Assessment of retinal neurodegeneration with spectral-domain optical coherence tomography: a systematic review and meta-analysis

Evaluation of thickness of individual macular retinal layers in diabetic eyes from optical coherence tomography

The effect of diabetes mellitus (DM) on individual retinal layers remains incompletely understood. We evaluated the intra-retinal layer thickness alterations in 71 DM eyes with no diabetic retinopathy (DR), 90 with mild DR, and 63 with moderate DR without macular edema, using spectral-domain optical coherence tomography (SD-OCT) and the Iowa Reference Algorithm for automated retinal layer segmentation. The average thickness of 10 intra-retinal layers was then corrected for ocular magnification using axial length measurements, and pairwise comparisons were made using multivariable linear regression models adjusted for gender and race. In DM no DR eyes, significant thinning was evident in the ganglion cell layer (GCL; p < 0.001), inner nuclear layer (INL; p = 0.001), and retinal pigment epithelium (RPE; p = 0.014) compared to normal eyes. Additionally, mild DR eyes exhibited a thinner inner plexiform layer (IPL; p = 0.008) than DM no DR eyes. Conversely, moderate DR eyes displayed thickening in the INL, outer nuclear layer, IPL, and retinal nerve fiber layer (all p ≤ 0.002), with notably worse vision. These findings highlight distinctive patterns: early diabetic eyes experience thinning in specific retinal layers, while moderate DR eyes exhibit thickening of certain layers and slightly compromised visual acuity, despite the absence of macular edema. Understanding these structural changes is crucial for comprehending diabetic eye complications.

Macular Neural and Microvascular Alterations in Type 2 Diabetes Without Retinopathy: A SS-OCT Study

PURPOSE

To identify specific markers indicative of macular neural and microvascular alterations in individuals with Type 2 Diabetes Mellitus (T2DM) without clinically observable retinopathy.

DESIGN

Prospective cross-sectional study.

METHODS

Using the PLEX Elite 9000, all eyes underwent swept-source optical coherence tomography (SS-OCT) angiography. Quantitative analysis of acquired images compared macular neural and microvascular alterations in T2DM patients without retinopathy to age-matched controls. Precise assessments encompassed measuring the thickness of each individual retinal layer and evaluating macular vascular indices within different capillary plexuses.

RESULTS

Forty-nine T2DM patients and 51 age-matched controls participated. T2DM patients exhibited a significant reduction in the mean macular thickness of the ganglion cell-inner plexiform layer (GC-IPL) (82.5 ± 5.5 µm vs 86.2 ± 5.0 µm, P = .001) and macular retinal nerve fiber layer (RNFL) (45.8 ± 3.0 µm vs 48.1 ± 3.7 µm, P = .001). Furthermore, macular full retinal thickness was significantly lower in diabetic eyes than controls (324.9 ± 16.3 µm vs 332.8 ± 13.7 µm, P = .009). Vascular measurements revealed subtle changes in macular vascular skeleton density within the total capillary plexuses in T2DM patients (0.132 ± 0.005 vs 0.135 ± 0.005, P = .019).

CONCLUSIONS

Metrics derived from SS-OCT, particularly macular RNFL and GC-IPL thicknesses, emerged as superior indicators for the early detection of diabetic retinal disease in individuals with T2DM without clinically observable retinopathy. Further investigations are warranted to comprehensively understand the clinical implications of these findings.

Reduced macular thickness and vascular density in abnormal glucose metabolism patients: A meta-analysis of optical coherence tomography (OCT) and OCT angiography studies

BACKGROUND

Alterations in macular thickness and vascular density before clinically visible diabetic retinopathy (DR) remain inconclusive. This study aimed to determine whether retinal manifestations in abnormal glucose metabolism (AGM) patients differ from those in the healthy individuals.

