Sialolithiasis in horses: A retrospective study of 25 cases (2002–2013)

Abstract

This study evaluated outcomes of surgical treatment of equine sialolithiasis, highlighting cases involving the proximal parotid salivary duct. Sialoliths in the proximal parotid duct were difficult to identify radiographically and more frequently associated with draining tracts and sialadenitis compared with sialoliths in the distal parotid duct. Ultrasonography confirmed the diagnosis of sialolithiasis in all cases in which there was no radiographic evidence of a sialolith. All cases of proximal parotid duct sialolithiasis required transcutaneous removal. A longer duration of illness was observed in cases of proximal parotid duct sialolithiasis compared with cases involving the distal parotid duct, and in cases requiring transcutaneous removal compared with cases requiring transoral removal. Recurrence of sialolithiasis was documented in 24% of cases, all of which were located in the distal parotid duct. The average time to recurrence was 2.8 years.

Introduction

Sialolithiasis is an infrequently encountered condition in equids (1). Sialoliths are more frequently observed within the distal parotid salivary duct with cases involving the proximal parotid duct or other salivary ducts rarely documented in the literature (2–4).

Typically, a sialolith in the parotid duct presents as a firm, freely movable mass near the rostral aspect of the facial crest (2). Disease may remain subclinical, but acute obstruction of the salivary gland may lead to painful parotid distention and localized facial swelling with subsequent sialadenitis (2,4,5). Diagnosis of sialolithiasis can be confirmed with radiographs and ultrasound of the swelling and surrounding salivary duct (4,6–8).

Both transcutaneous and transoral surgical techniques have been described for removal of sialoliths in the horse (2). Transcutaneous removal of sialoliths has been associated with increased incidence of post-operative infection at the surgical site and fistula formation compared with transoral removal, and also presents a risk for iatrogenic damage to the maxillary branch of the facial nerve (9). While transoral sialolith removal has been advocated in the literature, information about long-term outcome for only a small number of cases is available (4,6,7,9). The recurrence of sialoliths following surgical removal has not been reported in the horse. The purpose of this paper was to review cases of equine sialolithiasis describing diagnostic findings, surgical techniques, and outcomes for affected animals.

Materials and methods

Medical records of equine patients presenting to Steinbeck Country Equine Clinic were reviewed for cases of sialolithiasis from 2002 to 2013. In all cases, the diagnosis was confirmed by surgically removing the sialolith. Data recorded included signalment, clinical signs, and when available, oral examination and diagnostic imaging findings. Concurrent sialadenitis was diagnosed based on the presence of warm, swollen, and painful salivary glands with definitive diagnosis made with cytology and culture of the saliva. The type of surgical procedure performed in each case was recorded along with post-operative treatments, and complications. Information in the medical record and follow-up telephone conversation with owners were used to obtain information about long-term outcome and recurrence.

Sialoliths were classified by affected salivary duct and sub-categorized according to location within the salivary duct for parotid duct cases. The distal parotid duct was previously defined as the portion of the duct that passes along the rostral border of the masseter muscle and exits into the oral cavity at the level of the second or third maxillary premolar (1). This study defines the proximal duct as the remaining portion of the salivary duct as it courses with the horizontal ramus of the mandible and ascends along the vertical ramus of the mandible to the parotid salivary gland.

Surgical removal of sialolith through a transoral approach

Sialoliths located in the distal parotid duct were surgically removed via standing sedation with detomidine hydrochloride (Orion Corp., Espo, Finland), 0.01 mg/kg, body weight (BW), IV, or injectable anesthesia based on the horse’s demeanor and surgeon’s preference.

