Disorganization of the inner retinal layers in diabetic macular edema: systematic review

The objective of this article was to review the disorganization of inner retinal layers as a biomarker in diabetic macular edema. A systematic search was conducted in PubMed®/MEDLINE®, Cochrane and Embase until August 2021. The keywords used were: “disorganization of inner retinal layers (DRIL)”, “diabetic macular edema (DME)” and “biomarkers”. No restrictions were imposed on the types of study to be included. The studies selected for eligibility were those that included the diagnosis of diabetic macular edema (center involved, resolved), that were well documented with spectral domain optical coherence tomography, that included disorganization of inner retinal layers as one of the reported alterations, with a follow-up of at least 3 months, and those in which the best corrected visual acuity was evaluated pre and post. There were no limitations regarding the type of treatment established. References of identified studies were searched for additional relevant articles. Articles not published in peer review journals were excluded. All studies were evaluated by two investigators independently. When one of them was in doubt, it was assessed by a third evaluator. A total of seven studies were included. Four were retrospective, longitudinal cohort study and three cross-sectional observational. Regarding the population studied, 61.5% were men and 38.4% were women, most of them had diabetes mellitus type 2 (85.8%). Regarding the stage of diabetes, the percentage of patients with mild nonproliferative diabetic retinopathy was 28.2%, with moderate nonproliferative diabetic retinopathy was 28.5%, with severe nonproliferative diabetic retinopathy was 15.9% and with nonproliferative diabetic retinopathy was 27.4%. In 100% of the studies, the diagnosis of diabetic macular edema in the center involved was included by spectral domain optical coherence tomography (Heidelberg). In all the studies, the presence of disorganization of inner retinal layers was recorded and its association with best corrected visual acuity was evaluated. The measurement was carried out using the LogMAR scale. In all the studies, the presence or absence of disorganization of inner retinal layers was associated with the best corrected worse/better final visual acuity using p <0.05 as a statical significance. The disorganization of inner retinal layers as a biomarker and their presence have shown to be important predictors of visual acuity in the future in patients with diabetic macular edema. Histopathological studies are required to understand its mechanism of action.


RESUMO
O objetivo deste artigo foi revisar sobre a desorganização das camadas internas da retina como biomarcador no edema macular diabético. Uma busca sistemática foi realizada no PubMed®/ MEDLINE®, Cochrane e Embase até agosto de 2021. As palavras-chave utilizadas foram "disorganization of inner retinal layers (DRIL)", "diabetic macular edema (DME)" e "biomarkers". Não foram impostas restrições quanto aos tipos de estudo a serem incluídos. Os estudos selecionados para elegibilidade foram aqueles que incluíram o diagnóstico de edema macular diabético (centro envolvido, resolvido), que foram bem documentados com tomografia de coerência óptica de domínio espectral, que incluíram a desorganização das camadas internas da retina como uma das alterações relatadas, com acompanhamento de pelo menos 3 meses, e aqueles em que a melhor acuidade visual corrigida foi avaliada pré e pós. Não houve limitações quanto ao tipo de tratamento estabelecido. Referências de estudos identificados foram pesquisadas para artigos relevantes adicionais. Foram excluídos os artigos não publicados em revistas de revisão por pares. Todos os estudos foram avaliados por dois investigadores de forma independente. Quando havia dúvida com algum deles, a mesma era avaliada por um terceiro avaliador. Um total de sete estudos foram incluídos. Quatro eram estudos de coorte INTRODUCTION Diabetic mellitus (DM) is a global epidemic with significant morbidity and mortality. Diabetic retinopathy (DR) is a major complication of DM and a leading cause of vision loss in middle-aged working adults. (1) Diabetic macular edema (DME) is the most common cause of vision loss in DR. (2) Standard treatment for DME involves repetitive, invasive intraocular injections, which place heavy burdens on the patient, physician, and health care reimbursement. (3) Despite advances in treatment, there are no reliable methods to determine which individuals with DME will gain or lose vision. (4) Attempts have been made to analyze biomarkers to see the anatomical and functional response to the treatments available to date. Among the parameters studied to evaluate visual results there are: glycemic control, retinopathy severity, presence of DME, and extent of retinal thickness. All of them have been associated but cannot be future visual predictors and cannot be individualized to each patient. (4) The same occurs with circulatory, genetic, aqueous humor and vitreous humor biomarkers. (5) With the technological advance in imaging diagnosis, new biomarkers have been studied.
Optical coherence tomography (OCT) has been an indispensable tool to evaluate response to treatment in patients with DME. Optical coherence tomography is a rapid, non-invasive imaging modality that uses light waves to render a cross-sectional view of the retina in vivo. It creates high-resolution images of retinal morphology, allowing for volumetric quantification of retinal and choroidal thickness (CT) and DME. (4)(5)(6) In 2012, Soliman et al., using spectral domain optical coherence tomography (SD-OCT), found that the disorganization of the inner retinal layer (DRIL) was correlated with current visual acuity (VA) in patients with existing or resolved center-involved DME. (6) In 2014 Sun et al. studied the DRIL and found that the DRIL in the 1-mm foveal area was associated with VA, and that the change in DRIL predicts future change in VA. (4) Based on the findings of Sun et al., DRIL has been evaluated together with other anatomical characteristics by OCT, evaluating anatomical and visual response in patients receiving anti-VEGF or intravitreal corticosteroids. (7)(8)(9)(10) Identifying imaging findings and biomarkers capable of predicting risks of development and progression of DR is important to screen, monitor, and treat patients. (5) For this reason, we conducted a systematic review to evaluate all the existing information on DRIL in DME as a non-invasive biomarker.

