Claudia Zawinka

Reduced response of retinal vessel diameters to flicker stimulation in patients with diabetes

Background/aim: Stimulation of the retina with flickering light increases retinal arterial and venous diameters in animals and humans, indicating a tight coupling between neural activity and blood flow. The aim of the present study was to investigate whether this response is altered in patients with insulin dependent diabetes mellitus. Methods: 26 patients with diabetes mellitus with no or mild non-proliferative retinopathy and 26 age and sex matched healthy volunteers were included in the study. Retinal vessel diameters were measured continuously with the Zeiss retinal vessel analyser. During these measurements three episodes of square wave flicker stimulation periods (16, 32, and 64 seconds; 8 Hz) were applied through the illumination pathway of the vessel analyser. Results: In retinal arteries, the response to stimulation with diffuse luminance flicker was significantly diminished in diabetic patients compared to healthy volunteers (ANOVA, p<0.0031). In non-diabetic controls flicker stimulation increased retinal arterial diameters by +1.6% (1.8%) (mean, p<0.001 v baseline), +2.8% (SD 2.2%) (p<0.001) and +2.8% (1.6%) (p<0.001) during 16, 32, and 64 seconds of flicker stimulation, respectively. In diabetic patients flicker had no effect on arterial vessel diameters: +0.1% (3.1%) (16 seconds, p = 0.9), +1.1% (2.7%) (32 seconds, p = 0.07), +1.0% (2.8%) (64 seconds, p = 0.1). In retinal veins, the response to flicker light was not significantly different in both groups. Retinal venous vessel diameters increased by +0.7% (1.6%) (16 seconds, p<0.05), +1.9% (2.3%) (32 seconds, p<0.001) and 1.7% (1.8%) (64 seconds, p<0.001) in controls during flicker stimulation. Again, no increase was observed in the patients group: +0.6% (2.4%), +0.5% (1.5%), and +1.2% (3.1%) (16, 32, and 64 seconds, respectively). Conclusion: Flicker responses of retinal arteries and veins are abnormally reduced in patients with IDDM with no or mild non-proliferative retinopathy. Whether this diminished response can be attributed to altered retinal vascular reactivity or to decreased neural activity has yet to be clarified.

Influence of change in body position on choroidal blood flow in normal subjects

Aim: To compare subfoveal choroidal blood flow (ChBF) in sitting and supine positions in normal volunteers. Methods: ChBF was measured with laser Doppler flowmetry in 22 healthy volunteers of mean (SD) age 24 (5) years. Six independent measurements of ChBF were obtained in one randomly selected eye of each subject while seated. The subjects then assumed a supine position for 30 minutes and a new series of six measurements was obtained. The mean values of the two series were calculated. Systemic brachial artery blood pressure and intraocular pressure were measured in the sitting and supine positions. Ocular perfusion pressure (OPP) was calculated based on formulae derived from ophthalmodynamometric studies. The influence of changing OPP during change in body posture on the change in ChBF was assessed by linear regression analysis. Results: ChBF decreased by 6.6% (p = 0.0017) in the supine position. The estimated ophthalmic blood pressure in the supine position was adjusted to obtain a result of no change in OPP for no change in ChBF, yielding a mean decrease in the estimate of OPP of 6.7% (p = 0.0002). The necessary adjustment for the estimate of OPP in the supine position suggested a marked buffering of the change in perfusion pressure by the carotid system. The relative decrease in OPP correlated significantly with the relative decrease in ChBF (R2  =  0.20; p = 0.036) with a slope for the regression line of 1.04. Conclusions: The comparable degree of change in ChBF and OPP and the linear relationship between the two parameters suggest a passive response of the choroidal circulation to a change in posture. In contrast, the carotid system seems to control the gradient in perfusion pressure closely between the heart and its branches.

Effects of dopamine on retinal and choroidal blood flow parameters in humans

Aim: To investigate the effect of dopamine on retinal and choroidal blood flow in humans. Methods: We investigated the effect of two doses of intravenous dopamine (5 and 10 µg/kg/min) via a randomised double-masked crossover study in 12 healthy subjects chosen from a total of 16. Blood flow parameters in retina, optic nerve head and choroid were assessed with bi-directional laser Doppler velocimetry, laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude, respectively. Results: Intravenous dopamine dose-dependently increased retinal blood cell velocity and fundus pulsation amplitude (p<0.001). At the highest administered dose red blood cell velocity in retinal vessels increased by 37% and fundus pulsation amplitude by 24%. By contrast, optic nerve head blood flow did not change with dopamine administration. Conclusions: Our data indicate that dopamine has a pronounced enhancing effect on the retinal perfusion in humans. Further studies are required to establish the exact role of dopamine in the regulation of choroidal and optic nerve head blood flow.