Axillary nerve lesions

Causes

• Posterior cord of brachial plexus lesion.
• Traumatic lesion of the nerve by fracture, dislocation, or reduction of dislocated humerus.
• Compression injury during sleep or anesthesia with arm placed above head.
• Iatrogenic injury (injection).
• Parsonage-Turner neuralgic amyotrophy.
• Entrapment by fibrous band in quadrilateral space.

Symptoms & signs

• Sensorimotor deficit is more frequent in proximal lesions. Pure motor deficits can be seen in injury at humoral head level. Pure sensory deficit is rare.
• Deltoid weakness and atrophy causing flattening of the shoulder contour.
• Difficulty in shoulder abduction.
• Sensory deficit is most pronounced in outer aspect of upper arm above deltoid insertion.

Examination

• Abduction weakness can easily be tested against examiner’s resistance. Patients may compensate by using other shoulder girdle muscles.
• Inspect shoulder contour for signs of deltoid atrophy.
• Test for sensory deficit.
• Test for teres minor weakness: With the patient seated with elbow flexed 90o and arm placed at 90o angle to scapular plane, ask the patient to externally rotate the arm against resistance. Inability to externally rotate is called the Hornblower’s sign.