This study shows that retinal arteriolar diameter decreases significantly in patients with neovascular macular degeneration after the ITV injection of bevacizumab. In addition, the decrease in the arteriolar diameter was observed 1 week after the first injection, and this decrease continued until the end of the study, suggesting a long-term effect of bevacizumab on vascular tone.
Bonnin et al.
18 has recently shown a 10% decrease of time-averaged mean blood flow velocity in the central retinal artery 4 weeks after a single ITV injection of bevacizumab using ultrasound imaging. Arterial blood flow velocity assessed upstream of an organ provides an indirect estimate of the downstream vascular resistance.
19 Therefore, the arterial blood flow velocity recorded upstream of the retina can reflect its vascular resistance. If the central retinal artery diameter did not change during their study, the 10% decrease of the blood flow velocity in central retinal artery was in accordance with the 9.1% decrease of retinal blood flow observed in our study. A prospective study of Soliman et al.
20 on 10 eyes has also shown a decrease in retinal arterial and vein diameter of 0.2% and 1.9%, respectively, after three bevacizumab injections. However, these vessel diameter decreases did not reach statistical significance. The retinal vessel diameter was assessed at baseline and 1 month after the third injection using early phase fluorescein angiograms coupled with customized software. The modest and nonsignificant decrease observed by Soliman et al.
20 differs from our results that revealed a significant vasoconstrictive effect of bevacizumab. Another researcher revealed significant retinal arteriolar vasoconstriction in patients with neovascular AMD after three ITV injections of ranibizumab, an anti-VEGF molecule that is similar to bevacizumab.
21 Indeed, ranibizumab, like bevacizumab, neutralizes all isoforms of VEGF
2 and may potentially have similar ocular adverse effects. These researchers found a significant mean decrease of 8.1% and 17.6%, respectively, 4 weeks after the first injection and 4 weeks after the third injection. These changes, observed with the retinal vessel analyzer (Zeiss FF, Jena, Germany), were highly significant and represent an overall decrease that is more pronounced than the decreased observed in our study. These results taken as a whole might suggest a more pronounced effect of ranibizumab than bevacizumab on vascular tone. Prospective studies comparing both anti-VEGF molecules are needed to assess this hypothesis.
While the arteriolar diameter decreased significantly by a mean 4.3% during our study, the mean flow decreased by 9.1% over the same period but did not reach statistical significance, probably because of high variability. Blood flow is calculated according to Poiseuille's law, and because it depends directly on the square of the diameter (and on the velocity, which was virtually unchanged), we expected to observe a significant change in arteriolar blood flow. The absence of statistically significant changes in blood flow could be related to diverse limitations of the velocimetry technique; therefore, the most important and reliable result would be the decrease in arterial diameter.
13,22 The coefficient of variation for diameter (2.0%) is smaller than for velocity and flow (19.9% and 19.3%, respectively).
22 The variability in velocity would therefore be the primary source of the variability of flow, and is induced by the patient's eye motion and centerline displacement of the laser beam.
22 Because of parabolic blood flow—with faster blood flow in the center of the vessel—a small displacement of the laser beam will bias the velocity measurement, thereby decreasing the measured velocity and increasing measurement variability. Moreover, the accuracy of the eye movement detector that maintains the laser beam at the centerline of the vessel depends on the contrast of the retinal vessel against the local fundus background and also on the noise source picked up by the detector.
23 Imperfect media in our neovascular AMD patients could have decreased the accuracy of the eye movement detector and increased measurement variability in our study. Another source of measurement variability in our study relates to the ocular surface. Symptoms and signs of dry eyes, especially for the tear film break-up time,
24 increase significantly with aging.
24,25 A break-up of the tear film during the measurement can disrupt the anterior cornea power and cause a drift of the laser beam away from the center of the vessel. The decrease in fixation, media clearness, and tear break up time can contribute to an increase in the coefficients of variability, especially for velocity and flow, thereby contributing to a statistical assessment of the blood flow decrease as not significant. The vessel diameter is assessed by a separate laser segment integrated into the CLBF and placed perpendicularly to the target vessel. This technology is less sensitive to the above factors. These discrepancies could have been counteracted by increasing the sample size. A post-hoc sample size calculation for the variable retinal arteriolar blood flow indicated that 66 subjects would have been necessary to achieve adequate power. This sample size was calculated with a two-sided one-sample
t-test considering the Bonferroni correction to conserve a global alpha level of 0.05.