Injury to the gracilis muscle can cause acute, severe lameness in horses. Horses with gracilis muscle injuries have a good prognosis for returning to athletic performance after prolonged rest. Fibrotic myopathy can develop secondary to gracilis muscleinjury. A 16-year-old female Quarter Horse (horse 1) weighing 470 kg (1,034 lb) was evaluated because of left hind limb lameness of 5 days' duration. The horse was being used for barrel racing. The horse became severely lame after turning around a barrel in a competition 5 days prior to the evaluation and was initially hesitant to bear weight on the left hind limb. On physical examination, the horse had increased digital pulses in the left front foot, digital tendon sheath effusion in the left forelimb, and a soft fluid-filled swelling on the medial aspect of the gracilis muscle belly of the left hind limb. Lameness evaluation revealed a grade 3/5 left hind limb lameness. Flexion of the lower portion of the limb did not alter the grade of lameness, but a flexion test performed on the upper portion of the limb increased the degree of lameness to grade 4/5. Perineural diagnostic analgesia was performed to localize the source of lameness. Analgesia of the lower portion of the limb via a low plantar nerve block followed by a peroneal and tibial nerve block did not modify the lameness. Intra-articular analgesia of the left medial and lateral femorotibial, femoropatellar, and hip joints did not change the lameness. Local infiltration of 30 mL of mepivacaine anesthetic into the medial aspect of the gracilis muscle at the site of the fluid-filled swelling resulted in considerable improvement in the left hind limb lameness. Ultrasonographic evaluation of the gracilis muscle of the left hind limb revealed an anechoic fluid space containing fibrous trabeculation within the muscle, which was suggestive of muscle tearing with seroma or hematoma formation (Fig 1).
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