2023 Rioux MHS Mediastina.pptx

The Mediastinum Lise Rioux, PhD Touro University Nevada [email protected] Modified lectures from Dr Findlay and Dr Congdon Lecture Objectives • List regions of the mediastinum and their contents. • Label the vasculature in the superior mediastinum. • List the branches of the thoracic aorta, in ord...

The Mediastinum Lise Rioux, PhD Touro University Nevada [email protected] Modified lectures from Dr Findlay and Dr Congdon Lecture Objectives • List regions of the mediastinum and their contents. • Label the vasculature in the superior mediastinum. • List the branches of the thoracic aorta, in order. • Recognize the differences between the phrenic and vagus nerves. • Describe the azygos system. The Mediastinum • The central area of the thorax, located between the two pleural cavities • Borders: • superior thoracic aperture to diaphragm S-I • sternum to vertebral bodies A-P • bordered laterally by the two pleural cavities Regions of the Mediastinum • The mediastinum is divided into: • Superior Mediastinum • Inferior mediastinum • Anterior mediastinum • Middle mediastinum • Posterior mediastinum Superior Mediastinum • Borders: • Manubrium of sternum to the 1st 4 thoracic vertebrae • Superior thoracic aperture to the plane between sternal angle and T4-T5 disc Superior Mediastinum • Contents: ▫ thymus ▫ R & L brachiocephalic veins ▫ superior vena cava ▫ arch of aorta and major branches ▫ trachea ▫ esophagus ▫ phrenic nerves ▫ vagus nerves ▫ thoracic duct T4/T5 level Thymu s • The thymus is an asymmetric, bilobed structure located immediately posterior to the manubrium of the sternum. • It is a lymphoid organ involved in the early development of the immune system (T-cells) . • The thymus increases in size until puberty, and then starts to decrease slowly through a process of fatty involution. • Most adults will have little remaining active thymus tissue • Exception possible with some types of cancer and some autoimmune conditions Brachiocephalic Veins • The right and left brachiocephalic veins are located immediately posterior to the thymus and are formed from the junction of the internal jugular and subclavian veins. Superior Vena Cava • The brachiocephalic veins join at the lower edge of the right first costal cartilage to form the superior vena cava. Thoracic Aorta T4/T5 level • The thoracic portion of the aorta can be divided into the ascending aorta, the arch of the aorta, and the thoracic (descending) aorta. Only the arch of the aorta is located in the superior mediastinum. Arch of Aorta and Major Branches  The arch of aorta has three large branches arising from its superior border: the left subclavian artery, the left common carotid artery, and the brachiocephalic trunk, which gives rise to the right subclavian and right Trach ea • The trachea is a flexible tube formed by a series of C-shaped transverse cartilaginous rings. It forms the section of the respiratory tract inferior to the larynx. • At the T4-T5 vertebral level, the trachea divides into the right and left main bronchi. Phrenic Nerves • The phrenic nerves arise from spinal cord segments C3-5 and provide innervation to the diaphragm and its associated membranes. • They descend through the superior mediastinum and then along the pericardial sac, within the fibrous pericardium, anterior to the root of the lung. • Here they innervate the mediastinal pleura, the fibrous pericardium, and the parietal layer of serous pericardium. Vagus Nerves • The vagus nerves (CN X) provide parasympathetic innervation to the thoracic and abdominal viscera. • They descend through both the superior and posterior mediastinum on their way to the abdomen. Vagus Nerves • The right and left vagus nerves give rise to the right and left recurrent laryngeal nerves, which innervate the muscles of the larynx. • The right recurrent laryngeal nerve passes under the right subclavian artery before ascending to the larynx. • The left recurrent laryngeal nerve passes under the arch of aorta artery before ascending to the larynx. Recurrent Laryngeal Nerves • Due to the inferior position of the left recurrent laryngeal nerve, it is highly susceptible to compression by a pathologic mass, which can lead to vocal cord paralysis and hoarseness of the voice. • Lymph node enlargement, often due to the spread of lung cancer, is a common cause of this problem. • Chest x-rays are often performed on patients who present with a hoarse voice. Inferior Mediastinum • Divided into • Anterior Mediastinum • Body of sternum to pericardial sac • Middle Mediastinum • Pericardial sac and heart • Posterior mediastinum • Pericardial sac to vertebral bodies T5-T12 Anterior Mediastinum • Small space