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Laryngeal Ultrasound in Yearling Thoroughbreds with Two Year Old Follow Up – 2019

April 21, 2020

Laryngeal Ultrasound in Yearling Thoroughbreds with Two Year Old Follow Up

B Hunter DVM MS MANZCVS DACVS-LA

D P Keenan BVSc MANZCVS

A Ritmeester BVSc (Hons) MS DACVS

D Hanlon BVMS MVSc PhD DACT

Reason for performing study: Echogenicity of the left cricoarytenoideus lateralis muscle (CALM) has been reported to be 90% sensitive and 98% specific for diagnosing left recurrent laryngeal neuropathy in racing Thoroughbreds ≥ 2 years of age (1), but assessment of this same muscle in yearlings has not been reported.

Objectives: To determine the echogenicity of both the left and right CALM in clinically normal yearling Thoroughbreds intended for racing and correlate echogenicity with grade of arytenoid function on standing endoscopy. To repeat ultrasound and endoscopy assessments in the same population as two year olds. Also, to determine the incidence of laryngeal cartilage anatomic abnormality and correlate with endoscopic abnormality.

Hypothesis: As left arytenoid function on endoscopy decreases, echogenicity of the left CALM will increase.

Methods: Thoroughbreds born in 2014 and 2015 underwent standing endoscopy when 12-14 months of age and left laryngeal function was graded 1-5 using the Lane system. Following endoscopy, horses were sedated with xylazine (0.3-0.4 mg/kg) and standard, previously described (2) transverse and longitudinal views of the CALM and associated laryngeal cartilages were acquired with a Sonoscape S2V ultrasound unit using a linear probe at 6-7 MHz and a depth of 5.5 cm. All images were collected by one individual blinded to endoscopy grade (B. Hunter). All endoscopies were performed by one individual with an Olympus uniocular 1m endoscope (D. Keenan). Echogenicity of the CALM was compared to the ipsilateral thyrohyoid muscle and subjectively judged as normal or increased. The arytenoid, thyroid and cricoid cartilages were described as normal or abnormal. Still images were assessed by two reviewers blinded to horse age and endoscopy findings (A. Ritmeester and D. Hanlon). Multivariable statistical analyses were used to determine the effect of age and gender on left laryngeal function and CALM echogenicity. All analyses were performed using R version 3.3.2. Means are presented as mean ± SD. Significance was determined at P < 0.05.

Results: Endoscopy grade incidence in yearlings (n = 254) was 54.3% grade 1, 37.4% grade 2, 8.3% grade 3. There were no yearlings with a grade of 4 or 5. On re-assessment as two year olds (n = 53), incidence was 75.5% grade 1, 13.2% grade 2, 11.3% grade 3. There were no two year olds with a grade of 4 or 5. The mean age of yearlings at the time of endoscopy (1.2 ± 0.13 years) was not significantly different from the mean age of yearlings at the time of ultrasound (1.2 ± 0.12 years). Mean two year old age at endoscopy (2.3 ± 0.17 years) was not significantly different from mean two year old age at ultrasound (2.3 ± 0.18 years). Yearlings comprised 139 colts and 115 fillies. Two year olds comprised 8 colts and 45 fillies. Gender was not significantly associated with endoscopy or ultrasound findings. Twenty-nine two year olds showed no change from yearling endoscopy grade. Seventeen improved in grade and seven degenerated in grade. CALM echogenicity was not significantly associated with endoscopy grade for either observer. Reviewer one found significantly more ultrasound images to be non-diagnostic than reviewer two (P<0.05). Reviewer two found significantly more yearlings to have increased left CALM echogenicity than reviewer one (P<0.05), however only two of these had increased echogenicity as two year olds. Both of these horses were endoscopy grade 2 or less as yearlings and two year olds. Three horses had an abnormally shortened right thyroid cartilage on ultrasound with only one having an endoscopic abnormality (rostral displacement of the palatopharyngeal arch), making an incidence of 1.2% of laryngeal dysplasia in a clinically normal yearling Thoroughbred population and a 0.4% incidence in endoscopically normal yearling Thoroughbreds.

Discussion: Left CALM echogenicity does not correlate with endoscopy grade in clinically normal yearling Thoroughbreds that are grade 3 or less using the Lane grading system. Although all images were collected by one individual, subjective assessment of CALM echogenicity on still images has high inter-observer variability. Thus, laryngeal ultrasound is not reliable as a screening tool for subclinical left recurrent laryngeal neuropathy in clinically normal yearlings but can be used to screen for laryngeal dysplasia. The two year old gender distribution had a disproportionately high number of fillies, as colts are more commonly sold at yearling sales while fillies are kept home to race in this population.

References:

1. 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;43:365-371.
2. Garrett KS, Embertson RM, Woodie JB, Cheetham J. Ultrasound features of arytenoid chondritis in Thoroughbred horses. Equine Vet J 2013;45:598-603.