Glucose Extremes and Cognitive Function: A Review of the Neurological Impacts of Hypoglycemia and Hyperglycemia in Type 1 Diabetes

Abstract

This article explores the neurological impacts of hypoglycemia and hyperglycemia on cognitive function in type 1 diabetes. It reviews how fluctuating glucose levels disrupt critical brain metabolic pathways, impairing various cognitive processes such as memory, attention, and executive functioning. The authors highlight both the acute and long-term effects of glycemic variability on critical brain regions, including the hippocampus, prefrontal cortex, and occipital lobes. Notably, both hyperglycemia and hypoglycemia contribute to cognitive dysfunction but through different mechanisms. Hypoglycemia induces an energy crisis in the brain, triggering increased oxidative stress and neuronal vulnerability, with repeated episodes leading to cumulative hippocampal and prefrontal damage. Hyperglycemia results in vascular compromise, disrupting cerebral blood flow and leading to various states of neurotransmitter dysregulation, with chronic exposure being associated with structural changes such as reduced gray matter volume. Diabetes technology devices such as continuous glucose monitoring systems may reduce cognitive impairments associated with glucose fluctuations, but their benefits underscore the limitations of A1C alone and the need for metrics that better capture glycemic variability. This review underscores the need to prioritize glucose control to protect cognitive health in patients with type 1 diabetes.

Type 1 diabetes is an autoimmune disorder characterized by the progressive destruction of insulin-producing β-cells in the pancreas, leading to absolute insulin deficiency. Insulin is a critical hormone for regulating blood glucose levels, and, without it, individuals with type 1 diabetes must rely on exogenous insulin to prevent acute complications such as diabetic ketoacidosis and long-term complications such as nephropathy and retinopathy (1,2). Although the disease can be diagnosed at any age, it most commonly presents in children and young adults, making it a lifelong condition (1). Globally, an estimated 1.1 million children and adolescents live with type 1 diabetes, with this incidence increasing by ∼3% annually. Although the drivers of this trend remain unclear, potential causes include genetic susceptibility and environmental triggers that initiate autoimmune responses in predisposed individuals (3). The strongest genetic associations are in the human leukocyte antigen class II locus on chromosome 6 (4). The autoimmune nature of type 1 diabetes involves autoreactive CD4+ and CD8+ T cells that target and destroy pancreatic β-cells (5). Although T cells are central to this process, B cells also contribute by producing autoantibodies against insulin, glutamic acid decarboxylase, islet antigen-2, and other β-cell proteins (6,7). These autoantibodies often appear in circulation prior to clinical diagnosis and are used as predictive biomarkers for disease development (6,8). Early stages of the disease, often referred to as presymptomatic type 1 diabetes, are categorized as stage 1 and stage 2 and precede the traditional, symptomatic presentation known as stage 3 (clinical) type 1 diabetes (9). In stage 1, individuals are positive for one or more autoantibodies targeting β-cell proteins but maintain normal glucose levels; stage 2 is marked by both autoantibodies targeting β-cell proteins and dysglycemia without overt symptoms. Although no insulin treatment is required during these earlier stages, the presence of multiple autoantibodies indicates a high likelihood of progression to stage 3 disease. In contrast, stage 3 type 1 diabetes—the focus of this review—is defined by persistent hyperglycemia and the need for exogenous insulin therapy, with or without classic symptoms such as polydipsia, polyuria, and weight loss.

Glycemic variability is a significant factor influencing the management and outcomes of the disease and refers to the fluctuations in blood glucose levels that occur within a day (intra-day variability) or across multiple days (inter-day variability). These fluctuations are particularly pronounced in individuals with type 1 diabetes because of their inability to produce endogenous insulin, making glucose control challenging and highly dynamic (10). High glycemic variability has been associated with an increased risk of hypoglycemia and can be a strong predictor of diabetes complications, including both microvascular and macrovascular conditions (11). Managing glycemic variability is thus a critical component of ongoing diabetes care and aims to minimize extreme glucose outliers and maintain glucose levels in the target range over time.

