Sustained elevation of cerebral blood flow after hypoglycaemia in normal man

Effect of lactate administration on cerebral blood flow during hypoglycemia in people with type 1 diabetes

INTRODUCTION

Impaired awareness of hypoglycemia, clinically reflected by the inability to timely detect hypoglycemia, affects approximately 25% of the people with type 1 diabetes. Both altered brain lactate handling and increased cerebral blood flow (CBF) during hypoglycemia appear to be involved in the pathogenesis of impaired awareness of hypoglycemia. Here we examine the effect of lactate on CBF during hypoglycemia.

RESEARCH DESIGN AND METHODS

Nine people with type 1 diabetes and normal awareness of hypoglycemia underwent two hyperinsulinemic euglycemic-hypoglycemic (3.0 mmol/L) glucose clamps in a 3T MR system, once with sodium lactate infusion and once with sodium chloride infusion. Global and regional changes in CBF were determined using pseudocontinuous arterial spin labeling.

RESULTS

Lactate (3.3±0.6 vs 0.9±0.2 mmol/L during lactate infusion vs placebo infusion, respectively) suppressed the counter-regulatory hormone responses to hypoglycemia. Global CBF increased considerably in response to intravenous lactate infusion but did not further increase during hypoglycemia. Lactate also blunted the hypoglycemia-induced regional redistribution of CBF towards the thalamus.

CONCLUSIONS

Elevated lactate levels enhance global CBF and blunt the thalamic CBF response during hypoglycemia in patients with type 1 diabetes, mimicking observations of impaired awareness of hypoglycemia. These findings suggest that alteration of CBF associated with lactate may play a role in some aspects of the development of impaired awareness of hypoglycemia.

TRIAL REGISTRATION NUMBER

NCT03730909.

Dynamic response of cerebral blood flow to insulin-induced hypoglycemia

The dynamics of cerebral blood flow (CBF) at the onset of hypoglycemia may play a key role in hypoglycemia unawareness; however, there is currently a paucity of techniques that can monitor adult CBF with high temporal resolution. Herein, we investigated the use of diffuse correlation spectroscopy (DCS) to monitor the dynamics of CBF during insulin-induced hypoglycemia in adults. Plasma glucose concentrations, cortisol levels, and changes in CBF were measured before and during hypoglycemia in 8 healthy subjects. Cerebral blood flow increased by 42% following insulin injection with a delay of 17 ± 10 min, while the onset of hypoglycemia symptoms was delayed by 24 ± 11 min. The findings suggest that the onset of CBF increments precedes the appearance of hypoglycemia symptoms in nondiabetic subjects with normal awareness to hypoglycemia, and DCS could be a valuable tool for investigating the role of CBF in hypoglycemia unawareness.

Cerebral blood flow response to hypoglycemia is altered in patients with type 1 diabetes and impaired awareness of hypoglycemia

It is unclear whether cerebral blood flow responses to hypoglycemia are altered in people with type 1 diabetes and impaired awareness of hypoglycemia. The aim of this study was to investigate the effect of hypoglycemia on both global and regional cerebral blood flow in type 1 diabetes patients with impaired awareness of hypoglycemia, type 1 diabetes patients with normal awareness of hypoglycemia and healthy controls ( n = 7 per group). The subjects underwent a hyperinsulinemic euglycemic-hypoglycemic glucose clamp in a 3 T MR system. Global and regional changes in cerebral blood flow were determined by arterial spin labeling magnetic resonance imaging, at the end of both glycemic phases. Hypoglycemia generated typical symptoms in patients with type 1 diabetes and normal awareness of hypoglycemia and healthy controls, but not in patients with impaired awareness of hypoglycemia. Conversely, hypoglycemia increased global cerebral blood flow in patients with impaired awareness of hypoglycemia, which was not observed in the other two groups. Regionally, hypoglycemia caused a redistribution of cerebral blood flow towards the thalamus of both patients with normal awareness of hypoglycemia and healthy controls, consistent with activation of brain regions associated with the autonomic response to hypoglycemia. No such redistribution was found in the patients with impaired awareness of hypoglycemia. An increase in global cerebral blood flow may enhance nutrient supply to the brain, hence suppressing symptomatic awareness of hypoglycemia. Altogether these results suggest that changes in cerebral blood flow during hypoglycemia contribute to impaired awareness of hypoglycemia.

