Pharyngeal neuromuscular dysfunction associated with bilateral guttural pouch tympany in a foal

Abstract

A 2-month-old warmblood filly was presented for a 1-week history of a large, nonpainful, fluctuant swelling of the parotid and laryngeal area. Bilateral guttural pouch tympany was diagnosed. Surgical correction resolved the guttural pouch tympany; however, postoperative pharyngeal neuromuscular dysfunction developed.

A 2-month-old, warmblood filly with a 1-week history of fluctuant, nonpainful bilateral swelling of the parotid and laryngeal region was referred for surgical consultation. Physical examination confirmed that the foal was bright, alert, and responsive. Temperature, heart rate, and respiratory rate were within normal ranges. Auscultation of the heart and respiratory and gastrointestinal systems identified no abnormalities. Mucous membranes were pink and capillary refill time < 2 s. The foal was of average body condition and weighed 100 kg. The filly was observed to have no difficulty nursing from the mare and no nasal discharge was evident. No abnormalities were detected on neurological examination. A large, nonpainful, resonant, fluctu-ant swelling of the parotid and laryngeal area was evident. The swelling was slightly more prominent on the right side.

The foal was tranquilized with xylazine (Rompun; Bayer, Toronto, Ontario), 0.5 mg/kg bodyweight (BW), and diaz-epam (Diazepam; Sabex, Boucherville, Quebec), 0.05 mg/kg BW, administered IV. The mare was sedated with aceproma-zine (Atravet; Ayerst, Mississauga, Ontario), 0.01 mg/kg BW, IV, detomidine (Dormosedan; Pfizer, Kirkland, Quebec), 0.01 mg/kg BW, IV, and butorphanol (Torbugesic; Ayerst, Mississauga, Ontario), 0.01 mg/kg BW, IV.

The foal’s pharynx, trachea, and both guttural pouches were examined with a videoendoscope. The dorsal wall of the pharynx appeared to be compressed into the lumen with mild asymmetry to the right side. The mucus membranes were grossly normal, with mild lymphoid hyperplasia present in the walls. Eustachian tubes appeared normal and no redundant plica could be appreciated when viewed from the pharynx. The trachea did not show any abnormalities. The left guttural pouch appeared to be normal but was slightly distended, suggestive of tympany. The right guttural pouch was obviously distended. The eustachian tube appeared mildly inflamed. A urinary catheter was placed via the ventral meatus of the right nostril and guided with the aid of a videoscope into the right guttural pouch to relieve the tympany. The foal was observed overnight and administered ceftiofur (Excenel; Pharmacia Animal Health, Orangeville, Ontario), 2.2 mg/kg BW, IM, q12h, vedaprofen (Quadrisol; Intervet, Whitby, Ontario), 1 mg/kg BW, PO, q24h, and raniti-dine (Gen-Ranitidine; Genpharmacia, Darmstadt, Germany), 6.6 mg/kg BW, PO, q12h.

The foal remained afebrile, bright, alert, and responsive 24 h later; however, the tympany had recurred. The foal was sedated, using the same protocol as before, and an endoscope was passed into both guttural pouches. Their appearance was consistent with previous findings. The right guttural pouch was flushed with sodium penicillin (Penicillin G Sodium; Novopharm, Toronto, Ontario), 1 × 10⁶ IU diluted in 100 mL 0.9% sodium chloride (NaCl) solution (0.9% NaCl; Baxter, Mississauga, Ontario). The left guttural pouch was flushed with 100 mL 0.9% NaCl solution (0.9% NaCl; Baxter, Mississauga, Ontario). Both pouches were passively drained. A urinary catheter was passed into the right guttural pouch as before and the tympany was relieved.

On day 3, the foal continued to have signs of predominately right-sided guttural pouch tympany. The foal and mare were sedated as before. The right guttural pouch was flushed via endoscope with sodium penicillin (Penicillin G Sodium; Novopharm, Toronto, Ontario), 1 × 10⁶ IU diluted in 200 mL 0.9% NaCl solution (0.9% NaCl; Baxter, Mississauga, Ontario). A coil tipped urinary catheter was passed into the right pouch and sutured in place at the nares. The tympanic pouch deflated. The foal was discharged with instructions to administer ceftiofur (Excenel; Pharmacia Animal Health), 2.2 mg/kg BW, IM, q12h, vedaprofen (Quadrisol; Intervet), 1 mg/kg BW, PO, q24h, and ranitidine (Gen-Ranitidine; Genpharmacia), 6.6 mg/kg BW, PO, q12h for 7 d.