To better characterize these fluctuations, researchers and clinicians increasingly rely on continuous glucose monitoring (CGM) systems, which collect high-frequency glucose data and allow for the differentiation between short-term intra-day patterns (e.g., postprandial glucose spikes and nocturnal hypoglycemia) and long-term inter-day glycemic trends (e.g., variability across weekdays vs. weekends or during periods of illness). CGM-derived metrics provide a comprehensive glycemic profile and include, among others, glycemic time in range (the percentage of readings with blood glucose typically in the range of 70–180 mg/dL), a glucose management indicator (an estimate of A1C), and glycemic variability (expressed as a percent coefficient of variation [standard deviation divided by mean glucose] or as mean amplitude of glycemic excursions [MAGE; the average size of glucose spikes and dips over a specific period]). These metrics not only enhance clinical understanding of glycemic control, but also inform individualized treatment decisions and lifestyle modifications aimed at minimizing glucose excursions and optimizing metabolic outcomes. Various studies consistently highlight the importance of managing glycemic variability. For example, Siegelaar et al. (12) demonstrated that reduced glycemic variability, as measured by MAGE, is associated with fewer episodes of hypoglycemia and hyperglycemia. More recent studies have also highlighted the neuroprotective effects of tighter glycemic control and emphasized that stable glucose levels may prevent neuronal damage and cognitive decline trends traditionally associated with type 1 diabetes (13,14).

Further elucidating the effects of glycemic variability in people with type 1 diabetes, the acute manifestations of hypoglycemia and hyperglycemia (Figure 1) significantly affect patient health outcomes. In this review, hypoglycemia is typically defined as a blood glucose level <70 mg/dL, whereas hyperglycemia is generally defined as a blood glucose level >125 mg/dL (Figure 1). Although thresholds may vary slightly across studies, these values are widely accepted within clinical practice (15). The cognitive effects discussed in this synthesis relate to both acute and prolonged episodes of glycemic extremes, with durations ranging from several minutes to hours, depending on the cited study.

Figure 1.

Schematic of blood glucose variability: cross-section of a human blood vessel, emphasizing the quantity of glucose (distinguished by white dots) and red blood cells (distinguished by red concave shapes). Focus is placed on the three primary states of blood glucose: hypoglycemia, normal levels, and hyperglycemia. Created in BioRender (https://BioRender.com/k67o158).

Hypoglycemia, or abnormally low blood glucose, is a frequent complication of diabetes management and is especially prevalent in type 1 diabetes, for which insulin therapy is a constant factor. Hypoglycemia generally occurs when insulin doses are not well matched to nutritional intake and physical activity. The acute symptoms of hypoglycemia include confusion, profuse sweating, and palpitations; in severe cases, hypoglycemia can lead to loss of consciousness, seizures, or even death. The fear of hypoglycemia may lead to increased glycemic variability because patients often overtreat suspected low glucose levels, inadvertently causing blood glucose levels to swing into the hyperglycemic range (16). Moreover, research has suggested that repeated hypoglycemia may blunt hormonal responses to hypoglycemia, making future episodes more dangerous and less noticeable to the patient (17).

Conversely, hyperglycemia is characterized by abnormally high blood glucose levels and often results from insufficient insulin relative to metabolic needs. Acute hyperglycemia may lead to symptoms such as polyuria, polydipsia, fatigue, and blurred vision. More severe cases of acute hyperglycemia may also lead to extreme pain, internal injuries to various organs, and death. Chronic exposure to high glucose levels is a well-established risk factor for the development of long-term diabetes complications, including retinopathy, nephropathy, neuropathy, and cardiovascular disease (18).

Hyperglycemia is generally more prevalent across the type 1 diabetes patient population, particularly in individuals with newly diagnosed or poorly managed diabetes. Hypoglycemia, while less common overall, tends to be more prevalent in children and adolescents because of fluctuating insulin sensitivity, inconsistent dietary patterns, and variable physical activity levels (19).

