In this neuroimaging study of men with type 1 diabetes, differences in symptomatic and hormonal responses to hypoglycaemia between participants with preserved (HA) or impaired (IAH) awareness of hypoglycaemia were accompanied by subtle differences in brain responses. These occurred not only in regions associated with the generation and subjective awareness of stress responses, but also in regions associated with executive control, reward, memory and emotional salience.
Defective hormonal responses to hypoglycaemia in IAH are well recognised [
2,
8,
16]. They are inducible by exposure to hypoglycaemia [
7,
8] and restored by avoiding it [
2,
20]. The IAH group showed the expected diminution of symptomatic and catecholamine responses to our hypoglycaemic challenge.
Responses in men with type 1 diabetes and intact awareness
Our neuroimaging data in HA participants are largely consistent with studies in people without diabetes and extend earlier observations, particularly by describing the evolution of responses during development of and recovery from hypoglycaemia. Our ‘early’ scans were made as arterialised plasma glucose level was falling, and include data collected at glucose values between 3.5 and 2.2 mmol/l; the ‘established’ data were collected at 2.4 mmol/l. Thalamic activation seen in both early and established hypoglycaemia is consistent with data from individuals without diabetes, and with the role of the thalamus in relaying sensory signals to cortical areas [
30]. Novel findings include insula activation, seen in all three phases of hypoglycaemia and not previously described in diabetes. Using similar techniques, we previously described insular activation in established hypoglycaemia only in people without diabetes [
14]; we speculate that earlier and more persistent activation in type 1 diabetes relates to heightened sensitivity to changes in plasma glucose or prior experience of more fluctuating glucose concentrations. Wiegers et al did not find insular activation in people without diabetes and described reduced insular perfusion using functional MRI (fMRI) in seven HA individuals with type 1 diabetes [
19]. Comparing direction of signal change across studies using different technologies is complex; however, [
15O]water PET is less susceptible to low signal to noise ratios and movement than fMRI. It also allows more quantitative measurement of regional responses and may be better at detecting differences between groups in studies of similar size [
31].
Activation of the GP in established hypoglycaemia and recovery, and the ACC in all three phases, is consistent with some reports involving individuals without diabetes at similar glucose concentrations [
13,
14] but has not been described in type 1 diabetes. The GP and ACC, involved in reward, might be reacting to the hypoglycaemic stress responses. Activation of orbital cortex is likewise consistent with non-diabetic responses to comparable hypoglycaemia [
13,
14,
18] but not previously observed in type 1 diabetes [
19]. The orbital cortex encodes stimulus value or salience [
32], and the lateral orbital cortex forms a ‘salience’ network with the ACC [
33]. Activation of the DLF cortex in response to hypoglycaemia has not previously been described [
12‐
14,
18,
19]. It has a role in working memory—the short-term recall and processing of information necessary for complex task performance, including learning and reasoning [
34]—and is involved in inhibitory control [
35,
36]. Changing activity during hypoglycaemia is consistent with clinically observed changes in cognition and behavioural disinhibition during hypoglycaemia, and recall after it. The report of activation of the precuneus and angular gyrus during established hypoglycaemia and recovery is also novel [
12‐
14,
19]. The precuneus is part of the ‘default mode network’, showing reduced activity compared with the resting state when undertaking tasks [
33]; activation may reflect lesser ability to perform tasks during hypoglycaemia. The angular gyrus, linked to the DLF cortex [
37] and showing parallel responses, plays a role in regulating shift of attention to more salient stimuli [
38]. The amygdala encodes the predicted biological relevance of a stimulus [
39]; its activation in recovery may be a key determinant of responses to subsequent hypoglycaemic events.
Deactivation of the inferior temporal gyri in all three phases, of parietal regions during established hypoglycaemia and recovery, and of parahippocampal regions during established hypoglycaemia, described in some studies of individuals without diabetes but not previously in type 1 diabetes, provide a neurological correlate of failure to form memory during hypoglycaemia: temporal gyri for semantic or conceptual memory [
40], and parahippocampal gyrus and lateral parietal cortex for episodic memory [
41].