SEARCH STRATEGY AND STUDY SELECTION
Two investigators performed an electronic search, following the recommendations of Preferred Reporting Items for Systematic review and Meta-Analyses (PRISMA) (Figure 1). An electronic search without date or language restrictions was carried out in the following databases: PubMed®/MEDLINE®, Scopus and Embase for articles published until August 2021. The keywords used were: "disorganization of inner retinal layers (DRIL)", "diabetic macular edema (DME)" and "biomarkers". No restrictions were imposed on the types of study to be included. The studies selected for eligibility were those that included the diagnosis of DME (center involved, resolved), that were well documented with SD-OCT, that included DRIL as one of the reported alterations, with a follow-up of at least 3 months and whose best corrected VA (BCVA) was pre and post evaluated. There were no limitations regarding the type of treatment established. References of identified studies were searched for additional relevant articles. Articles not published in peer review journals were excluded. All studies were evaluated by two investigators independently. When one of them was in doubt, it was assessed by a third evaluator, it was assessed by a third evaluator.

Operational definitions
Center involved DME was defined as SD-OCT central subfield thickness (CST), ≥320μm for men and ≥305μm for women. (11) Resolved DME was defined as SD-OCT CST <320μm for men or <305μm for women. (11) discrepancies resolved after discussion. According to the Oxford Center for Evidence-Based Medicine Levels of Evidence, the methodological quality of the studies was graded. Data included first author, year of publication, number of eyes, gender, type of diabetes, stage of the DR, follow-up (months), diagnosis, type of OCT, treatments used, pre and post BCVA, and the biomarkers found associated with gain or loss of vision were recorded on a standardized data collection sheet. Descriptive statistics were performed using mean and standard deviation (SD) for continuous variables, and numbers (n) and percentages (%) for categorical data, using the Stata software version 15.1 (StataCorp. 2015, Texas, United States).

RESULTS
Seven studies were included in the analysis. Regarding the characteristics of the studies included (Table 1), three studies were conducted in the United States, one was multicentered, one in the United Kingdom, one Japan and one in Turkey. Four were retrospective, longitudinal cohort study and three were cross-sectional observational studies.
All demographic information including number of eyes, gender, type of diabetes, stage of DR was found in four of seven studies. Regarding gender, which was specified in five of seven studies, the proportion of men was 61.5% and there were 38.4% women. The type of diabetes was specified in five of seven studies, and type 2 DM was found in 85.8% of the participants and type 1 DM in 14.1%. Regarding the stage of diabetes, it was recorded in five of eight studies, being fully detailed in four of seven. The percentage of patients with mild nonproliferative diabetic retinopathy (NPDR) was 28.2%, with moderate NPDR was 28.5%, with severe NPDR was 15.9% and with proliferative diabetic retinopathy (PDR) was 27.4%.   Disorganization of inner retinal layers was defined (measured by SD-OCT) as the horizontal extent in microns for which any boundaries between the ganglion cell-inner plexiform layer complex, inner nuclear layer, and outer plexiform layer could not be identified. (3) Data extraction and analysis In 100% of the studies the diagnosis of center involved diabetic macular edema (DME-CI) was included by SD-OCT (Heidelberg). In the totality of the studies, the imaging technique was detailed and performed by an experienced ophthalmologist. The diagnosis of DME-CI according to our operational definition was recorded in five of seven studies and the definition of DRIL was made according to the description of Sun et al. in all the studies. (4) Regarding the history of previous treatments, this information was possible to obtain in six of seven studies. The treatments described were: intravitreal injections in 62.5%, macular laser in 12.5%, PRP in 25%, corticosteroids (DEX-I) in 25% and combined treatment in 12.5%.