The effects of type 1 diabetes on cognitive functions are multifold, with significant attention given to how hypoglycemia and hyperglycemia influence cognitive decline and how glycemic variability plays a role in neurovascular integrity. Both chronic hyperglycemia and severe hypoglycemia can damage blood vessels, impede blood flow, and exacerbate inflammatory processes throughout the nervous system. This vascular damage is particularly concerning within the central nervous system, where it may lead to cerebral vascular events and contribute to the progression of various neurodegenerative diseases (20). Acute fluctuations in glucose levels may also cause immediate cognitive impairments, affecting memory, attention, and executive functions. Hyperglycemia has been shown to reduce cognitive flexibility and slow psychomotor speed, and hypoglycemia may also impair decision-making and critical-thinking abilities, both acutely and over time (21). Studies have shown that children diagnosed with type 1 diabetes have a higher risk of developing cognitive deficits, particularly in intelligence quotient, executive functions, and processing speed (22). These effects may be further exacerbated in adults, in whom prolonged exposure to dysregulated blood glucose levels has been linked to a greater incidence of cognitive decline and an increased risk of developing dementia in later life stages (23).

The primary objective of this review is to examine how irregular high and low blood glucose levels disrupt normal brain metabolic pathways that are essential for higher reasoning capabilities. Specifically, it seeks to examine whether, in the context of hypoglycemia, the brain deprioritizes certain cognitive functions as a result of insufficient glucose, leading to impairments in higher reasoning abilities and disrupted neural pathways. Conversely, the review will also explore whether, during hyperglycemia, the overabundance of glucose leads the body to prioritize restoring homeostasis, consequently negatively influencing the metabolic pathways that facilitate optimal functioning of critical brain regions involved in higher reasoning, alertness, and overall cognitive function. Although much of the clinical data on cognitive outcomes in type 1 diabetes come from observational studies in human populations, often relying on neuroimaging, cognitive testing, or retrospective analysis, more detailed mechanistic insights, such as those related to hypothalamic neuron activation, neurotransmitter alterations, and structural vulnerability, are primarily derived from rodent models. These animal studies often involve induced hypoglycemia or hyperglycemia under controlled conditions, allowing for precise mapping of neural responses. However, caution must be used when extrapolating these findings to humans because metabolic demands, compensatory responses, and brain maturation timelines differ across species.

Type 1 Diabetes and Neural Structure

The complex interplay between type 1 diabetes and cognitive function reveals critical insights into the impact of glycemic extremes on the brain, including region-specific alterations in the frontal, parietal, temporal, and occipital lobes (Figure 2). Both hypoglycemia and hyperglycemia have been shown to influence neural structure and cognitive performance, with particularly pronounced effects observed in pediatric and geriatric populations (24,25). In children and adolescents, ongoing brain development, particularly in white matter tracts and prefrontal regions, may increase vulnerability to glycemic extremes during critical neurodevelopmental windows. In older individuals, longer disease duration may compound these effects, as cumulative glycemic insults contribute to structural brain changes. Age-related vulnerability, such as increased gray matter loss and reduced neuroplasticity, may further interact with chronic dysglycemia to accelerate cognitive decline. Additionally, the structural consequences of glycemic extremes may differ by brain tissue type. Hypoglycemia has been consistently associated with white matter disruption, particularly in pediatric populations, where active myelination increases vulnerability to glucose deprivation (26). Repeated episodes may impair axonal integrity and contribute to long-term structural changes. In contrast, chronic hyperglycemia is more frequently linked to gray matter atrophy, especially in regions such as the prefrontal cortex and hippocampus. These structural alterations are thought to result from oxidative stress, inflammation, and impaired insulin signaling within the central nervous system (27).

Figure 2.

Glycemic variability and lobe specificity: lateral view of the human brain divided into specific lobes, indicated by color. Focus is placed on the frontal, parietal, temporal, and occipital lobes, with a detailed explanation of how each lobe’s function is distinctly affected by fluctuations in glucose levels, specifically during hypoglycemia and hyperglycemia. This figure represents studies across both pediatric and adult populations, whereas age-specific vulnerability is addressed in the article text. Created in BioRender (https://BioRender.com/e76f272).