Impact of IAH
The subtle differences in hypoglycaemia responses in IAH are potentially important. In early hypoglycaemia, deactivation seen in parts of the central operculum, MO cortex and posterior and lateral DLF cortex in HA was replaced by activation. Operculum activation changes in response to food cues, modulated by feeding state and degree of liking the food [
32]: differences between IAH and HA may relate to differences in the drive to eat to treat. These were paralleled by different responses in the MO cortex, encoding stimulus value and salience [
32], in early and established hypoglycaemia; this may underlie differences in perceived importance of hypoglycaemia, including lack of aversion. Lack of activation of the GP, with its role in memory of unpleasant experiences or aversion [
42], is also consistent with not finding hypoglycaemia unpleasant. These are key findings, as IAH is clinically associated with reduced motivation to avoid hypoglycaemia [
21,
22], with reduced incentive to treat hypoglycaemia as important [
43].
Parts of the dorsal and posterior DLF cortex responded from early hypoglycaemia through to recovery with activation responses in IAH compared with deactivation in HA. Hypoglycaemia is associated with impaired inhibitory control; perhaps the deactivation in HA represents conscious attempts to maintain inhibitory control of behaviour during hypoglycaemia.
The ACC is involved in decision-making and conflict resolution between options, and is key in monitoring performance, evaluating actions and detecting events that require behavioural modification and re-evaluation [
44]. Lack of ACC activation only in IAH fits with views of IAH as a habituation response. A similar lack of activation in IAH in the somatosensory post-central (somatosensory) and pre-central (motor) gyri, persisting in recovery, may reflect reduced somatic sensations (e.g. warmth, shakiness) and motor responses (e.g. tremor) experienced by the IAH group in hypoglycaemia.
Minimal responses in IAH, vs deactivation in HA, in the left posterior middle temporal gyrus during established hypoglycaemia, and the bilateral posterior middle and left inferior temporal gyri in recovery, are also consistent with different memory formation during hypoglycaemia and recovery [
40,
41]. The same is true of differences in the left parietal lobule/angular gyrus in established hypoglycaemia, with activation in IAH but minimal response in HA, and in recovery in the left angular gyrus and supramarginal gyrus, with activation in IAH and deactivation in HA. The lateral parietal cortex shows functional connectivity with the hippocampal formation and is associated with recollection of experiences [
41,
45].
In recovery, in addition to persisting differential responses in somatosensory and memory networks, we found activation in IAH and deactivation in HA in part of the medial frontal cortex; this was in a cluster corresponding to regions of the dorsal-medial prefrontal cortex identified as having a role in self-referential mental activity, such as making judgements about unpleasantness/pleasantness [
46]. It may also have a role in episodic or experiential memory [
41]. This may provide a correlate for individuals with type 1 diabetes with IAH and HA forming differently valenced memories of the experience of hypoglycaemia. However, the medial frontal cortex, along with the lateral parietal regions discussed above, is also a component of the default mode network [
33], and these differences may represent hypoglycaemia being a different ‘task’ for the brain in IAH than HA.
Our participants were matched well for age, diabetes duration and diabetes control but imperfectly for BMI. Obesity alters brain responses to food and food cues, including the responses of some frontal regions described as different in their response to hypoglycaemia here [
47]. However, none of our participants was obese, so it is unlikely our observed differences in response to hypoglycaemia were related to this. The recruitment of only men facilitates research involving radio-isotopes and importantly reduces variability of responses due to sex differences in counterregulation [
48]. The clinical picture of IAH is not sex-specific so our data interpretation probably also applies to women; however, adaptation to antecedent hypoglycaemia may vary by sex, at least in individuals without diabetes [
49], and studies in women would be of interest. Right-handedness was chosen as many brain functions are lateralised.
The strengths of our study include pre-study determination of awareness status on clinical grounds, so individuals defined as having IAH were representative of those with clinically problematic hypoglycaemia. In addition, the analysis did not require a preconception of brain regions that might respond differently to hypoglycaemia by awareness status. Although less powerful than a region of interest analysis, in which data are compared between groups only in prespecified brain regions, this enabled us to identify areas not traditionally associated with stress responses. That differences between the two groups (the effect of awareness status on rCBF responses) were identified at lower thresholds than those used to find differences within groups (the effect of hypoglycaemia) is statistically explicable: within-group comparison is always more powerful than between-group comparison, where differences between participants come into play. It is also biologically plausible as hypoglycaemia is a large stress stimulus whereas differences between HA and IAH are probably an order of magnitude less. It is, however, possible that other brain regions responding differently were missed.