Detecting Nasopalatine Duct Cysts

The nasopalatine duct cyst (NPDC), also known as the incisive canal cyst, is the most common nonodontogenic cyst of the oral cavity.¹ First described by Meyer in 1914,² these developmental cysts typically occur in the midline of the anterior maxilla near the incisive foramen. While the presentation may appear to be characteristic, other benign conditions can mimic the NPDC. An overview of the NPDC will allow oral health professionals to become familiar with the characteristics of this cyst.

Epidemiology and Etiology

The NPDC comprises 12% of jaw cysts with a slightly higher frequency in men. Most present in the fourth through sixth decades with an average age of 46.2. No racial predilection has been identified.³

The etiology of NPDC is unknown. Most cases appear to be idiopathic.^4,5 Possible causes may include trauma, infections, ill-fitting dentures, and retention phenomenon of blocked mucous glands.⁶ Spontaneous cystic degeneration of epithelial remnants has been suggested as a likely etiology.^4,7 More recently, cystic development after placement of a dental implant has been reported.⁸

Pathophysiology 

The origin of the NPDC is believed to arise from embryonic epithelial remnants of the nasopalatine duct.⁹ This duct plays an important role in fetal development as it is the connection between the nasal cavity and the premaxilla. As the fetus continues to develop, this connection narrows as the anterior bones of the palate fuse. This results in the formation of the incisive canal that houses nerves and blood vessels.⁹ The nasopalatine canals usually fuse together prior to exiting the incisive foramen.¹⁰ These canals can exit the incisive foramen in three different patterns: type 1 which is a single canal, type 2 where the canals remain separated (parallel canals), or type 3 where the canals fuse prior to exiting forming a “Y” type of canal.¹⁰

Cell types within the nasopalatine canals are variable and change as the canal moves downward to exit the incisive foramen.^4,9,11 Histologically, the upward portions of the canals near the nasal area contain respiratory epithelial cells. As the canal moves downward cuboidal cells are found in the lining. Squamous cells are found near the oral cavity. In addition, mucous secreting glands are also found in the canals. The branches of the descending palatine and sphenopalatine arteries are also housed within these canals.^4,9,11 The nasopalatine cells undergo degeneration; however, remaining epithelial cells and mucous secreting cells can create a foundation for NPDC.¹¹ 

Clinical Presentation

The NPDC can vary in size and presentation. In the early stages, NPDCs are frequently small and asymptomatic. Case reports have demonstrated that these lesions are more likely to be identified on routine radiographic examination.^7,9,12–14

The most commonly reported clinical symptom of larger cysts is swelling in the anterior part of the palate.^15,16 Another symptom includes pain in the anterior part of the maxilla that radiates into the bridge of the nose. The pain can present as an intermittent burning sensation and may be associated with infection. Tooth displacement, nasal obstruction, or discharge into the nasal cavity or oral cavity are other symptoms associated with NPDC.^1,12,17,18 The vitality of teeth is variable.¹⁸

Differential Diagnosis 

Due to radiographic similarities and location of this particular cyst, a differential diagnosis for the NPDC may include odontogenic cysts, such as periapical granuloma and radicular cyst, or median palatine cyst.^19,20 Nonodontogenic tumors, osteitis, enlarged nasopalatine duct, and nevoid basal carcinoma syndrome have also been considered as possible differential diagnoses.⁴ 

Odontogenic cysts can be inflammatory, such as periapical granulomas and radicular cysts, or developmental. The periapical granuloma is considered the first stage of inflammation as granulation tissue is created.¹⁹ Chemical mediators induced by chronic inflammation stimulate epithelial rests, which transform the granuloma into a radicular cyst. Both the granuloma and the radicular cyst create a well-defined round or ovoid radiolucency around the apical portion of the tooth; they are confirmed through a histological examination. Differential diagnoses between a periapical granuloma, radicular cyst, and NPDC is determined by testing tooth vitality.^19,21,22 

The median palatine cyst—which is usually located in the same region as the NPDC, but will be more posteriorly placed and centered toward the midline of the palate—is an example of a developmental cyst.¹⁹ This lesion is lined by stratified squamous epithelium and is believed to develop from the entrapped epithelial cells in the embryonic fusion of the lateral maxillary process to form the hard palate. Pain is often reported with this cyst as it expands and compresses the nasopalatine nerve. There is also a male predilection similar to the NPDC.¹⁹

Nonodontogenic tumors such as the central giant cell granuloma (CGCG), are more commonly found in the mandible (65% to 75%), however, these lesions can also be located in the maxilla.^19,20 CGCG is most often found anterior to the molar region. Although painless, CGCG will appear as a radiolucent lesion with expanding margins. If the lesion extends through the maxillary or mandibular bone structure, it will appear intraorally as a soft, flat-based nodule with a purple blue color. Teeth and roots may also be divergent with the CGCG’s enlarged growth.¹⁹ 

Osteitis can also be considered a differential diagnosis especially with a fistula tracking toward the palate.⁴ A bucconasal or buccosinusal communication might also be present with similar manifestations like that of a NPDC. An enlarged nasopalatine duct less than 6 mm in diameter would be an additional differential diagnosis.⁴ 

