HBF-II LEC 13 Superior Posterior Mediastina Notes 2024

Learning Objectives I. Course Objective: Describe the anatomy of the region under study. Session Objectives:  Describe the subdivisions of the mediastinum including boundaries and major contents.  Describe the boundaries and contents of the superior mediastinum including...

Learning Objectives I. Course Objective: Describe the anatomy of the region under study. Session Objectives:  Describe the subdivisions of the mediastinum including boundaries and major contents.  Describe the boundaries and contents of the superior mediastinum including thymus, great vessels, nerves, and trachea.  Describe the boundaries and contents of the posterior mediastinum including thoracic aorta and its branches, esophagus, thoracic duct, azygos system of veins, and thoracic sympathetic trunk. II. Course Objective: Relate the anatomy of structures to their function(s). Session Objectives:  Relate the anatomy of the thymus to its function.  Relate the anatomy of the great vessels to their functions.  Relate the anatomy of the vagus and phrenic nerves to their functions.  Relate the anatomy of the trachea to its function.  Relate the anatomy of the thoracic aorta to its functions.  Relate the anatomy of the esophagus to its functions.  Relate the anatomy of the thoracic duct to its function.  Relate the anatomy of the azgos system of veins to their function.  Relate the anatomy of the thoracic sympathetic trunk to its functions. III. Course Objective: Apply knowledge of anatomy to evaluate clinically relevant problems. Session Objectives:  Apply the anatomy of the thymus to ectopic parathyroid hormone production.  Apply the anatomy of the great vessels to guidewire/catheter procedures.  Apply the anatomy of the aortic isthmus to traumatic aortic injury.  Apply the anatomy of the recurrent laryngeal nerve to injury.  Apply the anatomy of the esophagus to sites of narrowing, constrictor functions, and bleeding in portal hypertension. Lecture Content Outline Superior & Posterior Mediastinum A. Subdivisions of the Mediastinum B. Superior Mediastinum 1. Thymus 2. Great Vessels 3. Vagus and Phrenic Nerves 4. Trachea C. Posterior Mediastinum 1. Thoracic Aorta 2. Esophagus 3. Thoracic Duct 4. Azygos System of Veins 5. Thoracic Sympathetic Trunks Mediastinum The mediastinum is the interval between the pleural sacs. It extends from the thoracic inlet to the diaphragm. A. Subdivisions An Figure 1. imaginary line drawn between the sternal angle and the disc between TV4-TV5 demarcates the superior mediastinum from the inferior mediastinum. The inferior mediastinum is further divided into: Middle mediastinum – containing the heart, beginning or termination of great vessels, pericardium Anterior mediastinum – posterior to the body of sternum, anterior to the heart; it contains fat, remnants of the thymus gland, lymph nodes and sternopericardial ligaments Posterior mediastinum – posterior to the heart and pericardium, anterior to TV5-TV12; contents reviewed under subheading C. Posterior Mediastinum The anterior and posterior mediastina are in direct continuity with the superior mediastinum. The imaginary plane between superior and inferior mediastinum passes through the bifurcation of the trachea, the concavity of the arch of the aorta and just above the bifurcation of the pulmonary trunk. B. Superior Mediastinum Located behind the manubrium  In front of TV1-TV4 Principle contents: Arch of the aorta and the roots of its branches Brachiocephalic veins and SVC Vagus and phrenic nerves, left recurrent laryngeal, sympathetics Trachea, esophagus, thoracic duct Thymus gland Figure 2.  Figure 3. Figure 4.  1. Thymus Bi-lobar organ that lies posterior to the manubrium, extends into the anterior mediastinum, anterior to the arch of aorta and major vessels. Important in development of the immune system; greatest size at puberty then begins to regress and is largely replaced by fat. A rich arterial supply comes from internal thoracic artery by way of anterior intercostal and anterior mediastinal branches. Venous drainage is usually into the left brachiocephalic vein. Lymphatic drainage returns to parasternal nodes along the internal thoracic artery; tracheobronchial nodes at the tracheal bifurcation; or in the root of the neck. Clinical Notes: - The parathyroid glands develop from the 3rd pharyngeal pouch – same as the thymus. As such, the thymus is a common site for ectopic parathyroid glands and, potentially, ectopic parathyroid hormone production. Figure 5.  