Retinal Capillary Damage Is Already Evident in Patients With Hypertension and Prediabetes and Associated With HbA_1c Levels in the Nondiabetic Range

Prediabetes is often considered as “benign” without the presence of comorbid conditions. However, several studies have demonstrated the risk for cardiovascular disease in patients with impaired glucose tolerance and even in patients with an HbA_1c below the prediabetes cutoff (1–3). A more gradual increase in micro- and macrovascular complications has been described with glycemic measures, suggesting a continuous relationship (4–6). The Maastricht study supported the concept that microvascular dysfunction occurs before the diagnosis of type 2 diabetes mellitus (T2DM) and may play a role in disorders that are of microvascular origin (7). The aim of our study was to analyze whether a difference in retinal capillary density assessed by optical coherence tomography-angiography (OCT-A) could be observed between patients with hypertension with and without prediabetes.

We performed a cross-sectional study in patients with arterial hypertension from February 2016 to October 2019 at the Dobney Hypertension Centre, Royal Perth Hospital. The patient groups were defined using HbA_1c based on American Diabetes Association guideline recommendations. The details of the clinical workup and definitions for prediabetes, normal glucose metabolism, and HOMA for insulin resistance (HOMA-IR) are presented in the Supplementary Material.

As previously described in detail (8), we used an AngioVue spectral-domain OCT-A device (Optovue, Inc., Fremont, CA) (Fig. 1F). Details on OCT-A scans and analyses can be found in the Supplementary Material.

Data were compared using unpaired Student t test. ANCOVA was performed as a multivariable linear model. Adjustment was made for sex and clinical variables that were significantly different between cohorts, including age, 24 h diastolic blood pressure (BP) average, and estimated glomerular filtration rate (eGFR). Bivariate correlation analyses were performed using Pearson test. Additional discussion of the statistical approaches can be found in the Supplementary Material.

Written informed consent was obtained from each patient before study inclusion. The study protocol of each trial was approved by the East Metropolitan Health Service Ethics and Governance Committees, Perth, Australia (EC00270, Reg. No. RGS1040), and the studies were conducted in accordance with the Declaration of Helsinki and the principles of the Guideline for Good Clinical Practice.

The clinical characteristics and additional details of the patients with hypertension with normal glucose metabolism and prediabetes are presented in Supplementary Table 1.

Capillary density in the foveal (CDF) retinal area was lower in patients with prediabetes than in patients with normal glucose metabolism (32.0 ± 7.0 vs. 36.7 ± 6.4%; raw P = 0.001). When potential confounders such as age, sex, 24 h diastolic BP, and eGFR were entered in the covariance analysis, CDF was still reduced in patients with prediabetes (adjusted P = 0.045) (Fig. 1A). No difference in capillary density in the parafoveal (CDPF) retinal area was evident between the groups (53.1 ± 3.1 vs. 52.6 ± 4.3%; raw P = 0.510; adjusted P = 0.184).

An inverse relation existed between CDF and HbA_1c when the entire cohort was considered (r = −0.388; P < 0.001) (Fig. 1C). A similar correlation has been noticed with fasting glucose (CDF: r = −0.245; P = 0.015) (Fig. 1D). No correlation was observed between CDPF and glucose or HbA_1c. Correlation coefficients of CDF and CDPF with other clinical characteristics are provided in Supplementary Table 2.

In an analysis of patients with HbA_1c <6.5% (48 mmol/mol), CDF was lower in patients with HOMA-IR ≥2.5 (n = 40) than in those with HOMA-IR <2.5 (n = 56) (33.7 ± 6.7 vs. 36.3 ± 6.5%; raw P = 0.059; adjusted P = 0.034) (Fig. 1B). Such a difference was not found with respect to CDPF (52.6 ± 4.1 vs. 53.3 ± 3.6%; raw P = 0.374). In six patients, insulin levels could not be measured; thus, the HOMA-IR value could not be determined. We found no correlation between HOMA-IR and CDF (r = −0.150, P = 0.146) (Fig. 1E).

The principal finding of our study is that the CDF retinal region is reduced in patients with hypertension and prediabetes compared with patients with hypertension and normal glucose metabolism. In addition, measures of glycemia were linearly associated with microvascular damage. Our data support the concept that microvascular damage may already occur before the diagnosis of T2DM.