Nevoid basal cell carcinoma syndrome is an inherited autosomal dominant disorder primarily affecting men with a median age of 41, which can manifest as jaw cysts.¹⁹ This disease will often present with odontogenic keratocysts (OKC). OKC can be round and ovoid in appearance with well-defined borders and appear lateral to a tooth radiographically. Although OKC is predominantly located in the mandible (80%), it can also manifest in the maxilla.¹⁹

Diagnosis 

Imaging studies are used to identify the NPDC. Radiographic examination may include panoramic, occlusal, and periapical radiographics, and typically discloses a well-defined, ovoid, round, pear-shaped, or heart-shaped radiolucency located in the midline of the maxilla between the central incisors.^3,9,23 The diameter of the incisive canal is expected to be 6 mm or less; therefore, radiolucencies larger than this size should warrant further investigation for pathologic lesions such as the NPDC.^3,9

Other radiographic techniques used to diagnose NPDC include multimodal tomography, magnetic resonance imaging, and computed tomography scanning. Three-dimensional images are highly reliable in differentiating NPDCs from other anatomical features or other cysts of odontogenic origin. In addition, these techniques facilitate planning the best surgical approach for removing the lesion.^3,4,15

Tooth vitality testing and percussion testing may be performed to avoid unnecessary endodontic treatment when there is concern about the vitality of the maxillary central incisors.^22,24 Aspiration of radiolucencies may be useful in distinguishing solid masses from cystic lesions, but is not diagnostic per se. For example, a clear or straw-colored aspirate is suggestive of a NPDC. However, this type of aspirate is also suggestive of a lateral radicular cyst or cystic ameloblastoma.³

Histopathologic examination of NPDCs reveals squamous cell epithelium surrounded by a connective tissue wall. Other types of epithelium may be present, including columnar, cuboidal, and ciliary cylindrical cells. Melanin remains and erythrocytes have been found in the cyst wall as well as an inflammatory infiltrate of polymorphonuclear leukocytes and plasma cells. Immunohistochemistry profiles have also been used for diagnosis including profiles for K7, K13, MUC-1, and P63.^3,4,16,25

Treatment and Prognosis

Treatment of the NPDC is by surgical enucleation; marsupialization has been applied for larger cysts.^1,4  Bone regeneration within the bony defect is anticipated; however, options for treatment of large cystic defects when both cortical plates are perforated include fibrin powder, thrombin, patient’s blood and antibiotics, platelet-rich fibrin, xenograft bone with aspirate bone marrow, recombinant bone morphogenic protein-2 in conjunction with rib graft, fibrin and collagen sponges, hydroxyapatite, calcium sulphates, and synthetic materials.¹⁵ Paresthesia of the anterior palate has been reported as a complication in 10% of cases if portions of the long sphenopalatine nerve are removed or damaged during the surgical procedure.^17,26,27 Recurrence of the NPDC is rare. Malignant transformation has been reported in two cases indicating this is an extremely rare complication.^28,29

Case Report 

A 50-year-old man presented to the clinic for a routine examination and a full-mouth series of radiographs. The health history revealed the presence of seasonal allergies and a positive COVID-19 test during the second week of January 2022. The dental history included a high caries risk and clinical gingival health on a reduced periodontium in a nonperiodontitis patient. The patient reported sensitivity along the face around the area of the maxillary left central incisor when he had COVID-19, so initially the symptoms were blamed on his illness. Once he recovered from his illness, the pa­tient complained that the maxillary left central and lateral incisors (teeth #9–10) felt mobile when blowing his nose. He also reported symptoms of pain, tenderness, and swelling associated with the left maxillary central incisor (tooth #9).

Upon completion of the full-mouth series, the anterior periapical radiograph revealed a large radiolucent lesion above tooth #9, the left maxillary central incisor (Figure 1). The patient was referred to a periodontist for further evaluation. Although the patient did have mobility associated with the tooth, tooth vitality testing was negative. The patient was then referred to an endodontist who performed root canal therapy on the left maxillary central incisor (tooth #9) and referred the patient back to the periodontist for removal of the cyst.

A cone-beam computed tomography scan of the cyst was recommended and performed (Figure 2). A NPDC was suspected. The cyst was surgically removed (Figure 3) and the biopsy report confirmed the finding of the NPDC. The patient was informed that the prognosis was very good and recurrence was rare. He was advised to return for follow-up to monitor wound healing.

This case resulted in a favorable outcome. There are examples in which a patient can be misdiagnosed and improper treatment can occur. Hilfer et al³⁰ noted instances in which NPDC lesions have an inverted pear or heart shape superimposed on the nasal spine, which are misdiagnosed as a nonhealing periapical granuloma, resulting in unnecessary extraction of one or more teeth. It is important to use multiple diagnostic modalities to identify these lesions and ensure appropriate treatment is rendered.

Conclusion 

The NPDC is one of the most common nonodontogenic cysts of the oral cavity and can be commonly misdiagnosed. Although the etiology of NPDC is unknown, NPDC can vary in presentation. The NPDC is most often detected through radiographs and can be confirmed through multiple imaging studies. Patients will often report swelling of the anterior maxilla and intermittent pain that radiates across the bridge the bridge of the nose. Other symptoms might include nasal obstruction or discharge into the nasal cavity or oral cavity. Tooth displacement can also be present. A differential diagnosis must be considered before undergoing permanent and unnecessary treatment. Surgical enucleation and marsupialization are considered effective treatment options for NPDC. 

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From Dimensions of Dental Hygiene. September 2022; 20(9)34-37.