Transnasal endoscopic marsupialization of nasopalatine duct cysts: A novel ‘Mickey Mouse’ sign and a septum-sparing surgical technique to reduce paresthesia risk

Abstract A nasopalatine duct cyst (NPDC) is the most common type of non-odontogenic cyst in the maxillary region and oral cavity. Recently, a novel transnasal endoscopic approach has been described to allow marsupialization of NPDCs, with good outcomes. However, one common complication appears to be paresthesia of the palate around the upper incisors, which is thought to occur due to damage to the nasopalatine nerve following the septal incision. In this case report, we introduce a variant of this endoscopic technique that attempts to minimize damage to this nerve by adopting a septum-sparing flap. We report no complications, including no paresthesia, at the 3-month follow-up. We also report a novel ‘Mickey Mouse’ sign on MRI imaging associated with this NPDC.


Introduction
A nasopalatine duct cyst (NPDC) is the most common type of non-odontogenic cyst of the maxillary region, occurring in 1% of the population [1].During development, the nasopalatine duct connects the floor of the nasal cavity to the incisive papilla of the hard palate, leading to direct communication between the oral and nasal cavities [2].This duct typically obliterates in utero.However, incomplete regression can lead to cyst formation later in life from epithelial remnants along the duct line.This usually occurs during the fourth to sixth decades of life [3].While the trigger for cyst formation is largely unknown, possible causes include trauma and infection [1].
NPDCs are often asymptomatic but surgical excision can be indicated to treat clinical symptoms such as swelling, drainage and pain.The most common surgical technique used is complete enucleation via open surgery through a sublabial or palatine approach [4].An alternative technique has recently been described that involves trans-nasal marsupialization via an endoscopic approach.While offering advantages such as shorter operation times and reduced incidence of complications such as fistula formation, it has been associated with paresthesia of the palatal areas.This is thought to be due to damage to the nasopalatine nerve [5,6].Here we describe our findings and results from treating a patient with this new technique.We describe a septum-sparing flap that aims to reduce paresthesia risk by minimizing damage to the nasopalatine nerve.

Case presentation
A 34-year-old male presented with a 12-month history of sharp bilateral pain in his incisors.There was no prior history of trauma, no dental disease, and no nasal obstruction or discharge.There was no family history of any otolaryngological or oromaxillofacial issues.The patient had sought multiple opinions from dental and oral surgical specialists, but no underlying cause was found; it had been suggested that the teeth could be devitalized to relieve the pain.The patient noted that pressure on the nose exacerbated the pain, prompting him to seek review with the Ear, Nose and Throat (ENT) medical team.
Endoscopic examination demonstrated bilateral medially based cystic swellings in the floor of the nose.There was no visible swelling of the gums, sublabially or of the palate.An orthopantomogram performed at another hospital did not report any abnormalities.Magnetic Resonance Imaging (MRI) was then performed at our center which demonstrated a cyst with bilobed extension on either side of the septum into the nasal cavity (Figure 1(A and B)).
The patient opted to undergo transnasal endoscopic marsupialization of the cyst and the operative steps are noted below.The patient was positioned as per the standard transnasal endoscopic surgery in the reverse Trendelenburg position.Moffett's solution, consisting of cocaine, adrenaline, and bicarbonate, was pre-applied into the nasal cavity immediately after induction of anaesthesia in order to decongest and vasoconstrict the operative field.Endoscopic visualization using a 0-degree 4 mm rigid Hopkins rod enabled a magnified, unrestricted view of the cyst within the nasal cavity (Figure 2(A and  B)).The cyst was decompressed from the left nasal cavity only as follows.After injection and hydro-dissection with Lignospan onto the nasal floor, a mucosal flap was elevated at the anterolateral aspect of the cyst and extended posteriorly along the nasal floor with a size 15 blade (Figure 2(C)).The cyst was decompressed, and the thick serous contents were suctioned out.With decompression, a clear plane was developed between the mucosal flap and the cyst wall, which was removed (Figure 2(D)).Curettage was performed on the remaining cyst lining.The mucosal flap was then replaced into the medial aspect of the cavity and Bismuth Iodoform Paraffin Paste (BIPP) ribbon was applied to help maintain the rotation of the flap into the cavity (Figure 2(E)).The right nasal cavity view after cyst decompression from the contralateral side is shown in (Figure 2(F)).The histological report confirmed a nasopalatine duct cyst with no evidence of malignancy.
The patient returned to the outpatient clinic 10 days post-operatively where the BIPP ribbon was removed to reveal the mucosa intact and healed with no immediate post-operative complications.The patient reported lasting improvement in his symptoms with no evidence of recurrence 3 months postoperatively.The endoscopic view at this time is shown in Figure 3 and illustrates the wide opening into the well-healed cyst cavity containing no debris.The patient was completely symptom-free and was discharged from routine follow-up 3 months post-operation.

