A nerve injury results in loss of abduction of the arm and loss of sensation over the lateral shoulder. Which nerve of the brachial plexus is most likely injured?

Explanation

## Axillary Nerve Injury: Clinical Presentation **Key Point:** The axillary nerve is a terminal branch of the posterior cord. It exits the axilla through the quadrangular space and supplies the deltoid (abduction) and teres minor (external rotation) muscles, plus sensory innervation to the lateral shoulder. ### Motor and Sensory Distribution of the Axillary Nerve | Function | Details | | --- | --- | | **Motor supply** | Deltoid (arm abduction), Teres minor (external rotation) | | **Sensory supply** | Lateral shoulder (regimental badge area), upper lateral arm | | **Reflex** | Shoulder abduction reflex (C5–C6) | ### Clinical Features of Axillary Nerve Injury **High-Yield:** The classic triad of axillary nerve injury: 1. **Loss of arm abduction** (deltoid paralysis) — arm cannot be raised above shoulder 2. **Loss of external rotation** (teres minor paralysis) — subtle, often overlooked 3. **Sensory loss over lateral shoulder** — "regimental badge" area (C5–C6 dermatome) **Mnemonic:** **DELTA** = **D**eltoid paralysis, **E**xternal rotation loss, **L**ateral shoulder sensory loss, **T**eres minor weakness, **A**xillary nerve. **Clinical Pearl:** Axillary nerve injury commonly occurs after anterior shoulder dislocation, fracture of the surgical neck of the humerus, or axillary artery puncture. The nerve is vulnerable in the quadrangular space (bounded by teres major, teres minor, long head of triceps, and humerus).