Brain imaging and hypoglycaemia
Exploring the brain has come a long way since the 19th century when Angelo Mosso, a prominent Italian physiologist, posited that ‘blood flow to the brain followed function’. Despite recent major advances in neuroimaging techniques his findings remain relevant. We have moved from static, anatomical imaging modalities such as X-ray and computerised tomography (CT) to functional techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) which measure cerebral blood flow (CBF) as a surrogate marker of regional brain activity. Modern neuroimaging techniques offer insights into functional anatomy allowing detailed evaluation of structure, assessment of brain function and quantification of neuronal reaction to stimuli.
Hypoglycaemia remains the most common side effect of insulin therapy and is universally stated to be the critical limiting factor in achieving the optimum glycaemic control required for prevention of macrovascular complications. The brain is reliant on glucose for optimum performance, so lack of glucose (hypoglycaemia) affects cognitive function, but is also perceived as a physiological stress. It also has an effect on mood and thinking processes thereby affecting decision making and emotional responses to subsequent hypos. Neuroimaging has been key in unravelling these different facets of the effect of hypoglycaemia on the brain.
Table 1 (Click to enlarge). Positron emission tomography (PET) and function magnetic resonance imaging (fMRI) contrast agents and methodology.Changes in regional cerebral blood flow during hypoglycaemia were originally assessed using PET. Water PET has previously been heralded as the most widely accepted gold standard modality for quantitative perfusion imaging in vivo . PET imaging can directly quantify blood flow, glucose utilisation, oxygen consumption and receptor binding in a silent environment that is not affected by electromagnetic interference . However significant limitations to this method include radiation exposure, expense and restricted availability. Functional MRI techniques, namely arterial spin labelling (ASL), have been developed to comparably demonstrate changes in regional cerebral blood flow during hypoglycaemia . This method is more economical, non-invasive and has no known radiation risk lending itself well to hypoglycaemia research.
Glucoregulation and the brain
The brain relies heavily on an ongoing supply of glucose as its main energy substrate though animal and human studies have demonstrated the presence of glycogen, glucose in its storage form, present in the brain . As peripheral blood glucose falls essential glucose sensing cells within the brain initiate and coordinate a neuroendocrine response designed to restore appropriate circulating glucose levels. This measure also prevents a decline in cognitive function due to fuel deficit .
Use of functional neuroimaging allowed us to investigate the theory that brain glucose transport was upregluated during hypoglycaemia ; this hypothesis was not supported. Both [11C]3-O-methyl-D-glucose (CMG) PET and magnetic resonance spectroscopy demonstrated that as blood glucose falls there is a reduction in global glucose concentration . There is however a relative increase in regional blood flow to particular brain regions. ASL MRI revealed that the healthy brain detects a drop in glucose even before peripheral levels fall into what we would classify as the hypoglycaemic range. Regional blood flow to the hypothalamus is doubled as blood glucose falls from 5.3 – 4.2 mmol/L . Though other glucose sensing units have been located centrally and peripherally
Table 2 (Click to enlarge). Cerebral regions of interest and their response during hypoglycaemia in healthy individuals as demonstrated by PET and fMRI. An increase in cerebral blood flow implies an increase in neuronal activity. the hypothalamus has emerged as the principal region responsible for detecting and synchronising the internal management of hypoglycaemia . Increased regional blood flow is also seen in the medial and orbital prefrontal cortex, thalamus and thalamic pulvinar, hypothalamus, anterior cingulate cortex, globus pallidum, periaqueductal grey matter, ventral striatum and insula  . In contrast there are relative decreases in areas such as the hippocampus, temporal cortex and cerebellum .
Figure 1 (Click to enlarge). Effect of hypoglycemic stress on cerebral perfusion. Brain images showing significant rise (red-yellow) and fall (blue-white) in regional perfusion during early hypo, late hypo and recovery to euglycaemia. Teh MM, Dunn JT, Choudhary P, Samarasinghe Y, Macdonald I, O'Doherty M, et al. Evolution and resolution of human brain perfusion responses to the stress of induced hypoglycemia. Neuroimage 2010;53(2):584-92.These dynamic changes in regional blood flow can be further subcategorised into early and late hypoglycaemia and right and left hemispheres supporting the hypothesis that there is a complex, hierarchical temporal pattern of regional responses to falling blood glucose. Though the correct neuropathological interpretation can be difficult there are robust clinical correlates with the neuroimaging data. Areas activated during hypoglycaemia stimulate hunger and food-seeking behaviour and individuals describe the state as unpleasant and stressful. At lower blood glucose levels deactivation occurs in areas involved in memory, speech and coordination.
