Anatomy, Thorax, Phrenic Nerves


Article Author:
Kaitlin Oliver


Article Editor:
John Ashurst


Editors In Chief:
Susan Jeno
Sarah Fabiano


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Frank Smeeks
Kristina Soman-Faulkner
Radia Jamil
Patrick Le
Sobhan Daneshfar
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Pritesh Sheth
Hassam Zulfiqar
Navid Mahabadi
Steve Bhimji
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Mark Pellegrini
James Hughes
Beata Beatty
Nazia Sadiq
Hajira Basit
Phillip Hynes
Tehmina Warsi


Updated:
12/9/2018 7:53:36 PM

Introduction

The phrenic nerve originates from the anterior rami of C3 through C5 and traverses the neck, heart, and lungs to reach the diaphragm. From its origin, the phrenic nerve descends vertically caudad and adjacent to the internal jugular vein. In the neck and upper thorax, the left phrenic nerve tracts proximal to the subclavian artery. The right phrenic nerve runs superficial to the anterior scalene muscle and the second part of the right subclavian artery. In the thorax, the right and left phrenic nerve will continue to descend anteriorly to the root of the lung and between the mediastinal surface of the parietal pleura and fibrous pericardium. The right phrenic nerve passes lateral to the right atrium and right ventricle and will continue to descend through the vena cava hiatus in the diaphragmatic opening at the level of T8. The left phrenic nerve descends anterior to the pericardial sac of the left ventricle and terminates at the central tendon of the diaphragm.[1][2][3]

Structure and Function

The phrenic nerve originates from the anterior rami of the C3 through C5 nerve roots and consists of motor, sensory, and sympathetic nerve fibers. It provides complete motor innervation to the diaphragm and sensation to the central tendon aspect of the diaphragm. The left phrenic nerve innervates the left diaphragmatic dome, and the right phrenic nerve innervates the right diaphragmatic dome, with the majority of nerve branching occurring on the inferior aspect of the diaphragm. The motor innervation activation will cause the diaphragm to contract with inspiration, resulting in a flattened diaphragm and increased intrapleural space. During exhalation, the diaphragm relaxes and returns to the dual dome shape. The phrenic nerve also provides touch and pain sensory innervation to the mediastinal pleura and the pericardium in addition to the intercostal nerves.

Embryology

The phrenic nerve is a peripheral nerve that originates from the neural crest cells and is derived from the neural plate. Neurulation, or the development of the neural plate, begins after the third week of fertilization. At weeks 5 through 6, the septum transversum, forming the thoracic diaphragm, descends from the cervical vertebrae to the thoracolumbar vertebrae. The phrenic nerve descends along with the septum transversum, carrying innervation from the ventral rami from C3 through C5.

Blood Supply and Lymphatics

The phrenic nerve is accompanied by the pericardiophrenic artery and superior phrenic vein throughout its course. The nerve, artery, and vein originate at the neck root and descend, parallel, along the lateral aspects of the pericardial sac, ultimately terminating at the superior aspect of the diaphragm. The pericardiophrenic artery is a branch of the internal thoracic artery. The superior phrenic vein drains into the azygos vein on the right and left.

Muscles

The phrenic nerves provide motor innervation to the diaphragm and work in conjunction with secondary respiratory muscles (trapezius, pectoralis major, pectoralis minor, sternocleidomastoid, and intercostals) to allow respiration.

The accessory phrenic nerve, if present, may provide motor innervation to the subclavius muscle. The subclavius muscle originates at the costochondral junction of the first rib and inserts at the subclavian groove of the clavicle. The subclavius muscle stabilizes the clavicle.

Physiologic Variants

In a small number of people, there may be an accessory branch of the phrenic nerve. An accessory phrenic nerve will follow the true phrenic nerve down its course to the diaphragm but often terminates at the pericardium. This variation will be located laterally and posteriorly to the main phrenic nerve and anteriorly to the subclavian vein. This variant is mostly C5 contribution and will branch off proximally at the root of the neck to provide motor innervation to the subclavius muscle.

Surgical Considerations

The phrenic nerve originates at the C3 through C5 nerve roots, which exits the spinal canal at the neck root and descends caudally, parallel to the pericardial sac to provide the motor innervation to the diaphragm. The phrenic nerve must be identified in cervical and thoracic surgical dissection to preserve the neuromuscular pathways. Superior to the clavicle, the phrenic nerve can be located in the posterior triangle of the neck, superficial to the anterior scalene. In the mediastinum, the phrenic nerves can be identified with lateral retraction of the lungs to reveal the pericardial sac where the phrenic nerves pass anteriorly to the root of the lung. The phrenic nerves are located on the lateral aspects with accompanying pericardiophrenic arteries and superior phrenic veins.