between sternum and pericardium • Contents: fat connective tissue lymph nodes mediastinal branches of internal thoracic vessels • sternopericardial ligaments • remnants of thymus • • • • Middle Mediastinum • Borders: • The pericardium • Contents: • pericardial sac • heart • origins of great vessels • phrenic nerves • C3,4,5 • pericardiacophrenic vessels Posterior Mediastinum • Borders: • Pericardium to vertebral bodies ~T5-T12 • Contents: • • • • • Esophagus Thoracic aorta Azygos system of veins Thoracic duct Sympathetic trunks & thoracic splanchnic nerves T4/T5 level Esophagus • The esophagus is a muscular tube that runs from the pharynx to the stomach. • It descends through both the superior and posterior mediastinum on its way to the abdomen. • It receives part of its innervation from the vagus nerves. The left vagus nerve continues as the anterior vagal trunk, whereas the right vagus nerve continues as the posterior vagal trunk. Sites of Normal Esophageal Constriction • The esophagus can be compressed by surrounding structures in 4 places: ▫ Junction of esophagus and pharynx in the neck (C6) ▫ Where the arch of aorta crosses esophagus ▫ Where esophagus is compressed by left main bronchus ▫ At esophageal hiatus of the diaphragm (T10) Sites of Esophageal Constriction Not on exam Esophagus & Left Atrium • The esophagus constitutes the primary posterior relationship of the base of the heart. Enlargement or pathology of the left atrium can be monitored or evaluated using contrast study in an upper GI series (barium swallow) due to indentation or displacement of the esophagus by a posteriorly enlarging LA or using a (trans)esophageal echocardiogram or doppler monitoring. Sympathetic Trunks & Thoracic Splanchnic Nerves • The sympathetic trunks and thoracic splanchnic nerves are major components of the sympathetic division of the autonomic nervous system. Thoracic Aorta • As the thoracic aorta descends through the posterior mediastinum, it gives off: • the posterior intercostal arteries, which supply the lower nine intercostal spaces (3-11). • the bronchial arteries, which supply the bronchi and lungs. • the esophageal arteries, which supply the esophagus. esophagus Azygos System of Veins • The azygos system of veins is formed by: • the azygos vein on the right and the hemiazygos and accessory hemiazygos veins on the left. • Together, they drain blood from the body wall to the superior vena cava. Azygos veins Drainage of posterior intercostal and ascending lumbar veins • RIGHT side • Azygos vein • T9-T12 • T5-T8 • Drains in superior vena cava • Right superior intercostal vein • T2-T4 • (drains into azygos vein) • Right T1 intercostal vein • (drains into right brachiocephalic vein) • LEFT side • Hemiazygos vein • T9-T12 • (drains into azygos vein) • Accessory hemiazygos vein • T5-T8 • (drains into azygos vein) • Left superior intercostal vein • T2-T4 • (drains into left brachiocephalic vein) • Left T1 intercostal vein • (drains into left brachiocephalic vein) Azygos System of Veins • In the case of an obstruction of the inferior vena cava, blood may be rerouted through the azygos vein in order to return to the heart. This typically results in substantial dilation of the azygos vein. Collateral Venous Circulation • During blockage of the inferior vena cava or the common iliac veins due to thrombosis or compression by a growing fetus or enlarging tumor, venous return from the lower extremities is accomplished through: • Anastomosis between the superior and inferior epigastric veins bringing venous blood to the internal thoracic vein, subclavian vein, and heart. • Anastomosis between the superficial epigastric vein (normally drains into the femoral vein) and the lateral thoracic vein brings blood to the axillary vein, subclavian vein and heart; sometimes the two form a large vein on the lateral abdominal wall called thoracoepigastric vein. Thoracic Duct • The thoracic duct is the channel through which lymph from most of the body is returned to the venous system. • It begins as a saccular dilation in the abdomen, the cisterna chyli (chyle* cistern), extends superiorly into the thorax through the aortic hiatus of the diaphragm, and empties into the junction of the left subclavian and left internal jugular veins. • The right arm and the right side of the head and neck are drained *Chyle: A fluidright consisting of a mixture of lymphatic by the lymphatic duct.fluid (lymph) and chylomicrons that has a milky appearance. Chylomicrons are small fat globules composed of protein and lipid (fat) which are combined in the lining of the intestine. Chylomicrons are found in the blood and in lymphatic fluid where they serve to transport fat from its