Successful treatment of persistent dorsal displacement of the soft palate and evaluation of laryngohyoid position in 15 racehorses

Authors


Summary

Reasons for performing study: Caudal descent of the larynx has been proposed to be associated with intermittent dorsal displacement of the soft palate (DDSP) as it leads to a loss of the seal between the epiglottis and soft palate but further investigation of this theory is required.

Objectives: To evaluate laryngohyoid position of horses with persistent DDSP in comparison to horses with intermittent DDSP and evaluate the outcome of treatment.

Hypotheses: Horses with persistent DDSP have a different laryngohyoid position compared to those with intermittent DDSP. Horses with persistent DDSP can be returned successfully to racing with a laryngeal tie-forward procedure.

Methods: Medical records of 15 racehorses presented for persistent DDSP between 2002 and 2007 were reviewed. Age, sex and breed matched horses diagnosed with intermittent DDSP were used as a comparison group. Treatment of all horses was performed by laryngeal tie-forward, 8/15 horses with persistent DDSP had a subsequent laser staphylectomy. Preoperative laryngohyoid position was compared between the 2 groups using a radiographic reference system. Surgical effect on position was assessed by comparing pre- and post operative radiographic measurements. Outcome was assessed by return to racing and comparison of pre- and post operative race earnings ($).

Results: Thirteen of 15 horses with persistent DDSP returned to racing. Seven of 15 horses were treated with laryngeal tie-forward alone while 8/15 horses were also treated with a laser staphylectomy. Horses with persistent DDSP had a more caudal larynx (ossification of the thyroid cartilage) (13 mm, P = 0.014), a more caudal (10 mm, P = 0.044) and dorsal (7 mm, P = 0.01) basihyoid bone, and a more dorsal thyrohyoid-thyroid articulation (10 mm, P = 0.002) than horses with intermittent DDSP.

Conclusions and potential relevance: Racehorses with persistent DDSP can be treated successfully by laryngeal tie-forward or by laryngeal tie-forward followed by laser staphylectomy. Horses with persistent DDSP have a more caudal larynx and more caudal and dorsal basihyoid bone than horses with intermittent DDSP.

Introduction

Dorsal displacement of the soft palate (DDSP) is a common cause of poor performance and abnormal upper respiratory noise in racehorses with an estimated prevalence of 10–20% (Parente et al. 2002; Ducharme 2006; Lane et al. 2006). The most common presentation is intermittent in which the free border of the soft palate becomes dislodged from its normal subepiglottic position and obstructs the nasopharyngeal lumen at exercise. DDSP has been shown to increase tracheal expiratory impedance, reduce minute ventilation and produce hypoxaemia (Rehder et al. 1995; Holcombe et al. 1998; Franklin et al. 2002; Ducharme et al. 2003). A definitive diagnosis of intermittent DDSP is often made using high speed treadmill examination when the caudal border of the soft palate is seen dorsal to the epiglottis for >8 seconds.

The aetiology of persistent DDSP is not completely understood. Ducharme et al. (2003) showed that resection of the thyrohyoideus muscles led to intermittent DDSP during exercise, suggesting that dysfunction of these muscles may lead to nasopharyngeal instability. Holcombe et al. (1998) demonstrated that blockade of the pharyngeal branch of the vagus nerve (Xp) induces reversible, persistent DDSP that may be due to dysfunction in the levator veli palatini, palatinus and palatopharygeus muscles. It has also been proposed that epiglottic shape and position play a role in development of persistent DDSP (Tulleners et al. 1997). However, Holcombe et al. (1997) showed that experimentally-induced retroversion of a normal epiglottis does not lead to DDSP. Persistent DDSP has been reported in association with a persistent subepiglottic frenulum (Yarbrough et al. 1999; Moorman et al. 2007) and hypoplastic epiglottis (Haynes 1981; Tulleners et al. 1997). Some authors have reported persistent DDSP as a complication following surgery for epiglottic entrapment (Ross et al. 1993; Tulleners et al. 1997). Further investigation of this theory was performed by comparing the radiographic position of the larynx and hyoid apparatus in horses with intermittent DDSP vs. persistent DDSP before and after laryngeal tie-forward (Woodie et al. 2005).