METHODS

PubMed, Embase, and Web of Science were searched between 2000 and 2021. The eligibility criteria were AGM patients without DR. Primary and secondary outcomes measured by optical coherence tomography (OCT) and OCT angiography (OCTA) were analyzed and expressed as standardized mean differences (SMDs) with 95% confidence intervals (CIs). A random-effects model was used in the data synthesis. The potential publication bias for the variables was evaluated using Egger's test.

RESULTS

A total of 86 observational studies involving 13,773 participants and 15,416 eyes were included. OCT revealed that compared to healthy controls, the total macular thickness of AGM patients was thinner, including the thickness of fovea (-0.24, 95% CI [-0.39, -0.08]; P  = 0.002, I2  = 87.7%), all regions of parafovea (-0.32, 95% CI [-0.54, -0.11]; P  = 0.003; I2  = 71.7%) and the four quadrants of perifovea; the thickness of peripapillary retinal nerve fiber layer (pRNFL), macular retinal nerve fiber layer (mRNFL), and ganglion cell layer (GCL) also decreased. OCTA indicated that the superficial and deep vascular density decreased, the foveal avascular zone (FAZ) area enlarged, and the acircularity index (AI) reduced in AGM individuals.

CONCLUSIONS

Retinal thinning and microvascular lesions have occurred before the advent of clinically detectable DR; OCT and OCTA may have the potential to detect these preclinical changes.

REGISTRATION

PROSPERO; http://www.crd.york.ac.uk/prospero/ ; No. CRD42021269885.

Current understanding of subclinical diabetic retinopathy informed by histology and high-resolution in vivo imaging

The escalating incidence of diabetes mellitus has amplified the global impact of diabetic retinopathy. There are known structural and functional changes in the diabetic retina that precede the fundus photography abnormalities which currently are used to diagnose clinical diabetic retinopathy. Understanding these subclinical alterations is important for effective disease management. Histology and high-resolution clinical imaging reveal that the entire neurovascular unit, comprised of retinal vasculature, neurons and glial cells, is affected in subclinical disease. Early functional manifestations are seen in the form of blood flow and electroretinography disturbances. Structurally, there are alterations in the cellular components of vasculature, glia and the neuronal network. On clinical imaging, changes to vessel density and thickness of neuronal layers are observed. How these subclinical disturbances interact and ultimately manifest as clinical disease remains elusive. However, this knowledge reveals potential early therapeutic targets and the need for imaging modalities that can detect subclinical changes in a clinical setting.

A New Approach to Staging Diabetic Eye Disease: Staging of Diabetic Retinal Neurodegeneration and Diabetic Macular Edema

Topic

The goal of this review was to summarize the current level of evidence on biomarkers to quantify diabetic retinal neurodegeneration (DRN) and diabetic macular edema (DME).

Clinical relevance

With advances in retinal diagnostics, we have more data on patients with diabetes than ever before. However, the staging system for diabetic retinal disease is still based only on color fundus photographs and we do not have clear guidelines on how to incorporate data from the relatively newer modalities into clinical practice.

Methods

In this review, we use a Delphi process with experts to identify the most promising modalities to identify DRN and DME. These included microperimetry, full-field flash electroretinogram, spectral-domain OCT, adaptive optics, and OCT angiography. We then used a previously published method of determining the evidence level to complete detailed evidence grids for each modality.

Results

Our results showed that among the modalities evaluated, the level of evidence to quantify DRN and DME was highest for OCT (level 1) and lowest for adaptive optics (level 4).

Conclusion

For most of the modalities evaluated, prospective studies are needed to elucidate their role in the management and outcomes of diabetic retinal diseases.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Retinal age gap as a predictive biomarker for future risk of clinically significant diabetic retinopathy

AIMS

Retinal age derived from fundus images has been verified as a novel ageing biomarker. We aim to explore the association between retinal age gap (retinal age minus chronological age) and incident diabetic retinopathy (DR).

METHODS

Retinal age prediction was performed by a deep learning model, trained and validated based on 19,200 fundus images of 11,052 disease-free participants. Retinal age gaps were determined for 2311 patients with diabetes who had no history of diabetic retinopathy at baseline. DR events were ascertained by data linkage to hospital admissions. Cox proportional hazards regression models were performed to evaluate the association between retinal age gaps and incident DR.