An oral speculum (McPherson) was fitted and 2% lidocaine hydrochloride (Henry Schein Animal Health, Dublin, Ohio, USA) was injected into the buccal mucosa surrounding the sialolith. Malleable retractors were used to retract the buccal mucosa away from the maxillary cheek teeth. A #12 scalpel blade was used to create a linear incision through the buccal mucosa over the length of the sialolith using it as a guard to keep the blade from penetrating into deeper tissues. Sharp dissection continued through the fibrotic capsule surrounding the sialolith within the duct. Extra oral pressure over the sialolith aided in its removal into the oral cavity (Figure 1). The incision was lavaged with 0.12% chlorhexidine gluconate (Vet One Boise, Idaho, USA), and left open to heal by second intention. Mandibular duct sialoliths were removed in a similar fashion as described.

Figure 1.

Sialoliths in the parotid and mandubular salivary ducts. A — A single sialolith located in the distal parotid duct adjacent to the first maxillary molar prior to the creation of an incision over the buccal mucosa. B — Digital pressure over the sialolith extra-orally aids in its removal into the oral cavity. C — An example of a sialolith in the mandibular salivary duct at the level of the sublingual caruncle prior to its removal.

Surgical removal of sialoliths through a transcutaneous approach

All horses were treated under general anesthesia to reduce the chance of iatrogenic damage to the facial artery and vein, which course beside the proximal aspect of the parotid duct and the linguofacial vein, which lies adjacent to the parotid duct as the duct exits the parotid salivary gland. Horses were sedated with xylazine hydrochloride (Akorn, Decatur, Illinois, USA), 1.1 mg/kg BW, IV, followed by induction with diazepam (Hospira, Lake Forest, Illinois, USA), 0.05 mg/kg BW, IV, and ketamine (Bioniche Pharma, Lake Forest, Illinois, USA), 2.2 mg/kg BW, IV. Anesthesia was maintained with a constant rate infusion of triple drip composed of a combination of 500 mg xylazine hydrochloride and 1000 mg ketamine hydrochloride mixed in a 1 L bag of 5% guaifenesin (Wedgwood Pharmacy, Swedesboro, New Jersey, USA), and administered at a rate up to 2 mL/kg BW per hour.

If a draining tract was present it was surgically debrided and lavaged. Concurrent cannulation of the parotid papilla facilitated retrograde lavage of the parotid duct with saline to aid in dislodgement of the sialolith while confirming communication of the salivary duct to the draining tract. Post-operative catheterization of the duct was not maintained in these cases. Due to extensive contamination of the surgical sites and difficulty in reconstruction of the parotid duct from the chronic infection, draining tracts were left open to heal by second intention.

In cases where no draining tract was present prior to surgery, an incision was made over the sialolith with a #15 scalpel blade through the skin and subcutaneous tissues extending into the parotid salivary duct. Following retrieval of the sialolith the tissues were lavaged with saline, and primary closure of the parotid duct was facilitated by a preplaced 5-French polypropylene urinary catheter through the parotid papilla extending beyond the surgical incision to the more aborad portion of the duct. The parotid duct was re-apposed with 4-0 polydioxanone (Ethicon, Somerville, New Jersey, USA), in a simple interrupted pattern. The subcutaneous tissue was closed with a 2-0 polydioxanone in a simple continuous pattern, and the skin was closed with skin staples. The catheter was maintained in the parotid salivary duct for 7 d after surgery to maintain duct patency and to reduce salivary flow over the surgical site. A stab incision through the skin rostral to the parotid papilla was made facilitating passage of the catheter out of the oral cavity where it was secured to the skin with simple interrupted sutures of 0 polypropylene (Ethicon), to prevent premature dislodgement by the horse.

Statistical analysis

Continuous data were expressed as a mean with a range, and a median was provided for the variable duration of clinical signs due to outliers in this data set. Categorical data were presented with the number of cases identified with a corresponding percentage.

Results

A total of 25 cases of sialolithiasis were identified. Breeds included American Quarter Horses (n = 9), Thoroughbreds (n = 8), Arabians (n = 3), Warmbloods (n = 3), an Appaloosa (n = 1), and a Tennessee Walking Horse (n = 1). There were 12 mares and 13 geldings. The horses ranged from 4 to 29 years of age (mean age = 16.8 y).

