History and Physical Examination Findings

A 3-year-old 10.9-kg (24-lb) spayed female mixed-breed dog was examined because of an oral mass in the right rostral maxillary region. The owner reported that the swelling, described as an intermittent enlargement of the muzzle, had appeared 10 days before the physical examination. Prior to this event, the dog had no apparent health problems.

The results of a general physical examination were unremarkable. Extraoral examination revealed facial asymmetry with swelling over the right rostral maxillary region and mild bilateral epiphora with no nasal discharge. Intraoral examination revealed a smooth, fluctuant mass that measured approximately 20 × 20 × 20 mm and involved the gingiva and alveolar mucosa from the level of the right maxillary first incisor tooth to the level of the right maxillary canine tooth. Mobility of all involved teeth was noted. Additionally, the right maxillary first incisor tooth was discolored but had no loss of crown integrity (Figure 1). Other findings included mild generalized gingivitis with no visual evidence of periodontitis. The results of routine preanesthetic clinicopathological tests, including a CBC and a serum biochemical analysis, were within the respective reference ranges. The patient was anesthetized, and intraoral radiographs were obtained (Figure 2).

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Photograph of the rostral aspect of the maxilla of a 3-year-old spayed female mixed-breed dog that was evaluated because of an intraoral mass. The patient is in dorsal recumbency. Notice the subtle intrinsic discoloration of the right maxillary first incisor tooth and the distended soft tissues extending to the level of the right maxillary canine tooth.

Citation: Journal of the American Veterinary Medical Association 257, 11; 10.2460/javma.2020.257.11.1121

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Selected intraoral dental radiographs for the dog in Figure 1. A—Right lateral view centered on the right maxillary canine tooth. B—Occlusal view of the rostral aspect of the maxilla. The dog's right is to the left in panel B.

Citation: Journal of the American Veterinary Medical Association 257, 11; 10.2460/javma.2020.257.11.1121

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Diagnostic Imaging Findings and Interpretation

Intraoral radiography revealed geographic bone loss at the rostral aspect of the right maxilla, extending from the level of the left maxillary first incisor tooth to the right maxillary canine tooth, with total or near-total loss of attachment of the associated teeth (Figure 3). The right maxillary canine tooth was laterodistally displaced, and bone loss extended past the palatal midline of the rostral portion of the maxilla. The pulp cavity of the right maxillary first incisor tooth was visibly wider, compared with the pulp cavities of the contralateral and immediately adjacent incisor teeth, consistent with a nonvital tooth.

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Same intraoral radiographic views as in Figure 2. There is geographic bone loss and loss of the turbinate pattern at the rostral aspect of the right maxilla, affecting the left maxillary first incisor tooth; right maxillary first, second, and third incisor teeth; and right maxillary canine tooth (arrows). The right maxillary first incisor tooth is nonvital, as evidenced by failure of the pulp cavity to narrow (arrowhead) in the occlusal view (B).

Citation: Journal of the American Veterinary Medical Association 257, 11; 10.2460/javma.2020.257.11.1121

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To further assess the nature and extent of the lesion, CT imaging of the head was performed before and after administration of contrast medium (iohexol [350 mg of I/mL], 2 mL/kg [0.91 mL/lb], IV). Images were acquired with a 16-slice CT scanner^a in 0.5-mm transverse-plane scans, reconstructed in 1-mm contiguous slices, and viewed with bone (window width, 4,500 HU; window level, 1,100 HU) and soft tissue (window width, 320 HU, window level, 30 HU) settings. The CT scan revealed a large (21 × 17 × 17-mm), well-defined, homogenous, fluid-attenuating, expansile lesion characterized by geographic bone lysis that involved the right incisive bone and rostral portion of the right maxillary bone and extended to the right conchae and across the midline toward the left incisive bone (Figure 4). The lesion encompassed the roots of the left maxillary first incisor tooth and the right maxillary first, second, and third incisor teeth as well as the right maxillary canine tooth. The periphery of the mass was strongly contrast enhancing. Mild enlargement of the right mandibular lymph node was also detected.