Table1. Characteristics of the studies included in the review
In all the studies, the presence of DRIL was recorded and its association with BVCA was evaluated ( Table 2). The BVCA was registered in all the studies. The measurement was carried out using the LogMAR scale. In all the studies, the presence of DRIL was associated with a worse final BVCA using p <0.05 as a statical significance.

DISCUSSION
The literature analysis enables us to highlight the importance of DRIL as a biomarker of DME. Numerous studies have discovered a variety of biomarkers for patients with DME, but what is worth mentioning is that reliability differs. Many of them are obtained invasively and not all of them are eligible to predict the effectiveness of the treatment that is ultimately the main objective. (14) When looking for a biomarker, it is extremely important that it be easily obtained, that it is measurable over time, that it is obtained non-invasively and that it is capable of predicting a response over time in order to evaluate the response to treatment reliably and efficiently. Disorganization of the inner retinal layers has been considered a robust biomarker in DME. (3,4) The presence of DRIL has been shown to be associated with VA and is a predictor of VA in the future. The exact mechanism by which DRIL affects VA has not been fully demonstrated and the need for histological studies becomes important. (3) Nevertheless, studies have shown that edema causes an interruption of the visual pathway due to the inability of the axons of the bipolar cells because their elasticity has exceeded. (15) Disorganization of the inner retinal layers could represent disorganization or destruction of cells within inner retinal layers, including bipolar, amacrine, or horizontal cells, and possibly indicates a disruption of pathways that transmit visual information from the photoreceptors to the ganglion cells. (3) Studies using OCT-A in resolved DME have shown that in patients with DRIL, the loss of the superficial (SCP) and deep plexuses (DCP) is greater. This has also been explained due to the relationship between the thickness of the retinal layers and the degree of ischemia of the capillary plexuses. (16) Other studies have shown that, in the presence of foveal DRIL, the vessel density (VD) at the SCP and DCP is lower, and it is associated with worse VA. This highlights the role of retinal vascular ischemia in the pathogenesis of DRIL. (17)  The characteristics and importance of DRIL has been evaluated using SD-OCT. Sun et al have shown that centrally located DRIL was correlated with VA in eyes with DME-CI. (3) In DME-CI, VA change over time was best associated with DRIL change over time, showing a dynamic association of DRIL morphology in these patients. (12) On the other hand, the location of the central DRIL is not solely important since it was found that its horizontal extension was also involved as a predictor of VA in the future. (3) Radwan and et al found that greater horizontal DRIL extent was associated with worse baseline VA. (12) Das et al demonstrated the association between the horizontal extent of the DRIL and VA, finding that, for every 100μm increase in DRIL, there is a negative impact of approximately 6 letters (ETDRS score). (7) DRIL by itself does not explain the findings in VA since this biomarker is strongly associated with other alterations such as the interruption of the external limiting membrane (ELM) and ellipsoid zone (EZ) layers. (7) Artificial intelligent (AI) based in deep learning algorithm in the actual "Big-Data" era allows us to construct useful therapeutic protocols for managing retinal diseases. (18)(19)(20) Biomarkers are part of the necessary information to feed the AI data networks. The results presented in this systematic review shows that DRIL can be one of those biomarkers. Although information relies on an OCT image acquisition, processed and medical human interpretation, some concerns must be described. In the present study, all the imaging comes from the same OCT device. This is good for our purpose, avoiding differences in this aspect, however there are more OCT devices in the market, and it will be necessary to confirm our present results with more OCT equipment. Also, it is difficult to confirm the clinical use of our results until an AI software is available for obtaining an objective and reproducible imaging interpretation, avoiding the potential human mistakes that could arise doing manually retinal OCT measurements, where a few microns means a big difference between normal and pathological.
In conclusion, DRIL as a biomarker and its presence has been shown to be an important predictor of VA in the future in patients with DME. Histopathological studies are required to understand its mechanism of action.