2. Great Vessels Right and left brachiocephalic veins – brachiocephalic veins (BCV) are located immediately posterior to the thymus. They form on each side at the junction between the internal jugular and subclavian veins. Figure 6. The left BCV crosses to the right behind the manubrium and joins the right BCV to form the superior vena cava (SVC). SVC – returns blood from all structures superior to the diaphragm, except for the heart and lungs. It also receives the azygos vein just before entering the pericardial sac (middle mediastinum). Clinical Notes: - SVC and IVC are oriented along the same vertical axis, allowing for guidewires/catheters/ lines to be passed from the SVC through the right atrium, into the IVC. This is a common route for access for procedures such as insertion of an IVC filter to catch emboli dislodged from veins in the lower limb/pelvis (DVT patients). Arch of the aorta – the ascending aorta emerges from the pericardial sac, extends to the level of the sternal angle and continues as the arch of the aorta. The arch runs upward, to the left in front of the trachea, and the turns backward and down over the left main bronchus and pulmonary trunk. The arch of the aorta occupies an almost sagittal plane behind the lower part of the manubrium. Branches of the arch: Brachiocephalic trunk (BT) Left common carotid artery (LCCA) Left subclavian artery (LSA) Figure 7. Clinical Notes: - The ligamentum arteriosum connects the pulmonary trunk to the arch immediately distal to the origin of the LSA – see Fig. 6. 3. Nerves Vagus nerve – right vagus nerve crosses anterior to the 1st part of the subclavian artery, descends on the right side of the trachea, and then posterior to the root of the right lung. Here it gives branches to supply the lung. The right recurrent laryngeal nerve loops posterior to the subclavian artery and ascends in the neck. Left vagus nerve descends anterior to the arch of the aorta and then posterior to the root of the left lung. The left recurrent laryngeal nerve leaves the left vagus at the arch of the aorta, hooks below the arch to the left of ligamentum arteriosum, and then ascends at the right side of the arch, between the trachea and esophagus. Phrenic nerve – right phrenic descends on lateral surface of the SVC, left nerve crosses arch of aorta. Both nerves descend anterior to the roots of the lungs between the fibrous pericardium and the mediastinal pleura, accompanied by the pericardiophrenic arteries of the internal thoracic artery. Phrenic nerves provide motor to the diaphragm and sensory to the pericardium and diaphragmatic pleura. Avoid injury when incising pericardium. Figure 8. 4. Trachea Sits midline and can be palpated in the jugular notch as it enters the superior mediastinum. It is anterior to the esophagus. The trachea divides into left and right main bronchi near the transverse plane between the sternal angle and TV4/TV5. Figure 9.  C. Posterior Mediastinum Located behind the pericardial sac and diaphragm In front of TV5-TV12 Principle contents: Thoracic aorta and branches Esophagus and its associated nerve plexus Thoracic duct and lymph nodes Azygos venous system Sympathetic trunks and thoracic splanchnic nerves 1. Thoracic aorta This is the thoracic portion of the descending aorta and is continuous with the arch. It starts at TV4, first to the left of the midline, then slants to the right and leaves the thorax near the midline at TV12, passing through the aortic hiatus of the diaphragm. A number of parietal (posterior intercostal, subcostal, superior phrenic) and visceral (bronchial, esophageal, pericardial) branches are given off throughout its course (summarized in Table 1). Branches Origin and course Pericardial A few small vessels to the posterior surface of the pericardial sac branches Bronchial Vary in number, size, and origin—usually, two left bronchial arteries from branches the thoracic aorta and one right bronchial artery from the third posterior intercostal artery or the superior left bronchial artery Esophageal Four or five vessels from the anterior aspect of the thoracic aorta, which branches form a continuous anastomotic chain—including esophageal branches of the inferior thyroid artery superiorly, and esophageal branches of the left inferior phrenic and the left gastric arteries inferiorly Mediastinal Several small branches supplying lymph nodes, vessels, nerves, and areolar branches tissue in the posterior mediastinum Posterior Nine pairs of vessels branching from the posterior surface of the thoracic intercostal aorta—usually supply lower nine intercostal spaces arteries Superior Small vessels from the lower part of the thoracic aorta supplying the phrenic arteries posterior superior diaphragm Subcostal The lowest pair of branches from the thoracic aorta located inferior to rib artery XII Table 1. Branches of the thoracic aorta Clinical Notes: - The part of the aorta between the origin of LSA and the attachment of the ligamentum arteriosum is referred to as the aortic isthmus. It is the most common site of traumatic aortic injury, resulting from sudden deceleration at high speed as in motor vehicle accidents. - The left recurrent laryngeal may be impinged upon by abnormal dilation (aneurysm) of the arch of the aorta or by enlarged lymph nodes in this area. The patient will present with hoarseness. 