Capillary rarefaction is a hallmark of essential hypertension (9) and has been demonstrated in patients with early-stage T2DM (10,11). Capillary rarefaction has been demonstrated recently in mice fed a high-fat diet, which is a useful model for examining the effect of prediabetes on the retina (12). High-fat diet–fed mice developed higher body weight as well as a reduced ISI without developing overt hyperglycemia compared with low-fat diet–fed controls. At 12 months of high-fat diet, retinal vascular density was reduced in the central and peripheral regions of the retina.

Chua et al. (13) showed that retinal capillary density is reduced with higher BP and poorer eGFR in adults with treated systemic hypertension. In our study, patients in both cohorts had arterial hypertension as an underlying comorbidity. However, the difference in capillary density in the foveal region between the cohorts remained significant after adjustment for 24 h diastolic BP and eGFR along with age and sex.

Reduced capillary density could be observed only in the foveal region of the patients with hypertension and prediabetes, not in the parafoveal region. In support of our findings, no significant change in CDPF was seen in a study of patients with T2DM with a mean disease duration of 4.8 years (14). Zeng et al. (15) observed reduced CDF and CDPF in retinal regions of patients with T2DM; these patients were in the early stage of their disease without detectable retinopathy, but the mean HbA_1c was 9.21% (77 mmol/mol), which possibly explains the more severe nature of retinal disease in their study.

Insulin resistance possibly underlies the reduction in microvascular density observed in our patients with prediabetes. In the patients with prediabetes, HOMA-IR was significantly higher than in those with normal glucose metabolism. We also compared capillary density between patients with high and low HOMA-IR and again found lower values in patients with higher HOMA-IR, similar to the patients with prediabetes.

The differentiation between patients with hypertension with normal glucose metabolism and those with prediabetes was based on HbA_1c. No difference in capillary rarefaction could be found if the differentiation into two groups was based on the fasting plasma glucose (data not shown). However, both fasting plasma glucose and HbA_1c correlated negatively with capillary density. It is notable that different tests for the diagnosis of prediabetes do not always give matching results for the same individual, reflecting the imperfect correlation between HbA_1c and plasma glucose. Five patients with prediabetes had a mean glucose level of 7.8 mmol/L with a mean HbA_1c value of 6.2% (Fig. 1D).

The main limitations of this study are the relatively small sample size and the cross-sectional design. Furthermore, the data are only valid for patients with a combination of prediabetes and hypertension. We cannot conclude that the same relationship between glycemia and retinal capillary damage would exist in the absence of hypertension. No well-established parameter with a cutoff level for insulin resistance exists, which applies to HOMA-IR; therefore, a HOMA-IR value of ≥2.5 to define insulin resistance is arbitrary.

In summary, our data support the concept that microvascular damage may already occur before the diagnosis of T2DM.

Acknowledgments. The authors thank all the participants of the study and Derrin Brockman and Luca Schlaich (University of Western Australia, Perth, Australia) for expert administrative and technical assistance.

Funding. L.M.L.-G. is supported by a National Council on Science and Technology, Mexico, scholarship. R.C. is supported by an Australian National Heart Foundation postdoctoral fellowship. M.P.S. is supported by a National Health and Medical Research Council research fellowship.

Duality of Interest. M.P.S. has received consulting fees and/or travel and research support from Medtronic, Abbott, Novartis, Servier, Pfizer, and Boehringer Ingelheim. No other potential conflicts of interest relevant to this article were reported.

Author Contributions. D.K. designed the study, analyzed all data, and wrote the manuscript. J.M.N. rechecked the data analysis, contributed to the discussion, and reviewed data and the manuscript. M.G.K., R.C., L.M.L.-G., J.C., A.Joy., A.Jos., S.R., V.B.M., L.Y.H., and O.A. contributed to the discussion and reviewed the manuscript. S.F. analyzed retinal data, contributed to the discussion and methods, and reviewed manuscript. M.P.S. designed the study, contributed to the discussion, and reviewed data and manuscript. D.K. and M.P.S. are the guarantors of this work and, as such, had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.