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Newer Diagnostic Modalities: Laryngeal Ultrasound and Neurodiagnostics
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Newer diagnostic modalities: laryngeal ultrasound and neurodiagnostics
The status of the laryngeal function in horses is generally determined through video endoscopy of the upper airway. This allows accurate assessment of the structure of the luminal side of the laryngeal cartilages and is a reasonable, but imperfect, predictor of laryngeal function at exercise. More recently, laryngeal ultrasound has been introduced as a methodology to assess the morphological status of the non-luminal aspect of laryngeal cartilages and associated muscles (Chalmers et al., 2006), using a high resolution linear array transducer (12.5MHz broadband, 38mm) and a convex transducer (8.5 MHz broadband 14mm) and HDI 3000 and HDI 5000, Philips Medical Systems, Bothell WA, 98041, USA.
Five acoustic windows of the laryngohyoid area can be obtained. 1) Rostro-ventral window imaged the rostral aspect of the basihyoid bone including the lingual process; the depth of rostral aspect of the basihyoid bone measured at the level of the caudal aspect of the lingual process correlates with the prevalence of DDSP (Chalmers et al., 2009). 2) Mid-ventral window which allows identification of the position of the thyroid cartilage in relation to the basihyoid bone. In addition, laryngeal tie-forward sutures, when present, can be imaged for their integrity. 3) Caudo-ventral window (cricothyroid notch), allows identification of the vocal folds and anomalies such as prior surgical intervention, mucocele or abscess. 4) The lateral acoustic window is most helpful in identifying the status of the cricoarytenoid lateralis (CAL) muscle and the lateral surface of the arytenoid cartilage and thus the presence or absence of chondritis or abcessation. Detection of abnormality in the echogenicity of the CAL muscle is evidence of recurrent laryngeal neuropathy because the laryngeal adductors are affected earlier and more severely than the abductors for which dysfunction causes the clinical signs. 5) The dorso-lateral window allows imaging of the lateral compartment of the cricoarytenoid dorsalis muscle (CAD). Increase in echogenicity is harder to evaluate here because of the prominence of the CAD tendon. Instead size of the muscle can be assessed and measured.
The exam is performed with the horse in standing position usually without sedation. Head extension facilitates imaging of the lateral and dorso-lateral window. Alcohol or proper echogenic gel is used.
Electro-diagnostic tests were reported many years ago as a methodology to assess laryngeal function (Hahn et al., 1998). This was a logical extension of the discovery of the slap test. Traditionally a laryngeal muscle response is evoked by striking the skin over the withers and measuring the compound motor action potentials (CMAP) using surface electrodes over the larynx or an intramuscular (CAD) electrode (Steiss et al., 1989; Hawe et al., 2001). The time between stimulation and CMAP recording is called errors. First, the latency period transmission time includes transmission across the nerve and motor end-plates, which is not equivalent to conduction velocities. Second, the length of the nerve is an estimate based on animal height or poll-thorax length (Hahn et al., 1998; Hawe et al., 2001), so it is probably either inaccurate or not precise enough. [...]
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