What is the pathophysiology of choroidal melanoma?

Updated: Dec 12, 2018
  • Author: Enrique Garcia-Valenzuela, MD, PhD; Chief Editor: Andrew A Dahl, MD, FACS  more...
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Answer

Answer

Primary choroidal melanoma arises from melanocytes within the choroid. Most choroidal melanomas are believed to develop from preexisting melanocytic nevi, though de novo growth of choroidal melanomas also occurs. Three distinct cell types are recognized in choroidal and other uveal melanomas: (1) spindle A, (2) spindle B, and (3) epithelioid. The last 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 dome-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 last of these 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 1 or both eyes.

Choroidal melanomas affect the retinal pigment epithelium as they push against it and deprive it of normal choroidal circulation. Overlying retinal pigment epithelium usually develops areas of atrophy, drusen, and localized pigment epithelial detachments.

Areas of phagocytic activity, where cellular debris from melanocytes is digested, 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. [2] The tumor’s disruption of choroidal circulation and consequent ischemia typically cause 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 the tumor’s origin is from the optic nerve and fovea, the larger the tumor can become 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, can lead to vitreous hemorrhage or hyphema.

Choroidal melanoma ultimately causes death, practically always secondary to distant metastases rather than local spread. Its metastatic potential depends on the histopathologic aggressiveness of the tumor cells. Unfortunately, it not infrequently metastasizes before diagnosis. If the melanoma does not show extraocular extension, it can only spread hematogenously, because there are no lymphatic vessels in the eye. It most often metastasizes to the liver; other organs of dissemination include the lung, bone, skin, and central nervous system (CNS).

Less frequently, choroidal melanoma can grow transsclerally, through emissary channels, and metastasize locally into the orbit or rarely the conjunctiva. Choroidal melanoma almost never extends through the optic nerve; when it does, it is usually in juxtapapillary tumors or in diffuse choroidal melanomas.


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