Long Thoracic Nerve Anatomy

Root Origin

• The long thoracic nerve is a pure motor nerve arising from the anterior (ventral) rami of C5, C6 and C7.
• The C5 and C6 contributions typically pierce the scalenus medius muscle; the C7 contribution usually joins distal to the muscle.
• It arises in the root of the brachial plexus (proximal to the trunks), which is why it can be spared or selectively involved independently of the rest of the plexus.

Mnemonic: "C5, 6, 7 raise your arms to heaven" - serratus anterior upward-rotates the scapula to allow overhead elevation.

Course

• After formation, the nerve passes behind the brachial plexus and the axillary vessels and enters the axilla.
• It then descends on the superficial (outer) surface of serratus anterior along the mid-axillary line of the lateral chest wall, giving branches to each digitation of the muscle.
• This course is long and relatively superficial, with little soft-tissue protection - the anatomical basis for its vulnerability.

Anatomical Vulnerability

The nerve's length and superficial position on the chest wall expose it to:

• Traction / stretch (sudden shoulder depression, repetitive overhead activity, carrying loads).
• Iatrogenic injury during axillary lymph node dissection, first-rib resection, chest-tube placement, and thoracic procedures.
• Direct blunt trauma to the lateral chest wall.
• Neuralgic amyotrophy (Parsonage-Turner syndrome) and viral/post-immune causes, which have a predilection for the long thoracic nerve.

Presentation

• Medial scapular winging - prominence of the inferomedial scapula, accentuated by forward elevation or pushing against a wall.
• Difficulty elevating the arm overhead and a feeling of shoulder-girdle fatigue/weakness.
• Periscapular pain and aching are common, particularly early.
• May follow trauma, surgery (axillary dissection), heavy/repetitive activity, or a viral/neuralgic amyotrophy prodrome.

Examination

• Wall push-up test: ask the patient to push against a wall - medial winging becomes obvious.
• Forward flexion: elevating the arm forward accentuates the wing (in contrast to trapezius palsy, where abduction accentuates lateral winging).
• Assess active overhead elevation and scapulohumeral rhythm.
• Examine for the differential (trapezius/spinal accessory - lateral winging; rhomboid/dorsal scapular).

Investigations

• Electrodiagnostics (EMG / nerve conduction studies): confirm serratus anterior denervation, localise the lesion to the long thoracic nerve, exclude a broader plexopathy, and assess for reinnervation - important before considering surgery.
• Imaging (radiographs/MRI): exclude bony/structural causes and traumatic muscular detachment.