Endoscopic Balloon Dilatation of the Eustachian Tube via the Soft Palate Approach in Miniature Pigs

ET, connecting the nasopharynx and the middle ear, plays a crucial role in regulating middle ear pressure and is implicated in the pathogenesis of otitis media²⁵. To comprehensively study the function of ET, researchers have expanded beyond small-sized animal models to other models such as rodents or rhesus monkeys. Currently, multiple studies have used sheep or miniature pigs. Through CT scans and three-dimensional structures, Miller et al. confirmed that the anatomy of the eustachian tube in Blackfaced Sheep is very similar to that in humans²². Pohl et al. verified the feasibility of treating dysfunction of ET by implanting a stent into the sheep's ET²¹.

Miniature pigs, characterized by small size, early sexual maturity, rapid reproduction, and ease of management, have become increasingly popular as large animal models in medical research²⁴. Previous studies from our team indicated that miniature pigs serve as excellent models for the study of the middle ear and inner ear^30,31,32, given their morphological and functional similarities to humans. Remarkable achievements have been made in investigating the morphology, electrophysiological characteristics, development of the inner ear^33,34, gene expression in the auditory system³⁵, gene regulation^36,37,38, and gene transformation^39,40. Simultaneously, in-depth research has been conducted in the realm of ET function. Therefore, to investigate ET structure and function more effectively, we explored the novel surgical approach using miniature pigs.

In the investigation of surgical approaches, we initially employed the transnasal approach²³, which is a conventional approach for all large animal surgeries involving ET²⁵. Subsequently, we attempted the hard palate and soft palate approaches, and the soft palate approach was finally selected. The advantages and disadvantages of each approach are as follows:

Transnasal approach
The transnasal approach is a conventional approach for clinical examinations and surgeries involving the ET. Due to the relatively short yet spacious human nasal cavity and the large space, adrenaline-soaked cotton can be used to constrict the nasal mucosa, facilitating the passage of an endoscope to the nasopharynx. The ample space allows for comfortable manipulation, making the surgery relatively simple. Naturally, when performing surgery on large experimental animals, this approach is preferred.

The advantages of this routine approach are that the anatomical structure is relatively simple, and the use of nasal endoscopy allows for clear visualization, making it a familiar choice for operators. The disadvantages of this approach are first that the nasal cavity in miniature pigs is elongated and narrow, which can be seen from the CT results of our previous research (Figure 8A)²³, restricting the mobility of instruments after deep insertion. Maneuvering becomes challenging, particularly when a rotation or angulated movement is required, making it notably difficult when attempting to reach outward and upward into the pharyngeal opening of ET.

Second, bleeding from the nasal mucosa may occur, leading to endoscopic lens contamination and an unclear view of the surgical field (Figure 8B). This issue significantly affects the surgery, and much time is spent during the process of inserting the endoscope into the nasal cavity, cleaning the lens after lens contamination, and then reinserting the endoscope. Additionally, prolonged surgery leads to congestion and swelling of the nasopharyngeal mucosa, exacerbating nasal cavity narrowing and increasing blood loss. Transnasal surgeries often take 3 h or longer; during this period, the nasal cavity is in a state of continuous bleeding, resulting in low surgical efficiency. Prolonged procedures may lead to operator fatigue and increased damage to the nasal cavity, nasopharynx, and pharyngeal opening of ET. In ET experiments, changes in pharyngeal opening morphology and function are critical points for observation. Excessive surgical damage to this area can impact observation results and, consequently, affect experimental conclusions, which should be avoided as far as possible during the operation.

Hard palate approach
The advantage of this approach is that the incision is positioned closer to the front during surgical procedures, allowing for a larger oral space. Instrument deployment is convenient, rendering the surgery relatively simple. The palatal mucosa is thin, and no instruments are needed.

The disadvantages of this approach are that the surgical steps are complex, and equipment such as a powered system drill is required to open the hard palate bone. This complexity hinders the widespread application of the approach. Most importantly, postoperative healing is often poor, leading to the formation of a hard palatal fistula, which may adversely affect ET function and even the nutritional status of the pigs. Due to these complications, this approach was discarded.

Soft palate approach
The advantages of this approach include clear anatomical layers with recognizable landmarks. The surgical incision is small, only 5-7 mm, and can be fully exposed for sufficient hemostasis so that blood loss is limited. The surgical procedures are simple, and after opening the soft palate, the nasal cavity from the level of the junction of the soft and hard palate to the level of the throat can be observed using a 0° endoscope, the pharyngeal opening of ET is directly exposed. The surgery takes approximately 20 min, and skilled personnel can complete it in just over 10 min, increasing surgical efficiency. Postoperative recovery is fast, and the wound heals within 1 week.

The disadvantages of this approach are that the incision position during surgery is posterior, resulting in a smaller oral space and making the operation more challenging. However, these challenges can be overcome. This approach involves cutting a portion of the levator veli palatini (LVP) muscle of the soft palate, which may affect the function of the ET.

