Axillary Nerve

Key Points:

  • Critical function provided is that of shoulder abduction; also contributes to lateral/external shoulder rotation.
  • Originates from roots C5, C6

EXAMINATION
Physical examination of the axillary nerve includes both motor and sensory examination. The motor examination involves testing the deltoid and teres minor muscles. The deltoid muscle is the easiest to examine — by testing shoulder abduction.

Detailed Examination Links:

  • Deltoid
  • Teres minor
  • Superior lateral cutaneous nerve

Summarized Examination:

  • Physical examination of the axillary nerve includes both motor and sensory examination.
  • The motor examination involves testing the deltoid and teres minor muscles.
  • The deltoid muscle has 3 components (anterior/middle/posterior) and is most quickly and easily tested by palpation at the shoulder while having the patient perform resisted shoulder abduction.
  • The teres minor muscle is tested by similar maneuvers as for those for the infraspinatus muscle (innervated by suprascapular nerve), as both provide lateral/external rotation of the shoulder.
  • The sensory examination involves testing sensation over the superior lateral cutaneous aspect of the shoulder which is innervated by the superior lateral cutaneous nerve.

Pathological Findings:

  • In patients with isolated axillary nerve injury, some shoulder abduction and external rotation is preserved through suprascapular nerve function (to the supraspinatus [should abduction] and infraspinatus [external rotation] muscles).  However, obvious atrophy of the deltoid is visible (especially in young, muscular patients) on the affected side, and sensation at the lateral shoulder is absent.
  • The axillary nerve can, in rare cases, be entrapped at the quadrangular space (bordered by teres minor, teres major the long head of the triceps and the humeral bone. This is the space through which the axillary nerve travels from the anterior to posterior aspect of the arm.

Clinical Relevance:

  • While axillary nerve function is important, if both axillary and suprascapular nerve function is absent (as is seen in upper brachial plexus injury patterns), suprascapular nerve function “trumps” axillary nerve function on the reconstructive ladder and should be preferentially restored.
  • In axillary nerve injuries when the denervation time permits (less than 3-6 months is ideal), restoration of elbow flexion can be accomplished by radial to axillary nerve transfer where the medial triceps motor branch is transferred to the deltoid muscle branch, and a radial innervated sensory branch is transferred to the axillary innervated sensory branch.
  • In delayed presentations or chronic upper brachial plexus injuries with loss of both axillary and suprascapular nerve function, trapezius muscle transfer provides an alternative option to shoulder arthrodesis for improved shoulder function.

Relevant Anatomy:
Innervation

  • Roots: C5, C6.
  • Nerve: Axillary nerve.
  • Muscles Innervated: Deltoid, teres minor.
  • Sensory Distribution Innervated: Superior lateral cutaneous nerve.
  • Innervation Route:
    • C5, C6 → posterior cord → axillary nerve → deltoid.
    • C5, C6 → posterior cord → axillary nerve → teres minor.
    • C5, C6 → posterior cord → axillary nerve → superior lateral cutaneous nerve.