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Diabetes is associated with abnormalities of red/green (protan) and blue/yellow (tritan) vision. 1 Protan and tritan colour thresholds are consistently raised in the presence of overt diabetic retinopathy. 2 Changes in colour perception occur in diabetic subjects before any detectable retinopathy by indirect funduscopy. 3 The pathological basis of these acquired colour vision defects in diabetes is unknown. The abnormal retinal perfusion and ischaemia that occurs early in diabetic retinopathy may contribute to the abnormalities in colour vision, perhaps as a consequence of relative retinal oxygen desaturation. 4 We examined, using computer graphics technology, 5-7 whether increasing circulating oxygen saturation improved the raised colour vision thresholds in diabetic subjects.
Materials and methods
Thirty seven, young, normotensive, insulin dependent diabetic (IDDM) patients were recruited from an inner city diabetic outpatient clinic. Assessment of diabetic control was obtained from their last outpatient HbA1c value. All subjects had no or minimal diabetic retinopathy (≤ grade 1). 8 No patient had received laser therapy, had previous ocular disease, or treatment likely to affect colour vision. Twenty seven non-diabetic hospital personnel of similar age without ocular pathology were recruited as controls. Patients and controls gave informed consent, the study having been previously approved by the hospital's medical ethics committee.
Studies were performed in the hospital's ophthalmology department. Corrected visual acuity was measured using the Snellen chart. The eye with the better visual acuity was tested for colour contrast sensitivity with the other eye occluded.
Colour contrast sensitivity was assessed measuring protan and tritan thresholds using a computer graphics system as previously described. 5-7 In order for the alphabetical letter displayed on the computer screen to subtend a constant angle on the retina, the subject is seated at a fixed distance from the screen. The size of the letter is set to create an image that tests the central 6 degrees of the retina. The letters displayed on the screen are of variable intensity on a background of equiluminance. The operator has no influence on the contrast of the test letter given and could only influence the result by indicating a wrong response. The computer finds the endpoint of the test by a double staircase method; this offers a darker letter that is easier to see following a wrong...