Duration of clinical signs prior to presentation as reported by the owner ranged from 2 d to 7 y (mean 19 mo, median 8 mo). The parotid duct was involved in 23/25 (92%) cases, with mandibular duct involvement observed in only 2 cases. Most cases involving the parotid duct were located in the distal 18/23 (78%) versus the proximal 5/23 (22%) duct. Both cases involving the mandibular duct were observed in the oral aspect of the duct near the sublingual caruncle (Figure 1).

Findings at presentation included facial swelling 20/25 (80%), quidding 7/25 (28%), cutaneous draining track 6/25 (24%), fever 4/25 (16%), halitosis 3/25 (12%), inappetence 3/25 (12%), and facial nerve paralysis on the ipsilateral side of the sialolith 1/25 (4%). Oral examination findings were recorded in 12 horses at the time of surgery. Of these 12 horses, 8 had significant dental disease including moderate to severe wave malocclusion 7/8 (88%), diastema associated with significant periodontal disease 5/8 (63%), loose cheek teeth 2/8 (25%), and a sagittal fractured upper third premolar 1/8 (13%).

Sialadenitis was observed more frequently in cases of proximal parotid duct sialolithiasis 2/5 (40%) compared with distal parotid duct sialolithiasis 1/18 (6%). Cutaneous draining tracts were also observed more frequently in cases involving the proximal parotid duct 4/5 (80%) compared with the distal parotid duct 2/18 (11%).

Radiographs were performed in 9 cases, confirming the diagnosis in 5 cases (Figure 2). Radiographs were more likely to confirm diagnosis of sialoliths in the distal parotid duct 5/6 (83%) than in the proximal parotid duct 0/3 (0%). Ultrasound was performed in 5 cases, all of which documented hyperechoic densities consistent with a sialolith within the affected duct (Figure 3). In the 4 horses with negative radiographic findings, ultrasonography confirmed the diagnosis.

Figure 2.

Dorso-ventral radiographic views of 2 sialoliths. Sialoliths are typically identified radiographically as round to ovoid radiodense bodies adjacent to the maxillary molars. A — They may display concentric layers of mineralization surrounding a radiolucent core often associated with organic debris such as a plant awn. B — An example of multiple sialoliths (arrows) of various sizes located within the parotid salivary duct. The superimposition of the sialoliths upon each other and skull structures can make identification of all sialoliths difficult.

Figure 3.

Ultrasonographic evidence of sialolithiasis within the parotid salivary duct. A — A cross-sectional image through the salivary duct. Ultrasound probe orientation is indicated by the green dot which is rostral in this image. A hyperechoic mass consistent with a sialolith is identified with the wide arrow without tail. The facial artery is identified with the narrow arrow with tail to the left of the sialolith. B — A long axis view through the parotid salivary duct. Ultrasound probe orientation is indicated by the green dot which is dorsal in this image. The hyperechoic sialolith (arrow) is blocking salivary flow causing parotid duct distention (outlined by arrowheads).

Intra-oral sialolith removal was performed on 19 horses. Standing sedation was used in 16 cases, and general anesthesia was used in 3 cases. After surgery all horses were treated with flunixin meglumine (Merck Animal Health, Whitehouse, New Jersey, USA), 1.1 mg/kg BW, PO, q12h or phenylbutazone (Sparhawk Laboratories, Lenexa, Kansas, USA), 2.2 mg/kg BW, PO, q12h for an average of 5 d (range: 1 to 14 d), and most cases 12/19 (63%) were administered sulfamethoxazole trimethoprim (Amneal Pharmaceuticals, Hauppauge, New York, USA), 30 mg/kg BW, PO, q12h for an average of 8 d (range: 5 to 14 d). In 11 cases with follow-up data, the average healing time of the oral incision was 9 d. One horse developed a pronounced focal cellulitis 24 h after intra-oral sialolith removal with substantial packing of feed into the surgical wound. In addition to antibiotics, this horse received daily oral lavage and application of a warm compress to the region for 4 d. The cellulitis resolved within 72 h. Two horses had small cutaneous draining tracts on presentation that were not surgically debrided due to their small size. Both continued to drain small amounts of saliva at discharge from the hospital, but the drainage had discontinued by 10 d. At initial presentation, 1 of these 2 cases also had concurrent facial nerve paralysis on the ipsilateral side of the sialolith that did not improve after surgery.

