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A possible new preoperative indicator for postoperative visual acuity via optical coherence tomography
In this issue of the British Journal of Ophthalmology, Ota and his associates (see page 1644)1 report that the presence of the third high reflectance band (HRB) in images obtained by optical coherence tomography (OCT) postoperatively is correlated with the visual outcome after different treatments for macular oedema secondary to branch retinal vein occlusion (BRVO). They also investigated whether the appearance of the preoperative OCT images could predict the final visual acuity. Their results indicated that the presence of the third HRB in the parafoveal area preoperatively could indeed be a predictor of the postoperative visual acuity.
Grid laser photocoagulation has been the standard treatment for macular oedema secondary to BRVO.2 However, over the past decade, more interventional therapeutic options have emerged. Vitrectomy with or without arteriovenous sheathotomy,3,4,5 intravitreal triamcinolone injection6 and intravitreal injection of tissue plasminogen activator7 have been used to treat macular oedema. The efficacies of these interventions, however, are controversial because of the lack of a randomised controlled studies.8 In a previous study, the authors reported the efficacy of intravitreal tissue plasminogen activator injection,7 and also demonstrated a correlation between the presence of the third HRB and visual outcome.9 The current report includes 46 eyes treated by the other interventions, with 19 of the 46 eyes treated by two or more interventions. Even after the resolution of macular oedema, some of the eyes did not achieve a significant improvement in visual acuity. Therefore, their retrospective study focused on eyes in which macular oedema was resolved, and the foveal thickness in the 46 eyes studied was <250 μm after the treatments. The significant correlation between the absence of the third HRB and poor postoperative visual acuity identified one of the factors accounting for the poor visual outcomes after successful anatomical results.
The recently developed high resolution OCT allows not only accurate quantitative measurements of macular thickness but also qualitative analysis of the different retinal layers. The third HRB is reported to represent the junction of the inner and outer segments of the photoreceptor cells,10,11 and thus the absence of the third HRB was suggested to indicate a degeneration or disorganisation of the photoreceptor cells. To explain the mechanisms leading to the absence of the third HRB, Ota et al argue that severe ischaemia or swelling of the foveal photoreceptor layer could lead to significant disorganisation of the photoreceptors. Although there were no angiographic data to confirm this in the current study, fluorescein angiography can show macular ischaemia and vascular leakage, which are widely accepted as signs of the severity of macular oedema. Severe ischaemia often reduces the macular oedema without improving the visual function. By contrast, it has been reported that eyes with incomplete macular perfusion, the ischaemic type, had a higher incidence of an improvement in visual acuity than eyes with macular perfusion, the nonischaemic type.12 Therefore, fluorescein angiography could have provided additional information to explain the mechanism determining the visual outcome.
Investigations that attempt to determine whether the presence of the third HRB is a prognostic factor of the postoperative visual acuity are of great value. Thus, it would be of interest to use OCT images of the parafoveal area because retinal haemorrhages and severe thickening of the retina often prevent the acquisition of a clear image of the outer retina at the fovea before treatment. Indeed, the third HRB at the fovea could be detected in only 4 of the 29 eyes in the study by Ota et al, and the findings in these eyes were not correlated with the postoperative visual acuity. One of the reasons for this might be that the other 25 eyes included eyes with an intact photoreceptor layer but without the third HRB in their images due to retinal haemorrhage or severe swelling of the inner retinal layers. By contrast, the presence of the third HRB in the parafoveal (eg, at 500–1000 μm) unaffected area superior to the fovea, was well correlated with the postoperative visual acuity. The findings in this area might reflect the severity of the macular oedema because the association between the absence of the third HRB at the unaffected area and the distance from the fovea indicated the extent of macular oedema or retinal haemorrhage. It is reasonable to assume that extensive macular oedema or retinal haemorrhage would be associated with poor visual improvement.
The preoperative visual acuity is a very important prognostic factor in eyes with clear media.5,13,14 It is well known that good preoperative visual acuity results in a better improvement of visual acuity not only in the natural course of the disease process but also after treatment. In the 46 eyes in the Ota et al study, there was no difference in the initial visual acuity between the groups with or without the third HRB after treatment. For their second investigation of prognostic factors, 29 of 46 eyes were examined using OCT3, which resulted in a different grouping. These eyes were divided into subgroups determined by the absence or presence of the third HRB. Therefore, the initial visual acuity in each group, with or without the third HRB, should be shown at each point from the fovea. The authors, however, did not mention the association between the preoperative visual acuity and the preoperative third HRB at 500 μm and 1000 μm from the fovea where the absence of the third HRB is well correlated with poor visual outcome. These findings would be more valuable as prognostic factors if there had been no difference in the initial visual acuity between the groups with or without the third HRB around the fovea.
The presence or absence of the third HRB around the fovea determined from the OCT image is supposed to be a predictor of the visual prognosis in eyes with macular oedema secondary to BRVO. The inclusion criterion of this study was eyes with absorbed macular oedema, and the retrospective style of the study could limit the conclusions. Because the unresolved macular oedema can also lead to poor prognosis, it is necessary to evaluate if the third HRB around the fovea can also predict refractory macular oedema. Prospective studies are needed to determine whether the presence of the third HRB around the fovea can be widely accepted as a prognostic factor after treatment for macular oedema secondary to BRVO.
Competing interests: None declared.