Phrenic Nerve

Phrenic Nerve[edit | edit source]

The phrenic nerve is among the most important nerves in the body due to its role in respiration. The phrenic nerve provides the primary motor supply to the diaphragm, the major respiratory muscle[1]. It passes motor information to the diaphragm and receives sensory information from it. There are two phrenic nerves, a left and a right one.[2]

Both of these nerves supply

  • Motor fibers to the diaphragm and
  • Sensory fibres to the fibrous pericardium, mediastinal pleura, and diaphragmatic peritoneum.[2]
Right phrenic nerve (highlighted in green) - anterior view

Image 1: The Phrenic nerve, showing course to the diaphragm.

Origin[edit | edit source]

Image 2: Origin of the phrenic nerve

The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 3rd and 5th cervical nerves (C3-C5) in humans.[3] Thus, the phrenic nerve receives innervation from parts of both the cervical plexus and the brachial plexus of nerves.

Course[edit | edit source]

In the neck, the phrenic nerve lies on the anterior surface of the anterior scalene muscle, passes over the dome of the pleura and enters the thorax posterior to the subclavian vein. The right and left phrenic nerves have a different course in the thorax but as a general rule, they descend as lateral as possible whilst keeping in contact with the mediastinal pleura. Both travel anterior to the hilum/bronchus on their respective side.[4]

The right Phrenic nerve descends in the thorax along the right side of the right brachiocephalic vein and the superior vena cava. It 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. It then descends on the right side of the inferior vena cava to the diaphragm. Its terminal branches pass through the canal opening in the diaphragm to supply the central part of the peritoneum on its under aspect. (Image 2)

Image 3: Right and left phrenic nerve supplying the ipsilateral dome of the diaphragm

The left Phrenic nerve descends in the thorax along the left side of the left subclavian artery. It crosses the left side of the aortic arch and here crosses the left side of the left Vagus nerve. It passes in front of the root of the left lung and then descends over the left surface of the pericardium which separates the nerve from the left ventricle. On reaching the diaphragm, the terminal branches pierce the muscle and supply the central part of the peritoneum on its under aspect. (Image 2)[2]

Supply[edit | edit source]

The phrenic nerve is the sole motor supply to each hemidiaphragm. It also provides sensory supply to:

  • diaphragm (except the most peripheral diaphragm, which is supplied by intercostal nerves)
  • mediastinal pleura
  • pericardium
  • central parts of diaphragmatic pleura and peritoneum[4]

Phrenic Nerve Palsy[edit | edit source]

Phrenic nerve palsy (also known as phrenic nerve paresis or paralysis) has many causes and 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.

  • One common etiology of phrenic nerve injury is primarily from thoracic and cardiac surgery. The left phrenic nerve descends anteriorly between the pericardium and mediastinal pleura and can be injured while dissecting near the area of an internal thoracic artery.
  • The phrenic nerve can also be damaged from blunt or penetrating trauma, metabolic diseases eg diabetes, infectious causes eg Lyme disease and herpes zoster, direct invasion by tumour, neurological diseases such eg cervical spondylosis and multiple sclerosis, myopathy and immunological disease (e.g., Guillain-Barre syndrome)[1].
  • Spinal cord injury above the level of C3 will impact breathing with nerve impulses being either completely disrupted or partially.

The diagnosis of phrenic nerve injury requires high suspicion due to nonspecific signs and symptoms including unexplained shortness of breath, recurrent pneumonia, anxiety, insomnia, morning headache, excessive daytime somnolence, orthopnea, fatigue, and difficulty weaning from mechanical ventilation. Diagnostic investigations include a chest radiograph which shows an elevated hemidiaphragm. This is usually followed by a fluoroscopy or a sniff test which looks at diaphragm excursion in real-time.[5] Additional investigations include pulmonary function testing, measurement maximal inspiratory pressures (MIP), as well as diaphragm ultrasonography. On physical examinations, 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.

Video providing an overview of Diaphragm Paralysis

Treatment options include:

  • Ventilatory or inspiratory muscle training which includes providing resistance to the inspiratory muscles through devices which provide a variable resistance training stimulus eg Threshold trainer or the PowerBreathe devices. Individuals post-cardiac surgery with diaphragm dysfunction saw significant improvements in their diaphragm mobility when treated with IMT vs the control group 12 months later.[6]
  • Plication of the affected site is a very useful treatment method that allows weaning from mechanical ventilation. Plication is preferably performed in unilateral diaphragmatic paralysis in non-morbidly obese patients. Surgical plication stabilizes the diaphragm to prevent the lungs from ballooning outward during expiration (breathing out).
  • Phrenic nerve stimulation.   Phrenic nerve stimulation is performed in intact phrenic nerve without evidence of myopathy. This procedure can be performed in patients with bilateral diaphragmatic paralysis with cervical spine injuries[1]
  • Phrenic nerve repair may also be considered in an attempt to restore the function of the paralyzed hemidiaphragm and it may be the optimal first-line treatment when feasible.[7]

References[edit | edit source]

  1. 1.0 1.1 1.2 Mandoorah S, Mead T. Phrenic nerve injury.2018 Available from: https://www.ncbi.nlm.nih.gov/books/NBK482227/(accessed 8.2.2021)
  2. 2.0 2.1 2.2 Prakash; Prabhu, L. V.; Madhyastha, S; Singh, G (2007). "A variation of the phrenic nerve: Case report and review"
  3. Moore, Keith L. (1999). Clinically oriented anatomy. Philadelphia: Lippincott Williams & Wilkins. ISBN 978-0-683-06141-3
  4. 4.0 4.1 Radiopedia Phrenic nerve Available from: https://radiopaedia.org/articles/phrenic-nerve (last accessed 8.2.2021)
  5. Hacking C, Yadegarfar M, Fayed I, et al. Sniff test. Reference article, Radiopaedia.org (Accessed on 23 Jan 2024) https://doi.org/10.53347/rID-35785
  6. etka Kodric, Roberto Trevisan, Chiara Torregiani, Rossella Cifaldi, Cinzia Longo, Fabiana Cantarutti, Marco Confalonieri, Inspiratory muscle training for diaphragm dysfunction after cardiac surgery, The Journal of Thoracic and Cardiovascular Surgery,
  7. Kaufman MR, L. Bauer T. Surgical Treatment of Phrenic Nerve Injury. March 2019. doi:10.25373/ctsnet.7863332.Available from: https://www.ctsnet.org/article/surgical-treatment-phrenic-nerve-injury(accessed 8.2.2021)