Measuring visual cortical oxygenation in diabetes using functional near-infrared spectroscopy

Diabetes mellitus affects about 6% of the world's population, and the chronic complications of the disease may result in macro- and micro-vascular changes. The purpose of the current study was to shed light on visual cortical oxygenation in diabetic individuals. We then aimed to compare the haemodynamic response (HDR) to visual stimulation with glycaemic control, given the likelihood of diabetic individuals suffering from such macro- and micro-vascular insult.

Thirty participants took part in this explorative study, fifteen of whom had diabetes and fifteen of whom were non-diabetic controls. The HDR, measured as concentrations of oxyhaemoglobin [HbO] and deoxyhaemoglobin [HbR], to visual stimulation was recorded over the primary visual cortex (V1) using a dual-channel oximeter. The stimulus comprised a pattern-reversal checkerboard presented in a block design. Participants' mean glycated haemoglobin (HbA_1c) level (± SD) was 7.2 ± 0.6% in the diabetic group and 5.5 ± 0.4% in the non-diabetic group. Raw haemodynamic data were normalised to baseline, and the last 15 s of data from each 'stimulus on' and 'stimulus off' condition were averaged over seven duty cycles for each participant.

There were statistically significant differences in ∆[HbO] and ∆[HbR] to visual stimulation between diabetic and non-diabetic groups (p < 0.05). In the diabetic group, individuals with type 1 diabetes displayed an increased [HbO] (p < 0.01) and decreased [HbR] (p < 0.05) compared to their type 2 counterparts. There was also a linear relationship between both ∆[HbO] and ∆[HbR] as a function of HbA_1c level (p < 0.0005).

Our findings suggest that fNIRS can be used as a quantitative measure of cortical oxygenation in diabetes. Diabetic individuals have a larger HDR to visual stimulation compared to non-diabetic individuals. This increase in ∆[HbO] and decrease in ∆[HbR] appears to be correlated with HbA_1c level.