Rotator cuff denervation syndromes may be due to by a variety of causes, most important are:

• viral or autoimmune neuritis: Parsonage-Turner syndrome
• fibrous bands
• paralabral cyst
• traumatic nerve injury (stretching/traction injury)

Typical muscular innervations are:

• suprascapular nerve
  • supraspinatus muscle
  • infraspinatus muscle
• axillary nerve
  • teres minor muscle (5th cervical nerve)
  • deltoid muscle (5th & 6th cervical nerves)
• upper and lower subscapular nerve
  • subscapularis muscle
  • teres major

In this case, the distribution of affected muscles doesn't follow a single myotomic innervation, consistent with a polyneuropathy such as Parsonage-Turner syndrome.

Please note that the chronic stage differs in MRI appearance.