The objective here was to test the hypothesis that horses with persistent DDSP have a different laryngohyoid position than horses with intermittent DDSP. It was also proposed that horses with persistent DDSP can be treated successfully with surgical advancement of the larynx.

Materials and methods

Study design and population

Medical records of all horses presenting for treatment of persistent DDSP at Cornell University Hospital for Animals, between January 2002 and December 2007, were reviewed. Subject details, history and treatment information were obtained. Diagnosis of persistent DDSP was made by upper airway endoscopy at rest, without sedation, if the epiglottis was positioned ventral to the soft palate and the horse was unable to, or only very briefly able to, replace the soft palate into the correct anatomical position. The subepiglottic tissue of all horses with persistent DDSP was examined endoscopically. This was achieved by applying topical anaesthetic (2% lidocaine; 50 ml) to the epiglottis and soft palate through the endoscope channel before inserting broncho-oesophageal forceps through the opposite nasal passage. Once the forceps were at the level of the epiglottis, the horizontally oriented forceps were held open, placed under the epiglottis and used to retrovert it, which allowed good visualisation of the ventral surface of the epiglottis and subepiglottic tissues. During the same study period, 336 horses were diagnosed with intermittent DDSP and underwent laryngeal tie-forward surgery. From this population, a control horse (using only horses with a confirmed diagnosis via high-speed treadmill examination and not lost to follow-up for at least 6 months) was identified and matched by age, sex and breed for each case of persistent DDSP.

Surgical technique

All horses operated before September 2005 (3 horses with persistent DDSP; 10 horses with intermittent DDSP) were treated with surgical advancement of the larynx (laryngeal tie-forward), as described by Ducharme et al. (2003) and Woodie et al. (2005). Briefly, a 15 cm ventral midline incision was made starting 1 cm caudal to the cricoid cartilage and extending 1.5 cm rostral to the caudal end of the basihyoid bone. The ventral aspect of the larynx was exposed with blunt dissection. A hole was made in the basihyoid bone using a 3.2 mm drill bit and 2 strands of No.5 USP polybend suture (Fiberwire) were passed through the hole. One strand was used to place 2 bites in the left wing of the thyroid cartilage while the other strand was used to place 2 bites in the right wing. After suture placement, the head was flexed approximately 90° and the sutures were tied, pulling the thyroid cartilage rostrally. The sternothyroideus tendon was then transected and a 2 cm section removed on both sides prior to routine closure.

A modified technique (Cheetham et al. 2008) was used in horses treated after September 2005 (12 horses with persistent DDSP; 5 horses with intermittent DDSP). This technique differed in that a curved wire passer was inserted rostral to the basihyoid on either side of the lingual process instead of creating a hole in the basihyoid bone with a drill bit. The suture strands were passed through the wire passer from rostral to caudal. Intraoperative measurements of the distance from the caudal aspect of the basihyoid bone to the rostral aspect of the thyroid cartilage (BT) and the rostral aspect of the cricoid cartilage (BC) were made in all horses as described previously (Woodie et al. 2005).

Eight of the 15 horses with persistent DDSP continued to have persistently displaced soft palates as noted on post operative endoscopy following laryngeal tie-forward and were treated subsequently with laser staphylectomy. This was performed with the horse standing using a diode laser and broncho-oesophageal forceps to grasp the caudal edge of the soft palate. The laser was used to resect 5–10 mm of the caudal edge of the soft palate.

Radiography

A lateral radiograph of the larynx was taken before and within 24 h after laryngeal tie-forward surgery. Horses were lightly sedated (xylazine; 100–150 mg i.v.) and stood with their heads in a neutral resting position. Two 50 mm reference pins were placed on the lateral aspect of each mandible. Radiographs were available for 14 of 15 cases of persistent DDSP; therefore, the radiographs of the 14 matched cases of intermittent DDSP were analysed.

Radiographs were analysed and measured using a PACS system1. A radiographic reference system (Cheetham et al. 2008) was used to determine preoperative laryngohyoid position and the effect of surgery on the laryngohyoid position (Fig 1). Briefly, 2 lines were drawn along the caudal aspect of each vertical ramus and along the ventral aspect of each horizontal ramus of the mandible. The intersection of the 2 horizontal and 2 vertical lines created a rectangle caudal to the angle of the mandible. The height and width of the rectangle were measured to determine dorsoventral and rostrocaudal obliquity. Dorsoventral and rostrocaudal obliquity were calculated to evaluate potential bias between pre- and post operative radiographs for anatomical points that do not lie on the midline of the horse such as the thyrohyoid-thyroid cartilage articulation. A line was extended horizontal and vertically from the centre of the rectangle to establish a midline from which dorsoventral and rostrocaudal measurements could be made.

