Optic disc melanocytoma can mimic melanoma
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Optic disc melanocytoma, also known as magnocellular nevus, is a rare benign tumor that often develops within or close to the optic nerve head and occasionally involves the retina and choroid.
The majority of melanocytomas do not result in loss of visual acuity; however, about 25% of affected eyes experience minor vision loss. Retinal exudation involving the macula is the principal factor that leads to visual loss. Other risk factors include retinal extension and subretinal fluid. Although it is uncommon, malignant transformation, spontaneous tumor necrosis and central retinal vein obstruction can all result in severely impaired vision; 1% to 2% of cases of optic nerve melanocytoma developing into malignant melanoma have been recorded.
Source: Dhivya Ashok Kumar, MD, FRCS, FICO, FAICO, Keziah Mary Thomas, MBBS, DNB, and Amar Agarwal, MS, FRCS, FRCOphth
In this column, we share a case of optic melanocytoma mimicking melanoma.
Case report
A 60-year-old male patient presented to our hospital with complaints of decreasing vision in his right eye for 1 month, insidious in onset and gradually progressing in nature. It was not associated with any pain or redness. He had hypertension that was treated with medication for the past 10 years. On clinical examination, visual acuity in the right was 20/60, improving to 20/30 with correction. Best corrected visual acuity in the left eye was 20/20. IOP was within normal limits in both eyes. Anterior segment findings were unremarkable. Pupils were reactive to light for both direct and consensual light reflex. On confrontation, right eye inferior fields were restricted, and right eye color vision was affected (6/17 on Ishihara test). Left eye findings were within normal limits.
Fundus examination of the right eye showed a 1.5 disc diopter size, a brown-pigmented elevated lesion with irregular margins on the optic nerve head, surrounding retinal thickening and circinate exudation reaching up to the macula (Figure 1a). The vitreous was clear. Fundus autofluorescence (FAF) image showed hypoautofluorescence of the tumor with well-defined outlines along with hypofluorescent patches corresponding to retinal exudates reaching up to the macula (Figure 1b). Fundus fluorescein angiography (FFA) showed blockage at the site of the tumor and hyperfluorescence around the tumor during the late venous phase (Figure 1c). Right eye visual fields with the Humphrey Field Analyzer 30-2 program showed inferior arcuate defect (Figure 2a). OCT showed a dome-shaped configuration with a brightly reflective anterior surface and a posterior optical shadow (Figure 2b). B-scan ultrasonography showed an elevated dome-shaped lesion on the optic nerve head with high reflectivity and tumor thickness of approximately 2.7 mm (Figure 3a). MRI of the orbit showed a small elevated enhancing lesion at the right optic nerve disc (Figure 3b) of 1.5 mm × 1.4 mm size that was hyperintense on T1 and hypointense on T2. MRI brain was within normal limits. The above features were consistent with melanocytoma of the optic nerve head. Periodic follow-up and serial observation were recommended for the patient.
2b. OCT of the disc showing a dome-shaped configuration with a highly reflective anterior surface.
3b. MRI of the orbit showing a small elevated enhancing lesion at the right optic disc.
Melanocytoma vs. melanoma of the disc
Historically, pathologists and ophthalmologists regarded melanocytoma as a malignant tumor with enucleation of the affected eye.
Reese coined the phrase “melanoma of the optic disc” in 1933 to refer to pigmented lesions of the optic nerve head that shared histological and clinical characteristics with melanocytomas.
Eventually, after conducting a clinicopathological correlation on enucleated eyes with suspected melanoma, Zimmerman and Garron proved its benign nature in 1962. They coined the word “melanocytoma” because the tumor cells resembled those found in ocular melanocytosis.
Primary melanoma of the optic nerve is rare, whereas subsequent invasion of the optic nerve by uveal melanoma is uncommon. According to Lindegaard and colleagues, between 5% to 7% of uveal melanoma cases involved optic nerve invasion. Juxtapapillary site of the uveal melanoma and elevated IOP were risk factors for optic nerve invasion. Due to the aggressive character of choroidal melanoma and to prevent erroneous enucleation in benign cases, the presence of malignancy should be ruled out with a fair degree of certainty.
Malignant transformation should also be taken into consideration if there is progressive growth or significant involvement of the optic disc combined with vision loss, even though 10% to 15% of melanocytomas will gradually enlarge over time. Clinical indicators such as its location on the optic nerve head, dark pigmentation and constancy in size over time are used to make the diagnosis of optic disc melanocytoma (ODM). However, when in doubt or to confirm the diagnosis, a biopsy may be required.
Discussion
In B-scan ultrasonography, there is high internal reflectivity, unlike other malignancies such as choroidal melanoma, which tend to present with low internal reflectivity. Melanoma will show collar button or mushroom growth with typical choroidal excavation and orbital shadowing. The changing A-scan spike or angle kappa of internal reflectivity is another sign noted in ultrasonography in choroidal melanoma. On the other hand, the homogenous nature of the lesion also corresponds to melanocytoma. The avascularity of the tumor is a common finding as opposed to melanomas, which are highly vascularized.
Due to the frequent alteration of fluorescence properties in ocular malignancy, FAF is helpful in the research of ODM. For instance, the orange pigment deposit that is typical of malignant tumors and appears as hyperfluorescent in FAF is caused by the accumulation of lipofuscin macrophages. ODM, on the other hand, exhibits in FAF as a completely hypofluorescent mass without any hyperfluorescent regions at the tumor or its margins. There is no retinal pigment epithelium or lipofuscin over the tumor, which as a result is seen as hypofluorescent lesion. Unfortunately, the strong optical shadowing in the OCT results in minimal internal tumor information, and most studies have labeled the ODM as an optically empty space. OCT angiography (OCTA) is also used to evaluate ODM because it is not influenced by the tumor’s pigmentation or leakage, unlike fluorescein angiography. Due to its dense pigmentation, which prevents light from penetrating, the tumor exhibits low signal intensity throughout all stages of fluorescein angiography, failing to offer accurate information on the vasculature of the ODM. Even though fundus examination and fundus photography remain the gold standard for diagnosis and follow-up, B-scan ultrasonography, OCT of the disc, OCTA and FAF have demonstrated promise in the evaluation of ODM.
Conclusion
Although optic disc melanocytomas tend to have benign behavior, they may adversely affect visual function. Based on fundus findings and a history of recent decreasing vision, concerns were raised regarding malignancy in our case. The decrease in vision was due to retinal exudation reaching up to the macula. There is ground for considerable concern regarding malignancy with a previously unreported pigmented lesion of the optic disc and peripapillary region. However, diagnosing the disease and distinguishing it from choroidal melanoma can be made easier by being aware of the significant ophthalmoscopic characteristics of an optic disc melanocytoma.
- References:
- Burgos-Blasco B, et al. J Fr Ophtalmol. 2020;doi:10.1016/j.jfo.2020.01.032.
- Lindegaard J, et al. Invest Ophthalmol Vis Sci. 2006;doi:10.1167/iovs.05-1435.
- Lisker-Cervantes A, et al. Invest Ophthalmol Vis Sci. 2016;57(12):5878.
- Shields JA, et al. Ophthalmology. 2004;doi:10.1016/j.ophtha.2004.02.016.
- Shields JA, et al. Surv Ophthalmol. 2006;doi:10.1016/j.survophthal.2005.12.011.
- For more information:
- Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre, is the author of several books published by SLACK Books, sister company of Healio publisher Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at aehl19c@gmail.com; website: www.dragarwal.com.
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