diagnostic Evaluation

Imaging plays a key role in the evaluation of mediastinal anatomy and pathology, guiding diagnosis and treatment planning.

• Chest X-ray: initial screening tool for mediastinal widening, masses, or pneumomediastinum
• Computed Tomography (CT): provides detailed cross-sectional images of mediastinal structures and pathology
• Magnetic Resonance Imaging (MRI): useful for soft tissue characterization and vascular involvement
• Ultrasound and echocardiography: evaluation of anterior mediastinal masses and cardiac structures
• Positron Emission Tomography (PET): assessment of metabolic activity in mediastinal tumors

Surgical and Interventional Considerations

Surgical and interventional procedures in the mediastinum are performed to diagnose or treat masses, infections, and vascular or cardiac conditions. Accurate knowledge of mediastinal anatomy is essential to minimize complications.

• Anterior mediastinotomy: used for biopsy of anterior mediastinal masses
• Mediastinal mass excision: approaches vary depending on location, including sternotomy or thoracotomy
• Management of mediastinitis: may require surgical drainage or debridement
• Minimally invasive procedures: video-assisted thoracoscopic surgery (VATS) for select masses or cysts

Developmental Anatomy

The mediastinum and its contents develop from various embryological origins, and developmental anomalies can have clinical significance.

• Embryological origin: structures derive from mesodermal, endodermal, and neural crest tissues
• Thymus: develops from the third pharyngeal pouch and descends into the anterior mediastinum
• Great vessels: aortic arches remodel to form the major arteries
• Congenital anomalies: vascular rings, cysts, or abnormal positions of mediastinal organs
• Clinical implications: congenital anomalies can cause respiratory or swallowing difficulties

References

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