Figure 1.

Lateral laryngeal radiograph showing the radiographic measurement system used to evaluate laryngohyoid position. B = basihyoid bone; Ossification = ossification of the base of the thyroid cartilage; Th-T = thyrohyoid-thyroid articulation.

The angle of head flexion was determined by measuring the angle from the trachea to the midline drawn along the horizontal rami of the mandible. Three points of interest were used: the caudal aspect of the basihyoid bone, the thyrohyoid-thyroid articulation, and the ossification of the base of the thyroid cartilage (i.e. larynx). The dorsoventral position of each anatomical structure was measured from the mandibular horizontal midline and the rostrocaudal position of each anatomical structure was measured from the mandibular vertical midline. In addition, epiglottic length was measured on preoperative radiographs from the base of the epiglottis to the tip as described by Linford et al. (1983). All radiographic measurements were corrected for magnification using the reference pins (Linford et al. 1983).

Outcome

Successful outcome of treatment was defined as restoration of normal epiglottic position and return to racing. In addition, race records were obtained for each horse from an online database (Brisnet.com). Performance was determined by reviewing pre- and post operative race placement and race earnings ($) for at least 6 months pre- and post operatively.

Statistical analysis

Differences in preoperative laryngohyoid position, epiglottic length and the effect of surgery between the 2 groups (persistent DDSP vs. intermittent DDSP) were determined using a Wilcoxon rank-sum test (nonparametric test used to compare independent groups that cannot be assumed to be normal due to small sample size). The effect of surgery on laryngohyoid position was assessed using a paired student's t test (used to compare 2 paired groups). A mixed linear model with the horse as the random factor and race number as the fixed effect was used to evaluate racing performance. A Y = log (earnings [$]+ 1) transformation of the race earnings was performed to normalise the data as the raw data was positively skewed (Bartlett 1947). A Tukey's post-hoc test was used to make multiple comparisons for difference in transformed earnings by race relative to surgery. A linear contrast was used to compare race earnings for the immediate preoperative race and immediate post operative race. Statistical analysis was performed using S-Plus2 and JMP3. The level of significance was set at P<0.05. All confidence intervals (CI) were 95% CI.

Results

Study population

Fifteen racehorses were diagnosed with persistent DDSP between January 2002 and December 2007. There were 8 Thoroughbreds and 7 Standardbreds; 3 geldings, 6 mares and 6 colts with a mean ± s.d. age 3.2 ± 1.0 years. These consisted of 5 two-year-olds, 2 of which had raced prior to presentation and 3 of which were in training only; 3 three-year-olds, 2 of which had raced prior to presentation; 6 four-year-olds, all of which had raced prior to presentation and one 5-year-old, which had also raced prior to presentation. Inability to train or race due to the condition of persistent DDSP was reported in all horses prior to admission. Fourteen of these 15 horses had pre- and post operative radiographs available for analysis.

Eleven of 15 horses with persistent DDSP had a history of subepiglottic surgery. Of these horses, 10 had subepiglottic surgery within 12 months prior to presentation and one had subepiglottic surgery within 24 months of presentation. Ten horses had at least one surgical treatment for epiglottic entrapment (EE): 9 had laser transection of the aryepiglottic fold (with one of these having a repeat laser transection) prior to admission and one had a hook transection of the aryepiglottic membrane followed by 2 successive laryngotomies with partial aryepiglottic fold resection. One horse had a history of laser resection of a subepiglottic cyst.

Five of the eleven horses returned to racing or training following surgery prior to development of persistent DDSP and 6 did not return to racing or training following surgery due to development of persistent DDSP. Exact time between subepiglottic surgery and development of persistent DDSP could not be obtained from medical records.

All 15 horses were treated with a laryngeal tie-forward procedure. Eight of these continued to have intermittent or persistent DDSP following surgical advancement of the larynx and were subsequently treated with laser staphylectomy.

