At what thoracic spine level is the phrenic nerve (phrenic nerve) located?

The Phrenic Nerve is NOT Located at the Thoracic Spine Level

The phrenic nerve originates from the cervical spine at levels C3, C4, and C5—not from the thoracic spine. This is a fundamental anatomical principle often remembered by the mnemonic "C3, 4,5 keeps the diaphragm alive" 1.

Anatomical Course of the Phrenic Nerve

Cervical Origin

• The phrenic nerve is formed from C3, C4, and C5 nerve fibers in the neck 1
• It arises from the cervical nerve roots, specifically the third to fifth cervical roots 2
• The nerve is typically found underneath the posterior border of the sternocleidomastoid muscle, at the level of the cricoid cartilage in the neck 2

Descending Path Through the Thorax

• After forming in the cervical region, the phrenic nerve descends along the anterior surface of the scalenus anterior muscle before entering the thorax 1
• The nerve travels through the thoracic cavity but does not originate from thoracic spinal levels 1
• It continues inferiorly through the mediastinum to reach and innervate the diaphragm 3

Clinical Relevance

Stimulation Techniques

• For cervical magnetic stimulation (CMS), the coil is centered over the spinous process of the seventh cervical vertebra (C7), which stimulates the third to fifth cervical roots that form the phrenic nerve 2
• The phrenic nerve can be stimulated transcutaneously at the neck level, not at thoracic levels 2

Injury Patterns

• Phrenic nerve injury occurs from cervical spine trauma at C3-C5 levels, not thoracic levels 4, 1
• Patients with high cervical spine injury at C3 to C5 suffer axonal loss in the phrenic nerve, leading to diaphragm paralysis 5
• The anatomic location in the neck makes the nerve vulnerable to cervical trauma, including whiplash, neck compression, and strangulation 1

Common Pitfall to Avoid

Do not confuse the phrenic nerve's path through the thoracic cavity with its origin. While the nerve does travel through the thorax to reach the diaphragm, it originates exclusively from cervical nerve roots C3-C5 1, 3. The thoracic spine does not contribute to phrenic nerve formation.