In about a third of patients with type 1 diabetes (T1DM), hypoglycemic symptoms appear only at very low plasma glucose levels or do not appear at all. Cerebral adaptations, including alterations in regional or global cerebral blood flow (CBF) may play a role in the development of this impaired awareness of hypoglycemia (IAH)^1,2. The aim of the current study was to investigate the effect of hypoglycemia on both regional and global CBF in T1DM patients with IAH, as compared to T1DM patients with normal awareness (NAH) and healthy controls.

Subjects: Six subjects with T1DM and IAH (3M/3F, age: 25.2±3.6 yrs, diabetes duration: 15.3±4.6 yrs), seven subjects with T1DM and NAH (4M/3F, age: 26.2±2.2 yrs, diabetes duration: 12.6±3.5 yrs) and seven healthy, non-diabetic subjects (3M/4F, age: 27.6±2.6 yrs) were enrolled in this study. After an overnight fast, the subjects underwent a hyperinsulinemic euglycemic-hypoglycemic glucose clamp, while lying in a 3T MR system (TIM Magnetom Trio, Siemens). Arterial plasma glucose levels were determined every 5 minutes and were maintained at 5.0 mmol/l and 2.8 mmol/l during the euglycemic and hypoglycemic phase, respectively.

Data acquisition: Prior to initiating the euglycemic glucose clamp (baseline), after 30 minutes of stable euglycemia and after 45 minutes of stable hypoglycemia, CBF-weighted images were obtained with pseudo-continuous arterial spin labeling (pCASL) with a 3D-GRASE readout and background suppression (FOV: 230x173mm; 26 slices; post-labeling delay: 1.8s; labeling duration: 1.8s; TE: 30.88ms; TR: 4.8s; total acquisition time 5.1min; 16 pairs of label and control images). A 17-mm thick labeling plane was placed 2.0-3.5cm below the cerebellum, perpendicular to the brain feeding arteries. After each ASL series, two M₀ images were obtained (TR: 7s) with opposing in-plane phase-encoding directions.

Post-processing of ASL data: ASL data were analyzed using FSL. Before subtraction of label and control images, ASL and M₀ images were motion corrected. Each series was then averaged to generate one perfusion-weighted image. CBF was quantified, voxel wise, using the equation and parameters described by Alsop et al.³. Global CBF values were determined by averaging the CBF in gray matter. To assess hypoglycemia-induced regional CBF changes, each CBF map was normalized to its global gray matter mean and smoothed with a Gaussian filter (FWHM: 6mm). Subsequently, voxel-wise statistical analysis with cluster significance correction was performed (FEAT, Version 6.0⁴).

All data are expressed as mean±SEM.

Plasma glucose levels were similar between the three groups during both glycemic phases.

There was no change in global CBF between baseline and the end of the euglycemic phase in any of the groups. Global CBF increased in response to hypoglycemia in T1DM IAH subjects (+8±3%, p<0.05), increased numerically, but not significantly in T1DM NAH subjects (+5±2%, p=0.08) and decreased slightly, but not significantly in healthy controls (-2±2%, p=0.70) (figure 1). The CBF enhancement in T1DM IAH subjects was significantly higher compared to healthy subjects (p<0.05), but not compared to T1DM NAH subjects (p=0.19).

The regional distribution of CBF was altered in response to hypoglycemia in all three groups (figure 2). Hypoglycemia caused significant relative increases in regional CBF in the thalamus of both T1DM NAH and healthy subjects, and in the frontal lobes of T1DM NAH subjects. No such increases in regional CBF were found in T1DM IAH subjects, who only showed a small decrease in regional CBF in the left lateral occipital lobe.

Hypoglycemia induced an increase in global CBF in T1DM patients with IAH and a trend towards such an increase in patients with NAH, but not in healthy controls. The increase in global CBF in T1DM IAH subjects may serve as a neuroprotective response to hypoglycemia, as it enhances glucose supply to the brain.⁵ In turn, one can speculate that this enhanced glucose supply to the brain during hypoglycemia delays hypoglycemia sensing by the brain and therefore the onset of counterregulatory responses, as seen in patients with IAH.

A trend towards an increase in global CBF as seen in T1DM NAH subjects may reflect prior exposure to hypoglycemia, whereas only the healthy subjects are completely naïve to hypoglycemia. Remarkably, in T1DM NAH subjects we saw a clear redistribution of CBF to the frontal lobes during hypoglycemia. Since the frontal lobes are among the most vulnerable cortical regions, it could be that this is the first region to be protected against hypoglycemia.

The hypoglycemia-induced redistribution of CBF to the thalamus seen in patients with T1DM and NAH and in healthy controls may reflect activation of brain regions associated with the autonomic response to hypoglycemia, which is blunted in T1DM subjects with IAH.