Brain glucose metabolism during hypoglycemia in type 1 diabetes: insights from functional and metabolic neuroimaging studies

Hypoglycemia is the most frequent complication of insulin therapy in patients with type 1 diabetes. Since the brain is reliant on circulating glucose as its main source of energy, hypoglycemia poses a threat for normal brain function. Paradoxically, although hypoglycemia commonly induces immediate decline in cognitive function, long-lasting changes in brain structure and cognitive function are uncommon in patients with type 1 diabetes. In fact, recurrent hypoglycemia initiates a process of habituation that suppresses hormonal responses to and impairs awareness of subsequent hypoglycemia, which has been attributed to adaptations in the brain. These observations sparked great scientific interest into the brain's handling of glucose during (recurrent) hypoglycemia. Various neuroimaging techniques have been employed to study brain (glucose) metabolism, including PET, fMRI, MRS and ASL. This review discusses what is currently known about cerebral metabolism during hypoglycemia, and how findings obtained by functional and metabolic neuroimaging techniques contributed to this knowledge.

Potential non-hypoxic/ischemic causes of increased cerebral interstitial fluid lactate/pyruvate ratio: a review of available literature

Microdialysis, an in vivo technique that permits collection and analysis of small molecular weight substances from the interstitial space, was developed more than 30 years ago and introduced into the clinical neurosciences in the 1990s. Today cerebral microdialysis is an established, commercially available clinical tool that is focused primarily on markers of cerebral energy metabolism (glucose, lactate, and pyruvate) and cell damage (glycerol), and neurotransmitters (glutamate). Although the brain comprises only 2% of body weight, it consumes 20% of total body energy. Consequently, the ability to monitor cerebral metabolism can provide significant insights during clinical care. Measurements of lactate, pyruvate, and glucose give information about the comparative contributions of aerobic and anaerobic metabolisms to brain energy. The lactate/pyruvate ratio reflects cytoplasmic redox state and thus provides information about tissue oxygenation. An elevated lactate pyruvate ratio (>40) frequently is interpreted as a sign of cerebral hypoxia or ischemia. However, several other factors may contribute to an elevated LPR. This article reviews potential non-hypoxic/ischemic causes of an increased LPR.

Changes in extracellular hypothalamic glucose in relation to feeding

The aim of the present in vivo microdialysis study was to investigate the relation between feeding and changes in glucose concentrations in the rat ventromedial hypothalamus (VMH). Absolute ambient glucose concentrations in VMH were 1.43 mm in non-deprived rats as compared to 0.94 mm after 24-h food deprivation. To examine whether feeding influences hypothalamic glucose, changes of glucose concentration over time were determined relative to a baseline. Experiments were conducted in relation to both, nutritional state (food-deprived rats vs. non-deprived rats) and feeding conditions throughout the experiment (freely feeding rats vs. rats without access to food). The results of this microdialysis study show clearly that glucose concentration in the VMH of rats increases significantly in relation to food intake. The data demonstrate that a 24-h food deprivation before the experiment further augments this increase (up to 350% from baseline) as compared to non-deprived conditions (up to 60% from baseline). However, the magnitude of food related increase in VMH glucose does not correlate with the individual amount of food eaten. In conclusion, the present study shows for the first time that VMH glucose concentrations increase with food intake in the early dark phase, indicating that such changes do not only occur after pharmacological treatment, but also under physiological feeding conditions. The results further indicate that the feeding related increase in VMH glucose depends on the nutritional state of the organism.

