Summary
Highlights
The video introduces the thoracic inlet, also known as the superior thoracic aperture, located at the narrow upper end of the thoracic cavity, connecting it with the neck. It is kidney-shaped due to the inward projection of the first thoracic vertebra. Its dimensions are a transverse diameter of 10-12 cm and an anterior-posterior diameter of 5 cm.
The anterior boundary of the thoracic inlet is the upper border of the manubrium sterni, laterally by the inner border of the first rib and its costal cartilage, and posteriorly by the body of the first thoracic vertebra (T1). The thoracic inlet is obliquely placed, with its posterior boundary (T1 vertebra) approximately 1.5 inches higher than its anterior boundary, allowing the apex of the lung to extend into the neck.
The suprapleural membrane, or Sibson's fascia, is a fibrous structure that covers and protects the apex of the lung and the cervical pleura as they extend into the neck. It's triangular, attaching posteriorly to the transverse process of the C7 vertebra and anteriorly/laterally to the inner border of the first rib and costal cartilage. Its function is to protect the underlying pleura and resist intrathoracic pressure, preventing the neck from puffing up during respiration.
The thoracic inlet allows the passage of viscera, blood vessels (arteries and veins), nerves, and some muscles. Key viscera include the apex of the lung (covered by cervical pleura), remnants of the thymus, trachea, esophagus, and thoracic duct. Major arteries are the brachiocephalic trunk, left common carotid, and left subclavian artery. Main veins are the brachiocephalic veins. Nerves include the vagus and phrenic nerves, T1 spinal nerve, and sympathetic trunk.
The video summarizes structures passing through the thoracic inlet using a 'rule of five' categories: 5 viscera (apex of lung, trachea, esophagus, thoracic duct, thymus), 2 muscles (sternohyoid, sternothyroid), 5 nerves (right/left vagus, right/left phrenic, right/left sympathetic trunks, right/left ventral ramus of T1, left recurrent laryngeal nerve), 5 arteries (brachiocephalic trunk, left common carotid, left subclavian, right/left internal thoracic, right/left superior intercostal arteries), and 3 veins (right/left brachiocephalic, right/left inferior thyroid, right/left first posterior intercostal vein).
A step-by-step guide to drawing a cross-section of the thoracic inlet is provided. Beginning with the boundaries (manubrium, first rib, T1 vertebra), it then illustrates the placement of viscera (lung apex/cervical pleura, thymus, trachea, esophagus, thoracic duct), muscles (sternohyoid, sternothyroid), arteries (brachiocephalic, common carotid, subclavian, internal thoracic), veins (brachiocephalic, internal thoracic venae comitantes), and nerves (phrenic, vagus, recurrent laryngeal). Finally, structures near the neck of the first rib – sympathetic trunk, first posterior intercostal vein, superior intercostal artery, and first intercostal nerve (VAN) – are added.
Thoracic inlet syndrome (also known as scalene syndrome) occurs when structures in the scalene triangle (formed by scalenus anterior, scalenus medius, and the first rib) are compressed. Causes include hypertrophy or spasm of the scalenus anterior muscle, or the presence of a cervical rib. This compression affects the lower trunk of the brachial plexus and the subclavian artery, which pass through this space, but not the subclavian vein which is anterior to the scalenus anterior muscle.
Symptoms of thoracic inlet syndrome can be neural or vascular. Neural symptoms, due to compression of the lower trunk of the brachial plexus (C8, T1), include numbness, tingling, pain (paresthesia) along the medial side of the hand and forearm, and wasting/atrophy of small intrinsic hand muscles. Vascular symptoms, due to subclavian artery compression, include pallor and coldness of the upper limb, and a feeble radial pulse, as less blood reaches the arm.
A cervical rib, present in 0.5% of the population, is another cause of thoracic inlet syndrome. It articulates posteriorly with the transverse process of the C7 vertebra and may attach anteriorly to the first rib or via a fibrous structure. The presence of a cervical rib can cause similar neural and vascular symptoms due to traction on the lower trunk of the brachial plexus and compression of the subclavian artery, leading to paresthesia, muscle wasting, pallor, coldness, and a feeble radial pulse.