Page 1036 - Adams and Stashak's Lameness in Horses, 7th Edition

1002 Chapter 9

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A B

Figure 9.44. (A) 50‐mile endurance horse with grade 3 (B) Medial heel bruise that was evident after shoe removal. The
lameness of the left front limb. Note the displacement of the medial medial heel was floated and the shoe straightened and replaced.
branch of the shoe and the compression of the heel (arrow). Subsequently, the horse was sound and continued competing.

COMMON CAUSES OF LAMENESS Subclinical myopathy may also predispose to the
development of compensatory lameness. In a study of
Lameness is by far the most common indication for 36 elite horses competing in a 160‐km FEI 3‐star com­
elimination from endurance competition. Based on AERC petition, 22 horses that successfully completed had
post‐ride statistical reports, of 21,933 starts during the abnormally high mean creatine kinase (CK) activity
2007–2008 ride season, 1,410 horses were eliminated for (22,473 ± 41,192 IU/L) compared with pre‐ride concen­
lameness, accounting for 44% of all eliminations. Of trations (702 ± 720 IU/L). Although there was no differ­
these, the majority were unlocalized front limb (45.9%) ence in CK activity between finishers and nonfinishers,
and hindlimb (23.4%) lameness conditions. Keep in mind two horses eliminated for lameness had the highest CK
that diagnoses at the vet gate are based on brief examina­ values (240,000–400,000 IU/L). 6
tion only, usually without even the benefit of hoof testers Exertional rhabdomyolysis usually develops early in
and definitely without the use of diagnostic nerve blocks. the ride (fewer than 15 miles), is often associated with
Considering the defined causes of lameness, hindlimb anxiety, and may be influenced by cold weather and
muscle comprised 9.4% of all lameness problems, fol­ inadequate warm‐up. Myopathy also develops late in
lowed by front suspensory (4.9%), front foot (4.4%), the ride (more than 50 miles) and is associated with
front tendon (3.3%), front muscle (2.5%), front joint exhaustion or abrupt changes in footing (i.e. muddy hill
(2.1%), hind tendon (1.1%), hindfoot (0.78%), other work). Most commonly, the gluteals and lumbar epaxial
front ligament (0.71%), other hind ligament (0.64%), muscles are affected. The gluteal and/or lower lumbar
hind joint (0.57%), and hind suspensory (0.28%). 8 epaxial muscles may be affected asymmetrically, result­
Many causes of lameness in endurance athletes are ing in pronounced unilateral swelling or firmness in a
transient, leading to elimination from competition on specific area (e.g. middle gluteals). The most common
ride day but resolving after rest with or without the gait abnormality is a shortened anterior stride phase at
benefit of anti‐inflammatory treatment. Often, the exact a walk. Further exercise is contraindicated with signifi­
cause of lameness in these situations is not specifically cant rhabdomyolysis, and aggressive treatment is war­
determined. Recurrent problems or significant acute soft ranted in most affected horses. The primary goal of
tissue or orthopedic injuries are more likely to result in treatment is fluid diuresis until gross pigmenturia is
a full diagnostic and therapeutic approach similar to resolved. Analgesics may be necessary in some horses.
that performed for any other sport horse. Doses of flunixin meglumine and other toxic NSAIDs
should be minimized in the face of dehydration. Portable
Muscle Disorders biochemistry analyzers are commonly used by treatment
veterinarians, at even small local endurance rides. Field
Certain occupational conditions are common in the evaluation of serum electrolytes, muscle enzymes, and
sport of endurance, including exertional rhabdomyoly­ renal parameters in these patients helps to guide therapy.
sis and other muscle disorders primarily of the hindlimbs, Occasionally, endurance horses develop rhabdomyolysis
back, and neck. While data regarding the prevalence of upon resuming work after long‐distance transport (road
exertional rhabdomyolysis in endurance horses is lim­ and air).
ited, a recent prospective study of 101 endurance horses Although the efficacy of dantrolene in the treatment
competing in four 50‐mile distance races in the of acute rhabdomyolysis in horses has not been well
Northwestern United States identified a 4% prevalence studied, its role in preventing recurrent episodes is estab­
of exertional rhabdomyolysis. 7 lished. In treadmill studies, dantrolene sodium (4 mg/kg PO)