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Representative CT images of the maxilla of the dog in Figure 1. A—A 3-D rendered image of the rostral portion of the maxilla viewed from the ventral aspect. A large area of geographic bone loss affecting the palatine process of the maxilla and the incisive bone is evident on the right side. B—A 3-D rendered image of the rostral portion of the maxilla seen from the right lateral aspect. Geographic bone loss has resulted in severe attachment loss of the right maxillary third incisor and canine teeth. C—A coronal reconstructed image viewed in a bone window at the level of the hard palate. Bone loss extends from the level of the left maxillary first incisor tooth to the level of the right maxillary canine tooth. D—A postcontrast transverse-plane image viewed in a soft tissue window at the level of the maxillary canine teeth. A large (21 × 17 × 17-mm), well-defined, homogenous, fluid-attenuating, expansile lesion is evident (arrows).

Citation: Journal of the American Veterinary Medical Association 257, 11; 10.2460/javma.2020.257.11.1121

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Collectively, the radiographic and CT findings were suggestive of an odontogenic cyst. Given that a nonvital tooth was associated with the lesion, a periapical cyst was suspected. Neoplasia could not be ruled out on the basis of diagnostic imaging findings.

Treatment and Outcome

Periodontal probing, charting, and a complete periodontal treatment, including supra- and subgingival ultrasonic scaling, were performed with the patient under general anesthesia. Bilateral infraorbital regional nerve blocks were administered via the infraorbital canals with 0.5 mL of bupivacaine (5 mg/mL)/site. A full-thickness mucogingival flap was elevated to allow for surgical extraction of the left maxillary first incisor tooth and the right maxillary first, second, and third incisor teeth as well as the right maxillary canine tooth. Once exposed, the lining of the lesion was released and enucleated meticulously with a periosteal elevator. Enucleated tissues and extracted teeth were preserved in neutral-buffered 10% formalin solution and submitted for histologic analysis. The surgical site was copiously rinsed with sterile saline (0.9% NaCl) solution, and the surgical site was closed with 5-0 poliglecaprone 25 suture in a simple interrupted pattern. The patient recovered uneventfully and was hospitalized overnight for monitoring. During the hospitalization period, lactated Ringer solution (60 mL/kg/d [27.3 mL/lb/d], IV) was administered, and a constant rate infusion of fentanyl (3 μg/kg/h [1.4 μg/lb/h], IV) was instituted for analgesia. The patient was discharged from the hospital the following morning; meloxicam (0.1 mg/kg [0.05 mg/lb], PO, q 24 h for 3 to 5 days) and tramadol (2 mg/kg [0.91 mg/lb], PO, q 8 to 12 h for 3 to 5 days) were prescribed for analgesia, with amoxicillin–clavulanic acid (15 mg/kg [6.8 mg/lb], PO, q 12 h for 7 days) for antimicrobial coverage. The owner was instructed to feed soft food and restrict access to chew toys for the following 2 weeks and was advised of the possibility of blood-tinged oral or nasal discharge for a few days after surgery.

Histologic examination of the submitted soft tissues revealed a fibrous wall lined by nonkeratinized stratified squamous epithelium with small amounts of mononuclear inflammation. The incisor teeth had moderate chronic, locally extensive suppurative pulpitis and apical periodontitis. On the basis of histopathologic, clinical, and diagnostic imaging findings, a definitive diagnosis of periapical cyst was established.¹

A recheck examination was performed 2 weeks after surgery. The owner reported that the dog had recovered completely and uneventfully, and oral examination revealed appropriate healing of the surgical site. An oral examination 4 weeks after surgery was recommended, to be followed by a recheck CT examination 6 months after surgery to monitor for lesion persistence or recurrence.

