What are the characteristics and functions of the phrenic nerves, including their derivation, anatomical course, and distribution?

Phrenic Nerve Characteristics: True/False Analysis

A. Are predominantly C5 - FALSE

The phrenic nerve is predominantly derived from C3, C4, and C5, with C4 being the most consistent and dominant contributor, not C5. 1

• The classic mnemonic "C3, 4,5 keeps the diaphragm alive" accurately reflects the nerve's origin from these three cervical roots 2
• C4 is present in virtually all cases (all except one in a large anatomical study), making it the most constant component 3
• C5 contribution is variable and often forms accessory phrenic nerves rather than the main trunk 3
• In some anatomical variations, the phrenic nerve arises from C4-C7 or C4-C6, but C4 remains the predominant root 4

B. Passes anteriorly to the scalenus anterior - FALSE

The phrenic nerve descends along the anterior surface of the scalenus anterior muscle, not anterior to it. 2, 5

• The nerve forms in the neck and specifically "descends along the anterior surface of the scalenus anterior muscle before entering the thorax" 2
• In the supraclavicular triangle, the phrenic nerve crosses the anterior border of the anterior scalene muscle near Erb's point, approximately 2-3 cm superior to the clavicle 5
• This anatomical relationship is critical during neck dissection procedures to avoid iatrogenic injury 5
• The nerve is located underneath the posterior border of the sternocleidomastoid muscle at the level of the cricoid cartilage for transcutaneous stimulation 1

C. Has afferent fibers from the mediastinal pleura - TRUE

The phrenic nerve provides both motor and sensory innervation, including sensory input from mediastinal structures. 2

• The phrenic nerve supplies "motor and sensory input to the diaphragm" 2
• While the evidence provided focuses primarily on motor function, the phrenic nerve carries sensory fibers from the mediastinal pleura, pericardium, and peritoneum (general medical knowledge)
• This sensory function explains referred pain patterns to the shoulder in diaphragmatic irritation

D. Supplies the fibrous pericardium - TRUE

The phrenic nerve provides sensory innervation to the fibrous pericardium. 2

• The nerve supplies "motor and sensory input" to structures it courses alongside 2
• The phrenic nerve's anatomical course brings it into direct contact with the pericardium as it descends through the thorax (general medical knowledge)
• This innervation is clinically relevant in pericarditis, where irritation causes referred pain to the shoulder via C3-C5 dermatomes

E. Divides into 3 branches on the superior surface of the diaphragm - FALSE

The phrenic nerve divides on the inferior (abdominal) surface of the diaphragm, not the superior surface, and the branching pattern is variable. 4

• The phrenic nerve "terminated in the costosternal and lumbar branches, and less frequently as the lumbocostal trunk and sternal branch" 4
• The lumbar branch innervates the crura of the diaphragm, the costal branch serves the entire costal region, and the sternal branch distributes to the ventrolateral costal area and sternal portion 4
• The number of fascicles varies: "3 ± 2 fascicles in the neck that merged to form a single fascicle in the thorax and split again into 3 ± 3 fascicles above the diaphragm" 6
• The branching occurs as the nerve penetrates and passes through the diaphragm, not on its superior thoracic surface

Clinical Pitfall

The phrenic nerve's vulnerable anatomical position makes it susceptible to injury during cardiac surgery, neck trauma, and even minor whiplash injuries, potentially causing diaphragmatic paralysis with delayed symptom onset. 2 Recognition of Phren's sign (paradoxical abdominal movement during inspiration) should prompt immediate evaluation with phrenic nerve stimulation studies 7