Root Origin
- Spinal nerve roots: C3, C4, C5 (primary contribution from C4)
- Plexus: Cervical plexus
- Mnemonic: "C3, 4, 5 keeps the diaphragm alive"
- Type: Mixed (primarily motor to the diaphragm; sensory to the central diaphragm, mediastinal pleura, pericardium, and peritoneum covering the diaphragm)
Course
- Cervical plexus. The nerve forms from the ventral rami of C3, C4, and C5 at the lateral border of the anterior scalene. C4 provides the largest contribution.
- Anterior scalene. The nerve descends on the anterior surface of the anterior scalene muscle — this is a key anatomical relationship. It runs vertically on the scalene from its superior origin to the muscle's insertion on the first rib. The nerve lies between the anterior scalene and the prevertebral fascia, deep to the sternocleidomastoid.
- Thoracic inlet. The nerve passes between the subclavian artery (posterior) and the subclavian vein (anterior) to enter the thorax. At this level it crosses the internal thoracic (internal mammary) artery.
- Mediastinum. Each phrenic nerve descends through the mediastinum alongside the pericardium — the right phrenic nerve lateral to the right atrium and the superior vena cava, the left phrenic nerve lateral to the left ventricle and crossing the aortic arch. The nerves descend anterior to the lung roots.
- Diaphragm. Each phrenic nerve reaches the diaphragm and penetrates its superior surface, dividing into motor branches that supply the ipsilateral hemidiaphragm. The right phrenic nerve pierces the diaphragm at or near the caval opening (for the inferior vena cava). The left phrenic nerve pierces the muscular diaphragm lateral to the pericardium.
Motor Distribution
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/diaphragm (ipsilateral hemidiaphragm) | Contraction flattens the dome of the diaphragm, increasing thoracic volume (inspiration) | Each phrenic nerve supplies its own side. Unilateral injury = one hemidiaphragm paralyzed (reduced breathing capacity but compatible with life). Bilateral injury = both hemidiaphragms paralyzed (ventilator-dependent). |
Sensory Distribution
- Central diaphragm. The phrenic nerve carries sensory fibers from the central tendon and the central muscular portion of the diaphragm. This explains referred pain to the shoulder.
- Mediastinal pleura and pericardium. Sensory fibers from the pleura lining the mediastinum and the pericardial sac.
- Referred pain to the shoulder tip. This is the clinically critical sensory finding. Because the phrenic nerve originates from C3-C5 — the same spinal cord levels that receive sensory input from the shoulder (via the suprascapular nerve C5-C6, axillary nerve C5-C6) — irritation of the diaphragm is perceived as shoulder tip pain (Kehr's sign). The brain cannot distinguish C4 afferents from the diaphragm and C4 afferents from the shoulder, so it "refers" the pain to the shoulder. Common causes of phrenic irritation that produce shoulder tip pain: subdiaphragmatic abscess, splenic rupture (left shoulder), hepatic abscess or gallbladder pathology (right shoulder), post-laparoscopic surgery (residual CO2 irritating the diaphragm), and pneumonia affecting the base of the lung.
Entrapment Sites
1. Anterior Scalene
- Location: Where the nerve descends on the anterior surface of the anterior scalene
- Structure: Anterior scalene hypertrophy, spasm, or fibrous bands can compress the phrenic nerve against the muscle. This is clinically relevant in thoracic outlet syndrome, where the scalenes are frequently involved — phrenic nerve compression can coexist with brachial plexus compression.
- Presentation: Dyspnea (shortness of breath) that is disproportionate to the clinical picture. Unilateral diaphragm paralysis may be subclinical at rest but symptomatic with exertion — the patient feels they "cannot get a deep breath." Hiccups (rhythmic diaphragmatic spasm) can result from phrenic nerve irritation at this level.
- MT relevance: When treating the anterior scalene for thoracic outlet syndrome, cervical dysfunction, or breathing pattern disorders, be aware that the phrenic nerve runs on the anterior scalene surface. Sustained deep pressure on the anterior scalene can temporarily irritate the phrenic nerve — producing hiccups, an urge to breathe deeply, or a referred ache at the shoulder tip. These are signs to modify your technique, not to press harder.
2. Mediastinal Compression
- Location: Along the mediastinal course, from the thoracic inlet to the diaphragm
- Structure: Mediastinal tumors, enlarged lymph nodes, pericardial effusion, or cardiac surgery can compress or damage the nerve
- Presentation: Unilateral or bilateral diaphragm paralysis with dyspnea. Post-cardiac surgery phrenic nerve palsy (from cold-induced injury during cardioplegia or direct surgical trauma) is a recognized complication.