Radiographic head position and magnification

Radiographic obliquity and head angle were analysed to assess possible effects on laryngohyoid position. Pre- and post operative measurements were compared between horses with intermittent and persistent DDSP and pre- and post operative measurements within each group were compared. There was a significant difference in post operative dorsoventral obliquity between the 2 groups (P = 0.046) with a mean difference of 6 mm (95% CI 2–10 mm). There were no other significant differences between any other comparison group and no significant differences in head angle between groups. The mean magnification in the midline was 1.3 mm (95% CI 1.1–1.4 mm) preoperatively and 1.2 mm (95% CI 1.1–1.4 mm) post operatively.

Preoperative differences between intermittent DDSP and persistent DDSP

The epiglottis of horses with persistent DDSP was significantly shorter than horses with intermittent DDSP (P = 0.02). Mean epiglottic length was 66 mm (95% CI 62–69 mm) for horses with persistent DDSP and 72 mm (95% CI 75–69 mm) for horses with intermittent DDSP.

Figure 2 depicts the preoperative position of all 3 anatomical points. Prior to surgery the basihyoid bone was significantly more caudal (10 mm, 95% CI 4–17 mm, P = 0.044) and more dorsal (7 mm, 95% CI 3–11 mm, P = 0.01) in horses with persistent DDSP compared to those with intermittent DDSP. The thyrohyoid- thyroid (Th-T) articulation was significantly more dorsal (10 mm, 95% CI 6–15 mm, P = 0.002) in horses with persistent DDSP. The ossification of the thyroid cartilage was significantly more caudal (13 mm, 95% CI 6–20 mm, P = 0.014) in horses with persistent DDSP.

Figure 2.

a) Mean ± s.e. preoperative rostrocaudal position of the laryngohyoid apparatus including the basihyoid bone, ossification of the thyroid cartilage and thyrohyoid-thyroid (Th-T) articulation in horses with intermittent and persistent DDSP. b) Mean ± s.e. preoperative dorsoventral position of the laryngohyoid apparatus including the basihyoid bone, ossification of the thyroid cartilage and Th-T in horses with intermittent and persistent DDSP. * Indicates a significant difference between the 2 groups with P < 0.05. ** indicates a significant difference between the 2 groups with P < 0.01.inline image = Persistent DDSP;inline image=Intermittent DDSP.

There were no significant differences in the preoperative laryngohyoid position between horses with persistent DDSP that were treated with laryngeal tie-forward alone and those that were also treated with laser staphylectomy.

Effects of surgery on laryngohyoid position

Figure 3 depicts the movement of the laryngohyoid apparatus achieved by surgery. In horses with persistent DDSP, the laryngeal tie forward procedure moved the basihyoid bone significantly dorsally (5 mm, 95% CI 2–9 mm, P = 0.005). The caudal movement of the basihyoid bone was not significant (4 mm, 95% CI -11–4 mm, P = 0.335). The ossification of the body of the thyroid cartilage was moved significantly rostrally (17 mm, 95% CI 9–26 mm, P<0.001) and dorsally (25 mm, 95% CI 20–29 mm, P<0.001). The Th-T was moved significantly more dorsal (12 mm, 95% CI 8–17 mm, P<0.001). There were no significant changes in the caudal position of the Th-T (1 mm, 95% CI −7–6 mm, P = 0.88).

Figure 3.

a) Mean ± s.e. rostral or caudal movement of the laryngohyoid apparatus following laryngeal tie-forward surgery. b) Mean ± s.e. dorsal movement of the laryngohyoid apparatus following laryngeal tie-forward surgery. * Indicates a significant difference in laryngohyoid movement with P < 0.05. ** Indicates a significant difference in laryngohyoid movement with P < 0.01.inline image=Persistent DDSP;inline image=Intermittent DDSP.

In horses with intermittent DDSP, the procedure moved the basihyoid bone significantly caudally (8 mm, 95% CI 2–14 mm, P = 0.01). There was no significant change in the dorsal position of the basihyoid bone following surgery (3 mm, 95% CI −2–7 mm, P = 0.25). The ossification of the body of the thyroid cartilage moved significantly rostrally (8 mm, 95% CI 2–14 mm, P<0.02) and dorsally (21 mm, 95% CI 17–25 mm, P<0.001). The Th-T was moved significantly dorsally by surgery (11 mm, 95% CI 4–17 mm, P = 0.003). There were no significant changes in the caudal position of the Th-T (1 mm, 95% CI −6–3 mm, P = 0.53).

