Learning objectives: Being able to recognize the normal and abnormal structures of the equine larynx and understanding what the abnormal images mean in the context of case management.

Technical considerations: Laryngeal ultrasound can be performed with the linear ultrasound probe (12.5 MHz) and, as such, is technically accessible to many practitioners. Horses with a sport clip frequently don’t require additional clipping, but horses with thick hair will need clipping. The hair and skin are saturated with alcohol and then ultrasound gel is applied. A microconvex probe (8.5 MHz) can also be used, providing a smaller footprint and better penetration in horses with thick skin.

The laryngeal anatomy is relatively complex and identifying normal structures on the ultrasound exam can be challenging at first. Although this presentation focusses on the abnormal findings in clinical cases, a standardized approach including each of the ventral and lateral views is recommended for each ultrasound examination of the larynx.

Laryngeal ultrasound should be used in addition to other diagnostic modalities such as upper airway endoscopy and/or overground endoscopy. In the following cases, the ultrasound exam provides crucial information that changes the treatment plan or surgical approach:

Chondritis: Although the diagnosis can be made endoscopically, ultrasound examination allows a better evaluation of the extent of the damage to the arytenoid and surrounding tissues.¹
View: in the lateral window evaluate the arytenoid cartilage in the longitudinal and transverse planes.
Abnormalities: enlarged, irregular cartilage, fluid or gas within the arytenoid, surrounding tissue or other cartilages.

Right-sided laryngeal hemiplegia: For this uncommon presentation, laryngeal dysplasia is an important differential diagnosis.² The prognosis and surgical approach in cases of laryngeal dysplasia is quite different from cases of recurrent laryngeal neuropathy.
View: in the lateral window, evaluate the articulation of the cricoid and thyroid cartilages.
Abnormalities: lack of articulation of the cricothyroid articulation (lack of overlap of these cartilages in a transverse plane); in the longitudinal view, the dorsal edge of the thyroid cartilage overrides de cricoid.
Comparison with the unaffected side is helpful.

Left-sided laryngeal hemiplegia: Cases with little neurogenic atrophy are better candidates for a nerve graft.³ Laryngeal ultrasound has good sensitivity (90%) and specificity (98%)⁴ for diagnosing laryngeal hemiplegia but doesn’t replace the overground endoscopy which can evaluate the functional effect of the hemiplegia and the concurrent presence of other dynamic upper airway abnormalities.
View: in the lateral window, evaluate the cricoarytenoid lateralis muscle (CAL) in the longitudinal and transverse planes, and if possible the cricoarytenoid dorsalis muscle (CAD)
Abnormalities: changes associated with neurogenic atrophy of the CAD are a reduction in size and increase in echogenicity.^4,5 Compare the left and right CAD muscles. Although the CAD muscle is the one involved in abduction of the arytenoid, it is technically somewhat more difficult to image than the CAL. The CAD and CAL are innervated by the same nerve and several publications have shown changes in the CAD to be associated with recurrent laryngeal neuropathy.^4,5

In conclusion, laryngeal ultrasound is technically accessible to most practitioners and the skills required to perform a diagnostic evaluation can be acquired with practice. When used as part of a multimodal approach to the evaluation of the upper airway, it provides crucial information which can change the prognosis and treatment plan.

References

1.  Garrett KS, Embertson RM, Woodie JB, Cheetham J. Ultrasound features of arytenoid chondritis in Thoroughbred horses. Equine Vet J. 2013;45(5):598–603. Available from: http://doi.wiley.com/10.1111/evj.12037

2.  Garrett KS, Woodie JB, Embertson RM, Pease AP. Diagnosis of laryngeal dysplasia in five horses using magnetic resonance imaging and ultrasonography. Equine Vet J. 2009;41(8):766–71. Available from: http://doi.wiley.com/10.2746/042516409X434080

3.  Rossignol F, Brandenberger O, Perkins JD, Marie J-P, Mespoulhès-Rivière C, Ducharme NG. Modified first or second cervical nerve transplantation technique for the treatment of recurrent laryngeal neuropathy in horses. Equine Vet J. 2018;50(4):457–64. Available from: http://doi.wiley.com/10.1111/evj.12788

4.  Garrett KS, Woodie JB, Embertson RM. Association of treadmill upper airway endoscopic evaluation with results of ultrasonography and resting upper airway endoscopic evaluation. Equine Vet J. 2011;43(3):365–71. Available from: https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.2042-3306.2010.00216.x

5.  Chalmers HJ, Caswell J, Perkins J, Goodwin D, Viel L, Ducharme NG, et al. Ultrasonography detects early laryngeal muscle atrophy in an equine neurectomy model. Muscle and Nerve. 2016;53(4):583–92. Available from: http://dx.doi.org/10.1002/mus.24785.