Trauma to the point of the shoulder is a common injury of horses. It can occur from a kick or a horse passing too close to a solid object. Often this results in damage to the suprascapular nerve that traverses over the top of the neck of the scapula, the big wide flat bone that makes up the shoulder and the top half of the shoulder joint. This nerve innervates the two large muscles of the shoulder blade, the supraspinatus and infraspinatus. If damaged severely enough the nerve quits functioning and the result is loss of function and atrophy of the scapular muscles. This article discusses clinical signs, diagnosis, treatment, and prognosis of this problem.

Pain and Inflammation

The first sign of shoulder sweeny is most often pain and inflammation for the trauma: lameness, swelling, heat, pain of palpation, and possibly a wound to the point of the shoulder or surrounding areas. Pain in the shoulder and shoulder joint will appear as a reluctance to bear weight on the leg, a head bob while trotting, and a reluctance to advance the leg forward.

Muscular Weakness

Very focal injury to the nerve may leave the horse with no signs of injury other than that of denervation to the muscles to the shoulder: outward rotation of the shoulder as the horse bears weight on the leg and possibly an inability to advance the leg normally. If injury to the shoulder is not apparent, it should be kept in mind that damage to the axillary nerve in the arm pit can give you similar clinical signs and the area should be examined for signs of trauma. Because of the altered gait and unwillingness to bear weight on the leg, it may be difficult to distinguish the lameness caused by pain and the gait changes caused by nerve damage early in the disease.

Denervation Atrophy

In order for muscles to remain healthy they require stimulation from the motor nerves that activate them. If nerve conduction to the muscles is completely interrupted, the muscles begin to atrophy rapidly. Within two weeks the shoulder muscles become noticeably shrunken and the central ridge of the shoulder blade bone becomes prominent. Muscular denervation atrophy happens much quicker and more pronounced than with atrophy from disuse which may take weeks or months to develop.