Sternothyroideus Tenectomy Combination Surgery for Treatment of Dorsal Displacement of the Soft Palate in 96 Thoroughbred Racehorses (1996-2004)

Sternothyroideus tenectomy combination surgery has a favorable prognosis for Thoroughbred racehorses affected by intermittent dorsal displacement of the soft palate. Success rates of the combination techniques described here are similar to, and in some cases better than, what the literature suggests.

1. Introduction

Intermittent dorsal displacement of the soft palate (DDSP) is a significant cause of poor performance in Thoroughbred racehorses [1]. The etiology of the condition is not completely clear [2], and diagnosis of the dynamic condition is often presumptive. As such, there are a plethora of surgical and non-surgical treatments described [1,3-11]. The numerous techniques and varied success rates described in the literature suggest that, to date, there is no consistently effective treatment strategy for this condition in Thoroughbred racehorses. Surgical techniques currently in use include sternothyrohyoideus myectomy (with reported 58 - 60% [1,3] success rates) and staphylectomy (with a 59% [3] success rate). Sternothyroideus myotomy used in combination with staphylectomy was reported in a group of 41 Standardbred horses to result in 70% of horses improving their race times [5].

In cases where sternothyrohyoideus myectomy or tenectomy is used in conjunction with staphylectomy or soft-palate thermoplasty, the procedure is performed adjacent to the laryngotomy incision. Reports of the length of the musculotendinous structure removed at the level of the larynx vary and range from a simple tenotomy at the insertion to a myotomy, tenectomy, or myectomy of the sternothyroideus muscle [5,6]. A radical myectomy technique, which resects a portion of the sternothyrohyoideus and omohyoideus muscles at the junction of the proximal and middle thirds of the neck and removes 2.5 - 8 cm of each muscle at this level, reportedly resulted in 70% of horses increasing their earnings per start [7].

Combination surgeries, using two or more techniques, are also reported to infrequently achieve >60% improvement in performance. A recently reported combination of staphylectomy, ventriculectomy, and sternothyrohyoideus myectomy found a 93% return to racing; however, compared with pre-surgery, performance assessed by race earnings was only improved in 60% of horses [4]. Staphylectomy, soft-palate thermoplasty, and sternothyroideus myotomy using a CO2 laser improved mean earnings in 63% of horses that raced at least one time before and after surgery [6].

Standing endoscopic laser soft-palate thermoplasty has been reported as an alternative to staphylectomy after a sternothyroideus tenectomy is performed under general anesthesia [8]. Success rate, determined by return to racing and trainer satisfaction, was 80%. In a study evaluating the usefulness of high-speed treadmill endoscopy for diagnosing DDSP, one or a combination of techniques of either epiglottic augmentation, staphylectomy, sternothyroideus tenectomy, or sternothyrohyoideus myectomy resulted in 64% of horses improving their average earnings [12]. Epiglottic augmentation in combination with sternothyrohyoidmyectomy and staphylectomy in 59 cases (40 Thoroughbreds and 19 Standardbreds) improved racing performance in 73% of Thoroughbred racehorses and in 66% of the horses in the study overall [13]. More recently, a technique involving prosthetic implants to "tie-forward" the larynx has been described experimentally [11] with clinical outcome data yet to be published.

With the recent introduction of the "tie-forward" surgery to achieve rostral traction on the larynx, it is important to establish a baseline for efficacy of existing techniques using indicators of success previously described in the literature. There are difficulties in determining the success of a procedure by objective means, particularly when analyzing racehorse performance. Measures of success are not consistent throughout the literature, making comparisons between studies difficult.

Using return to racing as an indicator of success, prognosis is excellent with certain combination techniques, and they result in >90% [4,7] success. Outcome does not seem to be as favorable when performance indices, such as improvement in class or earnings, are evaluated with ~60 - 70% prognosis for successful surgery.

In summary, published retrospective studies on surgical treatment of DDSP report success rates ranging from 58% to 73% [1,3-5,12,13]. It is the clinical impression of one of the authors (C.W.M.) that sternothyroideus tenectomy combination surgery results in a better prognosis for Thoroughbred racehorses affected by intermittent DDSP than previously reported.

The purpose of this retrospective study was to objectively determine if sternothyroideus tenectomy in combination with either staphylectomy, soft-palate thermoplasty, or aryepiglottic fold resection (if epiglottic entrapment was present) has a greater efficacy than the currently reported 60 - 70%.