- MT relevance: Not an MT treatment target. Relevant for differential diagnosis: a post-cardiac surgery patient with persistent unexplained dyspnea may have iatrogenic phrenic nerve palsy.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| Diaphragmatic breathing observation | Observe the patient's breathing pattern. Place one hand on the chest, one on the abdomen. Normal breathing should show abdominal expansion (diaphragmatic descent) with inspiration. | Paradoxical breathing — the abdomen draws IN during inspiration while the chest expands. The paralyzed hemidiaphragm is sucked upward by the negative intrathoracic pressure. | Diaphragmatic paralysis (unilateral or bilateral). Paradoxical breathing is the hallmark clinical sign. May be subtle at rest and more obvious when the patient is supine (gravity loads the diaphragm). |
| Sniff test | Patient performs a sharp sniff through the nose while you observe or palpate the abdomen. | The paralyzed hemidiaphragm moves paradoxically upward during the sniff (detected by fluoroscopy or ultrasound, but palpable as paradoxical abdominal retraction on the affected side). | Quick screen for hemidiaphragm paralysis. A sharp sniff normally causes a brisk diaphragmatic descent — on the paralyzed side, the diaphragm is pulled up instead. |
| Supine versus seated dyspnea | Ask the patient if breathing is harder lying down or sitting up. | Dyspnea significantly worse when supine (orthopnea-like presentation). | In the supine position, abdominal contents push against the paralyzed diaphragm, further reducing lung volume. Patients with phrenic nerve palsy often report "I can't breathe lying flat." |
Clinical Notes
- Shoulder tip pain is a red flag when it occurs without shoulder pathology. If a patient reports shoulder tip pain — specifically the top of the shoulder at the trapezius/acromion level — without any shoulder dysfunction, trauma, or movement-related pattern, consider phrenic nerve irritation from subdiaphragmatic pathology. Right shoulder tip pain: gallbladder, liver, or right subdiaphragmatic abscess. Left shoulder tip pain: splenic rupture, left subdiaphragmatic pathology. This referred pain pattern (Kehr's sign) is medically significant — splenic rupture with left shoulder pain is a surgical emergency.
- Post-laparoscopic shoulder pain is phrenic. After laparoscopic surgery (gallbladder removal, gynecological procedures), patients commonly report bilateral shoulder tip pain for 24-72 hours. This is caused by residual CO2 gas from insufflation irritating the diaphragm. It is self-limiting and resolves as the gas absorbs. The MT may encounter these patients and can reassure them — this is not a new shoulder problem.
- Hiccups are phrenic nerve spasm. A hiccup is an involuntary diaphragmatic contraction followed by glottic closure. Transient hiccups are normal, but persistent hiccups (>48 hours) suggest phrenic nerve irritation — from gastric distension, GERD, mediastinal pathology, or central nervous system lesion. Refer persistent hiccups for investigation.
- Scalene treatment and the phrenic nerve. The anterior scalene is one of the most commonly treated muscles in clinical practice (cervical dysfunction, thoracic outlet syndrome, breathing pattern disorders). The phrenic nerve runs directly on its anterior surface. Deep, sustained pressure to the anterior scalene can temporarily compress the phrenic nerve. If the patient hiccups, takes a sudden deep breath, or reports shoulder tip discomfort during scalene work, you are likely on the nerve. Adjust position and depth.
- Breathing pattern assessment should include phrenic function screening. In patients with chronic hyperventilation, upper chest breathing, or persistent dyspnea without cardiopulmonary explanation, consider the possibility of subclinical unilateral phrenic nerve compromise. Observe for asymmetric abdominal wall movement during quiet breathing. If one side moves less than the other, the ipsilateral hemidiaphragm may be compromised.
Related Nerves
- anatomy/nerves/long-thoracic-nerve — C5-C7. Both nerves arise from the cervical region and have long courses to muscles of respiration/trunk function. Both pass near the scalenes at their origin.
- anatomy/nerves/suprascapular-nerve — C5-C6. Shares the C5 root level. The shoulder tip pain from phrenic nerve irritation overlaps with the area innervated by the suprascapular nerve — this convergence at C4-C5 is the basis for the referred pain pattern.
Key Takeaways
- Sole motor supply to the diaphragm — "C3, 4, 5 keeps the diaphragm alive." Bilateral loss is ventilator-dependent; unilateral loss produces dyspnea worse when supine.
- Runs on the anterior surface of the anterior scalene — deep scalene work can compress the nerve, producing hiccups, deep breathing urge, or shoulder tip discomfort.
- Shoulder tip pain without shoulder pathology is a red flag for subdiaphragmatic irritation (gallbladder, splenic rupture, post-laparoscopic CO2) — referred through the phrenic nerve's C3-C5 sensory fibers.
- Paradoxical breathing (abdomen draws in on inspiration) is the hallmark sign of diaphragm paralysis.