RESULTS

During the median follow-up period of 11.0 (interquartile range: 10.8-11.1) years, 183 of 2311 participants with diabetes developed incident DR. Each additional year of the retinal age gap was associated with a 7% increase in the risk of incident DR (hazard ratio [HR] = 1.07, 95% confidence interval [CI] 1.02-1.12, P = 0.004), after adjusting for confounding factors. Participants with retinal age gaps in the fourth quartile had a significantly higher DR risk compared to participants with retinal age gaps in the lowest quartile (HR = 2.88, 95% CI 1.61-5.15, P < 0.001).

CONCLUSIONS

We found that higher retinal age gap was associated with an increased risk of incident DR. As an easy and non-invasive biomarker, the retinal age gap may serve as an informative tool to facilitate the individualized risk assessment and personalized screening protocol for DR.

Early choroidal and retinal changes detected by swept-source oct in type 2 diabetes and their association with diabetic kidney disease: a longitudinal prospective study

BACKGROUND

To evaluate structural changes in retina and choroid in patients with type 2 diabetes (T2D) and their association with diabetic kidney disease (DKD).

METHODS

T2D patients with mild or no diabetic retinopathy (DR) were followed for 3 years using structural SS-OCT and OCT angiography (OCT-A) taken every 6 months. Parameters were compared longitudinally and according to the DKD status on baseline.

RESULTS

One hundred and sixty eyes from 80 patients were followed for 3 years, 72 with no DKD (nDKD) at baseline and 88 with DKD. Trend analysis of T2D showed significant thinning in GCL + and circumpapillary retinal fiber neural layer (cRFNL), choroid, and decreased vascular density (VD) in superficial plexus and central choriocapillaris with foveal avascular zone (FAZ) enlargement. Patients with no DKD on baseline presented more significant declines in retinal center and choroidal thickness, increased FAZ and loss of nasal and temporal choriocapillaris volume. In addition, the nDKD group had worse glycemic control and renal parameters at the end of the study.

CONCLUSION

Our data suggests the potential existence of early and progressive neurovascular damage in the retina and choroid of patients with Type 2 Diabetes (T2D) who have either no or mild Diabetic Retinopathy (DR). The progression of neurovascular damage appears to be correlated with parameters related to glycemic control and renal damage.

Influence of Diabetes Mellitus on Glaucoma-Relevant Examination Results in Primary Open-Angle Glaucoma

Primary open-angle glaucoma (POAG) is no longer considered an isolated eye pressure-dependent optic neuropathy, but a neurodegenerative disease in which oxidative stress and neuroinflammation are prominent. These processes may be exacerbated by additional systemic diseases. The most common are arterial hypertension, dyslipidemia, and diabetes mellitus. Using diabetes mellitus as an example, it will be shown how far-reaching the influence of such a systemic disease can be on both the functional and the structural diagnostic methods for POAG. This knowledge is essential, since these interferences can lead to misinterpretations of POAG, which can also affect therapeutic decisions.

Optical coherence tomography in the management of diabetic macular oedema

Diabetic macular oedema (DMO) is the major cause of visual impairment in people with diabetes. Optical coherence tomography (OCT) is now the most widely used modality to assess presence and severity of DMO. DMO is currently broadly classified based on the involvement to the central 1 mm of the macula into non-centre or centre involved DMO (CI-DMO) and DMO can occur with or without visual acuity (VA) loss. This classification forms the basis of management strategies of DMO. Despite years of research on quantitative and qualitative DMO related features assessed by OCT, these do not fully inform physicians of the prognosis and severity of DMO relative to visual function. Having said that, recent research on novel OCT biomarkers development and re-defined classification of DMO show better correlation with visual function and treatment response. This review summarises the current evidence of the association of OCT biomarkers in DMO management and its potential clinical importance in predicting VA and anatomical treatment response. The review also discusses some future directions in this field, such as the use of artificial intelligence to quantify and monitor OCT biomarkers and retinal fluid and identify phenotypes of DMO, and the need for standardisation and classification of OCT biomarkers to use in future clinical trials and clinical practice settings as prognostic markers and secondary treatment outcome measures in the management of DMO.

