Ultrasonography of the equine larynx

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Ultrasonography of the equine larynx
Tuesday, July 28, 2015
Ben Sullivan discusses indications, equipment requirements, patient preparation and scanning technique for ultrasonography of the equine larynx.

Indications:

Evaluation of the hyoid apparatus, laryngeal cartilages, and associated soft tissue structures for assessment of suspected upper airway disease
Specifically:
RLN
DDSP
Arytenoid chondritis
Laryngeal dysplasia (4-BAD)

Equipment requirements:

Ultrasound machine
High frequency (> 7.5MHz) linear transducer
Medium-high frequency micro-convex transducer (5-8MHz)
Surgical spirit

Patient preparation:

Sedate if required. (This is often not necessary, though some animals may not tolerate the technique particularly well if unsedated.)
Extend the horse’s neck slightly. (Using a head stand can be helpful.)
Apply surgical spirit liberally to the laryngeal region and wipe away any excess that may drip and distract the horse.
Set a single focal depth of approximately 2cm and adjust gain and time-gain compensation (TGC) to optimise the image.
Scanning Technique

First described by CHALMERS, CHEETHAM, YEAGER, et al., 2006.
Usually involves 4 acoustic windows:

Rostroventral (Assessment of the basihyoid bone including the lingual process, and of the ceratohyoid bones; you may also be able to visualize the base of the tongue, though a lower frequency curvilinear probe may be needed to get sufficient depth of penetration.)

start in a transverse plane
position the probe on the ventral midline, rostral to the base of the basihyoid bone, between the rami of the mandible
initially view the lingual process of the basihyoid bone, then slide caudally to view the body of the basihyoid bone; point more rostrally from here to assess the ceratohyoid bones
rotate into a longitudinal (sagittal or parasagittal) plane to view the same structures in longitudinal section

Midventral (Assessment of the caudal part of the basihyoid bone, the rostral part of the thyroid cartilage, and the insertion of the thyrohyoid muscles.)
in longitudinal (sagittal) section slide caudally to image the gap between the basihyoid bone (rostrally) and the thyroid cartilage (caudally)
slide laterally (parasagittal) to view the insertion of the thyrohyoid muscles on to the thyroid cartilage

Caudoventral (Assessment of the vocal folds, including their movement.)
Return to a transverse plane and move caudally to overlie the cricothyroid notch.
Occlusion of the nostrils promotes more pronounced respiratory effort, allowing better visualisation of the vocal fold movement

Caudolateral (Assessment of the thyroid, cricoid and arytenoid cartilages, and the left and right cricoarytenoideus lateralis muscles.)

Flex the neck away from the side to be scanned
Rotate back into a longitudinal (sagittal) plane
Slide dorsolaterally from the caudoventral window, following the cricoid cartilage as a guide

References
Chalmers H J, Cheetham J, Yeager A E & Ducharme N G (2006) Ultrasonography of the Equine Larynx. Veterinary Radiology & Ultrasound. 47(5), pp. 476–481.
Chalmers H J, Yeager A E, Cheetham J & Ducharme N G (2012) Diagnostic Sensitivity of Subjective and Quantitative Laryngeal Ultrasonography for Recurrent Laryngeal Neuropathy in Horses. Veterinary Radiology & Ultrasound. 53(6), pp. 660–666.
Chalmers H, Yeager A & Ducharme N (2009) Ultrasonographic Assessment of Laryngohyoid Position as a Predictor of Dorsal Displacement of the Soft Palate in Horses. Veterinary Radiology & Ultrasound. 50(1), pp. 91–96.
Kidd J A, Lu K G & Frazer M L (2014) Atlas of Equine Ultrasonography. New Jersey: Wiley-Blackwell.

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