The phrenic nerve arises from the anterior rami of C3,4,5 and passes down through the neck deep to the prevertebral fascia lying on the anterior surface of scalene anterior. As the nerve is so deep most important structures pass anterior to it. Of note the subclavian artery and brachial plexus pass deep to both scalene anterior and the phrenic nerve. The suprascapular and transverse cervical arteries and subclavian vein all pass over the phrenic nerve and finally, the phrenic nerve overlies the internal thoracic artery as they both enter the thorax.
Once in the thorax the phrenic nerves come to lie more anterior passing in front of the lung hila before piercing the diaphragm on the right with the IVC at the level of T8 and on its own on the left.
The phrenic nerve provides motor innervation to the diaphragm and sensory innervation to the mediastinal pericardium and pleura. There is no cross over between the two signs so damage to one phrenic nerve will result in paralysis of the ipsilateral hemidiaphragm.
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