The tympany did not improve. Five days after catheter placement, the foal was reevaluated via videoendoscopy. Sedation was completed as before. Both the right and left guttural pouches were distended and compressing the caudal part of the pharynx symmetrically. Catheterization resolved the tympany in both pouches. Bilateral guttural pouch tympany was diagnosed and surgical correction was recommended.

The mare was sedated with acepromazine (Atravet; Ayerst), 0.01 mg/kg BW, detomidine (Dormosedan; Pfizer), 0.01 mg/kg BW, and butorphanol (Torbugesic; Ayerst), 0.01 mg/kg BW, administered IV, and separated from the foal. Anesthesia was induced in the foal with xylazine (Rompun; Bayer), 0.5 mg/kg BW, IV, diazepam (Valium; Sabex, Boucherville), 0.1 mg/kg BW, IV, and ketamine (Ketaset; Wyeth Animal Health, Markham, Ontario), 3 mg/kg BW, IV, and maintained with halothane (Fluothane; AstraZeneca, Mississauga, Ontario) in a circle system with 4 L/min of oxygen.

The videoendoscope was placed in the left guttural pouch via the left nostril and the foal was rolled into dorsal recumbency. The right guttural pouch was entered through a modified Whitehouse approach (5). The flexible videoendoscope elevated and transilluminated the median septum, which was grasped in order to excise a 2.5- to 3-cm diameter segment, using Metzenbaum scissors. Hemorrhage was minimal. Air moved freely between the pouches. The medial aspect of the plica salpingopharyngeus was grasped in order to excise a triangular wedge of tissue 3 cm long by 2 cm wide. The cartilaginous flap of the medial lamina of the eustachian tube was identified with digital palpation, grasped with forceps in order to resect a 1- × 2-cm strip of the lamina with Metzenbaum scissors to further enlarge the pharyngeal orifice. Hemorrhage was minimal and the incision was left to heal by second intention. No gross abnormalities were observed to explain the development of the bilateral tympany in this foal. The foal recovered from anesthesia and returned to the mare.

Postoperatively, the foal was observed to have milk dripping from its right nostril after nursing. The foal’s vital signs were all within normal limits. The filly was given ceftiofur (Excenel; Pharmacia Animal Health), 2.2 mg/kg BW, IM, q12h, vedaprofen (Quadrisol; Intervet), 1 mg/kg BW, PO, q12h, and ranitidine (Gen-Ranitidine; Genpharmacia), 6.6 mg/kg BW, PO, q12h. The incision was hot packed with warm moist towels for 10 min twice daily and petroleum jelly was applied to the surrounding skin daily to prevent scalding. Milk drained from both nostrils and a green discharge from both nostrils was noted after 48 h. A cranial neurological examination confirmed pharyngeal paresis, indicating glossopharyngeal and hypoglossal muscle dysfunction. The tongue was flaccid and unable to resist manipulation. The foal was unable to swallow a bolus of food correctly and regurgitated milk and food from both nostrils.

The bilateral tympany was resolved; however, the foal was depressed but continued to nurse from the mare. The packed cell volume was 0.33 L/L and total protein was 62 g/L and considered normal 6 d postsurgery. The foal was bright, alert, and responsive. The ceftiofur was discontinued. The foal was discharged with instructions to administer vedaprofen (Quadrisol; Intervet), 1 mg/kg BW, PO, q12h and ranitidine (Gen-Ranitidine; Genpharmacia), 6.6 mg/kg BW, PO, q12h for 10 d. The pharyngeal paralysis did not resolve and was present 11 mo later on follow-up.

Discussion

Guttural pouch tympany most commonly manifests in foals 2 to 4 mo of age (1). The tympany is most commonly unilateral. Fillies are more often affected than colts with the ratio between 4:1 to 2:1 (2) and it manifests in several breeds: standardbred trotters, Thoroughbreds, Arabians, quarterhorses, Appaloosas, paint horses, American saddle horses, and warmbloods (2,3). A genetic predisposition for guttural pouch tympany in the Arabian horse has been reported (2).

