Superior Mediastinum and Root of Neck - LO 2

2. Describe a typical intercostal space. 

The intercostal spaces (spaces between the ribs) are occupied by layers of muscle and the neurovasculature which serves the muscle and overlying skin. Generally, the intercostal muscle is divided into three layers:

• External intercostal mm. - pull ribs upwards and outwards, to slightly increase the volume of the thoracic cavity (inspiratory)

  • 11 pairs

  • Distributed from tubercles of ribs to costal cartilages (only present anteriorly as aponeuroses)

  • Typically more robust than internal intercostal mm.

• Internal intercostal mm. - pull ribs downwards and inwards, to slightly decrease the volume of the thoracic cavity (expiratory)

  • 11 pairs

  • Distributed from sternum to costal angle (only present posteriorly as aponeuroses)

• Innermost intercostal mm. - function akin to internal intercostal mm.

Neurovascular supply of the intercostal spaces

Intercostal spaces are typically served by branches of three arteries:

• Highest (supreme) intercostal a., via posterior intercostal aa. (1st-2nd IC spaces)

  • From costocervical trunk (2nd part of subclavian a.)

  • Variably absent, especially on the left

• Thoracic part of descending aorta, via posterior intercostal aa. (3rd-11th IC spaces & subcostal space)

  • Dominant blood supply to IC spaces

• Internal thoracic a., via anterior intercostal brs.

  • From 1st part of subclavian a.

  • Anterior intercostal brs. 1-6 typically arise directly from internal thoracic

  • Anterior intercostal brs. 7-9 typically arise from musculophrenic br. 

  • Important anastomoses for IC spaces

Posterior intercostal brs. are typically the dominant source of blood to the intercostal spaces. Many typically openly anastomose (physically connect) to their smaller anterior intercostal br. counterparts, and these vessels are referred to as intercostal aa. 

Intercostal aa. are found in the subcostal groove of the rib forming the superior boundary of the intercostal space, and are accompanied by an intercostal v. and n. The arrangement of the intercostal neurovasculature is often conceptualized by the mnemonic ‘VAN,’ which places the Vein in closest (most superior) contact with the subcostal groove, then the Artery, and finally the Nerve. Often, intercostal neurovasculature has associated collateral brs. which are 1) much smaller, 2) found along the superior margin of the rib at the inferior boundary of the intercostal space, and 3) have the inverse spatial relationships to the lower rib (i.e. NAV - Nerve is most superior, then Artery, then the Vein is in closest contact with the rib). 

Blood is returned from the intercostal spaces through similar routes as it is supplied, chiefly via:

• Posterior intercostal vv.

  • Supreme intercostal vv.

    • 1st intercostal spaces

    • Drain into brachiocephalic vv. (variable)

  • Superior intercostal vv.

    • Intercostal spaces 2-4

    • Drain into azygos system (right), or brachiocephalic v. (left)

  • Azygos system (detailed in Posterior Mediastinum Session)

    • Ultimately drains into superior vena cava

• Anterior intercostal vv.

  • Internal thoracic vv.

Intercostal spaces are innervated by intercostal nn., which are typically ventral primary rami (VPR) of spinal nn. T1-T11. The VPR of T1 ramifies, with the bulk of the VPR joining superiorly with the brachial plexus (C5-T1) and the remaining fibers becoming the first intercostal n. The VPR of T12 is known as the subcostal n. 

For the greater part of their conveyance, neurovasculature serving the intercostal spaces may be found between the internal intercostal mm. and the innermost intercostal mm.