The use of ultrasonography for assessment of the equine intrinsic laryngeal muscles

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Date

2014-03-10

Authors

Chalmers-Chaudhry, Heather Jane

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Publisher

University of Guelph

Abstract

The application of ultrasonography to the intrinsic laryngeal muscles of horses was evaluated in two populations: 1) horses with poor performance (n=154) and 2) horses with disease induced via unilateral recurrent laryngeal neurectomy (n=28). In population 1, it was hypothesized that the ultrasonographic appearance of muscle would be different in horses clinically affected with recurrent laryngeal neuropathy (RLN) compared to unaffected horses. The cricoarytenoideus lateralis (CAL) muscle was assessed with ultrasound including echogenicity and thickness, and vocal fold and arytenoid cartilage movement were recorded. Increased left CAL muscle echogenicity yielded a sensitivity of 94.59% and specificity of 94.54% for detecting RLN, while muscle thickness and laryngeal movements were substantially less useful. Subsequently, in population 2, a right recurrent laryngeal neurectomy was used to model disease. Horses were serially evaluated with ultrasonography of the CAL and a novel trans-esophageal approach to the cricoarytenoideus dorsalis (CAD) muscles. Mean pixel intensity (MPI) and muscle appearance grade were recorded. For both CAL and CAD muscles, significant changes in muscle appearance grade and MPI were observed over time in the neurectomized side (p<0.0001) but not the control side (p>0.05). Differences in muscle echogenicity were first apparent at 2 weeks post neurectomy for CAL and 4 weeks for CAD, but the magnitude of these differences did not reach a clinically useful level until at least 8 weeks. Horses were sacrificed from 4-32 weeks post-neurectomy and muscles were collected. Mean fiber diameter, fiber density, percent collagen, percent fat, and fiber type profile were measured and compared to echogenicity parameters. A significant relationship was identified between collagen content and muscle appearance grade as early as 8 weeks following neurectomy. This effect was apparently mitigated by reduced mean fiber diameter, which was the only parameter to be significantly associated with muscle echointensity in a mixed model. Establishing which tissue characteristics lead to alterations in the ultrasound findings is a critical step towards using ultrasound for non-invasive tissue characterization in the diagnosis and monitoring of muscle disorders.

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Keywords

equine, larynx, muscle, ultrasound

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