2. Esophagus Extends from the cricoid cartilage, CV6, to the cardiac orifice of the stomach, TV10. It traverses the superior and posterior mediastina. Superiorly, it descends on the anterior surface of TV to the right of the aorta. In the lowermost part of the posterior mediastinum, it curves to the left crossing in front of the aorta. It enters the Figure 10. abdomen through the esophageal hiatus of the diaphragm at TV10. Relationships: Anterior (from superior to inferior): left main bronchus, left atrium and diaphragm Posterior: thoracic duct, right posterior intercostal arteries, aorta near diaphragm, veins Arterial supply and venous/lymphatic drainage Arterial blood supply is from inferior thyroid artery, thoracic aorta and left gastric artery (see esophageal branches in Table 1) Venous drainage to inferior thyroid veins, the azygos system and left gastric vein (LGV) Lymphatic draining of the esophagus returns to posterior mediastinal and left gastric nodes Figure 11. Clinical Notes: - The communication between LGV and azygos system links the portal system and systemic veins. In portal hypertension, blood backs up and distends the esophageal veins, which can bleed profusely (more later under Abdomen). Figure 13. Figure 12. Nerve supply Esophageal plexus of vagus nerve supplies muscle and glands, increasing peristalsis and secretion. After passing behind the roots of the lungs, the vagi divide into branches that form the anterior and posterior esophageal plexuses. Although there is some mixing, the anterior plexus is mainly from the left vagus, and the posterior plexus mainly from the right vagus. Just above the diaphragm, the fibers of the plexuses converge to form the anterior and posterior vagal trunks. Clinical Notes: - Sites of narrowing (where foreign material can lodge): the beginning of the esophagus at the upper esophageal sphincter, formed by the cricopharyngeus muscle; where the arch of the aorta and then the left main bronchus cross the esophagus; at the esophageal opening in the diaphragm and the site of the lower esophageal sphincter - Upper esophageal sphincter is a high pressure zone that prevents air from entering esophagus during breathing, and that prevents reflux of esophageal contents into the pharynx to guard against airway aspiration. Cricopharyngeus muscle is the major contributor to the high-pressure zone. - Lower esophageal sphincter (LOS) is a specialized thickened region of circular muscle of the distal esophagus. It generates a tonic pressure higher than the intragastric pressure preventing reflux of gastric contents. 3. Thoracic Duct Begins at the cysterna chili, as a lymphatic sac between the aorta and right crus of the diaphragm. The duct then passes through the aortic hiatus into the posterior mediastinum. Ascends behind the esophagus, to TV5-TV6, crosses to the left and enters the root of the neck. It courses laterally behind the left carotid sheath and enters the junction of the left subclavian and internal jugular veins. The duct drains the lymph of the whole body except that from the right side of the head and neck, right upper limb and right thorax. Figure 15. Figure 14.  4. Azygos system of veins Includes the azygos vein on the right and hemiazygos and accessory hemiazygos veins on the left. Azygos vein – begins in the abdomen by junction of right subcostal and right ascending lumbar veins. Ascends posterolateral to the esophagus, arches over root of the right lung and enters the SVC. Receives right posterior intercostal and hemiazygos veins. Figure 16. Hemiazygos vein – formed by union of left subcostal and left ascending lumbar veins. Receives left lower posterior intercostal veins, and crosses about TV10 to enter azygos vein. Accessory hemiazygos vein – receives upper posterior intercostal veins and joins hemiazygos vein or crosses to enter azygos vein. Clinical Notes: - The azygos system functions to return blood to the right side of the heart. There are numerous variations in pattern of the hemiazygos veins.  5. Thoracic sympathetic trunks Located on the necks of the ribs as the most laterally placed structure in the posterior mediastinum. 12 ganglia connected to thoracic spinal nerves by grey and white rami. First thoracic ganglion is often fused with inferior cervical ganglion to form stellate ganglion. Figure 17. Branches – detailed description in lecture on ANS Greater splanchnic nerve from thoracic ganglia 5-9/10 Lesser splanchnic from lower thoracic ganglia; typically 9/10 or 10/11 Least splanchnic from T12 ganglion These nerves course medial to the sympathetic trunks on the bodies of the vertebrae and pierce the crura of the diaphragm to enter the abdomen. Nerves most evident in the dissection are the splanchnics destined for the abdomen and the gastrointestinal tract. White and gray rami for thoracic spinal nerves are also readily identified. The splanchnic nerves are preganglionic fibers arising from the thoracic ganglia. Lymph nodes are present at many sites. They are prominent along the internal thoracic artery, in the hilum of the lung, and along the main bronchi and the bifurcation of the trachea. The lymphatic fluid ultimately drains into the thoracic duct on the left, and the right lymphatic duct.