There were no significant differences in the movements of the basihyoid bone, Th-T articulation or the ossification of the thyroid cartilage between horses with intermittent and persistent DDSP; however, there was a trend towards significance in the rostral movement of the ossification of the thyroid cartilage (P = 0.056) with persistent DDSP horses having a mean rostral movement of 17 mm (95% CI 9–26 mm) and intermittent DDSP horses having a mean rostral movement of 8 mm (95% CI 1.5–14 mm).

There were no significant differences in the dorsoventral or rostrocaudal movement of the laryngohyoid apparatus between horses with persistent DDSP that were treated with laryngeal tie-forward alone and those that were also treated with laser staphylectomy.

Intraoperative measurements

Table 1 depicts the intraoperative movements between the caudal aspect of the basihyoid bone and rostral aspect of the thyroid cartilage (BT) and caudal aspect of the basihyoid bone and rostral aspect of the cricoid cartilage (BC). Both measurements were changed significantly by surgery for both groups. The change in the BT distance was significantly greater in horses with persistent DDSP (P = 0.006) indicating that the larynx was moved significantly more rostral at surgery in horses with persistent DDSP compared to horses with intermittent DDSP.

Table 1. Mean ± s.d. intraoperative distances (mm) between the caudal aspect of the basihyoid bone and rostral aspect of the thyroid cartilage (BT) and caudal aspect of the basihyoid and rostral aspect of the cricoid cartilage (BC) prior to (Preop) and after suture tightening (Post op). A negative sign implies that the thyroid cartilage becomes rostral to the caudal aspect of basihyoid bone following suture tightening. Surgery moved the thyroid cartilage significantly (P = 0.006) more rostral with respect to the basihyoid bone in horses with persistent DDSP (*) compared to horses with intermittent DDSP
GroupPreop BT distancePreop BC distancePost op BT distancePost op BC distanceChange in BT distanceChange in BC distance
Intermittent DDSP33 ± 0.4103 ± 0.9−7 ± 0.661 ± 0.840 ± 0.641 ± 0.7
Persistent DDSP31 ± 0.3104 ± 0.6−12 ± 0.554 ± 0.6*50 ± 0.543 ± 0.7

Effect of breed, sex and age

In horses with persistent DDSP, the preoperative position of the Th-T articulation was significantly more rostral in Thoroughbreds than Standardbreds (P = 0.018) and significantly more rostral in males than females (P = 0.026). The preoperative position of the ossification of the thyroid cartilage was significantly more dorsal (P = 0.039) in males than in females and was significantly more dorsal in Standardbreds than in Thoroughbreds (P = 0.027). The preoperative position of the ossification of the thyroid cartilage was also significantly more rostral in Thoroughbreds than in Standardbreds (P = 0.005). In horses with intermittent DDSP, the preoperative position of the ossification of the thyroid cartilage was significantly more dorsal (P = 0.022) in Thoroughbreds than in Standardbreds. No other significant differences were noted when evaluating the effect of breed, sex and age.

Outcome

Persistent DDSP was resolved immediately by laryngeal tie-forward alone in 7 horses. Eight horses continued to have intermittent or persistent DDSP following laryngeal tie-forward and were treated subsequently with laser staphylectomy. A staphylectomy was performed within one week of tie-forward surgery in 6 horses. The staphylectomy resolved the persistent DDSP immediately in all 6 horses. One horse had a staphylectomy performed 3 months after tie-forward surgery. One horse required 2 staphylectomies, one 3 months and one 6 months post laryngeal tie-forward. In both of these horses, staphylectomy was performed at a later date due to apparent resolution of persistent DDSP following tie-forward with subsequent recurrence of disease.

Long-term endoscopic follow-up was available in 2 horses only. For one horse at one year post operatively, DDSP could be easily induced at rest but the horse was able to return to training and racing. The second horse was normal on endoscopic examination at rest 2 years after surgery.