Within the frontal lobe, acute hypoglycemia impairs executive functioning and decision-making as a result of energy deprivation and oxidative stress, whereas acute hyperglycemia disrupts neurotransmitter balance, affecting mood stability and cognitive impairments, particularly in tasks requiring a sustained attention span (28). The parietal lobe also experiences reductions in sensory processing and spatial awareness under hypoglycemic conditions and further causes difficulties in interpreting sensory information and navigating spaces. In parallel, during hyperglycemic episodes, vascular changes affect sensory integration and spatial reasoning, leading to impaired perception and coordination (29). The temporal lobe then encounters disruptions within acute hypoglycemic events, leading to memory impairments and language comprehension issues as a result of insufficient glucose for neuronal activity. Hyperglycemia further exacerbates these challenges, resulting in disruptions in auditory processing and memory functions, affecting cognitive tasks related to language and sound (30). Similarly, the occipital lobe faces significant challenges; hypoglycemia causes visual disturbances and impairments in visual processing fields as a result of insufficient glucose supply, whereas hyperglycemia leads to structural changes and vascular damage, affecting visual acuity and processing and contributing to long-term vision problems and visual field defects (31).

Transitioning from acute to long-term effects, hyperglycemia has been associated with diminished cognitive performance, notably in tasks necessitating sustained concentration and alertness, suggesting an adverse impact on cognitive domains essential for daily functioning. On the other hand, hypoglycemia has been linked to alterations in brain glucose metabolism, manifesting as impairments in executive cognitive functions such as working memory and attention. This impairment further underscores the vulnerability of cognitive processes to glucose availability and metabolic changes (32). Moreover, recurrent episodes of hypoglycemia have also been found to induce cumulative effects, resulting in permanent alterations in brain function and cognitive deficits, highlighting the chronic impact of glucose fluctuations (33). Exploring deeper into specific brain regions, the aftermath of hypoglycemic and hyperglycemic episodes reveals that brain regions such as the thalamus, cerebellum, prefrontal cortex, and hippocampus exhibit a spectrum of functional disturbances and imbalances. The thalamus faces neurochemical disruptions that transiently impair sensory processing and integration, leading to altered perceptual accuracy and responsiveness (29). Concurrently, the cerebellum experiences disruptions in neural activity patterns, manifesting as difficulties in balance and precise motor execution (33). The prefrontal cortex undergoes metabolic and neurotransmitter-related alterations that transiently compromise decision-making, attention, and emotional regulation (28). In parallel, the hippocampus, having been identified as particularly susceptible, experiences disruptions in metabolic pathways such as histidine metabolism, often implicated in cognitive dysfunction (34).

Over a prolonged period of glucose dysregulation, these regions grapple with enduring structural and functional modifications. The thalamus and cerebellum may exhibit synaptic degradation and altered neural connectivity, discernible through neuroimaging, reflecting the chronic impact of glucose fluctuations (31). In the prefrontal cortex and hippocampus, sustained glucose extremes can foster synaptic loss, neuronal vulnerability, and alterations in neurotransmitter systems, contributing to persistent cognitive deficits (34,35). Furthermore, research has demonstrated that hypoglycemia can contribute to cumulative neuronal damage and cognitive deficits, particularly affecting memory and spatial navigation functions in the hippocampus (36). This research aligns with findings indicating that repeated episodes of low glucose availability can induce persistent alterations in brain metabolism and cognitive capabilities (37). Moreover, vascular integrity across these regions is also compromised over time, affecting cerebral blood flow and nutrient delivery and further exacerbating the cellular and metabolic stress experienced by these critical brain regions (20). Emerging evidence points to the implications of adversely affected cerebral blood flow and brain metabolism, leading to enduring changes in brain structure and function, including altered white matter integrity and microvascular damage (13). In parallel, these subsequent structural changes further exacerbate and add to impairments in neural communication that are crucial for cognitive processes (37,38).

