Etiology

• Mild neuropraxia during weightbearing is a common injury in cattle; complete paralysis is less common.
• Trauma to front of shoulder such as charging into a yoke during the loading of a trim chute, hitting a vertical stanchion at speed, from leaning against a crush yoke with head outstretched for long periods during foot trimming.
• Fractures of neck of scapula or scapular tuberosity.
• Extremes of movement, especially caudal extension of shoulder.
• Once damage initiated (even if slight) further shoulder movement, especially sudden outward excursions, may perpetuate the damage to the nerve.
• Can also be caused by injections of irritant material into the caudal neck, resulting in inflammation in the nerve.

Pathophysiology

• Suprascapular nerve originates from the C6 and C7 spinal segments:
  • It innervates the shoulder extensors (supraspinatus muscle) and the forelimb abductors (infraspinatus muscle).
• The nerve passes across the cranial edge of scapula and is prone to stretching/bruising.
• Damage to nerve may be mild leading to neuropraxia (temporary impairment of function); or severe   leading to  neurotmesis (possibly permanent).
• Denervation atrophy of supraspinatus and infraspinatus   →   loss of lateral shoulder support   →   sudden lateral outward motion of shoulder during weightbearing and   →   prominent scapular spine.
• Often several other nerves in the brachial plexus appear to be involved.