In nineteen patients with diabetic retinopathy the appearance and life history of cotton-wool spots were studied by serial color photographs and fluorescence angiograms.

Cotton-wool spots are a feature of both mild and severe diabetic retinopathy but, while their appearance is similar to that in hypertension, the associated capillary abnormalities are more severe. On fluorescence angiograms cottonwool spots are associated with arteriolar occlusion with an area of capillary closure and are surrounded by abnormal dilated capillaries.

Capillary abnormalities precede the development of arteriolar occlusion which is necessary for the development of cotton-wool spots.

Cotton-wool spots persist for long periods. Their mean half-life is 8.1 months in patients under forty years of age and 17.2 months in patients over the age of forty. Even when cotton-wool spots disappear, capillary closure persists. Following yttrium90 implantation of the pituitary gland the half-life of cotton-wool spots was 2.3 months.

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