Eleven of the 15 horses diagnosed with persistent DDSP raced at least once preoperatively, with a mean 16.3 preoperative races. Thirteen of the 15 horses with persistent DDSP raced at least once post operatively, a mean of 11 post operative races. Eleven of the 15 horses with persistent DDSP that raced preoperatively and 2 of the horses that did not race preoperatively were noted to race post operatively. Eleven of the 15 horses diagnosed with intermittent DDSP raced preoperatively, with a mean of 14.1 preoperative races. Thirteen of the 15 horses with intermittent DDSP raced at least once post operatively with a mean of 12.6 post operative races. Ten of the 11 horses with intermittent DDSP that raced preoperatively went on to race post operatively while 3 of the horses that did not race preoperatively were noted to race post operatively.

There were no significant differences in preoperative or post operative log transformed earnings or race placement between horses with persistent DDSP and horses with intermittent DDSP. Horses with persistent DDSP had significantly less log transformed earnings (linear contrast P<0.05) for the immediate preoperative race (least squares mean = 1.51; 95% CI 0.6–2.4) compared to the immediate post operative race (least squares mean = 2.43; 95% CI 1.6–3.3). There was no effect of horse on any model.

Discussion

Persistent dorsal displacement of the soft palate (DDSP) is a rare clinical presentation and has not been clearly defined in the literature. In the present study, persistent DDSP was defined as horses with dorsally displaced soft palates at rest that were unable to, or only very briefly able to, replace the soft palate into correct anatomical position. Intermittent DDSP occurs usually during high-intensity exercise such that affected horses have decreased performance towards the end of a race; whereas persistent DDSP can be considered a career-ending disease due to the constant upper airway expiratory obstruction it creates.

Some surgical treatments for DDSP are aimed at either advancing the larynx rostrally (e.g. laryngeal tie-forward) (Woodie et al. 2005) or decreasing caudal retraction of the larynx (e.g. sternothyrohyoideus myectomy) (Holcombe et al. 1994; Smith and Embertson 2005). These treatments have been supportive evidence for the theory that DDSP is associated with caudal descent of the larynx (Ducharme et al. 2003) leading to loss of the seal between the caudal edge of the soft palate and epiglottis.

Preoperative lateral laryngeal radiographs in the present study provided evidence that horses with persistent DDSP have a significantly more caudal larynx (i.e. ossification of the thyroid cartilage), than horses with intermittent DDSP. In addition, horses with persistent DDSP had a more dorsal basihyoid bone when compared to horses with intermittent DDSP. Finally, the thyrohyoid-thyroid articulation was significantly more dorsal and caudal in horses with persistent DDSP. Interpretation of thyrohyoid-thyroid position is difficult as it is affected by both hyoid position and laryngeal position. It is likely that a caudally positioned larynx is a main factor in the development of all forms of DDSP. The data presented in this study show that horses with persistent DDSP have a significantly more caudal larynx than horses with intermittent DDSP which suggests that severity of nasopharyngeal instability may be related to caudal position of the larynx.

The effect of laryngeal tie-forward surgery on laryngohyoid position evaluated radiographically revealed that the larynx (i.e. ossification of the thyroid cartilage) was moved significantly rostrally and dorsally and the thyrohyoid-thyroid articulation was moved significantly dorsally. The basihyoid bone was moved caudally in both groups, with this movement being significant in horses with intermittent DDSP only. These findings support the results of a study that evaluated changes in laryngohyoid position in 106 horses that underwent laryngeal tie-forward (Cheetham et al. 2008). The intraoperative movement of the larynx also agreed with the radiographic findings, as the larynx was moved significantly more rostral in both groups. It should be noted that the larynx was moved more rostral, by approximately 1 cm, in horses with persistent DDSP than in horses with intermittent DDSP. The suture between the thyroid cartilage and basihyoid bone was purposefully pulled tighter by the surgeon in horses with persistent DDSP as it was theorised that these more severely affected horses may have a more caudal larynx than horses with intermittent DDSP.

