A 15-MONTH-OLD, 1,000-lb (453.6-kg) American quarter horse filly, which was intended for use as a halter horse, presented with bilateral hindlimb lameness and hock effusion of approximately 1 week's duration. Radiography revealed subchondral cystic lesions in both stifles. The patient underwent bilateral stifle arthroscopy, and microfracture of the subchondral bone was performed to stimulate cartilage repair. The patient had been previously diagnosed as heterozygous for hyperkalemic periodic paralysis (HPP). This disease can often be controlled by avoiding high-potassium feeds (see box on page 174). For this reason, the amount of grain was reduced and divided into several feedings because grain can stimulate insulin-mediated movement of potassium across cell membranes. The filly was asymptomatic, but it was important to monitor her for clinical signs of an attack (i.e., fasciculations, sweating, muscular weakness, respiratory stridor, and stress-induced elevations in heart and respiratory rates). The filly had been treated 9 months earlier for bilateral flexural deformity of the carpi. At that time, the owner had declined a lameness evaluation to investigate the underlying problem. Therapy consisted of oxytetracycline (44 mg/kg IV sid for 3 days), stall and small-paddock rest, and corrective trimming.
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