MRCS Part B (OSCE) - Online Resources

"The mediastinum is a potential space within the chest cavity, which includes all structures including the heart and pericardium except the lungs and pleurae. It is bounded by the thoracic vertebrae posteriorly, the sternum anteriorly and laterally by the left and right lungs with their pleurae. "

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The mediastinum may be subdivided into superior and inferior parts at an imaginary line which corresponds to the angle of Louis anteriorly and the T4-T5 intervertebral disc posteriorly.

• Superior mediastinum lies above the angle of Louis.
• Inferior mediastinum lies below the angle of Louis.

The inferior mediastinum is further subdivided into three parts in the coronal plane; anterior, middle and posterior.

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The contents of the superior mediastinum are:

• Muscles: Origin of the sternohyoid and sternothyroid, lower ends of longus colli.
• Vessels: Aortic arch, brachiocephalic trunk, left common carotid and left subclavian artery, brachiocephalic veins and SVC.
• Nerves: Vagus, phrenic and left recurrent laryngeal nerves.
• Viscera: Oesophagus, trachea.
• Others: Thoracic duct, thymus (in children) and lymph nodes.

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The borders of the posterior mediastinum are:

• Anterior :      Pericardium.
• Inferior :      Diaphragm.
• Posterior :      Thoracic vertebrae (T5-T12).
• Lateral :      Mediastinal pleura.

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The contents of the posterior mediastinum are:

• Vessels :      Thoracic part of the descending aorta, azygos and hemiazygos            veins.
• Nerves :      Vagus and splanchnic nerves (arising from the sympathetic                    trunk to supply viscera in thorax and abdomen) 

• Viscera :      Oesophagus.
• Others :      Lymph nodes and thoracic duct.

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The contents of the middle mediastinum are:

• Vessels :      Ascending aorta, lower half of SVC, azygos vein, pulmonary             arteries and veins.
• Nerves :      Phrenic nerves.
• Viscera :      Heart and Pericardium.
• Others :      Bronchial lymph glands.

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"The anterior mediastinum is bounded anteriorly by the posterior surface of the sternum and posteriorly by the pericardium. It contains loose areolar tissue, lymphatic vessels, mediastinal lymph nodes and remnants of thymus gland. "

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The subclavian artery is anatomically described in three parts in relation to the scalenus anterior muscle; the first part is medial to the muscle; the second part is behind the muscle and; the third part is lateral to the muscle up to the outer border of the 1^st rib.

The branches of the subclavian artery are:

• 1^st part :      Vertebral artery.

Internal thoracic artery.

                                Thyrocervical trunk.

• 2^nd part :      Costocervical trunk and Dorsal scapular artery.
• 3^rd part :      No branches, although occasionally the dorsal scapular artery                     can arise from the third part.

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" The 3^rd part of the subclavian artery is the longest and supplies the upper limb:

• Begins just superior to the clavicle, opposite the lateral border of the anterior scalene muscle.
• Lies in the inferior of the posterior triangle of the neck.
• Terminates at the lateral border of the 1^st rib (from where it continues as the axillary artery).

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• Subclavian steal syndrome refers to steno-occlusive disease of the proximal subclavian artery before the origin of the vertebral artery, with retrograde flow in ipsilateral vertebral artery and associated cerebral ischaemic symptoms.
• The arm on the affected side is supplied by blood flowing in a retrograde direction down the vertebral artery at the expense of the vertebrobasilar circulation hence the blood is ‘stolen’.
• Subclavian steal is frequently asymptomatic and may be discovered incidentally or it may be prompted by a clinical examination finding of reduced unilateral upper limb pulse or blood pressure.
• In some cases, patients may develop upper limb ischemic symptoms due to reduced arterial flow in the setting of subclavian artery occlusion, or neurologic symptoms due to posterior circulation ischemia associated with exercise of the ipsilateral arm (such as dizziness, vertigo, syncope, ataxia, visual changes, dysarthria, weakness and sensory disturbance).

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