learning objectives

By the end of this lesson, you should be able to:

Describe the origin, course, and relations of the phrenic nerve
Explain the motor and sensory functions of the phrenic nerve
Correlate phrenic nerve injury with clinical features of diaphragmatic paralysis

The phrenic nerve is a paired, mixed peripheral nerve that originates in the cervical region and descends through the thorax to innervate the diaphragm. It is the sole motor supply to the diaphragm, making it essential for respiration. In addition, it provides sensory innervation to several thoracic structures.

Overview

Nerve roots: Anterior rami of C3, C4, and C5
Mnemonic: “C3, 4, and 5 keep the diaphragm alive”
Primary motor function: Diaphragm
Sensory supply:
- Central diaphragm
- Fibrous pericardium
- Mediastinal pleura
- Central diaphragmatic peritoneum

Anatomical course

origin in the neck

The phrenic nerve arises primarily from C4, with additional contributions from C3 and C5 via the cervical plexus. It emerges at the lateral border of the anterior scalene muscle.

From here, it:

Descends vertically over the anterior surface of the anterior scalene
Lies deep to the prevertebral layer of cervical fascia
Passes posterior to the subclavian vein on both sides

right phrenic nerve

Crosses anterior to the right subclavian artery
Enters the thorax via the superior thoracic aperture
Descends anterior to the root of the right lung
Courses along the pericardium over the right atrium
Passes through the diaphragm at the caval opening (T8)
Supplies the inferior surface of the right hemidiaphragm

left phrenic nerve

Crosses anterior to the left subclavian artery
Enters the thorax via the superior thoracic aperture
Descends anterior to the root of the left lung
Crosses the aortic arch
Courses along the pericardium over the left ventricle
Pierces and supplies the inferior surface of the left hemidiaphragm

motor function

The phrenic nerve provides motor innervation to the diaphragm, the principal muscle of respiration.

Each phrenic nerve innervates the ipsilateral hemidiaphragm
Contraction results in diaphragmatic descent, increasing intrathoracic volume during inspiration

Sensory function

Sensory fibers from the phrenic nerve supply:

Central portion of the diaphragm
Central diaphragmatic pleura and peritoneum
Fibrous pericardium
Mediastinal pleura

Clinical correlation: Irritation of the diaphragm or pericardium may cause referred pain to the shoulder (C3–C5 dermatomes).

Clinical relevance: diaphragmatic paralysis

Causes

Phrenic nerve injury may occur due to:

Iatrogenic injury (e.g., cardiac, thoracic, or neck surgery)
Compression by tumors or mediastinal masses
Trauma
Neuropathies (e.g, diabetic neuropathy)

Clinical features

Unilateral paralysis:
- Often asymptomatic
- Incidental finding on chest X-ray
Bilateral paralysis:
- Dyspnoea
- Orthopnoea
- Reduced exercise tolerance
- Fatigue

Paradoxical movement:

The affected hemidiaphragm moves upward during inspiration and downward during expiration

Pulmonary function tests typically show a restrictive pattern.

Management

Identify and treat the underlying cause where possible
Supportive management for symptoms:
- Non-invasive ventilation (e.g. CPAP)
- Respiratory physiotherapy in selected cases