The iris itself often shows degenerative changes in longstanding diabetics. The pupil can react sluggishly and fail to dilate widely after the instillation of mydriatic drops. The surgeon can appreciate that pigment is easily lost from the iris when it is handled, and it is interesting that a characteristic vacuolation of the pigment epithelium lining the posterior surface of the iris is seen in histological sections. When diabetes seriously interferes with the circulation of the eye, the iris can become covered on its anterior surface by a fibrovascular membrane. To the naked eye, the iris takes on a pinkish colour, but examination with the slit-lamp biomicroscope or a magnifying lens soon reveals the minute irregular blood vessels on its surface. The appearance is known as "rubeosis iridis" or neovascularisation of the iris (Figure 21.1). Neo-vascular glaucoma occurs once the rubeosis involves the anterior chamber angle. If left untreated, few eyes with rubeosis iridis retain useful sight. The iris should be examined carefully before pupillary dilation.
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