axillary nerve

**Origin and course:** The axillary nerve arises from the posterior cord of the brachial plexus (roots C5–C6). It passes inferior to the glenohumeral joint capsule, then exits the axilla posteriorly through the quadrangular (quadrilateral) space with the posterior circumflex humeral artery. It wraps around the surgical neck of the humerus.

**Boundaries of the quadrangular space (mnemonic: "Teres minor, Teres major, Long head of triceps, Humerus"):** - Superior: teres minor - Inferior: teres major - Medial: long head of triceps - Lateral: surgical neck of humerus

**Motor function:** - Deltoid: abducts the arm from 15° to 90° (the supraspinatus, via the suprascapular nerve, initiates the first 0–15°). The deltoid also contributes to flexion (anterior fibers) and extension (posterior fibers). - Teres minor: lateral (external) rotation of the arm and a rotator cuff stabilizer.

**Sensory function:** The superior lateral cutaneous nerve of the arm supplies the skin over the lower deltoid and lateral upper arm (the "regimental badge" area).

**Clinical significance:** - **Common causes of injury:** - Anterior (subcoracoid) shoulder dislocation. The nerve sits inferior to the humeral head, so it is stretched or compressed. - Fracture of the surgical neck of the humerus. - Compression from improper crutch use or posterior cord injury. - Quadrangular space syndrome (compression by fibrous bands, often in overhead athletes). - Intramuscular injection into the deltoid, if placed too low. - **Presentation:** - Weak or absent arm abduction beyond 15° (the supraspinatus can still start abduction). - Weak lateral rotation. - Flattened shoulder contour from deltoid atrophy over time. - Loss of sensation over the lateral shoulder (regimental badge area). - **Associated structure:** The posterior circumflex humeral artery travels with the nerve and can be injured in the same events. - **Exam pearls:** Distinguish from a suprascapular nerve lesion (weak initiation of abduction and weak lateral rotation, with no sensory loss) and an upper trunk (Erb) palsy (also affects biceps and the musculocutaneous and suprascapular nerves, giving the "waiter's tip" posture). Axillary nerve injury has an otherwise preserved elbow and hand function. - **Management:** Most stretch or neurapraxia injuries recover with observation and physical therapy. Surgical exploration or nerve grafting or transfer is considered if there is no recovery by about 3–6 months.