Transcutaneous removal of sialoliths with primary closure was performed on 2 horses. Neither horse had cutaneous draining tracts at the time of presentation. One horse had a large sialolith in the distal parotid duct and was a candidate for transoral removal, but it was removed trancutaneously at the owner’s request due to concerns with using a speculum with their horse. The second horse had a sialolith removed from the proximal parotid duct that was inaccessible via the oral cavity. In both cases an intra-parotid catheter was kept in place for 7 d after surgery. Both horses were administered phenylbutazone (Sparhawk Laboratories), 2.2 mg/kg BW, PO, q12h for 5 d, and sulfamethoxazole trimethoprim (Amneal Pharmaceuticals), 30 mg/kg BW, PO, q12h for 10 d. None of these horses had developed any complications by the time of suture removal 14 d after surgery.

Cutaneous draining tracts involving the proximal parotid duct were observed in 4 horses. In these cases the draining tracts were explored surgically, sialoliths were identified and removed, but the surgical sites were left open to heal by second intention without maintaining a post-operative intra-parotid duct catheter. All 4 horses had persistent salivary drainage from a cutaneous fistula after surgery. One horse experienced recurrent intermittent drainage from the site for 3 mo after surgery, and was euthanized for an unrelated small intestinal colic. This horse was administered phenylbutazone (Sparhawk Laboratories), 2.2 mg/kg BW, PO, q12h for 10 d and sulfamethoxazole trimethoprim (Amneal Pharmaceuticals), 30 mg/kg BW, PO, q12h for 42 d, but these were discontinued by the owner due to financial constraints. In 2 of these cases 10% formalin was injected into the parotid gland via parotid duct cannulation causing gland involution with loss of saliva production before healing of the tract was observed. The fourth horse had slight drainage of saliva after surgery, which resolved after post-operative administration of antimicrobials and an anti-inflammatory drug. Excluding the horse that was euthanized, these 3 horses were treated with phenylbutazone (Sparhawk Laboratories), 2.2 mg/kg BW, PO, q12h for an average of 11 d (range: 10 to 14 d), sulfamethoxazole trimethoprim (Amneal Pharmaceuticals), 30 mg/kg BW, PO, q12h for an average of 56 d (range: 42 to 70 d). Average time until healing of these 3 horses was 47 d (range: 28 to 84 d).

Complications and failure of treatment included persistence of cutaneous draining tracts (short-term: 2; long term: 4), significant post-operative cellulitis in 1 horse, and prolonged facial nerve paralysis in 1 horse. Horses encountered post-operative complications more frequently if they had a sialolith in the proximal parotid duct 4/5 (80%) versus the distal parotid duct 2/18 (11%), had a cutaneous draining tract at presentation 6/6 (100%) versus not having a draining tract at presentation 1/19 (5%), or had transcutaneous surgery 4/6 (67%) compared with intra-oral surgery 3/19 (16%).

Recurrence was observed in 6 horses with sialoliths located in the distal parotid duct. All recurrence was observed in the same location and duct as the initial sialolith. Single recurrence occurred in 4 horses and multiple recurrences occurred in 2 horses. One horse had multiple sialoliths identified on radiographs at initial presentation, and again developed multiple sialoliths 5 y later. The other horse with multiple recurrences initially presented with bilateral sialoliths in the distal ducts, but on 3 subsequent recurrences developed a singular large sialolith in the left distal parotid duct at approximately 1 y intervals. Average time to recurrence was 2.8 y following the initial surgery (range: 1 to 5 y).