Lesions to either the left or right phrenic nerve will cause relative elevation of the ipsilateral dome of the diaphragm, ultimately paralyzing it from contraction and depression during inspiration. The right phrenic nerve is at risk of being severed with vena cava clamping as the phrenic nerve enters the caval diaphragmatic opening with the inferior vena cava at T8.[4][5][6]

Clinical Significance

The phrenic nerve supplies sensory innervation to the diaphragm. Pain arising from the diaphragm is often referred to the tip of the shoulder, also known as the Kehr sign. For example, a patient with a subphrenic abscess or a ruptured spleen may complain of pain in the left shoulder. The hiccup reflex is due to irritation of the phrenic nerve. It results from sudden spasms of the diaphragm which pull air against the closed fold of the larynx. The phrenic nerve must be identified during thoracic and open-heart surgery. It may be injured during the taking down of the internal mammary artery, which is used for coronary artery bypass. The phrenic nerve often is injured in infants undergoing congenital heart procedures. Once the phrenic nerve is injured, the diaphragm will become paralyzed. On a chest X-ray, the diaphragm will appear elevated. Ultrasound or fluoroscopy can be used to make the diagnosis of a paralyzed diaphragm. If only one side of the diaphragm is paralyzed, most patients can overcome the deficit and lead normal lives. If both sides are paralyzed, phrenic nerve stimulation, intercostal nerve transfer, or a permanent tracheostomy with ventilation dependence is required. Diaphragmatic plication is sometimes done in symptomatic patients when only one diaphragm is paralyzed. Patients who have spinal cord trauma may be able to breathe despite being paralyzed because the phrenic nerve has a higher origin at C3 through C5.[7][8]


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Anatomy, Thorax, Phrenic Nerves - Questions

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Which of the following about the phrenic nerve is incorrect?



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A 47-year old male with a history of chronic obstructive pulmonary disease presents with dyspnea. Which of the following is false about the nerve that innervates the primary respiratory muscle?



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A 42-year-old male presents to the emergency department complaining of hiccups for the past hour. Hiccups are due to the intermittent spams of the diaphragm. From what nerve roots does the diaphragm receive its motor innervation?



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A 56-year-old male is complaining of upper neck pain with shortness of breath. Which cervical segment does not play a role in the innervation of the diaphragm?

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What cervical nerve segments innervate the diaphragm?



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A 34-year-old female is complaining of difficulty breathing. Which nerve passes superficially to the anterior scalene in the neck and then descends to innervate the diaphragm?



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A patient presents with dyspnea. It is determined the primary respiratory muscle is dysfunctional. What nerve roots supply motor innervation to the primary respiratory muscle?



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A 23-year-old male is receiving a left lung transplant. To prevent damage to the diaphragm, what nerve must be identified that runs laterally to the pericardial sac and medially to the lung tissue?



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A patient presents with diaphragmatic dysfunction. The phrenic nerve supplies motor and sensory sensation to the diaphragm. Which muscle does the phrenic nerve pass superficially to in the neck?



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You are assisting the cardiothoracic surgeon with a thoracoabdominal procedure. The surgeon is attempting to get access to the structure labeled "C" in the image shown below. To avoid injury to the 'nerve' which incision will you avoid?

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What muscle does the accessory phrenic nerve innervate?



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You perform a 2-vessel coronary artery bypass procedure in a 71-year-old male. You use the left internal mammary artery to bypass the occluded LAD and a saphenous vein graft for an ostial lesion in the right coronary artery. The surgery was difficult because of the patient’s obesity (BMI 33). After the patient is extubated the next day, you note the chest x-ray shown. To rule out a nerve injury, you will next order?

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Which nerve travels along the left side of the pericardium?



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Anatomy, Thorax, Phrenic Nerves - References

References

Bains KNS,Lappin SL, Anatomy, Thorax, Diaphragm null. 2018 Jan     [PubMed]
Goizueta AA,Bhimji SS, Anatomy, Thorax, Lung Pleura And Mediastinum null. 2018 Jan     [PubMed]
Chaudhry R,Bhimji SS, Anatomy, Thorax, Lungs null. 2018 Jan     [PubMed]
Graves MJ,Henry BM,Hsieh WC,Sanna B,PĘkala PA,Iwanaga J,Loukas M,Tomaszewski KA, Origin and prevalence of the accessory phrenic nerve: A meta-analysis and clinical appraisal. Clinical anatomy (New York, N.Y.). 2017 Nov     [PubMed]
Wang J,Li J,Liu G,Deslauriers J, Nerves of the mediastinum. Thoracic surgery clinics. 2011 May     [PubMed]
Khong P,Lazzaro A,Mobbs R, Phrenic nerve stimulation: the Australian experience. Journal of clinical neuroscience : official journal of the Neurosurgical Society of Australasia. 2010 Feb     [PubMed]
Loukas M,Kinsella CR Jr,Louis RG Jr,Gandhi S,Curry B, Surgical anatomy of the accessory phrenic nerve. The Annals of thoracic surgery. 2006 Nov     [PubMed]
Morgan JA,Morales DL,John R,Ginsburg ME,Kherani AR,Vigilance DW,Cheema FH,Smith CR Jr,Oz MC,Argenziano M, Endoscopic, robotically assisted implantation of phrenic pacemakers. The Journal of thoracic and cardiovascular surgery. 2003 Aug     [PubMed]

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