The etiology of guttural pouch tympany is unknown. It is thought to be a defect of the plica salpingopharyngeus, pharyngeal orifice, or both, in which redundant mucosal tissue flaps act as a one-way valve into the guttural pouch. As air passes through the eustachian tube into the guttural pouch it is trapped by this valve and cannot escape, resulting tympany (4,5). The clinical signs include a nonpainful, resonant swelling of Viborg’s region, respiratory stertor, cough, and, when advanced, dysphagia. Because the distention results in compression of the caudal part of the pharynx, aspiration pneumonia must be ruled out. The differential diagnoses are guttural pouch empyema and chondroids.

The diagnosis is confirmed with clinical signs, and endoscopic and radiographic examinations. Radiographs reveal enlarged air-filled pouches without fluid lines. It is important to differentiate unilateral from bilateral involvement. Since the right and left pouches are separated by a noncommunicating septum, a foal with bilateral distention usually has a defect of both pharyngeal orifices.

Conservative management consists of antibiotics, anti-inflammatories, and decompression via catheterization of the affected guttural pouches. The tympany in this filly responded well initially to the indwelling catheterization but failed to resolve. The factor that may most affect the failure of conservative management is length of the treatment. It is difficult to determine that if given more time, this foal’s tympany may have resolved. There are currently no reports of the effects of growth and maturity on clinical and performance outcomes in foals with guttural pouch tympany.

Surgical intervention is the treatment of choice for correction of guttural pouch tympany, both unilateral and bilateral (4). The recommended correction of unilateral tympany is fenestration of the median septum. Fenestration of the median septum allows the trapped air in the defective pouch to exit through the normal contralateral pharyngeal orifice. Prognosis for surgical correction of unilateral tympany is good (4–6). Recurrence rates range from 30% to 33% (3,6). The recommended correction for bilateral tympany is fenestration of the median septum and unilateral excision of the plica salpingopharyngeus. This allows the trapped air to communicate between the 2 pouches and exit through the modified pharyngeal orifice. Prognosis for surgical correction of bilateral guttural pouch tympany is guarded (4,5). The recurrence rates range from 15.9% to 29% (3,6). Complications of this surgery involve iatrogenic injury to the cranial nerves that course through the guttural pouch. Clinical signs of neurological injury will include dysphagia, coughing, and aspiration pneumonia.

This foal developed a postoperative pharyngeal neuromuscular dysfunction, due mainly to the glossopharyngeal and hypoglossal nerve damage. However, damage to the accessory and pharyngeal branch of the vagus nerve cannot be ruled out. This filly developed severe dysphagia, which was treated by tube feeding. The foal had not improved over 11 mo, was smaller than her cohorts, and had a body condition score of 2/9. Aspiration pneumonia had not developed; however, the prognosis was grave and the filly was euthanized.

There are 4 common surgical approaches to the guttural pouch: Viborg’s triangle, Whitehouse, modified Whitehouse, and hyovertebrotomy. All 4 approaches are complicated by the cranial nerves in the pouches. The modified Whitehouse approach, although controversial, is usually the most appropriate (1,4,5). The advantages of this approach include access to the median septum and pharyngeal orifice, access to the roof of the guttural pouch, and superior ventral drainage. The glos-sopharyngeal, hypoglossal, and pharyngeal branch of the vagus nerves all course adjacent to the medial compartment of the guttural pouch directly within the surgical field of the modified Whitehouse and other approaches.

Alternative approaches have been reported (1,4,5,7,8). Minimally invasive techniques involve endoscopic surgery to trim the plica salpingopharyngeus via cannula placement transcutaneously into the pouch through Viborg’s triangle and the use of minimally invasive endoscopic procedures (4). Disadvantages include the inherent risk of neurological injury and lack of direct visualization. Another minimally invasive approach is the use of transendoscopic contact and noncontact Nd:YAG lasers to fenestrate the median septum for correction of unilateral tympany and to create a salpingopharyngeal fistula or create a fistula between the dorsal pharyngeal recess and the guttural pouch for both unilateral and bilateral tympany (4,7). The advantages of these techniques include standing sedation and topical anesthetic, direct visualization, and a decreased risk of damage to nerves and vessels. Disadvantages of the laser include expense/availability of laser equipment, postoperative necrosis, and the production of smoke, which obstructs the view of the surgical field (4,7,8).