This retrospective study included 27 eyes of 27 consecutive iERM patients who underwent vitrectomy with ICG-assisted membrane peeling with intraoperative photo documentation (Olympus, Visera, OTV-S7, Tokyo, Japan) in the Department of Ophthalmology of the Medical University of Vienna. All the research and measurements were performed in accordance with the tenets of the Declaration of Helsinki. The ERM diagnosis was made following standard methods including slit lamp fundus biomicroscopy, fundus photography and OCT examination with Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, California, USA).
Inclusion criteria: patients demonstrating distinct ICG staining with good quality intraoperative photos, no considerable elevation of the iERM from the underlying surface of the retina and HD-OCT images taken within 30 days preoperatively without any retinal thickness measurement errors were included.
Exclusion criteria: patients who had at least one of the following: patchy ICG staining, bad quality intraoperative photos, considerable elevation of the iERM from the underlying surface of the retina, HD-OCT images older than 30 days preoperatively or retinal thickness measurement errors.
Of all studied eyes, the operative decision was based on disturbing metamorphopsia, diplopia or reduced visual acuity. The level of the increase in thickness of the maculae was not a concern.
Histologically, ICG outlines the ERM by staining selectively the exposed ILM beyond the iERM margin.17
Thus, these photos illustrated a central unstained iERM surface surrounded by a green dyed iERM-free neighbouring retina (). To make these photos evaluable and comparable with the different thickness macular measurements obtained by RTM, each photo had to be edited (well rotated and resized) to fit exactly its corresponding enface OCT image, which in turn represents the definitive position and size of the corresponding RTM (). Therefore, the investigators were blinded to neither the OCT images nor the intraoperative photos.
Figure 1 Illustrates the editing steps of the images of two patients. A, The intraoperative photos before editing. B, The photos after being rotated and resized according to their en face image. Note the central unstained epiretinal membrane (ERM) surface surrounded (more ...)
We identified the iERM area on intraoperative images as the unstained central area outlined by ICG-stained retina. Similarly, each colour on OCT-RTM (white, red, orange and yellow coded areas), representing an area with a specific range of retinal thicknesses, was compared with the iERM contour.
Analysis of iERM shape
Subjectively, we assessed the shape similarity between each unstained central area on the intraoperative images and its corresponding thickness-indicating colours map. We evaluated it as yes if the whole unstained iERM area including its contour looks similar to its corresponding RTM, and no if there was no obvious similarity. In addition, the presence and direction of any retinal folds were also determined by two certified ophthalmologists.
Analysis of iERM size
The unstained macular surface (iERM) areas of the edited intraoperative photos as well as each pathological false colour coding (white, red, orange, yellow) on the (6×6 mm) RTM were measured with the assistance of Image J program (freeware, National Institutes of Health, Bethesda, Maryland, USA). Subsequently, we used a predefined excel program that converts the pixel into quadratic micrometre size. The correlation between both groups and the mean of all calculated areas were explored. The correlation between iERM and thickness-indicating colours was determined by the Pearson correlation test. The reported p value is a result of a two-sided test. A p value of p≤0.05 was considered significant.