Phrenic nerve

In addition to motor fibers, the phrenic nerve contains sensory fibers, which receive input from the central tendon of the diaphragm and the mediastinal pleura, as well as some sympathetic nerve fibers. Although the nerve receives contributions from nerve roots of the cervical plexus and the brachial plexus, it is usually considered separate from either plexus.

The name of the nerve comes from Ancient Greek phren 'diaphragm'.

Structure

The phrenic nerve originates in the phrenic motor nucleus in the ventral horn of the cervical spinal cord. It descends obliquely with the internal jugular vein across the anterior scalene, deep to the prevertebral layer of deep cervical fascia and the transverse cervical and suprascapular arteries. On the left, the phrenic nerve crosses anterior to the first part of the subclavian artery. On the right, it lies on the anterior scalene muscle and crosses anterior to the 2nd part of the subclavian artery. On both sides, the phrenic nerve usually runs posterior to the subclavian vein as it enters the thorax where it runs anterior to the root of the lung and between the fibrous pericardium and mediastinal parietal pleura.

• The right phrenic nerve passes over the brachiocephalic artery, posterior to the subclavian vein, and then crosses the root of the right lung anteriorly and then leaves the thorax by passing through the vena caval foramen opening in the diaphragm at the level of T8. The right phrenic nerve passes over the right atrium.
• The left phrenic nerve passes over the pericardium of the left ventricle and pierces the diaphragm separately.

The pericardiacophrenic arteries and veins travel with their respective phrenic nerves.

The phrenic nerve can be marked by a line connecting these two points:

1. 1st point can be labelled 3.5 cm at the level of the thyroid cartilage from the midsagittal plane.
2. 2nd point is at the medial end of the clavicle.

Variation

As with most nerves in the neck, multiple anatomic variants have been described. Notably, there may be variability in the course of the phrenic nerve in the retro-clavicular region such that the nerve courses anterior to the subclavian vein, rather than its typical position posterior to the vein (between the subclavian vein and artery). This variant may predispose the phrenic nerve to injury during subclavian vascular cannulation.

In addition, an accessory phrenic nerve is commonly identified, present in up to 75% of a cadaveric study.

In canines, the phrenic nerve arises from C5-C7 with occasional small contributions from C4. In the cat, horse, ox, and small ruminants the phrenic nerve arises variably from C4-C7.