This study showed that the arcade veins tended to adopt a more divergent path in eyes with IMH than the control eyes. This divergence in vascular paths was particularly evident temporal to the fovea. As a result, eyes with IMH might have a more chance to assume a V-shape arcade in contrast to the normal eyes which usually takes a U-shape path.
The results of our study are in line with Yoshihara et al. [10] who demonstrated that the eyes with IMH have a significantly wider retinal artery trajectory compared to the normal fellow eyes. Our study, however, has several differences from Yoshihara et al. in terms of the methods used to assess the arcade vessel trajectory. They picked retinal arteries, while we chose retinal veins. The logic behind choosing the veins over the arteries in our study was provided in the method section. Compared to Yoshihara et al., we also used a completely different and simpler way to measure retinal vascular paths. Moreover, their method mostly included the vascular path between the fovea and the disc, while we also analyzed the same distance temporal to the fovea. Anyway, with its distinct method, the present study confirmed the findings of Yoshihara et al. regarding a wider retinal vessel trajectory in eyes with IMH.
The fovea of the human eye is subjected to two main tractional forces from the posterior surface of the vitreous and tangential forces from the surrounding retina. The vitreous traction is supposed to be the most important cause of developing IMHs. [1, 11] The fovea would be torn off if the mentioned forces overcome its tensile strength. For eyes with a persistent attachment of the vitreous to the edges of the macular hole, vitrectomy is mandatory to resolve the vertical (z-axis) traction from the vitreous, and this step is considered the most important phase of the operation in these eyes. [12, 13]
It is not thoroughly understood that why some macular holes with released vitreous traction are spontaneously closed, while others would persist and ultimately need surgical intervention. The flow of liquid vitreous through the macular hole and tangential forces from the surrounding retina are the major culprits. [1, 2] A typical example of retinal tangential forces is the development of epiretinal membrane in the macular area. [14] A more subtle traction is introduced by the internal limiting membrane; hence, internal limiting membrane peeling is considered an essential part of modern macular hole surgery. [7–9]
It is difficult to measure retinal tangential forces in vivo. Intuitively, the wider path of arcade vessels might be correlated with a greater tangential force at the fovea. These forces should probably exert their effect via the y-axis (Fig. 1-B). In our study, we did not find any intergroup difference in the fovea-disc distance, which could be correlated with tangential forces in the x-axis. These justifications, however, need to be verified by future studies.
In addition, since the vitreous is usually attached firmly to retinal vessels, the larger distance between arcade vessels can generate stronger tractional forces (from the remaining vitreo-foveal attachment) on the fovea by inertia or momentary force during eye movements, leading to an increased chance for macular hole formation. [10]
We found no significant difference between eyes with IMH and normal fellow eyes in vertical arcade vein distances and proportions nasal to the fovea, while we showed a significant difference in D5 and D5/D3, indicating a wider vascular path for eyes with macular hole temporal to the fovea. This finding might emphasize the importance of vertical tractional forces that are exerted temporal to the fovea rather than the nasal tractions. Considering the temporal venous paths, and based on the D5/D3 measurements, we introduced a classification (i.e., V-shape vs. U-shape), which might be clinically useful in a rapid distinction between eyes with probably more y-axis tangential traction (V-shape; and though a greater risk of MH formation) versus those with less traction (U-shape).
This study is limited by its relatively small sample size. However, it seems acceptable from a statistical point of view, since we had matched controls (fellow eyes). Involving the fellow eye as control could be a power of the present study because it removes most of the confounding factors such as age, sex, and also most ocular biometric features, which are typically similar in fellow eyes. However, since the fellow eyes of IMHs are at greater risk of developing macular holes, [15] the findings of this study should be confirmed by future studies with larger sample sizes that include normal patients as well. Finally, we used Photoshop CS6 interface to calculate retinal distances; and therefore, the measured distances (in micrometers) are not exactly the actual values (and are somehow magnified). However, this issue affected all images in the same manner, and so it did not compromise the validity of statistical analyses or calculated proportions.