Post laryngoplasty ultrasonography of the horse larynx

Abstract

Laryngoplasty is commonly used to treat laryngeal hemiplegia in Thoroughbred racehorses and success and evaluation thereof has traditionally been determined using endoscopy with little emphasis on ultrasonography. Laryngeal ultrasonography can easily be performed in the standing unsedated horse and is relatively inexpensive. Ultrasonography of the normal horse larynx has been described, as have a number of pathological conditions that occur in the laryngeal and pharyngeal area. Postsurgical ultrasonographic evaluation of laryngoplasty has limited coverage in the literature and is limited to individual cases. The aim of the investigation was to evaluate and validate the use of transcutaneous ultrasonography in horses following laryngoplasty, as well as improving our understanding of the healing process and detecting postsurgical complications. Laryngeal hemiplegia is a relatively common condition and is of particular importance in Thoroughbred racehorses. Traditionally, postsurgical laryngoplasty management and success has been determined by, and monitored with, resting upper respiratory endoscopy. This modality is limited in that peri- and non-luminal laryngeal structures cannot be assessed. A prospective case series of 10 Thoroughbred racehorses with left recurrent laryngeal hemiplegia was examined prior to, 3-10 days, 30-50 days and 6-12 months after laryngoplasty and ventriculectomy, using transcutaneous ultrasonography and endoscopy. Arytenoid abduction was graded endoscopically at each of the examination times using the Havemeyer system for presurgery and the Dixon system postsurgery. The following measurements were made ultrasonographically: cricoarytenoideus lateralis muscle widest diameter, circumference and cross-sectional area; the distance between the vocal folds in exhalation and inhalation, basihyoid depth, rostral thyroid cartilage depth, and distance between the basihyoid and thyroid cartilage was also measured. The ultrasonographically visible presence or absence of the laryngeal ventricles was determined as well as the degree of laryngeal movement as absent, present or intermediate. Repeated means ANOVA was conducted on the ultrasonographic measurements at the four examination times with significance set at P < 0.05. Descriptive analyses on the vocal fold and arytenoid cartilage movement and visibility of the ventricles was performed. It was determined that the structures described in a normal laryngeal ultrasonographic examination could be seen postsurgically. Postsurgical complications of laryngoplasty in the extra-luminal structures were found in seven horses and included soft tissue swelling, seroma formation and one case of marked subcutaneous haemorrhage. In addition a luminal vocal fold abscess and vocal fold granuloma were detected. It was possible to visualise the presence or absence of a ventricular opening ultrasonographically in 70 % of postsurgical horses. The presence of the laryngeal prosthesis suture material was possible to determine in the healing tissues when metallic clips were used, but visualization of other material proved difficult. The distance between the vocal folds in exhalation over the long term was significantly larger than presurgically (1.00 ± 0.24 cm vs 0.67 ± 0.11 cm). The presurgical exhalation distance was also significantly increased from the 30-50 days mean. In addition, the comparison of vocal fold inhalation and exhalation distances at presurgery, 3-10 days postsurgery, 30-50 days postsurgery and at 6-12 months were all significantly different. When compared over time, the mean presurgical inhalation and exhalation means (1.09 ± 0.32 cm and 0.78 ± 0.28 cm, respectively) were significantly smaller than the 6-12 month inhalation and exhalation means, the 3-10 day means were significantly smaller than the 6-12 month inhalation and exhalation means, and the 3-10 day means were significantly smaller than the 30-50 day over time with an increase in the distance as healing progressed. Vocal fold and arytenoid cartilage movement was easy to evaluate ultrasonographically and appeared to correlate to the endoscopic grading, although the latter was not measured. Caudal basihyoid bone depth means in the medium term were significantly different between 3-10 days postsurgery (1.85 ± 0.28 cm) and 30-50 days postsurgery (1.64 ± 0.14 cm). Over the long term the presurgical mean (1.62 ± 0.24 cm) was significantly less than 3-10 days postsurgery mean (1.93 ± 32 cm) and the latter mean was significantly larger than the 30-50 days postsurgery mean (1.66 ± 0.13 cm). In addition the 3-10 days and 6-12 month postsurgery means were also significantly different (1.93 ± 0.24 cm vs 1.60 ± 0.14 cm). Rostral thyroid cartilage depth means in the medium term at 3-10 days postsurgery (3.67 ± 0.53 cm) were significantly larger than 30-50 days postsurgery (3.24 ± 0.48 cm). These differences were most likely as a result of tissue swelling related to the surgery because the laryngoplasty and ventriculectomy had no influence on these structures: the 6-12 month postsurgery values are very similar to the presurgical values. No significant differences in any of the measurements of the cricoarytenoideus lateralis muscle were found at any measurement. Future studies need to be performed with a larger sample size to establish a baseline of vocal fold excursion distance measurement to enable them to be used as a grading system for ultrasonographically evaluating the success of a laryngoplasty in the post laryngoplasty /ventriculectomy horse. Additional objective endoscopic measurements of vocal fold excursion with correlation to ultrasonographic measurements also need to be performed. Ultrasound is an easily accessible tool for most practitioners and can be used for, and will allow, easy evaluation of the post laryngoplasty larynx in the horse in assessing both the success (based on abduction grade) of the procedure, monitoring healing and detecting complications early in order to institute treatment early.