The phrenic nerve is a deep branch of the cervical plexus with neural contribution from the brachial plexus. It usually arises from the third, fourth, and fifth cervical nerves (Fig. 2-20). The nerve courses downward with a slight medial inclination over the anterior face of the anterior scalene muscle. It runs directly on the anterior surface of the muscle fibers, between them and the overlying fascia, which is the prevertebral layer of the cervical fascia (Figs. 5-21 and 4-40). The phrenic nerve provides motor function to the ipsilateral diaphragm.
The easiest way to avoid injuring the phrenic nerve is to stay superficial to the prevertebral layer of the cervical fascia at the level of the anterior scalene muscle. This will keep the phrenic nerve protected by the fascial layer. When approaching the carotid sheath, the anterior superficial branches of the cervical plexus must be transected distal to the exit of the phrenic roots to preserve the deep cervical contributions and maintain the anatomical and functional integrity of the phrenic nerve.
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