Anatomy of Intercostal Nerves

The twelve pairs of thoracic spinal nerves (T1-12) are divided into ventral and dorsal rami after they pass through the intervertebral foramina. The ventral rami of T1-T11 form the intercostal nerves that enter the intercostal spaces. The ventral ramus of T12 forms the subcostal nerve that is located inferior to the 12th rib. The dorsal rami of T1-T12 pass posteriorly to supply sensation to skin, muscles, and bones of the back.¹

Intercostal nerves are composed of dorsal horn sensory afferent fibers, ventral horn motor efferent fibers, and postganglionic sympathetic nerves. The major branches of intercostal nerves are anterior and lateral cutaneous branches (Fig. 31-1). These branches divide and innervate the skin and intercostal muscles of an individual segment along with variable collateral innervation of the adjacent segments. Because of such collateral innervation, it is necessary to block a level above and below the desired level when an intercostal nerve block is performed.

Figure 31-1 Anatomy of an intercostal nerve.

(Adapted from Ferrante FM, VadeBoncouer TR: Postoperative Pain Management. New York, Churchill Livingstone, 1993. Elsevier.)

Throughout its course, each intercostal nerve is associated with an artery and a vein. The intercostal arteries are derived directly from the aorta. The intercostal veins are derived from the confluence of venules along the chest and end in the azygous and hemiazygous veins. The intercostal nerve travels inferior to the vein and artery of the same segment (Fig. 31-2).

Figure 31-2 Intercostal nerve block: Cross-sectional anatomy.

(From Brown DL: Atlas of Regional Anesthesia, 3rd ed. Philadelphia, Saunders 2006, p 240.).

Pathophysiology of Intercostal Neuralgia

The pathogenesis of acute intercostal neuralgia involves multiple etiologies, such as skin and muscle injury, costochondral dislocation, costovertebral ligament damage, postherpetic neuralgia, postthoracotomy pain, or intercostal neuritis. The pathophysiology of acute pain involves chemosensitive ion channels in peripheral nociceptors that are activated by cytokines released from damaged tissue or inflammatory cells. Activation of these channels results in generation of action potentials that are conducted from the peripheral nociceptors in the chest area to the thoracic spinal cord via intercostal nerves.

The pathophysiology of chronic intercostal neuralgia pain is complex and involves a combination of peripheral, central, and psychological mechanisms. In some patients, a structural or inflammatory source can be identified; however, most patients encountered in clinical practice have primary pain disorders in which the etiologies are uncertain.

Indications for Intercostal Nerve Blocks

An intercostal nerve block is used for therapeutic and diagnostic purposes. Intercostal nerve blocks manage acute and chronic pain in the chest area. Common indications for an intercostal nerve block are described subsequently.

Chest wall surgery: An intercostal block with a single injection or an indwelling catheter offers analgesia for thoracotomy and percutaneous drainage tubes. It also helps relieve pain after upper abdominal or flank surgery.

Chest wall trauma: Single injections around the appropriate intercostal nerves or continuous infusion with intercostal catheters helps to control pain resulting from fractured ribs and other chest wall trauma.