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Long thoracic nerve

The long thoracic nerve is a slender, superficial motor nerve that supplies the serratus anterior muscle, crucial for scapular protraction and stabilization. It arises from the ventral rami of the C5, C6, and C7 spinal nerves and descends along the lateral chest wall to the serratus anterior.

Because of its long, exposed course on the thoracic wall, the nerve is highly vulnerable to injury from trauma, surgery, or repetitive shoulder strain. Injury leads to winging of the scapula, particularly noticeable when pushing against resistance.

Synonyms

  • External respiratory nerve of Bell

  • Posterior thoracic nerve

Origin and Root Value

  • Root origin: Ventral rami of C5, C6, and C7 spinal nerves (occasionally C4 or C8 contributions).

  • Origin level: From the brachial plexus roots in the posterior triangle of the neck.

Course

  • Neck:

    • Arises from C5–C7 roots.

    • Descends posterior to the brachial plexus and axillary vessels.

    • Passes through or behind the middle scalene muscle.

  • Axilla and Thoracic Wall:

    • Descends vertically along the lateral thoracic wall on the superficial surface of the serratus anterior muscle.

    • Lies deep to the fascia but superficial to the muscle, accompanied by lymphatic vessels and small branches of the lateral thoracic vessels.

  • Termination:

    • Ends by supplying the lower digitations of the serratus anterior near the 8th and 9th ribs.

Relations

  • Superiorly: Roots of the brachial plexus (C5–C7).

  • Anteriorly: Clavicle, deep cervical fascia, and skin.

  • Posteriorly: Serratus anterior muscle and thoracic wall.

  • Medially: Brachial plexus trunks and subscapular region.

  • Laterally: Axilla and lateral thoracic wall.

Branches

  • Muscular branches: To the serratus anterior muscle — upper (C5), middle (C6), and lower (C7) slips.

  • No cutaneous or sensory branches.

Nerve Supply

  • Motor: Serratus anterior muscle (C5–C7).

  • Sensory: None (purely motor nerve).

Function

  • Primary action: Innervates serratus anterior, which protracts and stabilizes the scapula against the thoracic wall.

  • Assists in overhead movement: Rotates scapula upward during arm elevation above 90°.

  • Maintains scapular alignment: Prevents winging and facilitates smooth shoulder mechanics.

Clinical Significance

  • Injury causes:

    • Trauma: Blunt chest trauma, traction injuries, or rib fractures.

    • Surgical: Iatrogenic injury during axillary dissection, mastectomy, or chest wall surgery.

    • Neuralgic amyotrophy: May affect the nerve idiopathically.

    • Sports-related strain: Common in athletes performing repetitive shoulder elevation.

  • Presentation:

    • Scapular winging — medial border and inferior angle of scapula protrude posteriorly.

    • Weakness in pushing or lifting overhead.

    • Shoulder fatigue and reduced elevation range.

  • Diagnosis:

    • Electromyography (EMG): Shows denervation of serratus anterior.

    • MRI: Evaluates nerve integrity and associated muscle atrophy or denervation edema.

  • Prognosis:

    • Many injuries recover spontaneously within 6–12 months; persistent cases may require nerve repair or muscle transfer.

MRI Appearance

  • T1-weighted images:

    • Nerve: Low-to-intermediate signal, thin linear structure along the lateral chest wall.

    • Serratus anterior: Intermediate muscle signal; bright fatty signal in chronic denervation.

    • Acute denervation: Normal size, intermediate signal with early muscle swelling.

    • Chronic denervation: Fatty infiltration — high T1 signal, muscle volume loss.

  • T2-weighted images:

    • Nerve: Intermediate-to-low signal (not clearly visualized unless inflamed).

    • Acute denervation: Bright hyperintense signal within serratus anterior (edema).

    • Chronic stage: Muscle becomes dark on T2 due to fatty replacement, minimal edema.

  • STIR:

    • Nerve: Faint intermediate signal tracing lateral thoracic wall.

    • Muscle: Bright hyperintensity in acute denervation (edema or inflammation).

    • Chronic denervation: Intermediate-to-dark signal as edema resolves.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Nerve appears as fine dark linear band.

    • Acute injury: Muscle shows bright hyperintense signal from denervation edema.

    • Chronic phase: Intermediate-to-dark muscle signal with fatty infiltration.

  • T1 Fat-Sat Post-Contrast:

    • Nerve: May show mild enhancement in neuritis or inflammation.

    • Serratus anterior: Patchy enhancement in acute denervation; absent in chronic fatty atrophy.

    • Useful in differentiating acute neuritis from long-standing paralysis.

CT Appearance

Non-Contrast CT:

  • Nerve not directly visible due to small caliber.

  • Secondary findings:

    • Muscle bulk asymmetry or atrophy of serratus anterior.

    • Scapular winging evident by increased scapulothoracic space.

    • May reveal chest wall trauma or rib fractures affecting the nerve pathway.

Post-Contrast CT (standard):

  • Nerve: Not typically enhanced, but surrounding soft tissue may show changes in trauma or inflammation.

  • Serratus anterior: Volume loss or fatty density in chronic denervation.

  • Excellent for evaluating bony deformities, fractures, and postsurgical complications along the nerve’s course.

MRI image

Long thoracic nerve axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Long thoracic nerve axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

MRI image

Long thoracic nerve axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002