Optic Disc Melanocytoma: A Case Report

Background: Optic disc melanocytoma is a rare tumor of the optic nerve that typically occurs on the surface of the optic disk. Our purpose was to identify a case of optic disc melanocytoma. Case presentation: A 51-year-old female patient presented with chief complaints of black shadows in the left eye, having normal visual acuity. Fundus examination of the left eye revealed a homogeneous black bulging mass located on the optic disc. The mass demonstrated a hypo ﬂ uorescence throughout an angiogram at all stages and had differentiated from a malignant melanoma. Conclusion: The patient presented with black shadows in the left eye. After careful examination, an optic disc melanoma in the left eye was found. Even if it is benign, malignant transformation may occur in the future. Therefore, a follow-up is recommended for a long time.


Introduction
Melanocytoma of the optic disc is a type of melanocytic nevus that occurs primarily in the optic disc, and sometimes, with the involvement of the adjacent choroid and retina [1] . It is considered to be a congenital and non-genetic lesion, without racial preference, and is usually unilateral to the eye. Its mean age of diagnosis is between 40 and 50 years, with a slightly greater predominance in female patients [2] . This melanocytoma may cause papilledema, as well as optic nerve damage, and always presents a dark brown or black pattern, being the tumor size of usually around 3 mm [3] . Melanocytoma of the optic disc is a pigmented benign tumor that is usually considered a quiescent lesion; however, this tumor grows slowly and gradually affects the visual eld and vision [4] . It has been described that in 47-86% of the cases, the adjacent choroid is involved, whereas retinal involvement is described in 30-67% of the cases [5] .
Furthermore, in 25% of the cases, a fundus examination may demonstrate papilledema, and 2% of tumors can progress to a malignant transformation. The tumor may be associated with plasmacytoid retinal detachment, retinal edema, lipid deposition as well as intraretinal hemorrhage. Vitreous pigment dispersion, in some cases, may extend into the anterior segment of the eye. About 75% of patients are asymptomatic, with incidental detection of the tumor. There are several common symptoms associated with this clinical condition including blurred vision, decreased visual acuity, visual perversion, and microsight [4] . Melanocytoma has been reported in association with ocular melanocytosis, congenital pigment dispersion hypertrophy, choroidal nevus, and retinitis pigmentosa.
Melanocytoma of the optic disc is clinically and histopathologically mistaken with uveal melanoma. The diagnosis of melanocytoma is usually made by examining the optic disc due to characteristic pigmented masses on this disc [7] . Ancillary tests include ultrasound, uorescein, and indocyanine green angiography, fundus auto uorescence, and optical coherence tomography (OCT) [8] . Fundus auto uorescence is a noninvasive imaging method. In rare cases, spontaneous necrosis or progression into a melanoma can occur, resulting in severe vision loss. Although the course of the disease is considered relatively stationary, it is known to increase slightly in 10-15% of the cases. However, increased tumor size and secondary loss of vision do not correlate with a malignant transformation [9] . The main biological mechanism, involved in this disease, is the ischemic necrosis of the optic disc and some portions of the retinal nerve ber layer, caused by the tumor's occupying effect. In early stages, malignant melanoma can mimic melanoma, leading to di culties in the diagnosis. Considering its rarity and diagnostic di culties, we report a case of melanoma to describe its clinical features and diagnostic approach.

Case Presentation
A 51-year-old female patient presented with chief complaints of black shadows in the left eye. For further diagnosis and treatment, the patient was referred to the Ophthalmology department, at Qingdao's municipal hospital, on May 31 st , 2021.
The ocular examination showed that her best-corrected visual acuity was 0.8 with normal pupillary reactions in both eyes. Slit-lamp biomicroscopy showed normal anterior segments in both eyes, and no abnormalities were found in the systemic examination. Furthermore, a fundus examination of her left eye showed a homogeneous black mass located on the optic disc, occupying the superior one-half, where the vessels in that area were signi cantly bulged ( Figure 1). It was observed that the superotemporal quadrant, adjacent to the retina and the macular fovea, did not re ect light.
The remaining portion of the posterior segment was normal in the left eye, and no abnormality was found in the right eye. Intraocular pressure was measured being 15 mmHg and 17 mmHg for the right and left Altogether, due to its clinical features and the tumor's location, the patient was clinically diagnosed with optic disc melanoma and is currently followed once a month. Since the patient's visual acuity has remained at 0.8 without any changes in lesion size or her visual elds.

