1. Trauma
- blunt trauma
- anterior shoulder dislocation
- proximal humeral fracture
2. Iatrogenic
- deltoid splitting approach
- deltopectoral approach
3. Quadrangular space syndrome
- pain in abduction and ER
- diagnose on dynamic angiogram
4. Parsonage Turner syndrome
- brachial neuritis
- can rarely affect AXN in isolation
5. Nerve compression from mass effect
- tumour / aneurysm