Association of Male Sex and Microvascular Alterations on Optical Coherence Tomography Angiography in Diabetes

Purpose

Epidemiologically, men have a higher incidence, severity, and progression of diabetic retinopathy (DR) than women. We investigated microvascular differences between men and women with diabetes on optical coherence tomography angiography (OCTA).

Methods

Three × 3 mm OCTA macula scans of non-diabetic and patients with diabetes were obtained. Vascular parameters included parafoveal vessel density (VD), vessel length density (VLD), and flow index (FI) of the superficial capillary plexus (SCP) and deep capillary plexus (DCP) as well as foveal avascular zone (FAZ) area and perimeter. Multivariable linear regression was used for statistical analysis.

Results

There were 1809 patients with diabetes and 217 non-diabetic participants that were included in this study. Diabetic individuals included those with no DR (n = 1356), mild non-proliferative DR (NPDR; n = 286), moderate NPDR (n = 126), and severe NPDR/proliferative DR (PDR; n = 41). Male sex was significantly associated with smaller FAZ area/perimeter and lower DCP VLD in both non-diabetic subjects and patients with diabetes. Male sex in the diabetic group was additionally associated with lower SCP VD/VLD and DCP VD. Addition of an interaction between male sex and diabetes status in the interaction analysis showed that being male and diabetic conferred increased reduction in DCP VD and VLD compared to sex-based changes in non-diabetics. Larger FAZ perimeter, lower SCP VD/VLD, and lower DCP VLD were associated with poorer visual acuity in diabetics.

Conclusions

On OCTA, male patients with diabetes may have more severe microvascular disease especially in the DCP compared to women.

Translational Evidence

Sex-based alterations in diabetic microvascular disease has the potential to influence future basic and clinical studies as well as the implementation of OCTA disease markers.

The cross-sectional and longitudinal relationship of diabetic retinopathy to cognitive impairment: a systematic review and meta-analysis

OBJECTIVES

To establish a potential relationship between diabetic retinopathy (DR) and different stages of cognitive impairment METHODS: Literature searches were conducted on PubMed and EMBASE, with keywords "diabetic retinopathy" and "cognitive impairment". Inclusion criteria were original human studies, and English language. Quality of studies was assessed by the Newcastle-Ottawa Quality Assessment (NOSGEN). The register number of this study on the International Prospective Register of Systematic Reviews (PROSPERO) is CRD42021236747. The main outcome measures were odds ratios (OR) and risk ratios (RR) for cross-sectional and longitudinal studies, respectively. Meta-regression was performed to evaluate the effects of potential moderator variables, including, age, onset age of diabetes mellitus (DM), duration of DM, and HbA1c.

RESULTS

Twenty-five studies (17 cross-sectional and 8 longitudinal studies) with a total of 1,963,914 subjects, were included. Among the cross-sectional studies, the pooled ORs of any cognitive impairment, early stage of cognitive impairment and dementia in subjects with DR (95% confidence interval) were 1.48 (1.08-2.02), 1.59 (1.01-2.51), and 1.13 (0.86-1.50), respectively. Among the longitudinal studies, the pooled RRs of any cognitive impairment, early stage of cognitive impairment, and dementia in subjects with DR (95% confidence interval) were 1.35 (1.12-1.65), 1.50 (1.06-2.12), and 1.31 (1.03-1.66), respectively. Meta-regression showed age, onset age of DM, duration of DM, and glycated hemoglobin (HbA1c) were not statistically associated with the outcomes.

CONCLUSIONS

The presence of DR in DM patients indicates both higher odds of prevalent cognitive impairment and escalated risks of developing cognitive impairment in the future.

