It was the purpose of the present study to examine dark adaptation in subjects with type 2 diabetes during transient hyperglycemia. Twenty-four subjects with type 2 diabetes and minimal diabetic retinopathy were randomized to undergo an oral glucose tolerance test (OGTT) or to remain fasting. Dark adaptometry was measured in one eye, chosen at random, using a computer-controlled dark adaptometer. Dark adaptation and capillary blood glucose were measured at baseline and 80 minutes into the OGTT/fasting test. Blood glucose remained stable throughout the examination in the 12 fasting subjects, whereas glycemia increased in the 12 OGTT subjects, from 8.6±2.1 at baseline to 21.1±3.6 mM after 80 min. In the OGTT group, four out of seven subjects with delayed dark adaptation at baseline reached normal values during hyperglycemia. All examined aspects of rod adaptation were accelerated by hyperglycemia (time to rod-cone break -26%; time to rod intercept -16%, rod sensitivity recovery slope (log units/min) +35%), whereas no measurable change in cone adaptation was seen. The results are consistent with rod adaptation being limited by glycemia and with rod adaptation being delayed in subjects with diabetes compared with healthy subjects, the delay being reversible in response to hyperglycemia.
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