Discussion
Optic disc melanocytoma is located on the optic disc and sometimes extends below the sieve plate. Tumor cells are uniform in size, with a pigmented cytoplasm and a low nucleoplasmic ratio. Ruptured specimens show ovoid to round cells with abundant cytoplasm [1] . The pathogenesis of optic disc melanocytoma remains unknown, but it is thought to be a congenital lesion.
In a retrospective analysis of 115 patients, Shields et al. [5] found that 99% of patients presented with unilateral lesions and 100% presented with hyperpigmentation. Tumors averaged 2 mm in diameter and 1 mm in thickness. In the present study, the patient showed a similar pattern, with a black mass of approximately 1/2 pupillary distance (PD) in the optic disc of the left eye, protruding from its' surface and obscuring some retinal vessels. The fundus of the right eye showed no signi cant abnormalities ( Figure  1). With a prominent choroidal component, a differential diagnosis of choroidal melanoma, extending into the optic disc, becomes di cult. Salvanos et al. [6] reported the effectiveness of discriminating optic disc melanoma from choroidal melanoma. Optic disc melanoma shows a completely hypoautofluorescent with clear boundaries, without hyperauto uorescence spots associated with orange pigment.
In most cases, uorescein angiography of optic disc melanomas shows low uorescence, however, regarding optic disc edema, dye leakage is common in these lesioned areas. Most melanomas submitted to indocyanine green angiographies also show low uorescence. In this study, early frames showed a hypofluorescent tumor, and hyperfluorescent optic disc tissue with few vessels, whereas late frames showed dye leakage from the optic disc tissue (Figure 2). Shields et al. [7] analyzed OCT of 15 optic disc melanocytoma cases, showing hyperre ectivity on the anterior surface of the tumor, and dense posterior shadowing in all cases with an optically cavernous appearance. Okubo et al. [4] identi ed hyperre ective spots around central retinal veins and arteries of the optic disc, and perhaps this mechanism may be related to phagocytosis or lipid deposition. In the present case, a single hyperre ective band was detected in the local eminence, and the overlying retinal tissue on the tumor was severely disorganized ( Figure   3). Additionally, OCT angiography depicts abundant capillaries at the surface of the tumor.
High-resolution B-scan was used by Gologorsky et al. [8] to evaluate signs of malignant transformation in benign lesions, including increased size and angiogenesis, and transformation of nodule appearance. In this case, this technic showed a dome-shaped mass lesion over the optic disc with high echogenicity (Figure 4). Melanoma are rarely associated with poor vision, which is related to the compression of the optic nerve, exudative retinal detachment, and malignant transformation of the tumor. Common visual eld defects include blind-spot enlargement and nerve ber bundle defects. It has been de ned that the enlargement of the blind-spot is related to the number of tumors extending to the edge of the intervertebral disc. However, in this case, the visual acuity of the patient's left eye was normal and no obvious visual eld defects were found ( Figure 5).
As referred above, optic disc melanocytoma is a benign tumor that may grow slowly, and rarely transforms into a melanomas, being the rate of malignant transformation of 2% [3] . There is no established treatment for melanocytoma, and observation is the only clinical tool for its management. However, there are some reports demonstrating that photodynamic therapy and vitreous cavity injections with bevacizumab could be used as a potential treatment of uveal melanoma with choroidal neovascularization [9] . Tumor necrosis can lead to a misdiagnosis of progressive tumor growth, suggesting its possible transformation into melanoma. In some cases where eyes presented tumor necrosis, due to temporary ischemic necrosis, vision is restored at the request of the patient according to early reports [10] . Optic disc melanoma and suspicious conditions are usually treated with enucleation since it has been described that plaque radiation therapy presents lower visual function as well as a higher recurrence rate. Altogether and taking into account the results obtained in this case report, we suggest that patients with optic disc melanocytoma should have a regular ophthalmic examination to monitor the tumor's size, growth rates and prevent its malignant transformation.

Conclusion
The reported case in this study highlights the importance of properly identifying optic disc melanocytoma. Furthermore, whereas these cases do not require treatment a long-term follow-up is needed to exclude and prevent malignant transformations.

Abbreviations
OCT: Optical coherence tomography; PD: pupillary distance Declarations Ethics approval and consent to participate All the subjects have signed written informed consent according to the study protocol approved by the Ethics Committee of Qingdao Municipal Hospital before participation.

Consent for publication
Written informed consent was obtained from the patient according to Ethics Committee Regulations.

Availability of data and materials
The datasets used during the current study are available from the corresponding author on reasonable request.

Funding
No funding was obtained for this study.