Impaired awareness of hypoglycaemia
Functional neuroimaging has been particularly useful in investigating the loss of subjective awareness of hypoglycaemia. Recurrent antecedent hypoglycaemia can lead to impaired awareness of hypoglycaemia which essentially strips individuals of their warning signs of falling blood glucose. This significantly increases risk of severe hypoglycaemia and thus morbidity and mortality.
PET and fMRI studies have looked at the brain responses to hypo in individuals with type 1 diabetes with good hypoglycaemia awareness (HA) in comparison to those with impaired awareness of hypoglycaemia (IAH). Parallels can be drawn between results seen in healthy subjects and HA individuals, however when comparing the hypoglycaemia aware and unaware significantly different brain responses to falling blood glucose have been reported.
Unaware subjects exhibit reduced activation in the amygdala, anterior cingulate cortex, ventral striatum and hypothalamus; there is a failure to trigger the stress or food-seeking response vital to restoring euglycaemia in these individuals. In addition deactivation of regional areas involved in reward and motivation shown in the aware are relatively unaffected or even mildly activated in the unaware  . In clinical terms those with impaired awareness may not find the experience of hypoglycaemia at all unpleasant thus have no inclination to avoid future episodes .
Functional neuroimaging allows us to map brain activity in space and time and as such has been utilised to improve our understanding of the role of the brain in hypoglycaemia. A better understanding of the cerebral mechanisms of hypoglycaemia and in particular impaired hypoglycaemia awareness will take us closer to developing novel therapeutic interventions.
^ Detre J et al. Arterial spin-labeled perfusion MRI in basic and clinical neuroscience. Current Opinion in Neurology 2009, 22:348-355
^ Detre J, Wang J. Technical aspects and utility of fMRI using BOLD and ASL. Clinical Neurophysiology 113 (2002) 621-634
^ Arbelaez AM, Su Y, Thomas JB, Haunch AC, Hershey T et al. Comparison of Regional Cerebral Blood Flow Responses to Hypoglycemia Using Pulsed Arterial Spin Labelling and Positron Emission Tomography. PLoS One 2013; 8(3): e60085
^ Tesfaye N, Seaquist E, Oz G. Non-invasive measurement of brain glycogen by NMR spectroscopy and its application to the study of brain metabolism. J Neurosci Res (2011); 89(12): 1905-1912
^ Amiel S. Hypoglycemia and the human brain. International Diabetes Monitor 2009;21: 219-25
^ Kumagai AK, Kang YS, Boado RJ, Pardridge VM. Upregulation of blood-brain barrier GLUT1 glucose transporter protein and mRNA in experimental chronic hypoglycaemia. Diabetes 1995; 44(12): 1399-404
^ Bingham EM, Dunn JT, Smith D, Sutcliffe-Goulden J, Reed LJ, Marsden PK, Amiel SA. Differential changes in brain glucose metabolism during hypoglycaemia accompany loss of hypoglycaemia awareness in men with type 1 diabetes mellitus. An [11C]3-O-methyl-D-glucose PET study. Diabetologia 2005; 48: 2080-2089
^ van de Ven KC, van der Graaf M, Tack CJ, Heerschap A, de Galan BE. Steady-state brain glucose concentrations during hypoglycemia in healthy humans and patients with type 1 diabetes. Diabetes 2012;61(8):1974-7
^ Page K, Arora J, Qiu M, Relwani R, Constable R, Sherwin R. Small Decrements in Systemic Glucose Provoke Increases in Hypothalamic Blood Flow Prior to the Release of Counterregulatory Hormones. Diabetes (2009); 58 (2) 448-452
^ Borg WP, Sherwin RS, During MJ, Borg MA, Shulman GI. Local ventromedial hypothalamus glucopenia triggers counterregulatory hormone release.
^ Teh MM, Dunn JT, Choudhary P, Samarasinghe Y, Macdonald I, O'Doherty M, et al. Evolution and resolution of human brain perfusion responses to the stress of induced hypoglycemia. Neuroimage 2010;53(2):584-92.
^ Teves D, Videen TO, Cryer PE, Powers WJ. Activation of human medial prefrontal cortex during autonomic responses to hypoglycaemia. Proc Natl Acad Sci USA, 2004; 101:6217-21
^ Dunn J, Cranston I, Marsden P, Amiel S, Reed L. Attenuation of Amydgala and Frontal Cortical Responses to Low Blood Glucose Concentration in Asymptomatic Hypoglycemia in Type 1 Diabetes. A New Player in Hypoglycemia Unawareness. Diabetes 2007; 56 (11) 2766-2773