2. Materials and Methods

Medical records of 118 Thoroughbred racehorses presented to the Equine Medical Center, Cypress, CA, for surgery for DDSP from November 1996 to November 2004 were reviewed. Horses with pre-operative historical, clinical and/or endoscopic diagnosis of DDSP were included in the study. Horses with additional upper airway conditions such as left laryngeal hemiplegia, arytenoid chondritis, or chondroma were excluded from the study.

Information obtained from the medical records included history, signalment, pre-operative upper respiratory tract endoscopy findings, surgical procedures performed, date of surgery, and post-operative care recommendations. Clinical diagnosis of DDSP was based on history, standing endoscopic exam, and, in selected cases, high-speed treadmill evaluation. The soft palate and epiglottis were subjectively evaluated for size, position, and presence of ulceration or entrapment at rest and during swallowing and nasal occlusion. Outcome was determined by analysis of lifetime race records obtained from The Jockey Club Information Systems. Parameters evaluated were the number of pre-operative and post-operative race starts and the total earnings for up to three starts before and after the date of surgery. Minimum follow-up time was 4 mo post-surgery. Success of surgical intervention was defined as an increase in total earnings in up to three races after surgery compared with before surgery. To evaluate if pre-race earnings influenced increases in total earnings after surgery, records were also evaluated based on total earnings for at least one and up to three starts (group A, >$25,000; group B, $5000-25,000; group C, <$5000) [3].

All surgeries were performed by a single surgeon (C.W.M.). Pre-operative antibiotics, anti-inflammatory medications, and tetanus toxoid were administered before induction of general anesthesia. Horses were positioned in dorsal recumbency with the head extended, and anesthesia was maintained using halothane in a semi-closed system.

A large area over the ventral larynx was clipped, aseptically prepared, and draped. An 8-cm midline incision was made over the ventral aspect of the larynx, extending caudally to 2 cm behind the caudal border of the cricoid cartilage. The incision was continued through the aponeurosis between the sternohyoid muscles. Each sternothyroid muscle and its tendinous insertion on the cricoid cartilage were located. The muscle was elevated with a Kelly forceps, and a 2.5-cm section, including the tendinous insertion and muscle, was excised bilaterally.

A laryngotomy incision was made through the cricothyroid membrane, and a staphylectomy was performed. The central caudal border of the soft palate was grasped with sponge forceps, and a 5 - 10 x 20-mm crescent-shaped section was removed. The laryngotomy incision was left open to heal by second intention. If a soft-palate thermoplasty was performed, the caudal border of the soft palate was cauterized after staphylectomy, using either an electrocautery or a radiofrequency probe. If a concurrent epiglottic entrapment was present, a 20 x 10-mm section of the aryepiglottic fold was excised without soft-palate thermoplasty.

Alternatively, a closed sternothyroideus tenectomy was performed in the same manner as described above. The sternohyoid muscles were apposed using 2-0 Vicryl in a simple continuous suture pattern. The skin was closed with simple interrupted sutures using 2-0 nylon.

All horses were discharged into the care of their referring veterinarians with recommendations for 5 days of antibiotic therapy for horses with a laryngotomy incision. The majority of horses received phenylbutazone (4 mg/kg, PO, SID), and anti-inflammatory throat spray was applied twice daily for 5 days after surgery. Exercise recommendations for horses with a laryngotomy incision included a rest period of 3 wk with hand-walking and a return to training after healing of the laryngotomy incision. There was minimal lay-up time recommended after closed tenectomy surgery with between 2 and 10 days rest.

The Wilcoxon signed rank test for non-parametric data was used to analyze total pre-operative and post-operative earnings per start for each horse. The Fisher's exact test was used to determine if outcome was associated with age, signalment, upper airway endoscopic exam findings (i.e., epiglottic entrapment, hypoplasia, or flaccidity), or earnings for up to three races before surgery. The results were considered significant if p < 0.05.

3. Results

One hundred eighteen Thoroughbred racehorses with DDSP were identified, and 96 met the criteria for inclusion in the study. There were 48 males, 24 females, and 24 geldings. The age distribution was 19 2-yr-old horses, 40 3-yr-old horses, 22 4-yr-old horses, 9 5-yr-old horses, 5 6-yr-old horses, and 1 10-yr-old horse. The mean age was 3.6 yr, and the median age was 3 yr.

