Excerpt from Melanoma, Choroidal

Synonyms, Key Words, and Related Terms: choroidal melanoma, anterior choroidal melanoma, posterior choroidal melanoma, choroidal malignant melanoma, intraocular tumor, primary malignant melanoma, uveal melanoma, anterior uveal melanoma, posterior uveal melanoma, intraocular melanoma, uvea, iris, choroid, ciliary body, uveal structure

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Background: Choroidal melanoma is the most common primary malignant intraocular tumor and the second most common type of primary malignant melanoma in the body. It is nevertheless an infrequently found tumor.

Choroidal melanoma is a subtype of uveal melanoma. Uveal melanomas can be classified in anterior uveal melanomas, when the tumor arises in the iris, and in posterior uveal melanomas, when it arises in either the choroid or the ciliary body. Intraocular melanomas simultaneously can involve more than one uveal structure.

The ocular tissue where these tumors arise, the uvea, is a densely pigmented layer that forms part of the wall of the eye. The uvea is subdivided into iris, ciliary body, and choroid. The choroid underlies the retina and its pigment epithelium throughout the ocular fundus. The main function of the uvea is to provide oxygen and other nourishment to the highly metabolically demanding retinal photoreceptors. It is primarily a vascular tissue, with fenestrated capillaries and stroma containing melanocytes.

Pathophysiology: Primary choroidal melanoma arises from melanocytes within the choroid. Most choroidal melanomas are believed to develop from preexisting melanocytic nevi. De novo growth of choroidal melanomas also occurs. Three distinct cell types are recognized in choroidal and other uveal melanomas, as follows: spindle A, spindle B, and epithelioid. The latter cell type usually has the most aggressive behavior and carries a poorer prognosis for the patient's long-term survival.

Choroidal melanomas may have variable coloration, ranging from darkly pigmented to purely amelanotic. They typically are domed-shaped. As they enlarge, if they break through the Bruch membrane, they can assume a mushroom configuration. Other shapes found for these tumors are bilobular, multilobular, and diffuse. The latter is characterized by lateral growth throughout the choroid with minimal elevation. It occurs in about 5% of cases. Rarely, choroidal melanomas may arise in a multicentric distribution in one or both eyes.

Choroidal melanomas affect the retinal pigment epithelium as they push against it and deprive it from normal choroidal circulation. Overlying retinal pigment epithelium usually develops areas of atrophy, drusen, and localized pigment epithelial detachments. Areas of phagocytic activity, digesting cellular debris from melanocytes, give the pigment epithelium patches of coloration change. Macrophages within these typically orange areas contain melanin and lipofuscin. These changes can lead to choroidal neovascularization over the tumor, with consequent subretinal exudation, hemorrhage, and fibrous plaque formation.

Growth of choroidal melanomas can occur silently until it produces enough visual loss through various mechanisms. The tumor's disruption of choroidal circulation and consequent ischemia typically causes degeneration of retinal photoreceptors and other retinal neurons. The retina overlying the tumor can separate into cystoid spaces and larger schisis cavities. There may be associated cystoid macular edema.

In general, the farther away the tumor's origin is located from the optic nerve and fovea, the larger size it can reach before the patient notices a visual field defect. Exudation of fluid into the subretinal space with consequent retinal detachment may enlarge the field loss. This exudation can lead to total retinal detachment. Rarely, choroidal melanomas can impinge into underlying posterior ciliary nerves, causing severe ocular pain. Other signs and symptoms can result if the tumor grows anteriorly, pathologically involving the ciliary body, trabecular meshwork, and lens, with consequent ocular hypotension or hypertension and cataract. Large choroidal melanomas can induce iris rubeosis. Erosion of the melanoma into blood vessels in adjacent tissues, or areas of necrosis within the tumor, ca .....