What is the primary function of the long thoracic nerve?

The Long Thoracic Nerve: Anatomy and Clinical Significance

The long thoracic nerve is a purely motor nerve that innervates the serratus anterior muscle, and its injury results in weakness of arm abduction due to scapular winging.

Anatomical Characteristics

• The long thoracic nerve (LTN) originates from the C5, C6, and C7 nerve roots and exclusively provides motor innervation to the serratus anterior muscle 1
• It is a purely motor nerve without sensory function, distinguishing it from mixed nerves like the recurrent laryngeal nerve which contains both sensory and motor fibers 2
• The LTN can be visualized using high-resolution ultrasound from its origin to its course along the thoracic wall 3
• The average diameter of the LTN is approximately 1.6 mm (range 1.1-2.1 mm) after formation of the main trunk 3

Innervation Pattern

• The serratus anterior muscle consists of three parts (upper, middle, and lower), each with distinct functions 4
• The upper part is primarily supplied by the C5 nerve root, with additional contributions from C4, C6, or C7 in most cases 4
• The middle and lower parts are innervated by the C6 and C7 components of the long thoracic nerve 4
• This innervation pattern enables the serratus anterior to perform its critical functions of stabilizing and moving the scapula 4

Clinical Significance of LTN Injury

• Injury to the long thoracic nerve results in paralysis or weakness of the serratus anterior muscle, causing scapular winging 5
• Patients with LTN injury present with:
  • Medial translation of the scapula
  • Rotation of the inferior angle toward the midline
  • Prominence of the vertebral border
  • Pain and weakness
  • Limitation of shoulder elevation 5
• LTN injury directly impacts arm abduction, as the serratus anterior is crucial for proper scapular positioning during this movement 5
• Scapular winging is the landmark manifestation of LTN neuropathy 1

Etiology of LTN Injury

• LTN injury may result from trauma or occur without obvious injury 5
• Iatrogenic causes include accidental injury during selective cervical nerve root or inter-scalene brachial plexus blocks 1
• There are rare cases of familial long thoracic nerve palsy as a manifestation of hereditary brachial plexus neuropathy 6

Management of LTN Injury

• Most patients experience return of serratus anterior function with conservative treatment, though recovery may take up to 2 years 5
• Bracing is often poorly tolerated by patients 5
• Surgical reconstruction may benefit patients with severe symptoms when 12 months of conservative treatment has failed 5
• The preferred surgical treatment is transfer of the sternal head of the pectoralis major tendon to the inferior angle of the scapula, reinforced with fascia or tendon autograft 5

Functional Importance

• The serratus anterior muscle plays a critical role in shoulder function 3
• The upper part stabilizes rotational motion of the scapula on the thorax during shoulder elevation 4
• The middle part facilitates scapular abduction 4
• The lower part contributes to upward rotation, abduction, and posterior tilting of the scapula 4

Understanding the purely motor nature of the long thoracic nerve and its exclusive innervation of the serratus anterior muscle is essential for proper diagnosis and management of conditions affecting shoulder function.