A history suggestive of DDSP was recorded for 96 horses. Recorded historical findings included referral endoscopic exam findings (33 of 96 horses), respiratory noise during exercise (3 of 96 horses), and poor performance (4 of 96 horses). All 96 horses in the study had pre-operative endoscopy performed at rest. Twenty-two of the 96 horses had an abnormal epiglottis with 6 subjectively being hypoplastic, 5 flaccid, and 11 with epiglottic entrapment (of the 11 with epiglottic entrapment, two had ulcerations and one had an ulcerated soft palate).

All 96 horses had sternothyroideus tenectomy performed. Sixteen of these were closed primarily, and 11 of the 16 had soft-palate thermoplasty performed in the recovery stall through the oral cavity. Eighty horses had a staphylectomy performed after a sternothyroideus tenectomy; 13 of these horses also concurrently had a soft-palate thermoplasty, and 11 had an aryepiglottic-fold resection.

Evaluation of lifetime race records at the time of publication revealed that 84 of 96 (88%) had raced after surgery. Seventy-seven of 96 horses (80%) raced at least one time before and after surgery, and 73 of 96 horses (76%) raced at least three times after surgery.

Of the 77 horses that raced at least one time before and after surgery, 48 (62%) improved their performance (based on total earnings) in up to three races after surgery. Ten of 13 (77%) of the horses that had closed sternothyroideus tenectomy improved their earnings. With combined tenectomy, staphylectomy, and soft-palate thermoplasty, 6 of 9 (67%) horses had increased earnings; with tenectomy and staphylectomy, 30 of 49 (61%) horses had increased earnings. Of those horses with epiglottic entrapment, one of six (17%) of horses improved earnings. Mean and median total earnings before surgery were $17,238 and $5480, respectively. After surgery, mean and median total earnings for starts increased to $18,596 and $9890, respectively.

When increase in total earnings was evaluated by grouping horses in pre-operative earnings categories (group A, >$25000; group B, $5000 - 25,000; group C, <$5000), 27 of 36 (75%) of the horses in group C increased total earnings. Fourteen of 24 (58%) horses in group B and 7 of 17 (41%) horses in group A improved their earnings. The most successful group was C with tenectomy and staphylectomy resulting in increased total earnings after surgery in 20 of 25 (80%) horses. The mean and median age for all three groups was 3 yr.

4. Discussion

Sternothyroideus tenectomy in combination with staphylectomy and/or soft-palate thermoplasty seems to be an effective treatment for DDSP in Thoroughbred racehorses.

Llewellyn and Petrowitz [5] compared pre-operative and post-operative race times in a group of 41 Standardbred racehorses and reported a 70% success rate after sternothyroideus myotomy and staphylectomyThe main limitations of the study were that there was no pre-operative endoscopy performed and that the diagnosis of DDSP was based mostly on history of respiratory noise and poor performance. The performance indices used for Standardbred racehorses may not be applicable in the Thoroughbred racehorse.

Studies in which pre-operative endoscopy was performed report success rates for return to racing and performance similar to our study (88% raced after surgery; 76% for at least three starts). In the study by Hogan et al. [8] describing transendoscopic laser cauterization of the soft palate, 8 of 10 Thoroughbreds raced at least three times after surgery. Subjectively, Thoroughbred and Standardbred trainer satisfaction was high (92%). Parente et al. [12] studied average earnings per start for three races before and after surgery in 45 horses (35 Thoroughbreds and 10 Standardbreds); 23 (66%) of the Thoroughbreds had improved average earnings after diagnosis and treatment with sternothyroideus tenectomy, staphylectomy, sternothyrohyoideus myectomy, or epiglottic augmentation.

In our study, the addition of soft-palate thermoplasty to the combination of sternothyroideus tenectomy and staphylectomy increased the success rate from 62% to 67%. The 77% success rate of the closed tenectomy technique gives rise to the question of if a staphylectomy is necessary to achieve improved function of the soft palate. Collagen contraction and fibrosis may be equally achieved by soft-palate thermoplasty [14] using either electrocautery, bipolar radiofrequency [15], or laser[8] energy.

Only one of six (17%) horses with DDSP and epiglottic entrapment that raced before and after surgery increased total earnings. This suggests that the prognosis with concurrent epiglottic entrapment is decreased.

The favorable success rate of sternothyroideus tenectomy in combination with staphylectomy and/or soft-palate thermoplasty in our study suggests that these techniques are useful for surgical treatment of intermittent DDSP. Management and treatment strategies for intermittent DDSP will be refined and developed as the etiology of the condition is elucidated.