There is an embryological explanation for retinal detachment in that the separating layers open up a potential space that existed during the early development of the eye, as described previously (Chapter 2). The inner lining of the eye develops as two layers. In its earliest stages of development, the eye is seen as an outgrowth of the forebrain, the optic vesicle, the cavity of which is continuous with that of the forebrain. The vesicle becomes invaginated to form the optic cup, and the two-layered cup becomes the two-layered lining of the adult eye. Anteriorly in the eye, the two layers line the inner surface of the iris and ciliary body. Posterior to the ciliary body, the outer of the two layers remains as a single layer of pigmented cells, known as the m • • • K • '

Figure 13.1. Histology of retinal detachment showing the location of subretinal fluid.This eye has an underlying choroidal melanoma. CO

pigment epithelium. The inner of the two layers becomes many cells thick and develops into the sensory retina. In the adult, the sensory retina is closely linked, both physically and metabol-ically, with the RPE and, in particular, the production of visual pigment relies on this juxtaposition. When the retina becomes detached and the sensory retina is separated from the RPE, the retina can no longer function and the sight is lost in the detached area. Both RPE and sensory retina are included in the term "retina" and in this sense "retinal detachment" is a misnomer.

The retina receives its nourishment from two sources: the inner half deriving its blood supply from the central retinal artery, and the outer half from the choroid. The important foveal region is supplied mainly by the choroid. When the retina is detached, the central retinal artery remains intact and continues to supply it because it is also detached with it. The outer half of the retina is deprived of nourishment, being separated from the RPE and choroid. Eventually degenerative changes appear, the fovea being affected at an early stage. It is interesting that after surgical replacement the retina regains much of its function during the first few days but further recovery can occur over as long a period as one or even two years.

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