To exclude the impact of muscle injury on the function of the eustachian tube, we conducted related studies. First, we severed the hamulus of the pterygoid process to weaken the contractile ability of the tensor veli palatini (TVP). Postoperative CT scans were conducted monthly, and no otitis media was formed at the end of 3 months, confirming that the eustachian tube function was not affected (Figure 9A and Figure 9D). Second, we performed CT scans on the miniature pigs 3 months after making the incision on the soft palate, and the results also showed no formation of otitis media, confirming that the eustachian tube function was not affected (Figure 9B and Figure 9E). However, eustachian tube cauterization can damage the function of the eustachian tube, leading to the formation of otitis media (Figure 9C and Figure 9F). The comparison of postoperative CT scans of the three pigs showed that the soft palate surgical pathway did not lead to the formation of otitis media in miniature pigs, indicating that this surgical method did not affect the function of the eustachian tube. Moreover, the length of the incision is only 5-7 mm, while the length of the soft palate exceeds 5 cm, providing an anatomical basis for not affecting the function of the ET (Figure 8A). We also sutured the incisions promptly, with the incision healing 1 week postoperatively. These findings, both from the anatomical structure and postoperative CT scans, confirm that the surgical incision did not affect the function of the ET. This may be due to the different effects of the pig's TVP and LVP on the ET compared to humans. Additionally, the miniature pigs were raised for four weeks after the surgery to observe the recovery of the eustachian tube. During this period, no complications such as difficulty in eating or breathing occurred. Therefore, the soft palate approach is safe and effective.

Key points of the surgical procedure for the soft palate approach include infection prevention and critical anatomical landmarks. For infection prevention, the primary focus is preventing the formation of a soft palate fistula. Once it occurs, it may severely affect the pig's water intake and nutritional status, and compromise ET function. It is imperative to follow strict aseptic procedures, ensuring thorough disinfection of the surgical area, which includes the entire upper palate mucosa, both sides of the gums and dental arches, portions of the tongue surface, and the skin surrounding the oral cavity. Additionally, antibiotics are prescribed during and after surgery, with daily oral antibiotics intake for one week postoperatively.

Critical anatomical landmarks are the posterior edge of the hard palate and the median suture of the soft palate. The posterior edge of the hard palate is located at the junction of the soft and hard palate and is palpable with the forefinger. Identification of this landmark is crucial to avoid cutting forward and risking damage to the hard palate bone, making it difficult to incise, or making it impossible to gain access to the nasopharynx after incision. Cutting backward may cause excessive damage to the soft palate, influencing the localization of the pharyngeal opening of ET. For the median suture of the soft palate, the incision should be approximately 2-3 mm to the surgical side of the median suture of the soft palate. Cutting too close to the median suture may inadvertently enter the nasal septum submucosa, hindering access to the pharyngeal opening of ET. If the incision is too far from the median suture, there is a risk of proximity to the lateral aspect of the nasopharynx, potentially damaging the pharyngeal opening of ET. In extreme cases, it may even enter the submucosa of the lateral wall, making it difficult to locate the pharyngeal opening of ET and causing potential damage to the surrounding tissues, leading to deep tissue infection postoperatively.

Fifty-three miniature pigs were involved in this research dedicated to the study of eustachian tube function. In addition to the soft palate pathway as a surgical method, we also studied the impact of balloon dilation on the structure and function of the eustachian tube and the mechanism of injury and repair process of the eustachian tube mucosa. Subsequent studies included dilation with balloons of different diameters and the histological characteristics of the eustachian tube mucosa at various time points after dilation. Fifty miniature pigs were successfully operated on for the experimental research described above.

Our surgical approach failed in 3 cases. These results could be caused by multiple factors, including some of the following. First, the location of the incision was not accurate enough, leaning towards the nasal septum side so that the position of the pharyngeal opening of the eustachian tube could not be observed effectively through the original incision. To complete the surgery, the incision was expanded, resulting in an unhealed wound. Second, the artery was injured during the cutting of the soft palate, surgical time was prolonged due to repeated hemostasis, and the residual blood stains adhered to the nasal cavity, serving as a culture medium for accelerating bacterial growth, which led to wound infection and affected healing. Third, the sutured depth was insufficient, and food friction during eating caused the suture to fall off in an untimely manner, resulting in the wound not healing. Although non-healing was presented in particular cases, the surgical success rate was still at a high level.

The limitation of this study is that we have not performed a statistical analysis on blood loss and surgical time between the transnasal and soft palatal pathways. We initially used the transnasal approach but found the operation quite challenging. The endoscope needed to be repeatedly removed to clean the blood, further aggravating the damage to the nasal mucosa and making the operation difficult. It took over 3 h before the balloon dilation was completed. Despite being able to complete the procedure, we did not continue to use this surgical method owing to the animal experiment ethics. Due to the limited number of early transnasal procedures, we just made preliminary calculations of the surgical time. However, it was precisely the disadvantages of long surgical time and excessive bleeding that inspired us to consider other pathways to avoid these issues and achieve the goal of Eustachian tube balloon dilation.

In conclusion, the surgical approach described in this study is highly suitable for performing ET procedures in large animals such as miniature pigs, sheep, and dogs. The technique is characterized by its simplicity, easy learning, short surgical time, high efficiency, minimal tissue damage, and rapid healing. Direct visualization of the pharyngeal opening of ET is possible during surgery, and the procedure has no adverse effects on the normal function of ET. This approach facilitates various interventions at the pharyngeal opening, including the balloon dilation mentioned in this study, and is equally applicable to other procedures such as ET stent placement in other literature.