Page 422 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Musculoskeletal system: 1.9 Muscle disorders of the horse 397

VetBooks.ir 1.765 horse-related breeds. The disease appears to have
a hereditary component. There is often a history of
respiratory disease or vaccination in the preceding
4–8 weeks, which appears to prompt an abnormal
autoimmune response to antigens within the host
muscle.

Clinical presentation
Horses present with rapid muscle atrophy, usually
affecting the gluteal and epaxial muscles. This may
occur in association with muscle stiffness, weakness
and depression.

Diagnosis
Haematology is usually within normal reference
intervals, but muscle enzyme activity is increased.
Biopsy of the affected muscles is required to confirm
the diagnosis. This will reveal lymphocytic inflam-
matory infiltrate between atrophying and regenerat-
Fig. 1.765 Ultrasound scan of the cranial cervical ing muscle fibres.
region of the horse shown in 1.763. There is extensive
subcutaneous oedema as shown by dissecting, Management
hypoechogenic lines along the fascial planes (red Immunosuppressive doses of corticosteroids are
arrows). Diffuse heterogeneous and hypoechogenic administered. If ongoing infection is present, antibi-
material within the muscle tissue represents exudate otics may also be indicated.
from the cellulitis (arrowheads). The exudate contains
numerous gas bubbles casting acoustic shadows and Prognosis
comet-tail artefacts (yellow arrow). Many horses make a full recovery, although it may
take months for the muscle mass to fully return.
Some horses may also experience recurrent bouts of
deep incisions through the skin into the affected muscle atrophy.
muscle. Supportive treatment with intravenous flu-
ids and NSAIDs is recommended. Daily lavage of INFARCTIVE PURPURA HAEMORRHAGICA
the wound with sterile saline is carried out to remove
debris and improve drainage. The wound is left to Aetiology/pathophysiology
heal by second intention. Horses with recent Streptococcus equi infection can
develop a type 3 hypersensitivity reaction that
Prognosis leads to a leucocytoclastic vasculitis. This results
Guarded to poor, particularly in acute cases, where the in vascular occlusions and infarction of the associ-
mortality rates are exceedingly high. Early, aggressive ated muscle. Quarter horses are most commonly
treatment can improve the prognosis and up to a 73% affected.
survival rate has been reported in one study.
Clinical presentation
IMMUNE-MEDIATED MYOSITIS Early clinical signs include firm focal muscle swell-
ing and pain, which can result in severe lameness and
Aetiology/pathophysiology stiffness. Often those muscles in contact with the
Immune-mediated myositis is an inflammatory ground in the recumbent horse are most susceptible.
disease most commonly identified in Quarter Other classic signs of purpura haemorrhagica such as