Anomalies in neurovascular coupling during early diabetes: A review

Diabetic retinopathy is the most feared complication for those with diabetes. Although visible vascular pathology traditionally defines the management of this condition, it is now recognised that a range of cellular changes occur in the retina from an early stage of diabetes. One of the most significant functional changes that occurs in those with diabetes is a loss of vasoregulation in response to changes in neural activity. There are several retinal cell types that are critical for mediating so-called neurovascular coupling, including Müller cells, microglia and pericytes. Although there is a great deal of evidence that suggests that Müller cells are integral to regulating the vasculature, they only modulate part of the vascular tree, highlighting the complexity of vasoregulation within the retina. Recent studies suggest that retinal immune cells, microglia, play an important role in mediating vasoconstriction. Importantly, retinal microglia contact both the vasculature and neural synapses and induce vasoconstriction in response to neurally expressed chemokines such as fractalkine. This microglial-dependent regulation occurs via the vasomediator angiotensinogen. Diabetes alters the way microglia regulate the retinal vasculature, by increasing angiotensinogen expression, causing capillary vasoconstriction and contributing to a loss of vascular reactivity to physiological signals. This article summarises recent studies showing changes in vascular regulation during diabetes, the potential mechanisms by which this occurs and the significance of these early changes to the progression of diabetic retinopathy.

Total retinal thickness: a neglected factor in the evaluation of inner retinal thickness

AIM

To determine whether macular retinal nerve fibre layer (mRNFL) and ganglion cell-inner plexiform layer (GC-IPL) thicknesses vary by ethnicity after accounting for total retinal thickness.

METHODS

We included healthy participants from the UK Biobank cohort who underwent macula-centred spectral domain-optical coherence tomography scans. mRNFL and GC-IPL thicknesses were determined for groups from different self-reported ethnic backgrounds. Multivariable regression models adjusting for covariables including age, gender, ethnicity and refractive error were built, with and without adjusting for total retinal thickness.

RESULTS

20237 participants were analysed. Prior to accounting for total retinal thickness, mRNFL thickness was on average 0.9 μm (-1.2, -0.6; p<0.001) lower among Asians and 1.5 μm (-2.3, -0.6; p<0.001) lower among black participants compared with white participants. Prior to accounting for total retinal thickness, the average GC-IPL thickness was 1.9 μm (-2.5, -1.4; p<0.001) lower among Asians compared with white participants, and 2.4 μm (-3.9, -1.0; p=0.001) lower among black participants compared with white participants. After accounting for total retinal thickness, the layer thicknesses were not significantly different among ethnic groups. When considered as a proportion of total retinal thickness, mRNFL thickness was ~0.1 and GC-IPL thickness was ~0.2 across age, gender and ethnic groups.

CONCLUSIONS

The previously reported ethnic differences in layer thickness among groups are likely driven by differences in total retinal thickness. Our results suggest using layer thickness ratio (retinal layer thicknesses/total retinal thickness) rather than absolute thickness values when comparing retinal layer thicknesses across groups.

Detecting retinal neurodegeneration in people with diabetes: Findings from the UK Biobank

Importance

Efforts are underway to incorporate retinal neurodegeneration in the diabetic retinopathy severity scale. However, there is no established measure to quantify diabetic retinal neurodegeneration (DRN).

Objective

We compared total retinal, macular retinal nerve fiber layer (mRNFL) and ganglion cell-inner plexiform layer (GC-IPL) thickness among participants with and without diabetes (DM) in a population-based cohort.

Design/setting/participants

Cross-sectional analysis, using the UK Biobank data resource. Separate general linear mixed models (GLMM) were created using DM and glycated hemoglobin as predictor variables for retinal thickness. Sub-analyses included comparing thickness measurements for patients with no/mild diabetic retinopathy (DR) and evaluating factors associated with retinal thickness in participants with and without diabetes. Factors found to be significantly associated with DM or thickness were included in a multiple GLMM.

Exposure

Diagnosis of DM was determined via self-report of diagnosis, medication use, DM-related complications or glycated hemoglobin level of ≥ 6.5%.

