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The left phrenic nerve or nervus phrenicus sinister in Latin (‘phrenicus’ meaning diaphragm, and ‘sinister’ meaning left) is a mixed nerve that arises mostly from the fourth cervical ramus, but also has contributions from the third and fifth cervical rami of the spinal segments (C3, C4, and C5) as well.
At the root of the neck, the left phrenic nerve descends from the medial edge of the scalenus anterior muscle along the first part of the left subclavian artery, and behind the thoracic duct. In some cases, its cervical course matches that of the right phrenic nerve, and so it runs across the second part of the left subclavian artery instead. Here, it is separated from said artery by the scalenus anterior, at the level of the thoracic inlet. It then crosses the left internal thoracic artery anteriorly, and passes the medial portion of the apex of the left lung and its pleura. Thereafter, it continues to the first part of the subclavian artery, where it crosses obliquely towards the groove found in between the left common carotid and the subclavian arteries. It will then run lateral and superficial to the left vagus nerve, right above the aortic arch, and posterolateral to the beginning of the left brachiocephalic vein. Afterwards, it passes superficial to the lateral surface of the aortic arch, across the left superior intercostal vein, to run along the fibrous pericardium covering the left ventricle and the mediastinal pleura. It leaves the thorax by piercing the diaphragm near the heart’s apex, accompanied by the pericardiophrenic vessels.
Both the right and left phrenic nerves are the diaphragm’s sole motor supply. The right phrenic nerve passes through the central tendon of the diaphragm, while the left phrenic nerve passes through the muscular portion. They also provide some sensory innervation to the diaphragm and its accompanying membranes. The intercostal nerves (lower 6-7) and the subcostal nerves provide the sensory innervation to the peripheral portion of the diaphragm.
Lesion of the left phrenic nerve leads to the paralysis of only the left half of the diaphragm, because each dome has its separate motor nerve supply. This will result in paradoxical diaphragmatic movements during inspiration and expiration. The gold standard for assessing diaphragm paralysis is through electric or magnetic stimulation of the phrenic nerves. Diaphragmatic pain can be referred and felt in the shoulders due to common nerve root origins in the neck.
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