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Left phrenic nerve

The left phrenic nerve is the longer of the two phrenic nerves, providing the sole motor supply to the diaphragm and sensory innervation to the pericardium, mediastinal pleura, and diaphragmatic peritoneum. It originates primarily from the C4 ventral ramus, with contributions from C3 and C5.

The nerve descends vertically along the left side of the mediastinum. After emerging from the cervical plexus, it passes posterior to the subclavian vein, then runs anterior to the aortic arch and over the pericardium covering the left ventricle. It continues caudally to pierce the diaphragm near the apex of the heart, where it branches to innervate the left hemidiaphragm.

Clinically, the left phrenic nerve is crucial in respiration, thoracic surgery, and cardiology. Injury can cause left hemidiaphragmatic paralysis or elevation, leading to dyspnea, reduced lung capacity, and paradoxical movement on inspiration. It is at risk during thoracic surgeries (CABG, pericardiectomy, lung resections), mediastinal tumor excision, and catheter ablation procedures.

Synonyms

  • Nervus phrenicus sinister

  • Left diaphragmatic nerve

Function

  • Sole motor innervation of the left hemidiaphragm

  • Provides sensory fibers to:

    • Central diaphragmatic pleura and peritoneum

    • Pericardium (fibrous and parietal layers)

    • Mediastinal pleura

  • Critical for respiratory mechanics and diaphragmatic motion

MRI Appearance

T1-weighted images:

  • Nerve appears as a thin, linear hypointense structure along the mediastinum and pericardium

  • Best seen with high-resolution sequences and fat contrast

T2-weighted images:

  • Appears as a low-to-intermediate signal linear structure against hyperintense lung and mediastinal fat

  • Pathological changes (inflammation, edema, tumor infiltration) appear hyperintense

STIR:

  • Suppresses fat, improving visibility of the phrenic nerve against adjacent mediastinal fat

  • Highlights edema, neuritis, or tumor involvement as hyperintensity

T1 Post-Gadolinium (Gd-enhanced MRI):

  • Normal nerve enhances minimally

  • Abnormal enhancement indicates neuritis, infiltration, or post-surgical changes

  • Useful for nerve mapping in oncologic imaging

MRI Non-Contrast 3D Imaging (Cardiac/Thoracic-gated):

  • Provides 3D visualization of nerve course relative to aortic arch, pericardium, and diaphragm

  • Important in pre-surgical planning to avoid nerve injury

CT Appearance

Non-contrast CT:

  • Direct visualization is difficult due to small size

  • Left phrenic nerve course inferred by landmarks: along pericardium, anterior to aortic arch, descending toward diaphragm

CT Post-Contrast:

  • Enhances delineation of adjacent vascular and mediastinal structures, indirectly outlining the nerve’s course

  • Useful for detecting secondary signs of phrenic nerve dysfunction, such as:

    • Hemidiaphragm elevation

    • Paradoxical diaphragmatic motion (dynamic CT)

    • Associated masses, fibrosis, or compression along its course

CT images

Left phrenic nerve ct axial image -img-00000-00000