Course
In the neck
The phrenic nerve forms at the upper lateral border of scalenus anterior, at about the level of the upper border of the thyroid cartilage. It descends almost vertically across the front of the muscle from its lateral to its medial border, held down by the prevertebral fascia. Superficial to it lie the internal jugular vein, sternocleidomastoid, the inferior belly of omohyoid, and the transverse cervical and suprascapular arteries, which cross in front of it. On the left, the thoracic duct arches forwards across it near the root of the neck.
At the root of the neck
The nerve leaves the medial border of scalenus anterior and enters the thorax by passing between the subclavian artery behind and the subclavian vein in front. It crosses the internal thoracic artery as it enters the superior mediastinum.
In the thorax
Both nerves pass anterior to the root of the lung, in contrast to the vagus nerves, which pass behind it. Each runs between the fibrous pericardium and the mediastinal pleura with the pericardiacophrenic artery and vein.
| Right phrenic nerve | Left phrenic nerve |
|---|---|
| Lies on venous structures: right brachiocephalic vein, superior vena cava, right atrium, inferior vena cava | Descends between the left common carotid and left subclavian arteries |
| Short and nearly vertical | Crosses the left side of the aortic arch, superficial to the left vagus nerve |
| Pierces the diaphragm at or near the caval opening (T8) | Runs over the pericardium covering the left ventricle and pierces the muscular diaphragm near the apex of the heart |
The left superior intercostal vein passes forwards between the left phrenic and left vagus nerves on the aortic arch.
Distribution
The phrenic nerve is a mixed nerve carrying motor, sensory and sympathetic fibres. It divides on or just below the diaphragm and supplies the muscle from its inferior surface.
- Motor: the whole of the diaphragm on its own side, including the crura. The intercostal nerves do not supply diaphragmatic muscle.
- Sensory: the fibrous pericardium and parietal layer of serous pericardium, the mediastinal pleura, and the pleura and peritoneum over the central part of the diaphragm. The lower intercostal nerves supply the peripheral rim.
- Abdominal branches: phrenico-abdominal branches reach the peritoneum under the diaphragm, and the right nerve contributes to the region of the inferior vena cava, liver and gallbladder.
Accessory phrenic nerve
An accessory phrenic nerve, usually carrying C5 fibres by way of the nerve to subclavius, joins the main nerve at the root of the neck or in the upper thorax in a proportion of people. It may keep the diaphragm working when the main nerve is divided above its junction.
Relations
The phrenic nerve is best identified in the neck by its fixed position on scalenus anterior deep to the prevertebral fascia. It is the only nerve that runs vertically down the front of this muscle. The brachial plexus trunks emerge lateral to scalenus anterior, and the subclavian artery arches behind the muscle, so a nerve on its anterior surface is the phrenic.
In the thorax the phrenic nerve is anterior to the lung root and the vagus is posterior. On the pericardium, the nerve is easily seen through the mediastinal pleura on a dissected specimen, running with the slender pericardiacophrenic vessels.
Clinical relevance
Pain from the diaphragm is referred to the shoulder tip. The phrenic nerve carries sensation from the central diaphragm to C4, the same segment as the supraclavicular nerves that supply skin over the shoulder. Blood under the diaphragm from a ruptured spleen (Kehr sign), gas after laparoscopy and gallbladder inflammation can all cause shoulder tip pain.
Phrenic nerve palsy paralyses one hemidiaphragm. The chest radiograph shows a raised hemidiaphragm, and on fluoroscopic sniff testing it moves paradoxically upwards. Causes include:
- interscalene brachial plexus block, which blocks the phrenic nerve in nearly all patients;
- invasion by lung cancer or mediastinal tumour;
- cardiac surgery, from topical cooling or stretch;
- neck surgery, central venous cannulation and trauma.
Spinal cord injury at or above C3 abolishes diaphragmatic breathing and requires long-term ventilation or phrenic nerve pacing. Injury below C5 spares the diaphragm. Irritation of the nerve causes persistent hiccups.