Page 684 - Adams and Stashak's Lameness in Horses, 7th Edition

650 Chapter 5

cases reported with SH joint luxation treated by closed tant to bear weight on the affected limb. As the pain
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reduction returned to soundness. 12,16,23,40,60,63,101,103 The subsides and the horse begins to bear weight, a
VetBooks.ir report of shoulder luxation in a Thoroughbred filly has der joint (shoulder slip) during weight‐bearing is
pronounced lateral instability (excursion) of the shoul-
prognosis for larger horses may also be good. One
observed. This sign is usually seen within 24 hours of
been described. The filly was sound for light work 8
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months after closed reduction and arthroscopic exami- injury and is most apparent as the horse is walked
nation of the joint. Long‐term successful outcome was slowly toward the examiner. The instability is the
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achieved in a racing Thoroughbred filly that luxated result of loss of the stabilizing function of the supraspi-
the joint during recovery from anesthesia. It has been natus and infraspinatus muscles, which serve as the
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suggested that the prognosis may be improved and major lateral support for the shoulder. It has been
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more accurately predicted if arthroscopy is used fol- suggested that this outward excursion of the scapula
lowing closed reduction to remove any subsequent during weight‐bearing may cause intermittent stretch-
bone fragments. Arthrodesis of the SH joint in a min- ing of the suprascapular nerve leading to continued
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1
ishetty stallion with chronic subluxation resulted in a trauma and perpetuation of the paralysis. The acute
good outcome with minimal lameness at a trot 6 weeks lameness may be followed by complete recovery over
post surgery. 8 a 2‐ to 3‐month period, or denervation atrophy of the
supraspinatus and infraspinatus muscles may become
evident. Following injury to the nerve, muscle atrophy
SUPRASCAPULAR NERVE INJURY (SWEENY) usually becomes apparent as early as 10–14 days after
injury. Once atrophy begins, the scapular spine
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Suprascapular nerve injury resulting in atrophy of becomes more prominent due to the loss of the mus-
the supraspinatus and infraspinatus muscles and shoul- cles cranial and caudal to it (Figure 5.58).
der joint instability can affect any age or breed of
horse. 14,34 The condition was originally reported as
most commonly affecting draft breeds and was believed Diagnosis
to be associated with repeated trauma to the shoulder A presumptive diagnosis of suprascapular nerve
region from poorly fitted harness collars. With the injury can be made from the clinical signs and the his-
decline in draft breeds, the condition is most commonly tory of trauma to the region. Radiographs of the region
seen in horses as result of trauma to the shoulder should be obtained to rule out a fracture or OA of the
region. The term “sweeny” has been defined as atrophy shoulder joint. Electromyographic evaluation (EMG) of
of the shoulder muscles in horses and is a commonly the supraspinatus and infraspinatus muscles confirms
used synonym for suprascapular nerve paralysis. 14 selective suprascapular nerve injury. For the EMG
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studies to be most meaningful, they should be performed
Etiology a minimum of 7 days after injury. EMG findings of
denervation of muscles supplied by other branches of
Trauma to the suprascapular nerve as it passes over the brachial plexus should prompt further neurologic
the cranial thin border of the scapula is believed to be evaluation to rule out injuries to the brachial plexus,
the cause. 33,34,62 The suprascapular nerve originates from spinal cord disease at the sixth and seventh cervical
the sixth and seventh cervical spinal segments and passes segments, and other diseases such as equine protozoal
via the brachial plexus to innervate the supraspinatus myelitis. 14,33
and infraspinatus muscles that overlie the scapula. As
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the nerve reflects around the cranial edge of the scapula,
it passes beneath a small but strong tendinous band. In
this position the nerve appears to be most susceptible to
direct trauma and compression against the underlying
bone. 33–35 In horses without clinical evidence of muscle
atrophy, there is also histologic evidence of chronic
demyelination and remyelination at the point where the
suprascapular nerve reflects over the cranial edge of the
scapula. These findings suggest there may be chronic
nerve compression caused by constriction of the small
tendinous band resulting in subclinical neuropathy. It
has been suggested that the chronic neuronal injury may
make the suprascapular nerve more susceptible to acute
trauma and the development of clinical signs or that
spontaneous development of the condition, without a
traumatic insult, may be possible. 33–35

Clinical Signs
A history of trauma to the shoulder region is rela-
tively common. The clinical signs vary depending on
the extent of the nerve damage and the duration of the
condition prior to examination. Shortly after the Figure 5.58. Prominent atrophy of the supraspinatus muscle is
injury, horses often exhibit severe pain and are reluc- evident in this horse with suprascapular nerve injury.