One should be cautious in extrapolating the data in this study because of the small sample size. In addition, head angle and obliquity may effect radiographic measurements; however, there was only a significant difference in post operative dorsoventral obliquity between the 2 groups. The basihyoid bone and the ossification of the thyroid bone lie on the midline and so estimates of their position should be unaffected by obliquity. The measured position of the Th-T articulation could be affected by obliquity; however, we can assume equal distribution of right and left obliquity in the dorsoventral and rostrocaudal plane, which would limit bias. This study suggests that horses with persistent DDSP have a significantly more caudal larynx and dorsal basihyoid bone than horses with intermittent DDSP only. Unaffected horses would make a more applicable control group; however, lateral laryngeal radiographs of normal horses were not available for comparison. Future evaluation of normal horses could yield further information regarding significant differences in the position of the larynx in relation to the development of DDSP.

It is also possible that structural defects of the epiglottic cartilage are a contributing factor to persistent DDSP. The mean epiglottic length of horses with persistent DDSP was significantly shorter than in horses with intermittent DDSP. In addition, horses with persistent DDSP had a mean epiglottic length that was 22 mm shorter than the previously reported average epiglottic length in 24 Thoroughbreds of 87 mm (Linford et al. 1983). Epiglottic hypoplasia has been commonly associated with both epiglottic entrapment (EE) and DDSP (Honnas and Wheat 1988; Tulleners 1990; Tulleners et al. 1997) although Redher et al. (1995) were unable to support this association in horses with intermittent DDSP at exercise. The study revealed that horses with persistently displaced soft palates have a shorter epiglottis when compared to horses with intermittent DDSP which may be a predisposing factor for DDSP or may be associated with previous subepiglottic surgery in which scarring of the epiglottis can lead to shortening of the cartilage.

It is interesting that 11 of 15 horses with persistent DDSP underwent sub- epiglottic surgery prior to development of DDSP. Several reports describe development of either intermittent or persistent DDSP following treatment for EE (Tulleners 1990; Ross et al. 1993; Lumsden et al. 1994). Epiglottic hypoplasia and/or excessive surgical resection of the aryepiglottic tissue have been proposed as a predisposing cause for development of DDSP following surgery (Jann and Cook 1985).

Successful treatment of persistent DDSP has been rare. Persistent DDSP was described in 2 Thoroughbreds in association with epiglottic hypoplasia both of which appeared refractory to staphylectomy via a laryngotomy (Haynes 1981). Moorman et al. (2007) described persistent DDSP in a 7-year-old Thoroughbred race horse attributable to a subepiglottic frenulum that prevented dorsal movement of the epiglottis whereas Yarbrough et al. (1999) described persistent DDSP in 4 neonatal foals due to a persistent subepiglottic frenulum. In both these reports, the horses were treated with transection of the frenulum until visualisation of the hyoepiglotticus muscle was achieved. This treatment was successful in the 4 foals but failed in the mature racehorse. In the present study of mature horses, those with persistent DDSP did not appear to have abnormal restrictive scar tissue when visualised endoscopically and the epiglottis was able to be retroverted completely in all horses using equine broncho-oesophageal forceps. Given the caudal position of the larynx in horses with persistent DDSP, it is logical that laryngeal tie forward surgery would correct this anomaly; however, the condition was corrected in only 7 of 15 horses. In the remaining horses, the epiglottic cartilage was noted to be bulging underneath the caudal free edge of the soft palate, which was resolved by a staphylectomy.

In this study, 13/15 horses with persistent DDSP were able to return to racing providing evidence that persistent DDSP is a treatable condition. Comparison of race placement and race earnings between the 2 groups showed that horses with persistent DDSP do not have significantly different post operative race earnings than horses with intermittent DDSP; however, the sample size was small and this may have precluded a true difference from being detected. In conclusion, persistent DDSP appears to be associated with a more caudally positioned larynx and a shorter epiglottic cartilage. Persistent DDSP is a treatable condition in racehorses with these horses returning to successful racing careers.

Acknowledgement

The authors thank Dr Eric Parente, New Bolton Center, University of Pennsylvania who performed a staphylectomy in one of the horses that did not respond to laryngeal tie-forward.

Manufacturers' addresses

1 Eastman Kodak Co., Rochester, New York, USA.

2 Insightful Corporation, Palo Alto, California, USA.

3 SAS Institute, Cary, North Carolina, USA.

Author contributions The initiation, conception, planning, statistics and writing for this study were by K.T.O., J.C. and N.G.D. All authors contributed to its execution.

Ancillary