Hypoglycemia: An Energy Crisis and Neuronal Vulnerability

Hypoglycemia in individuals with type 1 diabetes presents a profound metabolic crisis within the brain’s neural structure. Central to this crisis is the scarcity of glucose, an indispensable energy substrate that supports and maintains many neuronal activities. Glucose facilitates several essential cellular processes, including neurotransmission, maintaining ion gradients, and preserving cellular homeostasis (10,13). The brain contains specialized glucose-sensing neurons, primarily located in the hypothalamus and brainstem, that respond directly to fluctuations in peripheral glucose levels (39). These neurons are categorized as either glucose-excited or glucose-inhibited, adjusting their firing rates in response to rising or falling glucose concentrations. Within the ventromedial hypothalamus (VMH), these sensors play a critical role in initiating counterregulatory responses such as glucagon and epinephrine release during hypoglycemia. Importantly, disruptions in these glucose-sensing pathways may impair the brain’s ability to detect and respond to glycemic extremes, potentially contributing to cognitive symptoms and hypoglycemia unawareness in individuals with type 1 diabetes. Rodent studies have demonstrated that hypoglycemia predominantly activates glutamatergic neurons in the VMH, whereas hyperglycemia induces γ-aminobutyric acid (GABA) receptor activation in the arcuate nucleus, suggesting that distinct neuronal populations mediate the perception and regulation of opposing glycemic states (40). The spatial distribution of these glucose-sensitive neurons may also shape how various brain regions respond to glycemic extremes. For example, cortical regions with stronger input from glucose-excited neurons may exhibit more severe functional deficits under hypoglycemia, whereas other regions may show hyperglycemia-related disruption based on their metabolic demand and inhibitory input. Importantly, disruptions in these glucose-sensing pathways may impair the brain’s ability to detect and respond to glycemic extremes, potentially contributing to cognitive symptoms and hypoglycemia unawareness in individuals with type 1 diabetes.

During hypoglycemic events, neurons are forced to navigate an environment marked by glucose deficiency. Doing so necessitates a consequential metabolic adaptation, driving neurons toward utilizing alternative energy substrates such as lactate. However, these alternative metabolic pathways, while essential for cellular survival without glucose, are inherently less efficient. This inefficiency culminates in extensive energy deprivation throughout the brain, creating a cellular environment with the hallmarks of metabolic dysfunction (37,41). Compounding the challenges of this energy crisis is the emergence of oxidative stress, a detrimental state characterized by an increase in the production of reactive oxygen species levels. Essential cellular structures, including membranes, proteins, and DNA, bear the brunt of this oxidative stress, fostering an environment marked by cellular vulnerability and the potential for functional compromise (36,42). Moreover, certain cerebral regions exhibit a heightened susceptibility to the adversities of hypoglycemia. Notably, the hippocampus and the prefrontal cortex emerge again as critical regions of vulnerability. In the hippocampus, the convergence of energy deprivation and oxidative stress undermines the foundations of synaptic plasticity and precipitates neuronal loss (22,43). Similarly, the prefrontal cortex confronts energy metabolism and oxidative integrity disruptions manifesting as impairments in a spectrum of cognitive domains (21,37).

Building on these clinical observations, further statistical analysis provides deeper insights into the neurological implications of hypoglycemia. Ferguson et al. (43) examined the neurological implications of early-onset type 1 diabetes, revealing a 37% increase in lateral ventricular volumes and a more frequent occurrence of ventricular atrophy compared with cases with later onset. Toprak et al. (44) extended this understanding by exploring the microstructural integrity of the brain in individuals with type 1 diabetes through fractional anisotropy (FA) and apparent diffusion coefficient (ADC) measures. FA is a measure derived from diffusion tensor imaging (DTI) that quantifies the degree of directional water diffusion in tissue, indicative of fiber density, axonal diameter, and myelination in neural pathways. ADC then measures the magnitude of water diffusion within tissue, indicative of cellular health and integrity. Changes in FA and ADC values derived from DTI reflect early microstructural damage in white matter tracts, linking these alterations to potential disruptions in neural connectivity that underlie the cognitive deficits observed in hypoglycemic episodes. Toprak et al. (44) observed significant changes in FA and ADC values indicative of potential early damage to myelinated fibers or axonal degeneration. Adding to this understanding, Perantie et al. (45) reported that severe hypoglycemia experienced between brain imaging scans was associated with reduced growth in white matter volume in the parietal occipital cortex over a 2-year period. A study by Graveling et al. (46) examining the impact of acute hypoglycemia on cognitive functions in adults with type 1 diabetes further found significant impairments in executive functions under hypoglycemic conditions, with marked increases in autonomic, neuroglycopenic, and malaise symptoms during hypoglycemia. By utilizing the National Adult Reading Test (NART), a commonly used instrument to estimate premorbid intelligence levels in adults, as well as the Trail Making B Test and the Digit Symbol Substitution Test (DSST), which are both neuropsychological assessments that measure executive functions such as cognitive flexibility, processing speed, and working memory, Graveling et al. (46) showed that participants with type 1 diabetes had significantly lower baseline intelligence (NART scores) than those without diabetes. Moreover, tests such as the Trail Making B Test and the DSST showed significant impairments during hypoglycemic events, with executive function tests almost universally demonstrating significant impairments during hypoglycemia, with large effect sizes (Cohen d values >0.8) for hypoglycemia-induced detriments (39).

