A patient with shoulder pathology has a positive drop arm test, but MRI shows rotator cuff integrity. Which condition would be MOST likely to produce this finding?

• A. Multidirectional instability
• B. Thoracic outlet syndrome
• C. Bell's palsy
• D. Axillary nerve palsy

Answer: (D)

The drop arm test assesses whether a patient can eccentrically control the descent of the arm from 90 degrees of abduction. A true rotator cuff tear, especially involving the supraspinatus, often causes a positive sign because the torn cuff cannot control the lowering. But if MRI shows the rotator cuff is intact, the positive drop arm test points to weakness in the muscles that actually hold the arm up, most notably the deltoid supplied by the axillary nerve. When the axillary nerve is injured, the deltoid (and teres minor) are weak, so the arm cannot be held in abduction and will drop abruptly during the lowering phase. This explains a positive drop arm test even with an intact rotator cuff.

Clinically, axillary nerve palsy commonly results from shoulder dislocation or surgical trauma and presents with diminished deltoid contour and reduced ability to abduct beyond about 15 degrees, with possible sensory loss over the lateral shoulder. The other options don’t fit: multidirectional instability causes laxity and apprehension rather than isolated deltoid weakness; thoracic outlet syndrome produces vascular or neurogenic symptoms with specific provocative positions; Bell’s palsy affects facial muscles, not the shoulder.