Corpus: Phrenic nerve

The phrenic nerve is a peripheral nerve arising from the cervical plexus. It contains fibres from the spinal cord segments C3 and C4, with contributions from C5.

The phrenic nerve descends between the subclavian artery and subclavian vein, passing over the scalenus anterior muscle. In the mediastinum, it travels ventral to the pulmonary hilum, between the parietal pleura and the fibrous pericardium, continuing down to the diaphragm. Along its path, it gives off a pericardial branch. Both the superior and inferior surfaces of the diaphragm are supplied by its branches.

The nerve is accompanied by the pericardiacophrenic artery, a branch of the internal thoracic artery.

Some anatomical variations include an accessory phrenic nerve, which may contain fibres from different segments, such as C2, C5, C6, or even T1, and unites with the main phrenic nerve.

At the diaphragm, the phrenic nerve gives off right and left abdominal branches. These branches penetrate the diaphragm, innervating the parietal peritoneum and parts of the visceral peritoneum in the abdominal cavity. The right branch passes through the caval opening, while the left branch traverses the muscle fibres of the diaphragm or the oesophageal hiatus.

3.1. Motor

The phrenic nerve provides motor innervation to the diaphragm.

3.2. Sensory

Sensory innervation includes:

• Parietal pleura (mediastinal and diaphragmatic parts)
• Pericardium (via pericardial branches)
• Peritoneum, especially that covering the liver, gall bladder, and the upper stomach (via the phrenicoabdominal branches)

Irritation of areas innervated by the phrenic nerve can result in referred pain to the right shoulder (known as the Eiselsberg phenomenon).

The course of the phrenic nerve is derived from the caudal descent of the transverse septum, the mesodermal precursor of the diaphragm, which moves from the cervical region during early embryonic development.

Paralysis of the phrenic nerve, or phrenic nerve palsy, can occur due to various causes such as compression from a tumour, mediastinal fibrosis, or as a result of surgery. Unilateral phrenic nerve palsy may be asymptomatic or lead to difficulty breathing, while bilateral palsy can result in severe respiratory failure.

A useful mnemonic for remembering the spinal cord segments and function of the phrenic nerve is: "Three, four, five keep the diaphragm alive."

• Aumüller et al, Duale Reihe Anatomie: Mit Online-Zugang (5th updated edition), Thieme Verlag, 2020