Unilateral exophthalmos due to orbital metastasis from a contralateral intraocular melanoma

Progressive, left exophthalmos developed due to a left, retrobulbar mass in a 76-year-old Japanese woman. An open tumor biopsy was carried out, and both macro- and microscopic findings of the mass confirmed that it was a malignant melanoma. Orbital melanomas usually result from distant metastasis of cutaneous melanomas or from secondary extension of ipsilateral intraocular melanomas. Thorough physical, laboratory, and radiological examination, however, did not disclose any primary cutaneous or visceral melanoma, nor had the patient any previous history of excision or spontaneous regression of a pigmented lesion. Histopathologically, the left retrobulbar melanoma was rich in sinusoidal vessels which were surrounded by melanoma cells, oriented in a perpendicular array, a histological feature more characteristic of uveal melanomas than of cutaneous ones. The ophthalmological examination excluded development of a primary intraocular melanoma on the left side. Fundoscopic examination of the right eye was not feasible because of the complete opacity of the right vitreous body which had resulted from previous episode of idiopathic vitreous hemorrhage. Unexpectedly, CT and MR studies depicted retrobulbar masses of non-homogeneous densities in the bilateral orbits. These radiologic studies indicated the metastatic nature of the left retrobulbar melanoma, while suggesting the development of a primary, intraocular melanoma on the right side, extension into the right orbit, and involvement of the right optic nerve. All these clinical, radiological, and histological data suggested the development of a primary melanoma in the right eye and subsequent metastasis to the left orbit producing exophthalmos. The mechanism of such a peculiar mode of metastasis remains entirely unknown. This is a rare case of metastatic orbital melanoma, without visceral involvement, which originated in the contralateral eye. Development of the right ocular melanoma remained unrecognized due to atrophic degeneration of the right eyeball and complete opacity of the right vitreous body, until the contralateral orbital metastasis grew massive enough to cause exophthalmos.