Ultrasound-guided phrenic nerve block is useful in the management of intractable hiccups. This technique may also be utilized as both a diagnostic and therapeutic maneuver to help identify if the phrenic nerve is subserving subdiaphragmatic pain from tumor, abscess, or other pathology. The use of ultrasound imaging can identify the exact location and course of the phrenic nerve when surgical procedures in the posterior triangle of the neck are being contemplated. If destruction or sacrifice of the phrenic nerve is being considered, this technique is useful as a prognostic indicator of the degree of respiratory impairment from paralysis of the hemidiaphragm that the patient may experience. Neurodestruction of the phrenic nerve may be carried out by chemoneurolysis, cryoneurolysis, radiofrequency lesioning, surgical crushing, or resection of the nerve.

The fibers that comprise the phrenic nerve arise primarily from the fourth cervical nerve root, with the nerve also receiving contributions from the third and fifth cervical roots as well (Fig. 19.1). The left and right phrenic nerves contain motor, sensory, and sympathetic fibers, which provide the motor and sensory innervation to their respective hemidiaphragm as well as the diaphragmatic central tendon. The phrenic nerve provides sympathetic and sensory fibers to the pericardium and mediastinal pleura.

The phrenic nerve descends in proximity to the internal jugular vein with the phrenic nerve passing inferiorly beneath the sternocleidomastoid muscle. At the level of the cricoid cartilage (the level where classic interscalene brachial plexus block is performed), the phrenic nerve is in very close proximity to the brachial plexus (see Figs. 19.1 and 19.2). As the phrenic nerve courses downward, it moves in an inferior medial
trajectory away from the exiting nerves of the brachial plexus making selective blockade of the phrenic nerve a possibility. At the junction of the middle and lower posterior border of the sternocleidomastoid muscle, the phrenic nerve emerges from behind the sternocleidomastoid muscle and lies on top of the anterior scalene muscle where it is easily identified on ultrasound imaging (Figs. 19.3 and 19.4).

The phrenic nerves exit the root of the neck between the subclavian artery and vein to enter the mediastinum (Fig. 19.5). The right phrenic nerve follows the course of the vena cava to
provide motor innervation to the right hemidiaphragm. The left phrenic nerve descends across the pericardium of the left ventricle to provide motor innervation to the left hemidiaphragm in a course parallel to that of the vagus nerve.