Effects of Unilateral Laser-Assisted Ventriculocordectomy in Horses With Laryngeal Hemiplegia

Unilateral laser-assisted ventriculocordectomy can be performed in the standing, sedated horse, and it effectively reduces upper airway noise in laryngeal hemiplegia-affected horses exercising at maximum heart rate. Furthermore, the procedure returns upper airway function to baseline levels.

1. Introduction

Horses affected by laryngeal hemiplegia (LH) have exercise intolerance and make an inspiratory noise during exercise[1,2]. In sport horses, the use of bilateral ventriculocordectomy has been recommended as the technique of choice, because it effectively reduces respiratory noise, improves upper airway function, and has few serious post-operative complications [3]. However, the technique requires general anesthesia and a laryngotomy. Laser-assisted surgery performed through a fiberoptic endoscope is less invasive and does not require general anesthesia [4]. We recently showed, however, that laser-assisted vocal cordectomy is ineffective in reducing LH-associated respiratory noise [5]. Because the laryngeal ventricle is the most likely source of the noise [6], these results may be explained by failure of the surgical technique to remove this structure. In the current study, we evaluated the efficacy of a new surgical technique that results in removal of both the laryngeal ventricle and vocal fold without general anesthesia.

2. Materials and Methods

Animals

Six Standardbred horses, with a mean age of 9.6 ± 1.6 yr, that were free of upper airway abnormalities as determined by endoscopy at rest and during high-speed treadmill exercise were used in this study.

Experimental Protocol

Horses were exercised on a treadmill at a speed producing maximum heart rate before (baseline) and 14 days after induction of LH by left recurrent laryngeal neurectomy. At 60 days after laser-assisted, unilateral, left-sided ventriculocordectomy (LVC), the horses were exercised again.

Surgical Procedure

LH was induced under general anesthesia by left recurrent laryngeal neurectomy performed in the mid-cervical region as previously described [7].

LVC was performed [8]. Horses were restrained in stocks and sedated by use of a continuous infusion of detomidine HCl [a]. A specially constructed transnasal burr was inserted through the left nostril into the left laryngeal ventricle. The ventricle was everted, and the vocal fold and everted ventricle were resected with an endoscopically guided diode laser [b] applied in contact at 20 W.

Methods

Respiratory sounds were recorded using a dynamic unidirectional microphone positioned ~4 cm from the horse's nose. The microphone was connected to a cassette recorder. Tracheal pressure was measured by use of a catheter placed through the right nostril into the mid-cervical region. The catheter was attached to a pressure transducer and a physiograph. Trans-upper airway pressure (Pui) was measured as the pressure difference between tracheal and atmospheric pressure [9].

Data Analysis

Respiratory sounds were converted to digital files and analyzed using computer-based spectrogram and sound analysis programs. The inspiratory sound level (SL) and the sound intensity of formants 1 (F1), 2 (F2), and 3 (F3) were calculated as previously described [9,10]. Data were analyzed with a repeated measure analysis of variance (ANOVA). When treatment effects were significant (p < 0.05), means were compared with the Student-Newman-Keuls test.

3. Results

In horses exercising at maximum heart rate, induction of LH caused a significant increase in Pui (68%), SL (36%), and the sound intensity of F2 (30%) and F3 (19%). The sound intensity of F1 remained unchanged by induction of LH. After 60 days, LVC, Pui, SL, and the sound intensity of F3 had returned to baseline levels. LVC significantly decreased the sound intensity of F2, but 60 days after surgery, F2 remained significantly higher than baseline values.

4. Discussion

Unilateral laser-assisted ventriculocordectomy effectively decreased upper airway noise in horses affected by LH. All sound indices but one returned to baseline levels. These results are better than those reported after unilateral laser cordectomy [5], and they support the contention that the ventricle is an important source of the noise associated with LH [6]. Taken together, these data suggest that removal of the ventricle is necessary to effectively reduce noise associated with LH.

Upper airway inspiratory pressures returned to baseline 60 days after LVC. Although bilateral ventriculocordectomy and laser cordectomy also improved Pui, these techniques were unable to return Pui to baseline levels [3,5]. Therefore, LVC is more effective in improving upper airway function than ventriculocordectomy [3] and laser cordectomy [5], and it is as effective as prosthetic laryngoplasty [10].

The diode laser unit that was used in the current study was readily portable, and it created sufficient energy to perform the surgery in a timely manner in the standing horse.

We conclude that LVC is a minimally invasive surgical technique that effectively improves upper airway noise and function in horses affected by LH.

This study was funded by the Equine Performance and Health Funds at Michigan State University and the Matilda R. Wilson Equine Respiratory Disease Endowment. We acknowledge the support of DiodeVet, 825 Dawson Drive, Suite 2, Newark, DE 19713 and Pentax, Medical Endoscopy, a division of PENTAX of America, Inc. , 102 Chestnut Ridge Road, Montvale, NJ 07645-1856. We would like to acknowledge Cathy Berney and Sue Eberhart for their technical assistance.