Nerves - Phrenic Nerve

Question 1

Overview

Origin?

Sensory function?

Motor function?

Answer 1

Origin - anterior rami of C3, C4, and C5.

Sensory function - central part of the diaphragm, pericardium and mediastinal part of the pleura.

Motor function - diaphragm

‘C3, C4, C5 keeps the diaphragm alive’

Question 2

Anatomical course

Answer 2

Originates from C3, C4 and C5. C4 is the main contributor.

Arises at the lateral border of the anterior scalene. It then passes inferiorly and over the anterior surface of the anterior scalene, deep to the prevertebral layers of cervical fascia.

On both sides, the nerve runs posterior to the subclavian vein. From here the courses differ in the left and right phrenic nerves:

Right

Passes anteriorly over the lateral part of the subclavian artery

enters the thorax via the superior thoracic aperture

Descends anteriorly along the right lung root

Courses along the pericardium of the right atrium

Pierce the diaphragm at the IVC opening

Innervates the inferior surface of the diaphragm

Left

Passes anteriorly over the medial part of the subclavian artery

Enters the thoracic cavity, via the superior thoracic aperture

Descends anterior to the left lung root

Crosses the aortic arch and bypasses the vagus nerve

Courses along the pericardium of the left ventricle

Pierces and innervates the inferior surface of the diaphragm

Question 3

Motor functions

Answer 3

Diaphragm - main muscle of ventilation

Question 4

Sensory functions

Answer 4

Central part of the diaphragm, including the surrounding pleura, and peritoneum. It also provides sensory innervation to the mediastinal pleura and the pericardium.

Question 5

Clinical relevance - diaphragmatic paralysis

Answer 5

Damage to the nerve can result in paralysis.

Aetiology -

1) Mechanical trauma - iatrogenic
2) Compression - tumour in chest cavity
3) Neuropathies - e.g. diabetic neuropathy

Paralysis of the diaphragm produces paradoxical movement. The affected side moves upwards in inspiration and downwards in expiration.

Usually asymptomatic in unilateral diaphragmatic paralysis. Usually found on incidental x-ray.

Bilateral paralysis - poor exercise tolerance, orthopnea (breathlessness when lying down) and fatigue. Lung tests show a restrictive deficit.

Treatment - underlying cause should be treated. Symptomatic relief the second part of the treatment - e.g. CPAP.