Introduction

Melanoacanthoma of the oral mucosa is a rare condition indicative of a reactive process [1]. Oral melanoacanthoma was first reported in 1978 [2] and to the best of our knowledge, only 50 cases of melanoacanthoma have been reported in the literature to date (Table 1) [2–14]. The clinical presentation is a brown to brown-black macular lesion, predominantly solitary, encountered in the younger age group with a distinct female predilection [3, 12]. The most common site affected is the buccal mucosa. Melanoacanthoma has been reported in labial mucosa, palate, gingiva, alveolar mucosa and oropharynx (Table 1). The typical histological picture of melanoacanthoma is the proliferation of dendritic melanocytes throughout the epithelium. The epithelium exhibits acanthosis and spongiosis. A chronic inflammatory cell infiltrate with eosinophils may be noted. The lesion is benign and may regress following an incisional biopsy [1].

Table 1 List of reports of oral melanoacanthoma [2–14]
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Case presentation

A 24-year-old graduate dental student presented with a complaint of intra-oral pigmentation of the left buccal mucosa with duration of one and a half months. The patient had initially noted a small round area of pigmentation of about 5 mm in size which, to his concern, had rapidly increased to the present size (Figure 1). He did not report any discomfort associated with the lesion, except for an altered surface texture. Personal history revealed that the patient had infrequently (once a day) smoked filtered cigarettes over the previous 4 years. Intra-oral examination revealed carious 28, multiple teeth with glass ionomer cement (GIC) class V restoration (36, 37, 38, 46, and 47) and a brownish-black macular lesion in the left buccal mucosa. On further enquiry, the patient revealed that he had undergone multiple GIC restorations 3 months previously, during which procedure he had sustained a mild bur injury in the left buccal mucosa, which healed uneventfully.

Figure 1
figure 1

Brownish-black macular lesion on left buccal mucosa adjacent to molar teeth with Class V glass ionomer cement restorations (arrows).

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The brownish-black macular lesion on the left buccal mucosa was well demarcated from the surrounding mucosa with regular, well-defined borders. The lesion extended anteriorly from the region of the mandibular first molar (36) to the mandibular left canine region. It measured 25 mm antero-posteriorly and had a maximum width of 16 mm supero-inferiorly. The lesion was not tender, did not blanch under pressure and was not fixed to the underlying mucosa.

Diagnosis

Following incisional biopsy, the specimen was fixed in 10% neutral buffered formalin, routinely processed and paraffin embedded. Histopathological examination of the lesion revealed a stratified squamous surface epithelium exhibiting acanthosis, spongiosis, melanin pigmentation, inflammatory cell exocytosis and numerous dendritic melanocytes distributed diffusely in the suprabasal and spinous layers. A chronic inflammatory cell infiltrate was present in the subjacent connective tissue. The dendritic melanocytes were also demonstrated by Masson-Fontana silver impregnation stain (Figure 2). Based on the history, clinical features and histological presentation, the lesion was diagnosed as melanoacanthoma.

Figure 2
figure 2

Hematoxylin and eosin stained sections (A, B and C) revealed stratified squamous non-keratinized epithelium exhibiting acanthosis and numerous dendritic melanocytes throughout the entire thickness of the epithelium; Masson-Fontana (M-F) special stain reveals numerous melanocytes.

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Management

The lesion characteristically appeared to regress following the biopsy procedure. A regular follow-up of the patient was carried out to observe the progress of the lesion (Figure 3).

Figure 3
figure 3

Follow-up of lesion after 1 week (A), after 2 weeks (B) and complete resolution after 2 months (C).

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Discussion

The term melanoacanthoma refers to a lesion exhibiting a proliferation of dendritic melanocytes throughout the surface epithelium. Cutaneous melanoacanthoma is also known as pigmented seborrheic keratosis [15].

Oral melanoacanthoma is a benign, reactive process and is unrelated to cutaneous melanoacanthoma. The reported age of presentation ranges from 9 to 77 years, with a mean age of 29 years [3, 4, 12]. The lesion is most predominantly observed among black patients, though occurrences have been observed among Caucasians, Hispanics and Asians [1, 4, 12–14]. Oral melanoacanthomas show a female predilection, with a male to female ratio of 2:1 [1, 2, 14]. The etiology has been largely attributed to local irritation or even mild trauma [3, 14]. The intra-oral site most commonly affected is the buccal mucosa but involvement of other sites such as the mucosa of the lip, palate, gingiva and alveolar mucosa has also been reported (Table 1). Clinically, the lesion is a flat or slightly raised black or brown macule and may rapidly increase in size, ranging from a few millimeters to several centimeters [1, 12, 13]. The lesions are usually solitary and well circumscribed though a few authors have reported bilateral or multiple (Table 1) melanoacanthomas. Oral melanoacanthomas are usually asymptomatic and are not neoplastic. The other lesions to be considered in the differential diagnosis are smoker's melanosis, drug induced pigmentation, Addison's disease, melanotic macule, pigmented nevi – junctional, intramucosal, compound, Spitz nevus, postinflammatory melanosis and oral melanoma. A biopsy is mandatory to rule out melanoma and to alleviate patient apprehension. Histologically, melanocytes which are usually restricted to the basal layer are found distributed throughout the epithelium. These melanocytes exhibit prominent dendritic processes and are immunoreactive for S-100, Melan-A/Mart-1, HMB-45 and Tyrosinase [14]. Other dendritic cells in the oral mucosa are the Langerhans' cells which are antigen presenting cells of the immune system, usually distributed in the superficial epithelium and are demonstrated on immunohistochemistry by S-100 or CD1a. The adjacent connective tissue exhibits chronic inflammatory cell infiltrate. The presence of eosinophils among the inflammatory cells is not a universal feature and may not be essential for the diagnosis of oral melanoacanthoma. Once diagnosis is established, no further treatment is required, with some cases exhibiting spontaneous regression after biopsy [1]. It has been suggested that this entity be renamed melanoacanthosis or oral melanotic macule – reactive type, since the term melanoacanthoma is suggestive of a neoplastic process [11].

In our patient, the etiology of the lesion may be attributed to the incident of trauma during the restorative procedure. It may be safely assumed that GIC did not contribute to the cause of the lesion since the patient has multiple restorations with the same material and the adjacent sites did not exhibit any lesion.

Conclusion

To the best of our knowledge, this is the first case of oral melanoacanthoma in the Indian subcontinent and the second case of melanoacanthoma reported in an Asian Indian. In the present instance, a biopsy was performed to alleviate the patient's anxiety and as reported, the lesion regressed following biopsy. Thus, melanoacanthoma must be considered in the differential diagnosis of rapidly progressing pigmented lesions of the oral cavity and requires a histopathological diagnosis to rule out melanoma.

Consent

Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.