Many previous studies have analyzed the factors associated with visual outcome after vitrectomy for DME.
4,5,11–15 However, to the best of our knowledge, the present study is the first that analyzed the relationship between axial length and visual outcome and demonstrated a correlation between the two. This article reports that longer axial length predicts better postoperative BCVA after vitrectomy for nontractional macular edema secondary to diabetic retinopathy. Consistent with many reports showing better postoperative visual acuity in patients with good baseline vision,
4,5,12,15 our study also identified baseline visual acuity as an independent factor predicting postoperative visual outcome. ERM peeling
4 and eyes with signs of vitreomacular traction
13 are other factors reported to be associated with good postoperative visual acuity, whereas prior focal or grid macular laser is a factor significantly related to unfavorable postoperative visual outcome.
5,14 However, in our present study, ERM peeling and prior focal or grid macular laser was not significantly associated with postoperative visual acuity. A probable reason for the discrepancies in these reports is that the macular abnormalities of patients included in these studies differed to some extent. Among these reports, some included cases complicated by vitreomacular traction syndrome,
4,12,13 whereas others excluded these cases,
5,14,15 and one study included cases with concomitant ERM,
11 whereas another excluded them.
15 In the present study, 12% of our DME cases had ERM but patients with VMT were excluded. These baseline characteristics should be taken into consideration when interpreting the results.
Myopic foveoschisis is a macular complication of high myopia, and in these patients, vitrectomy and ILM peeling also improve visual acuity.
16 Thus, myopic foveoschisis may be an important confounding factor for the present study. The OCT characteristics of myopic foveoschisis are a marked increase in thickness of the retina in the posterior pole area, outer retinoschisis with intraretinal columns, and inner retinoschisis.
17 Furthermore, almost all cases have posterior staphyloma and the axial length is very long with a mean of 29 mm.
17 None of our patients had these characteristics. Therefore, myopic foveoschisis can be excluded as a factor that contributes to the favorable visual outcome after surgery observed in our patients.
We used partial coherence interferometry (IOLMaster) to measure axial length of all the eyes in the present study. Two reports confirm the accuracy of using partial coherence interferometry to measure axial length in eyes with macular edema, suggesting that IOLMaster is superior to A-scan ultrasonography in measuring axial length in eyes with macular edema and reducing postoperative refractive errors.
10,18 A double peak was observed in 4 eyes (8%) in the present study, and we used the posterior peak to calculate axial length. Kojima and colleagues
10 reported that when the posterior peak of the double peak was used in IOL calculation, 92% of eyes after IOL implantation were corrected within ±1.0 diopter of the postoperative refractive error. Therefore, the axial length is not likely to be affected by macular edema, and the present results have high reliability.
Our results showed that median axial length in eyes with a visible IS/OS line after surgery was significantly longer than that in eyes without a visible IS/OS line. And multivariate logistic regression analysis showed that short axial length increased the risk of poor integrity of the IS/OS line after PPV for DME. Moreover, a significant decrease in FTH at 6 months was found in eyes with an axial length more than 23.5 mm compared with eyes with axial length less than 23.5 mm. These results suggest that compared with eyes with shorter axial length, eyes with longer axial length show early improvement of macular edema after surgery and consequently milder photoreceptor dysfunction, which may explain the better visual outcome after vitrectomy for DME. Myopia has been suggested to have a protective effect against diabetic retinopathy.
6–8,19,20 Population-based, cross-sectional studies in persons with diabetes showed that myopic refraction and longer axial length were associated with a lower risk of diabetic retinopathy,
6,7,19 and were less likely to have mild and moderate DME.
8 Therefore, among eyes with diabetic retinopathy, those with longer axial length are less susceptible to develop DME, and even if DME occurs, vitrectomy results in early improvement of macular edema with a high probability of recovering visual acuity. However, the present study provides no physiological evidence for the mechanism by which axial length affects the visual outcome after vitrectomy for DME.
A past study in our department demonstrated that short axial length is significantly associated with more advanced proliferative diabetic retinopathy and a higher incidence of early vitreous hemorrhage after PPV, and also revealed a significant negative correlation between aqueous humor VEGF concentrations and axial length in eyes with proliferative diabetic retinopathy.
21 Diabetic eyes with shorter axial length tend to have higher intraocular fluid VEGF concentration. Therefore, assuming that VEGF continues to be overexpressed after surgery, some VEGF-induced effects may render eyes with short axial length more susceptible to prolonged macular edema, resulting in dysfunction of outer retinal layers and poor visual outcome after PPV for DME. On the other hand, diabetic eyes with long axial length have low basal VEGF production, and the intraocular VEGF concentration is further reduced after vitrectomy,
22 which may account for the early disappearance of macular edema.
In diabetic subjects, retinal blood flow increases with increasing severity of diabetic retinopathy, subsequently increasing retinal capillary pressure. This increased pressure may cause macular edema.
23–25 Several reports have demonstrated that ocular blood flow decreases as axial length increases,
26–28 and some reports speculated that the protective effect of increased axial dimension on diabetic retinopathy and DME could partially be due to reduced blood flow with axial elongation.
6–8 Thus, it has been hypothesized that the decrease in blood flow with increasing axial length plays a major role in this protective effect. Quigely and Cohen
29 postulated that elongation of the globe causes stretching and thinning of vessels, which reduces blood flow and consequently lowers the pressure exerted on vessel walls, thus providing a protective effect on diabetic retinopathy. Furthermore, Krepler and colleagues
30 suggested that vitrectomy induces significant reduction in ocular blood flow in patients with diabetic retinopathy. From these past reports and current results, it is possible that in DME cases, reduction in ocular blood flow after vitrectomy occurs earlier in eyes with longer axial length than in those with shorter axial length, and this phenomenon may promote early resorption of macular edema, minimizing dysfunction of photoreceptors, resulting in favorable postoperative visual outcome. Therefore, future studies should measure ocular blood flow after vitrectomy for DME, and analyze its relationship with axial length and postoperative visual acuity. In addition, because eyes with short axial length are at risk of delayed resorption of macular edema, it may be necessary to consider active postoperative treatments such as intravitreal triamcinolone injection and intravitreal injection of anti-VEGF agents in these eyes.
In conclusion, longer axial length predicts better postoperative BCVA after PPV for nontractional DME secondary to diabetic retinopathy. Earlier reduction of foveal thickness and preservation of the integrity of the IS/OS line are found in eyes with longer axial length, which may result in favorable visual outcome. Further research is needed to elucidate the mechanism by which axial length affects visual outcome after PPV for DME. In addition, ophthalmologists should take the axial length into account when assessing the visual prognosis of PPV for DME.