Hypoglycemic encephalopathy with extensive lesions in the cerebral white matter

Here we report an autopsy case of hypoglycemic encephalopathy with prolonged coma. Laboratory data obtained when the patient lapsed into a coma showed that she had a low level of serum glucose (27 mg/dL). Although the level of glucose returned to within the normal range rapidly after glucose infusion, the patient remained in a coma for 22 months. It was presumed that the state of hypoglycemia persisted for about 4 h. There was no evidence of hypotension or hypoxia. Magnetic resonance imaging was performed 3 h after glucose administration; diffusion-weighted images revealed hyperintensity in the cerebral white matter and in the boundary zone between the middle and posterior cerebral arteries. Post-mortem examination revealed superficial laminar necrosis throughout the cerebral cortex. Neuronal necrosis was also found in the hippocampus and dentate gyrus, although the CA3 region appeared normal. In addition to these lesions, which are consistent with hypoglycemia-induced brain damage, the cerebral white matter exhibited severe loss of myelin and axons with reactive astrocytosis and macrophage infiltration. Old infarcts were also present in the bilateral occipital lobes. Since the cerebral blood flow is reported to be decreased during severe hypoglycemia, the present findings suggest that white matter lesions and boundary-zone infarctions may develop primarily in uncomplicated hypoglycemia.

Activation of human medial prefrontal cortex during autonomic responses to hypoglycemia

Studies in humans implicate the medial prefrontal cortex (MPFC) in complex cognitive and emotional states. We measured regional cerebral blood flow (CBF) four times each during euglycemia (5.2 +/- 0.2 mmol/liter) and hypoglycemia (3.0 +/- 0.3 mmol/liter) in nine normal human volunteers. Autonomic responses during hypoglycemia were manifested by increases in neurogenic symptoms, heart rate, and plasma levels of epinephrine, norepinephrine, and pancreatic polypeptide. Typical symptoms of hypoglycemia were mild, and none reflected evidence of cognitive or emotional stress. Quantitative CBF fell 6-8% in the cerebrum, brainstem, and cerebellum. Analysis of regional CBF differences identified neuronal activation during hypoglycemia in bilateral MPFC (areas 24 and 32) and bilateral thalamus. These results provide evidence that the MPFC participates in the autonomic responses to simple physiological stimuli in humans.

Extracellular glucose in rat ventromedial hypothalamus during acute and recurrent hypoglycemia

The activity of neurons in the ventromedial hypothalamus (VMH) important for initiating compensatory responses to hypoglycemia is influenced by ambient glucose concentration. In the present study, we used in vivo microdialysis to evaluate interstitial glucose concentrations in rat VMH under various glycemic conditions. Using the zero-net-flux method, steady-state glucose concentration in the VMH was approximately 20% of blood glucose (approximately 1.4 mmol/l) in fed rats but approximately 14% of blood glucose (approximately 0.7 mmol/l) in overnight-fasted rats. During moderate hypoglycemia VMH glucose declined in parallel with blood glucose; however, VMH glucose decreased to a greater degree than blood glucose during a more severe hypoglycemic episode, falling to 10 +/- 1.2% of blood levels (P < 0.01). To determine whether VMH glucose concentrations were influenced by recurrent episodes of hypoglycemia a second zero-net-flux study was conducted. Steady-state glucose concentrations in the VMH were approximately 20% lower after three episodes of recurrent hypoglycemia, a value 17.8 +/- 0.8% of blood glucose, although the relative change in VMH glucose levels during the first and fourth hypoglycemic episodes were similar. From these results, we conclude that interstitial glucose concentrations in the VMH are not maintained at a constant level and are more dynamic than previously proposed.

Hypoglycemia in children with type 1 diabetes mellitus. Risk factors, cognitive function, and management

This article examines the relationship between hypoglycemia and brain function in children with type 1 diabetes. Hypoglycemic episodes occurring in the first 5 years of life may permanently disrupt cognitive function in a subset of children with diabetes, and a single acute episode of hypoglycemia may produce a transient reduction in mental efficiency, alter the electroencephalogram, and increase regional cerebral blood flow. Because iatrogenic development of hypoglycemic unawareness and autonomic failure are the most likely mediators of moderately severe hypoglycemia, medical management efforts should be directed at the prevention of frequently recurring, mild hypoglycemia.