Phrenic nerve

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The phrenic nerve is a mixed motor/sensory nerve which originates from the C3-C5 spinal nerves in the neck. 

 

It provides exclusive motor control of the diaphragm, the primary muscle of respiration. 

 

The right and left phrenic nerves are primarily supplied by the C4 spinal nerve, but there is also contribution from the C3 and C5 spinal nerves. 

 

It originates  in the neck, and travels downward into the chest to pass between the heart and lungs towards the diaphragm.

 

The phrenic nerve emerges from the cervical plexus, with the right brachial plexus.

 

In addition to motor fibers, the 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. 

 

The phrenic nerve originates in the phrenic motor nucleus in the ventral horn of the cervical spinal cord. 

 

It provides the primary motor supply to the diaphragm, the major respiratory muscle.

 

The phrenic nerve passes motor information to the diaphragm and receives sensory information from it. 

 

There are two phrenic nerves, a left and a right one.

 

The phrenic nerve has motor fibers to the diaphragm and sensory fibers to the fibrous pericardium, mediastinal pleura, and diaphragmatic peritoneum.

 

The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 3rd and 5th cervical nerves.

 

The  phrenic nerve receives innervation from parts of both the cervical plexus and the brachial plexus of nerves.

 

The phrenic nerve lies on the anterior surface of the anterior scalene muscle in the neck.

 

It then passes over the dome of the pleura and enters the thorax posterior to the subclavian vein. 

 

The right and left phrenic nerves descend in the thorax as lateral as possible while keeping in contact with the mediastinal pleura: It descends in the thorax along the right side of the right brachiocephalic vein and the superior vena cava. 

 

Both sided nerves travel anterior to the hilum/bronchus on their respective side.

 

The right phrenic nerve passes in front of the root of the right lung and runs along the right side of the pericardium, which separates the nerve from the right atrium, and then descends on the right side of the inferior vena cava to the diaphragm.

 

The right phrenic nerve’s terminal branches pass through the caval opening in the diaphragm to supply the central part of the peritoneum on its under aspect. 

 

The left phrenic nerve descends in the thorax along the left side of the left subclavian artery, crosses the left side of the aortic arch and here crosses the left side of the left Vagus nerve. 

 

The left phrenic nerve passes in front of the root of the left lung and then descends over the left surface of the pericardium.

 

The left phrenic nerve’s terminal branches pierce the muscle and supply the central part of the peritoneum on its under aspect.

 

The phrenic nerve is the motor supply to each hemidiaphragm, and also provides sensory supply to:

 

diaphragm

 

mediastinal pleura

 

pericardium

 

central parts of diaphragmatic pleura and peritoneum

 

Phrenic Nerve Palsy

 

Phrenic nerve palsy has many causes.

 

Phrenic nerve palsy can be caused by lesions anywhere along the course of the phrenic nerve, as it travels from the neck, to pierce the diaphragm adjacent to the pericardium.

 

Phrenic nerve injury may result from surgery, primarily thoracic or cardiac surgery. 

 

The phrenic nerve can also be damaged from blunt or penetrating trauma, metabolic diseases such as diabetes, infectious causes like Lyme disease and herpes zoster, direct invasion by tumor, neurological diseases such as cervical spondylosis, multiple sclerosis, myopathy and immunological disease like Guillain-Barré syndrome, and spinal cord injury.

 

Associated  with nonspecific signs and symptoms including: shortness of breath, recurrent pneumonia, anxiety, insomnia, morning headache, excessive daytime somnolence, orthopnea, fatigue, and difficulty weaning from mechanical ventilation. 

 

Physical examination with phrenic nerve paralysis findings may include decreased breath sounds on the affected side, dullness to percussion of the affected side of the chest and inward movement of the epigastrium during inspiration.

 

Most patients with asymptomatic unilateral diaphragmatic paralysis do not require treatment. 

 

Treatment options:

Plication is preferably performed in unilateral diaphragmatic paralysis in non-morbidly obese patients.

 

Plication stabilizes the diaphragm to prevent the lungs from ballooning outward during expiration.

 

Phrenic nerve stimulation is performed in intact phrenic nerve without evidence of myopathy, in patients with bilateral diaphragmatic paralysis with cervical spine injuries.

 

If feasible, phrenic nerve repair may restore function of the paralyzed hemidiaphragm.

 

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

 

There may be variability in the course of the phrenic nerve in the retroclavicular region such that the nerve courses anterior to the subclavian vein, rather than posterior to the vein, and such a change predisposes the phrenic nerve to injury during subclavian vascular cannulation.

 

Up to 75% of people have an accessory phrenic nerve.

 

Left phrenic nerve palsy leads to elevation of the paralytic left diaphragm.

 

Pain arising from structures supplied by the phrenic nerve can be referred to other somatic regions served by spinal nerves C3-C5: shoulder pain.

 

Irritation of the phrenic nerve can lead to the hiccup reflex. 

 

A hiccup is a spasmodic contraction of the diaphragm, which pulls air against the closed folds of the larynx.

 

During thoracic surgery, confirming the identity of the phrenic nerve can be achieved by manipulating it to move the diaphragm: the dartle response (diaphragmatic startle).

 

The right phrenic nerve may be crushed by the vena cava clamp during liver transplantation.

 

Severing the phrenic nerve, will paralyze that half of the diaphragm. 

 

Bilateral diaphragmatic paralysis causes: spinal cord injury, motor neuron disease, infection, pneumonia, sarcoidosis, multiple sclerosis, polyneuropathy, myopathy and amyotrophy, cardiac surgery, lung transplantation, or mediastinal tumors.

 

Diaphragm paralysis is best demonstrated by sonography.

 

Breathing is more difficult with unilateral diaphragmatic palsy.

 

Individuals suffering spinal cord injuries below the neck are still able to breathe effectively, despite any paralysis of the lower limbs, as the phrenic nerve originates higher.

 

Brachial plexus injuries with paralysis to various regions in the arm, forearm, and hand can be treated using the phrenic nerve as a donor for neurotization of the musculocutaneous nerve and the median nerve.

 

This procedure has a high success rate of >84% in partial to full restoration of the innervation to the damaged nerve.