Discussion
Nasopalatine duct cysts (NPDCs) are relatively well-known in the fields of oral surgery and dentistry.However, awareness and published reports in the field of otolaryngology is limited.NPDCs are usually asymptomatic but can present with palatal swelling, drainage, or oral pain.The feature of nasal pain can drive a patient to seek ENT review, as in this case, where the patient's pain was likely exacerbated by focal compression of the cyst on the nasopalatine nerve.Examination findings depend on the location of the cyst along the duct line.NPDCs that occur distally near the palate may present with palatal swelling.Alternatively, NPDCs forming more proximally may present as a swelling along the nasal cavity floor which is usually only visible with anterior rhinoscopy or endoscopic examination.

Imaging -a novel 'Mickey Mouse' sign
Cross-sectional imaging in the form of CT and/or MRI is typically used to further characterize cysts in this area.Differentials of midline cysts include NPDCs, granulomas, central incisor root cysts, and median palatine cysts [7].Nasolabial cysts usually appear laterally on either side of the midline [8].NPDCs are usually well-defined oval, round, or heart-shaped cystic lesions close to the midline of the anterior maxilla [9].In this patient's case, the appearance of the cyst on the coronal section MRI produced a characteristic shape which can be referred to as the "Mickey Mouse" sign [10] as the cyst expands on either side of the maxillary crest (Figure 1(B)).This is the first such description using this easy-to-remember term and highlights possible alternative radiological appearances of NPDCs.and will help in radiological differentiation between other pathologies at this site presenting in a similar way.For instance, to our knowledge, nasolabial cysts, which usually form in the soft tissues around the upper lip, have not been reported with this sign.Nasolabial cysts usually appear ovoid on either side of the midline and are unilateral [11].As they are soft tissue cysts [8], a lack of sharp bony structures in this area similar to the maxillary crest should preclude asymmetric expansion of a nasolabial cyst that could result in a bilobed appearance.

