The various types of peripapillary crescents which are observed clinically are generally interpreted to represent misalignments of the edges of the neural retina, retinal pigment epithelium (RPE), choroid, and sclera at the disc margin. In order to test the validity of conventional assumptions about the anatomic vasis for each type of crescent, the authors compared histologic sections to previously obtained clinical photographs and fluorescein angiograms of 21 eyes enucleated for choroidal melanomas. The authors' results define several configurations of peripapillary tissue, but also show that there can be more than one anatomic vasis for some appearances. A scleral lip, which consists of an anterior extension of sclera to separate the choroid from the optic nerve head, is nearly always present, and ophthalmoscopically appears as a white rim that marks the disc margin and accumulates fluorescein after the vascular transit in an angiogram. A chorioscleral crescent occurs when the RPE is retracted from the disc margin, most prominent when associated iwth a tilted exit canal for the axon bundles through the sclera. In such a crescent, the choroid may be thinned or absent next to the disc, exposing to view some of the underlying sclera. Malposition of the embryonic fold occurs when the boundary between the neural retina and the RPE does not coincide with the embryologically formed fold in the neuroectoderm that occurs at the disc. When this occurs, there is either a double layer RPE (forming a very dark pigment crescent) or a double layer of incompletely formed neural retina adjacent to the disc. Irregular pigmentation occurs when the RPE adjacent to the disc lacks pigment or is hyperpigmented, mimicking misalignment of the layers when viewed by ophthalmoscopy.
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