Fig. 5.16  CT scan demonstrating a meningioma of the left optic nerve. Note thickened appearance. Classic “tram” lines are not seen

ONSM, and often shows the tumor to be separated from the optic nerve proper on coronal views. The fusiform enlargement of the optic nerve is similar to optic nerve gliomas, but kinking (a classic radiographic feature of gliomas) is not seen with meningioma. The radiographic appearance of OHSMs may also be mimicked by idiopathic orbital inflammatory syndrome, optic perineuritis, sarcoid infiltration, and lymphomatous and carcinomatous meningitis. However, these conditions have more rapid onset than meningioma and are often associated with pain. Since the diagnosis of ONSM is established easily by clinical and radiographic findings, biopsy is almost never indicated.

5.6.6  Treatment

The management of this tumor is controversial. If visual function is good with mild field loss, observation is indicated unless progression is documented. Radiation is generally offered as primary therapy in patients with mild to moderate vision loss [126, 137, 138]. This is reserved for adult patients with salvageable vision. Surgical excision is used to treat blind, uncomfortable, or unsightly eyes or to reduce the risk of intracranial extension. Generally, more aggressive therapy is favored in pediatric patients with evidence of intracranial tumor spread. For these patients, surgical results are poor, although radiation has shown some benefit [139].

Fig. 5.17  Coronal MRI scan showing marked enlargement of the left optic nerve

5.7  Melanocytoma

5.7.1  Introduction

Melanocytoma is a nevus of the optic disc. Before the typical features of this tumor were recognized, it was frequently misdiagnosed as a malignant melanoma, and unnecessary enucleations were carried out [140, 141]. The patient is almost always asymptomatic, and the diagnosis is made as an incidental finding during a fundus examination. Clinically, it appears as an elevated pigmented lesion located over the optic disc (Fig. 5.18). The jet-black pigmentation is the most characteristic feature. It can extend into the nerve

Fig. 5.18  Melanocytoma of the optic nerve. Note black appearance and complete obsuration of the optic nerve

[140, 141, 143, 144]. It usually arises on or adjacent to the optic disc [145, 146]. The cytologic appearnce of individual cells in melanocytoma is quite distinct from the usual spindle cell type seen in most uveal nevi. Melanocytoma cells are large, usually round or slightly polyhedral with distinct cell borders. The nucleus within each tumor cell is small and round, and there is no variation in nuclear size and shape. The nuclei are normochromic, and mitotic activity is minimal in these slowly growing or stationary tumors. Spindle cells are rare, and there is little evidence of cellular pleomorphism, tumor giant cell formation, or other features suggestive of an epithelioid cell differentiation.

Fig. 5.19  Melanocytoma of the optic nerve. Note feathery edges with extension to the adjacent retina

fiber layer of the retina where it displays a feathery margin (Fig. 5.19). In 75% of cases, 50% of the disc or less is covered. In 12% of the eyes, over 90% is obscured [142]. Approximately, 90% are two disc diameters or less in diameter. Most disc melanocytomas do not project more than 1 mm forward toward the vitreous.

5.7.2  Pathophysiology

The melanocytoma is derived from uveal dendritic melanocytes, like uveal nevi and malignant melanomas

5.7.3  Natural History and Prognosis

It is a benign condition with a very low probability of evolving into melanoma [147]. Slight growth can be detected in about 15% of patient with long-term fol- low-up [142]. Most cases are unilateral although a bilateral case has been reported [148]. The absence of significant loss of visual acuity or severe field loss in melanocytoma helps differentiate it from malignant melanoma. However, melanocytoma can be associated with an afferent papillary defect and visual field change especially enlarged blind spot or arcuate scotoma [149]. Acute vision loss can occur secondarily to spontaneous necrosis of an optic nerve melano­ cytoma. Occasionally, this can also induce an obstruction of the central retinal vein and hemorrhagic retinopathy [150].

5.7.4  Diagnosis and Diagnostic Aids

In contrast to uveal malignant melanoma, which is unusual in black patients, melanocytoma is seen with equal frequency in blacks and whites [140]. In fluorescein angiography, the lesion blocks fluorescence. Ultrasonography typically shows a solid mass with high internal reflectivity (Fig. 5.18b). About 40% of melanocytoma cases may have a juxtapapillary choroidal component identical to a deeply pigmented choroidal neivi. Optic disc edema adjacent to the tumor is seen in 28% of cases, usually with moderate or large tumors. This may be secondary to a