Main outcomes and measures

Total retinal, mRNFL and GC-IPL thickness.

Results

74,422 participants (69,985 with no DM; 4,437 with DM) were included. Median age was 59 years, 46% were men and 92% were white. Participants with DM had lower total retinal thickness (-4.57 μm, 95% CI: -5.00, -4.14; p<0.001), GC-IPL thickness (-1.73 μm, 95% CI: -1.86, -1.59; p<0.001) and mRNFL thickness (-0.68 μm, 95% CI: -0.81, -0.54; p<0.001) compared to those without DM. After adjusting for co-variates, in the GLMM, total retinal thickness was 1.99 um lower (95% CI: -2.47, -1.50; p<0.001) and GC-IPL was 1.02 μm lower (95% CI: -1.18, -0.87; p<0.001) among those with DM compared to without. mRNFL was no longer significantly different (p = 0.369). GC-IPL remained significantly lower, after adjusting for co-variates, among those with DM compared to those without DM when including only participants with no/mild DR (-0.80 μm, 95% CI: -0.98, -0.62; p<0.001). Total retinal thickness decreased 0.40 μm (95% CI: -0.61, -0.20; p<0.001), mRNFL thickness increased 0.20 μm (95% CI: 0.14, 0.27; p<0.001) and GC-IPL decreased 0.26 μm (95% CI: -0.33, -0.20; p<0.001) per unit increase in A1c after adjusting for co-variates. Among participants with diabetes, age, DR grade, ethnicity, body mass index, glaucoma, spherical equivalent, and visual acuity were significantly associated with GC-IPL thickness.

Conclusion

GC-IPL was thinner among participants with DM, compared to without DM. This difference persisted after adjusting for confounding variables and when considering only those with no/mild DR. This confirms that GC-IPL thinning occurs early in DM and can serve as a useful marker of DRN.

Diabetic retinal neurodegeneration as a form of diabetic retinopathy

PURPOSE

To review the evidence supporting diabetic retinal neurodegeneration (DRN) as a form of diabetic retinopathy.

METHOD

Review of literature.

RESULTS

DRN is recognized to be a part of retinopathy in patients with diabetes mellitus (DM), in addition to the well-established diabetic retinal vasculopathy (DRV). DRN has been noted in the early stages of DM, before the onset of clinically evident diabetic retinopathy. The occurrence of DRN has been confirmed in animal models of DM, histopathological examination of donor's eyes from diabetic individuals and assessment of neural structure and function in humans. DRN involves alterations in retinal ganglion cells, photoreceptors, amacrine cells and bipolar cells, and is thought to be driven by glutamate, oxidative stress and dysregulation of neuroprotective factors in the retina. Potential therapeutic options for DRN are under evaluation.

CONCLUSIONS

Literature is divided on the temporal relation between DRN and DRV, with evidence of both precedence and simultaneous occurrence. The relationship between DRN and multi-system neuropathy in DM is yet to be evaluated critically.

Citicoline in Ophthalmological Neurodegenerative Disease: A Comprehensive Review

Cytidine 5'-diphosphocholine has been widely studied in systemic neurodegenerative diseases, like Alzheimer's disease, Parkinson's disease, and brain ischemia. The rationale for the use of citicoline in ophthalmological neurodegenerative diseases, including glaucoma, anterior ischemic optic neuropathy, and diabetic retinopathy, is founded on its multifactorial mechanism of action and the involvement in several metabolic pathways, including phospholipid homeostasis, mitochondrial dynamics, as well as cholinergic and dopaminergic transmission, all being involved in the complexity of the visual transmission. This narrative review is aimed at reporting both pre-clinical data regarding the involvement of citicoline in such metabolic pathways (including new insights about its role in the intracellular proteostasis through an interaction with the proteasome) and its effects on clinical psychophysical, electrophysiological, and morphological outcomes following its use in ophthalmological neurodegenerative diseases (including the results of the most recent prospective randomized clinical trials).