Discussion

Sialolithiasis may be a chronic problem in horses, with the mean duration of clinical signs prior to diagnosis being 18 mo in this study. The long duration of clinical signs prior to surgical removal may be multifactorial. Some owners did not seek veterinary attention after being made aware of the sialolith’s presence until their horse was significantly impacted by the sialolith. In some cases the horses were previously treated for abscesses associated with lymph nodes or the mandible prior to referral where the draining tracts were later identified to involve the salivary duct. Previous reports suggest sialoliths can occur over a wide age range, but are more frequently observed in older horses (4,9). This finding is supported by the current study in which the mean age of horses was over 16.8 y and the range 4 to 29 y.

Most of the sialoliths in this study were found in the distal parotid duct. This is in agreement with previous studies, and may be due to anatomic location of the parotid papilla in respect to the oral cavity and ease of diagnosis compared to sialolithiasis in other locations (4–7,9,10). Sialoliths in the proximal parotid duct presented a unique set of diagnostic and therapeutic challenges. There is limited information on the management of cases with sialoliths in the proximal parotid duct, possibly because they are misdiagnosed as abscesses or draining tracts associated with the mandible or other disease processes (3).

Sialoliths in the proximal parotid duct, in the current study, were more difficult to identify radiographically compared to those in the distal parotid duct. A previous case report was also unable to identify a sialolith radiographically in the proximal parotid duct (3). While the sensitivity and specificity of sialolith identification radiographically has not been established in horses, in humans roughly 20% of submandibular sialoliths are not radiopaque or too small for identification by radiography (11). The present study found 4/9 (44%) horses had no radiographic evidence of sialoliths. Use of open mouth or nonstandard oblique views may improve radiographic identification of sialoliths by decreasing the amount of superimposition of other structures of the skull. Sialography could also improve diagnosis of sialoliths that were not visualized with conventional radiographs, outlining radiolucent bodies within the duct or highlighting a source of obstruction (12). Regardless, this study confirmed that some sialoliths may not be readily identified radiographically in the horse.

Ultrasonography is considered the primary method for detecting salivary calculi in humans with computed tomography and magnetic resonance imaging reserved for cases with inconclusive or negative ultrasonographic findings (13). In humans the sensitivity of sialolith detection via ultrasonography is 94%, and overall diagnostic accuracy is approximately 96% (13). Ultrasonography has also been advocated for evaluation of the equid salivary ducts and glands (4,14). Sialoliths appear as curvilinear hyperechoic structures that can cast shadows, and are frequently associated with upstream dilation of the salivary duct (8). This study found ultrasonography useful in locating sialoliths in the proximal parotid duct, especially in cases with no radiographic evidence of the sialolith. It was also helpful in cases involving a cutaneous draining tract to confirm salivary duct involvement, and to differentiate between mandibular and parotid gland distention in 1 case. Based on our findings we feel that ultrasonography should be employed to evaluate and rule out sialoliths in cases with swelling or draining tracts in the mandible and throat latch.

Although oral examination findings were available for only 12 horses, it is worth noting that dental disease was documented in 8 of these horses. A recent retrospective study evaluating sialoadenitis theorized that dental pathology may contribute to sialoadenitis and sialolith development due to oral trauma and feed stagnation near the parotid papilla (4). Given the small number of cases for which data are available it is difficult to determine the role that dental disease has in sialolithiasis, but this finding warrants further examination.

