A 52-year-old woman from Mumbai undergoes elective left lower lobe lung resection for bronchogenic carcinoma. Intraoperatively, the surgeon notes that the left phrenic nerve is adherent to the tumor and must be sacrificed. Postoperatively, the patient develops orthopnea and dyspnea on exertion. On fluoroscopic examination of the diaphragm during respiration, the left hemidiaphragm moves paradoxically upward during inspiration. Which nerve was injured, and what is the embryological origin of the muscle it innervates?

Explanation

## Phrenic Nerve Anatomy and Diaphragmatic Innervation **Key Point:** The phrenic nerve (C3, C4, C5) is the sole motor innervation to the diaphragm. Unilateral phrenic nerve injury results in paradoxical diaphragmatic motion — the affected hemidiaphragm moves upward (toward the thorax) during inspiration instead of downward. ### Phrenic Nerve Origin and Course ```mermaid flowchart TD A["Phrenic Nerve Origin<br/>C3, C4, C5 nerve roots"]:::outcome --> B["Mnemonic: C3, C4, C5<br/>keeps the diaphragm alive"]:::action A --> C["Arises from cervical plexus<br/>anterior to anterior scalene"]:::outcome C --> D["Descends through thorax<br/>between pericardium and mediastinal pleura"]:::outcome D --> E["Right: passes lateral to SVC and IVC<br/>Left: passes lateral to left subclavian artery"]:::outcome E --> F["Enters diaphragm at T8 level"]:::outcome F --> G["Innervates motor fibers to diaphragm<br/>Sensory to parietal pleura & pericardium"]:::outcome ``` **High-Yield:** The phrenic nerve is the **ONLY** motor nerve to the diaphragm. Intercostal nerves provide only sensory innervation to the diaphragmatic pleura. ### Embryological Origin of the Diaphragm | Component | Embryological Origin | Innervation | |-----------|----------------------|--------------| | **Central tendon** | Septum transversum | Phrenic nerve (C3, C4, C5) | | **Crura** | Dorsal mesentery of esophagus | Phrenic nerve (C3, C4, C5) | | **Pleuroperitoneal membrane (Bochdalek)** | Pleuroperitoneal membrane | Phrenic nerve (C3, C4, C5) | | **Muscular ingrowth (body wall)** | Body wall mesoderm | Intercostal nerves (T1-T11) — **sensory only** | **Clinical Pearl:** All muscular components of the diaphragm are innervated by the phrenic nerve, regardless of embryological origin. The phrenic nerve carries motor fibers (C3, C4, C5) to the entire diaphragm. ### Pathophysiology of Phrenic Nerve Injury **Paradoxical Diaphragmatic Motion:** - **Normal inspiration:** Diaphragm contracts → moves downward (caudad) → increases vertical thoracic diameter → negative intrathoracic pressure → air flows in - **Phrenic nerve injury:** Diaphragm is paralyzed → during inspiration, negative intrathoracic pressure pulls the flaccid hemidiaphragm upward (cephalad) → **paradoxical motion** - **Fluoroscopy finding:** The affected hemidiaphragm moves **upward** during inspiration and downward during expiration (opposite of normal) **Mnemonic:** **C3, C4, C5 keeps the diaphragm alive** — the three cervical nerve roots that form the phrenic nerve. ### Clinical Consequences **Warning:** Unilateral phrenic nerve injury may be asymptomatic at rest but causes dyspnea on exertion because the contralateral diaphragm must work harder to maintain ventilation. Bilateral injury is life-threatening and requires mechanical ventilation. **High-Yield:** Orthopnea (dyspnea when lying flat) occurs because the paralyzed hemidiaphragm cannot assist in breathing when gravity is removed; the patient must rely entirely on intercostal muscles and the contralateral diaphragm.