This retrospective study of 26 patients with unilateral NAION found greater blood flow velocity of CCA in the affected eye than in the contralateral unaffected eye. The same findings were also seen in the subgroup analysis of the presence/ absence of diabetes mellitus and the presence of hypertension except in the subgroup of the absence of hypertension and the presence/ absence of hyperlipidamemia. There were no significant differences in the diameters of the CCA, ICA, ECA and the thickness of carotid artery plaque in the affected eyes versus those if the unaffected eyes.
NAION is a major cause of severe vision impairment or blindness in the middle-aged and elderly population [13]. Despite much study, the pathogenesis of NAION remains unclear, with contradictory findings. The main source of blood supply to the anterior part of the optic nerve is the PCA circulation via the peripapillary choroid and the circle of Zinn and Haller [14]. An ischemic disorder of the PCA circulation in the ONH is the most frequent cause of NAION [2]. The findings regarding the retrobular haemodynamics associated with ONH are controversial. We systemically reviewed the literatures on retrobular haemodynamics and carotid arteries in patients with NAION and the published results were summarized in Table 3. As shown in Table 3, the findings in the literatures on the blood flow velocity of the OA were inconsistent. Some of them found there was no difference on the blood flow velocity of OA between affected side and the unaffected side[7] or between the affected side and normal controls [5] ,while others found the blood flow velocity of OA were decreased in affected sided compared with the unaffected sided [6]. The findings of the blood flow velocity of CRA also were inconsistent [5, 7]. In individuals with NAION, most published studies reported increased IMT of the CCA on the affected side as compared to that on the unaffected side [6–8], with only one study finding no difference in the IMT of the CCA between the affected and unaffected sides [9].
In our retrospective analysis, we found no significant difference in the diameters of the ICA, ECA and CCA between the affected and unaffected sides of patients (n = 26) with NAION. However, the blood flow velocity of CCA in the affected side was increased as compared to that in the unaffected side. Our findings suppport carotid artery blood flow acting like laminar flow of a fluid through a tube, as described by Bernoulli’s equation [10], with the flow velocity of the CCA increasing as a result of vascular constriction or narrowing. A previous study detected no significant change in the blood flow velocity of the CCA on the affected side in those with NAION as compared to that in healthy controls [7]. No studies have investigated on comparison of blood flow velocity of the CCA between the affected side and the unaffected side. Based on our findings and those in the literature [5–9], we suppose that the increased blood flow velocity of the CCA might become a marker of NAION prior to changes in the IMT or vacular caliber. Further studies on larger scale are needed to confirm our assumption and to elucidate the normal range of differences in blood flow velocities of CCA between the two sides in a healthy population and in individuals with NAION.
Although we designed a self-control study to exclude potential confounders and enhance the validity of the findings, our study has some limitations. Among potential confounding factors were the differences in the haemodynamics of the affected and unaffected sides. It is possible that there were other unknown differences in the haemodynamics of vessesls of both sides other than that of CCA influenced measurements of the independent variables in the present study. In addition, due to the lack of data from matched normal controls, we were unable to determine whether the range of blood flow velocities or the differences between the affected and unaffected sides were within the range of those observed in a normal population.
In conclusion, increased blood flow velocity of the CCA might be associated with the development of NAION. Increased CCA blood flow velocity might be potentially serve as a marker of the development of NAION prior to other CCA-related abnormalities, such as changes in the IMT or vascular caliber if further study on larger scale is performed in the future.