A previous retrospective study found a complication rate of 50% with transcutaneous removal of sialoliths from the parotid duct, with fistula formation, incisional infection, and swelling of the surgical site observed in these cases (9). While that study did not document the use of stent catheterization of the parotid duct, other studies have advocated its use to aid in reconstruction during surgical closure and reduce complications (3,15). Maintenance of this stent after surgery may reduce the likelihood that stricture or a fistula will form by decreasing salivary flow over the surgical site (2,3). These findings are supported by our study as well, with no complications noted in either of the horses in which a stent was used after transcutaneous removal of a sialolith. Despite successful outcomes in 2 cases in this study, transcutaneous removal of sialoliths is not preferred by the authors due to concerns of fistula development. However, this may be required in some cases if the sialolith is inaccessible through the oral cavity.

In the present study primary closure was not attempted in cases with cutaneous draining tracts due to the contaminated surgical site and difficulty in apposition of the proximal and distal portions of the duct. All horses experienced delayed healing due to chronic fistulation at the surgical site. Resolution of the draining tracts of 2 horses was achieved following chemical ablation of the salivary gland with 10% formalin. Formalin was chosen as it has been suggested to cause the least amount of necrosis and suppurative inflammation compared to iodine, chlorhexidine, and silver nitrate (2). Resolution of the fistula of another horse was accomplished without surgery, but required nearly 3 mo of daily antibiotics and cleaning of the site by the owner. Spontaneous closure of salivary fistulas has been reported in the literature, but in our experience healing is unpredictable and requires a long convalescence period (1,16). To hasten resolution of a fistula, the authors recommend making an effort to reconstruct the salivary duct using an intra-duct catheter to divert salivary flow without primary closure, or ligating the duct proximal to the fistula (1,15). The latter may be less desirable as it prevents drainage of purulent salivary discharge from a concurrent sialadenitis.

Transoral approach to sialolith removal has been advocated to prevent development of external salivary fistula (2,9). Minimal complications were observed in the current study with only 1 horse developing post-operative local cellulitis. It is possible that the development of a localized cellulitis is more common than reported in this study, as most may remain subclinical. In addition, 2 horses with transoral removal of distal parotid duct sialoliths had small cutaneous draining tracts and concurrent regional cellulitis at presentation. Following surgery both horses experienced continued drainage in the immediate post-operative period, but unlike the cases of proximal duct sialolithiasis, these horses healed quickly without additional intervention. The faster resolution of these draining tracts was attributed to their initial smaller size and diversion of salivary flow into the oral cavity through the transoral incision. In addition, 1 horse had facial nerve paralysis on the ipsilateral side of the sialolith, cellulitis, and draining tract at initial presentation. Despite resolution of the draining tract and cellulitis the facial nerve paralysis persisted. A full workup of the facial nerve paralysis was not pursued by the owner, but the paralysis was speculated to be secondary to pressure necrosis from the sialolith and infection in the surrounding tissue from the ruptured parotid duct.

Recurrence of sialolithiasis has not been reported in horses. Recurrence rates in humans utilizing transoral removal of sialoliths range from 10% to 18% compared to 24% (6/25) in our study (17,18). In humans, recurrence is most commonly attributed to lack of identification and removal of all sialoliths at the time of surgery (18). Ultrasonography plays an important role in reducing recurrence of human sialolithiasis by identifying radiolucent sialoliths or sialolith fragments that could act as a future nidus (18). Ultrasonography was not performed in any of the horses that experienced recurrence, and failure to identify and remove all sialoths at the time of surgery may have contributed to recurrence. Additional causes of recurrence documented in humans are duct stricture at the surgical site and salivary composition (19). It is interesting that recurrence developed on the same side as was initially affected in all cases, making scar tissue and incomplete initial removal of the sialolith possible factors contributing to recurrence. Salivary composition was not evaluated in these cases, and warrants further investigation.

Based on our findings both radiography and ultrasonography are important in the diagnosis of sialolithiasis, and ultrasonography may help identify sialoliths not visible on radiographs. A longer morbidity should be expected in cases involving the proximal duct, especially when a cutaneous draining tract is present prior to surgery. Owners should be aware that recurrence of sialolithiasis is possible, occurring in 24% of cases in this study.