Ultrasonography of the equine larynx
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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.
