Suprascapular nerve injury (Sweeney)
Suprascapular nerve injury (Sweeney) resulting in atrophy of the supraspinatus and infraspinatus muscles and shoulder joint instability can affect any age or breed of horse.
The condition was originally reported as most commonly affecting draft breeds and was believed to be associated with repeated trauma to the shoulder region from poorly fitted harness collars. With the decline in draft breeds the condition is most commonly seen in horses as result of trauma to the shoulder region. The term “Sweeney” has been defined as atrophy of the shoulder muscles in horses and is a commonly used synonym for suprascapular nerve paralysis.
Etiology
Etiology of Suprascapular nerve injury (Sweeney) is trauma to the suprascapular nerve.
Clinical Signs
- The clinical signs vary depending on the extent of the nerve damage and the duration of the condition prior to examination.
- Shortly after the injury, horses often exhibit severe pain and are reluctant to bear weight on the affected limb.
- As the pain subsides and the horse begins to bear weight, a pronounced lateral instability (excursion) of the shoulder joint (shoulder slip) during weight-bearing is observed. This sign is usually seen within 24 hours of injury and is most apparent as the horse is walked slowly toward the examiner.
- The instability is the result of loss of the stabilizing function of the supraspinatus and infraspinatus muscles which serve as the major lateral support for the shoulder.
- It has been suggested that this outward excursion of the scapula during weight-bearing may cause intermittent stretching of the suprascapular nerve leading to continued trauma and perpetuation of the paralysis.
- The acute lameness may be followed by complete recovery over a 2- 3-month period, or denervation atrophy of the supraspinatus and infraspinatus muscles may become evident.
- Following injury to the nerve, muscle atrophy usually becomes apparent as early as 10 to 14 days after injury.
- Once atrophy begins, the scapular spine becomes more prominent due to the loss of the muscles cranial and caudal to it.
Diagnosis
- Clinical signs and History
- Radiograph to rule out fracture
- Electromyographic evaluation (EMGs) of the supraspinatus and infraspinatus muscles
Treatment
- Both conservative and surgical treatments have been described for management of horses with suprascapular nerve injury
- Initial treatment in all cases is directed toward reducing inflammation in the region of the suprascapular nerve.
- Stall rest with the administration of NSAIDs and the topical application of cold hydrotherapy or ice packs and topical anti-inflammatories may produce a resolution of the clinical signs.
Conservative treatment
- Stall rest is continued until the shoulder joint stability returns, followed by confinement to pasture for additional 2 to 4 months
- Injection of corticosteroids into the shoulder joint
- Anti-inflammatory therapy
- Therapeutic ultrasound
Surgical treatment
- Surgical decompression of the nerve can be considered in patients that continue to exhibit signs of suprascapular nerve dysfunction for 10 to 12 weeks.
- This time frame is based on the rate of peripheral nerve regeneration (1 mm/day) and the distance from the site of nerve injury on the cranial border of the scapula to the infraspinatus muscle (7 to 8 cm). Using these parameters nerve function should return in 10 to 12 weeks
- Surgical decompression involves removing a small piece of bone from the cranial border of the scapula underlying the nerve
- By removing the notch of bone it is thought that the nerve is decompressed, allowing for reinnervation.
- Complications of the surgery include the possibility of a glenoid fracture arising from the notched bone.
- Location of the skin incision (dotted line), centred over the suprascapular nerve and cranial to scapular spine.
- The suprascapular nerve is identified and a small crescent-shaped piece of bone is removed from the cranial border of the scapula (dotted line).
- The bone has been removed and tendinous band resected.