A septum-sparing surgical technique
The most common method of surgical removal is complete enucleation of the cyst via open surgery through a sublabial or palatine approach [4].For larger cysts, marsupialization is sometimes performed through the same approach (i.e.: palatine or sublabial) to reduce the size of the cavity to facilitate enucleation at a later date.Large retrospective series (>50 patients) have reported recurrence rates with surgical excision ranging from 2% to 11% [3,12].Complications reported with these approaches include paresthesia of the palate due to damage to the sphenopalatine nerve (<10%), and the formation of persistent oronasal fistulas (<3%) [4], as well as periods of disruption to normal oral intake and associated discomfort.
Recently, a new approach has been described to treat NPDCs, which involves endoscopic transnasal marsupialization of the cyst [5,6].This technique is less invasive than the transoral route, can be performed quickly as a day case and with shorter and less morbid recovery.These initial reports recorded no recurrence or fistula formation.However, temporary paresthesia around the upper incisors, improving over 3-6 months, has been recorded.In these cases, this is thought to be due to raising flaps in the midline along the septum, which damages the nasopalatine nerve [5,6].
The nasopalatine nerve runs through the nasal septum and courses through the mucoperichondrial, sloping downwards into the incisive canals to supply the hard palate, including the area adjacent to the upper incisors [13].Damage to this nerve can lead to paresthesia or pain in this area.However, there may still be sensations due to dual supply from the anterior palatine nerve [14].
The paresthesia rate around the upper incisors and palatine area following conventional septoplasty is estimated to be around 0.25% [15].In the presence of a medially based cystic mass, such as the NPDC described, the nerve will be lifted into the surgical field, increasing the risk of injury if making a septal incision.In this case, recognizing that the large cystic swelling likely displaced the nerve into the surgical field, a septal incision was avoided, and the incision was placed lateral to the cyst in the floor of the nose.A flap was then elevated towards the midline, aiming to preserve the nerve.This patient has not developed any complications from surgery.
Since NPDCs lie within the foramen of the nasopalatine nerve, large cysts may cause displacement of the nerve away from the midline and therefore the nerve may still be encountered during dissection via a medially based mucosal flap with a lateral incision along the nasal floor, as in this case.Endoscopic assistance mitigates this by equipping the surgeon with a magnified view so that the neurovascular bundle can be easily identified and avoided if necessary.

Strengths and limitations
Due to the relative rarity of NPDCs presenting to a hospital setting, it is challenging for a single center to amass enough cases to draw firm conclusions on outcomes of different surgical techniques.This case report adds to the sparse literature on transnasal endoscopic marsupialization of NPDCs and aims to allow the pooling of outcomes through a systematic review in the future.It also introduces a modified surgical technique avoiding a midline approach that aims to spare the nasopalatine nerve.However, further data are necessary to estimate true post-operative palatal paresthesia rates with this technique.While most NPDCs appear ovoid, this case report also highlights a novel radiological imaging sign -the 'Mickey Mouse' bi-lobed appearance -that can be associated with NPDCs and which may aid in the differentiation of NPDc and other similar pathology at this site, such as nasolabial cysts.

Conclusion
Otolaryngologists should consider NPDCs as a differential cause for those presenting with palatal and midline nasal pain.NPDCs typically appear as ovoid midline cysts in the anterior maxilla with radiological imaging.However, this report demonstrates that NPDCs can also appear bi-lobed on coronal section resembling a 'Mickey-Mouse' shape.Endoscopic transnasal marsupialization of NPDCs offers a less invasive surgical alternative for NPDCs close to the nasal cavity with favorable recovery rates.A laterally based septum-sparing flap along the nasal floor aims to reduce the risk of injury to the nasopalatine nerve.The report introduces the rationale for this technique to ENT and maxillofacial surgeons.More studies reporting outcomes with a septum-sparing flap will be required to see if paresthesia rates are indeed reduced with this technique.

Figure 1 .
Figure 1.(A and B) Mri imaging of the patient highlighting a high-intensity signal in the maxillary region on the t2 sagittal view (A) that has a bi-lobed 'Mickey Mouse' appearance on a coronal view (B).

Figure 2 .
Figure 2. endoscopic views during surgery.(A) endoscopic view of the left nasal cavity showing the npDc.(B) endoscopic view of the right nasal cavity showing the npDc.(c) A mucosal flap was elevated at the anterolateral aspect of the cyst and extended posteriorly along the nasal floor with a size 15 blade.this was performed unilaterally in the left nasal cavity.(D) A clear plane was developed between the mucosal flap and the cyst wall, which was removed.(e).A Bipp ribbon was applied to maintain the rotation of the flap into the cavity.(F) right nasal cavity view after cyst decompression from the contralateral side.npDc: nasopalatine Duct cyst; Bipp: Bismuth iodoform paraffin paste.

Figure 3 .
Figure 3. endoscopic view of the left nasal cavity at 3 months post-surgery showing the wide opening into the well-healed cyst cavity.