Given that impaired endothelial function has been observed to be an early feature in several systemic and ocular vascular-related diseases, much attention has been paid to the development of methods to noninvasively assess endothelial function in humans. As one of the most widely used techniques, the ultrasound-based FMD has been shown to give a reliable estimate of endothelium-dependent vasodilatation (10
). FMD is based on the capacity of blood vessels to self-regulate vascular tone in response to changes of shear stress caused by changes in blood flow. This regulation is dependent on endothelium-derived NO (14
) and can therefore be used as a marker for endothelial function.
Reduced FMD has been found in patients with mild systemic hypertension (15
), hypercholesterolemia (16
), and diabetes (17
), indicating for an impaired endothelial function in these patient groups. Additionally, it has been shown that FMD can predict future cardiovascular events (18
). However, the technique of FMD is hampered by the limited spatial resolution of the ultrasound systems currently available. In addition, measurement of FMD requires significant training and involves a subjective component when data are evaluated.
Flicker-induced vasodilatation may be another attractive noninvasive approach. It has been shown that flicker response is significantly diminished in patients with glaucoma or diabetes (5
). Even more importantly, a reduced response has also been observed in patients with systemic hypertension, indicating a potential insight into vascular function in general (7
), because an increase of blood pressure or IOP alone does not influence the flicker response (19
). These results support the hypothesis that the observed changes reflect long-term alterations of the vasculature. The hypothesis that flicker-induced vasodilatation may at least partially reflect endothelial function has also been encouraged by the observation that flicker-induced vasodilatation is mainly dependent on an intact NO synthesis (9
). Endothelial dysfunction due to abnormal release or action of NO is a well-recognized early feature of vascular damage, as it has been reported previously in vascular-related diseases like diabetes, hypercholesterolemia, systemic hypertension, and atherosclerosis (16
Our findings of greater baseline vessel diameters in patients with diabetes are in good accordance with earlier studies (23
). However, given that our measures were done only in one single artery and vein and not in all visible vessels, our data do not represent total cross-sectional retinal vessel diameters.
We observed a negative correlation between flicker-induced vasodilatation and blood cholesterol. This result is again a hint that endothelial dysfunction is involved in reduced flicker-induced vasodilatation, because cholesterol and oxidized LDL in particular are clearly associated with endothelial cell dysfunction (24
) and reduced bioavailability of NO. Evidence has been provided that reduction of serum cholesterol increases FMD and may therefore be beneficial for endothelial functions (22
). Whether this also holds true for flicker-induced vasodilatation has yet to be clarified. Correlations between FMD and age or plasma cholesterol, as observed earlier (16
), and between flicker-induced vasodilatation and age failed to reach level of significance after adjustment for multiple testing. Our study was, however, not designed for these outcome analyses, and a larger sample size may be required to investigate these issues. The present study provides evidence that in patients with type 1 diabetes and in patients with systemic hypertension and/or hypercholesterolemia, both FMD and flicker-induced vasodilatation are reduced compared with healthy volunteers. However, our study failed to show a strong correlation between FMD and flicker-induced vasodilatation.
What could be the reason for the differing responses between the two vascular beds? First and most importantly, FMD and flicker-light–induced vasodilatation differ in the method of stimulation. Whereas the diameter increase in FMD is caused directly by the augmented shear stress in the endothelium and the connected tissue, flicker response is basically the vascular answer to increased neural activity in the retina. This may be of special importance in patients with diabetes or glaucoma, since it cannot be ruled out that in these patients decreased neural activity may partially account for the decreased flicker response.
Second, it has to be noted that the properties of the vascular beds investigated differ significantly. Whereas flicker stimulation investigates arteries in an order of 150–250 μm, FMD reflects endothelial function in significantly larger vessels with different vessel wall properties. Thus, the weak correlation between FMD and flicker may indicate that the stimulation answer in the conduit arteries and in the smaller retinal arteries do not carry the same information, although both are diminished in patients with endothelial dysfunction. This phenomenon is also known from other experiments showing that FMD and endothelium-dependent vasodilatation assessed with an invasive technique that mainly reflects the endothelial function of resistance arteries are both independently related to the risk of coronary heart disease (25
). Flicker-induced vasodilatation may provide additional information to these techniques because of the smaller size of vessels assessed. This may particularly be interesting in diseases primarily affecting the microvasculature.
Flicker-induced vasodilatation offers a variety of significant advantages. On the one hand, it provides excellent reproducibility and sensitivity (12
). On the other hand, it is easily performed and quick, although pupil dilatation is required with the fundus camera used in the present experiments. Most importantly, the system does not include a subjective component once an optimal fundus image is achieved.
As a limitation of the study, no information is available about blood nitrate concentration. Although none of the subjects under study was under nitrate medication, we cannot fully exclude that a nitrate-rich diet may influence FMD or flicker-induced vasodilatation.
In summary, our data indicate that in both patient groups with endothelial dysfunction as assessed with FMD, flicker responses are diminished. The reason why no major correlation was found between FMD and flicker-induced vasodilatation needs to be the subject of further studies. Furthermore, whether flicker stimulation may also serve as a predictor for risk of systemic diseases, as it has been shown for FMD, has yet to be investigated in longitudinal studies. The system is, however, a candidate for assessing endothelial function in clinical routine because it induces minimum discomfort to the subject, provides good reproducibility and sensitivity, and does not include a subjective component.