Comments

The differential diagnoses for oral swellings in dogs include inflammatory or hyperplastic conditions, cysts, and benign or malignant neoplasms.² Both etiopathogenesis and characteristics differ fundamentally among these lesion types, and a precise diagnosis is critical for therapeutic and prognostic purposes. Reaching a definitive diagnosis requires a comprehensive oral examination in conjunction with imaging studies and histologic analysis.

The pattern of bone lysis in a lesion is among the criteria used to differentiate between aggressive and nonaggressive lesions.³ In order from least aggressive to most aggressive, patterns of bone lysis include geographic, moth eaten, and permeative. Geographic lysis typically has well-defined margins and may be associated with expansion but not destruction of the cortical bone. Moth-eaten lysis is defined as multiple, variably sized, discrete areas of lysis with or without cortical destruction. Permeative lysis is defined as focal areas of poorly defined lysis throughout a region of bone.³ The geographic lysis associated with the lesion in the dog of this report was consistent with a nonaggressive lesion.

The nonaggressive pattern of bone lysis in conjunction with the lesion's fluctuant clinical appearance and fluid-attenuating contents with peripheral contrast enhancement on CT were considered consistent with an odontogenic cyst. Odontogenic cysts arise from odontogenic epithelial rests in the periodontal ligament.⁴ Stimulation of these rests results in epithelial cell proliferation, followed by accumulation of fluid into a potential space as a result of osmotic pressure, gradually creating an enlarging, space-occupying lesion.⁵ Types of odontogenic cysts that have been identified in dogs include dentigerous cysts, lateral periodontal cysts, canine odontogenic parakeratinized cysts, and periapical cysts.¹ In the dog of the present report, a periapical cyst was considered the most likely diagnosis owing to the presence of radiographic signs consistent with pulp necrosis of the right maxillary first incisor tooth. A dentigerous cyst was ruled out because no unerupted tooth was present. The presence of an inflammatory infiltrate in the cyst lining, in conjunction with suppurative pulpitis of the incisor teeth, confirmed the diagnosis. The histopathologic findings were considered inconsistent with canine odontogenic parakeratinized cysts or lateral periodontal cysts owing to the lack of parakeratinization of the epithelial lining and absence of epithelial plaques, respectively.¹

In periapical cysts, also known as radicular cysts, the stimulation of epithelial cell rests arises from periapical inflammation associated with endodontal disease. Definitive treatment is achieved by extraction or root canal treatment of the initiating tooth in addition to meticulous enucleation of the cyst.⁵ In some cases, such as that described here, adjacent teeth may require extraction because of periodontal compromise resulting from cyst expansion.

The decision to proceed directly to definitive treatment of the cyst was made on the basis of the combination of signalment (a young patient), clinical features, and radiographic findings that resulted in a high degree of clinical suspicion for a periapical cyst. Careful consideration of the multitude of factors is critical, as malignant neoplasms can have a nonaggressive pattern of bone lysis,⁶ and reliance on intraoral radiographic findings alone is often insufficient. In the dog of the present report, CT examination results confirmed an internal structure and contrast-enhancement pattern consistent with a cystic lesion. These were details that intraoral radiographs alone could not provide. Computed tomography also overcame the superimposition of anatomic structures inherent during use of conventional radiographic techniques for imaging the maxilla, allowing more accurate surgical planning. Although an incisional biopsy could have been performed prior to definitive treatment, this approach would have necessitated a second general anesthesia episode, with the additional costs and risks entailed by the added procedure. However, it should be noted that histologic analysis is required for definitive diagnosis, and an incisional biopsy prior to definitive treatment may be prudent.

In the case detailed here, contrast-enhanced CT provided more diagnostic information than that provided by intraoral radiography alone, and the results were valuable for decision-making and surgical planning. Computed tomography should be considered in the diagnostic workup of suspected odontogenic cysts in dogs, particularly in the maxilla, where superimposition of anatomic structures complicates interpretation of conventional radiographic images. Computed tomography is also the imaging modality the authors prefer for follow-up of maxillary odontogenic cysts for these same reasons.