Further evidence of the direct effects of recurrent hypoglycemia in individuals with type 1 diabetes was provided by McCrimmon’s 2021 study (47), which describes a range of cerebral adaptations that occur in response to repeated hypoglycemic episodes. These adaptations include changes in regional glucose delivery, altered glucose transport across the blood-brain barrier, metabolic adjustments, and an increased reliance on nonglucose alternative fuels. Notably, recurrent hypoglycemia leads to an upregulation of glucose transporter proteins such as GLUT1 and GLUT3, enhancing cerebral glucose uptake in future hypoglycemic episodes (42). Although this adaptation may initially appear protective, it contributes to the development of impaired awareness of hypoglycemia (IAH), a condition in which typical autonomic warning symptoms diminish over time, thereby increasing the risk of severe hypoglycemic events resulting from delayed behavioral responses. Functionally, IAH reflects a blunted counterregulatory response and reduced hypothalamic sensitivity to falling glucose levels. Structurally, these adaptations may result in long-term changes such as regional brain atrophy, altered cerebral perfusion, and white matter disruption. Supporting this understanding, studies using functional neuroimaging have shown diminished brain activation during cognitive tasks under hypoglycemic conditions, as well as microstructural changes, including reduced white matter integrity and altered brain metabolism, in individuals with frequent hypoglycemia (48–50). These findings suggest that both metabolic reprogramming and neuronal desensitization underlie the progression of IAH, with direct implications for cognitive function.

Hyperglycemia: Vascular Compromise and Neurotransmitter Dysregulation

Hyperglycemia, in the context of type 1 diabetes, produces a diverse array of metabolic and cellular challenges within the neural environment. It is marked by various vascular modifications and disruptions in neurotransmitter homeostasis, each contributing to complex neuronal and cognitive challenges. The condition’s effects reverberate through critical cerebral regions, shaping the trajectories of neuronal function, cognitive integrity, and overall neurological well-being (14,48,51).

One of the initial challenges seen within acute and recurrent hyperglycemia is vascular modification. Elevated levels of glucose act as a catalyst for inflammatory processes and contribute to a reduction in the elasticity of cerebral blood vessels. This vascular alteration impedes the optimal flow of blood; consequently, the delivery of essential nutrients and oxygen to the brain’s neuronal cells is compromised (37,38). This vascular dysfunction is more pronounced in specific brain regions, including the hippocampus, prefrontal cortex, thalamus, and cerebellum, and directly contributes to an environment marked by neuronal distress and the manifestation of further cognitive impairments (20,52). In addition to the cumulative effects of chronic hyperglycemia, acute hyperglycemic fluctuations have been shown to induce distinct inflammatory and oxidative stress responses that may independently damage neural structures. Recent research by Bahrami et al. (53) demonstrated that rapid glycemic swings produced greater elevations in proinflammatory biomarkers such as interleukin-6 and C-reactive protein compared with sustained hyperglycemia. These acute spikes may amplify oxidative stress and endothelial dysfunction, contributing to transient cognitive deficits and, over time, structural damage, particularly in brain regions with high metabolic demand. This finding suggests that glycemic variability may play a more critical role than average glucose levels alone in determining neurological outcomes. In addition to vascular and inflammatory challenges, hyperglycemia further disrupts various neurotransmitters, such as serotonin and dopamine, throughout the brain. Hyperglycemia-induced disruptions in the levels and activity of these neurotransmitters manifest in a range of cognitive and mood-related disturbances (34,54). These subsequent alterations are observable symptoms, including mood instabilities, deficits in attention, and a broader array of cognitive impairments.

Building on these clinical observations, statistical analysis provides deeper insight into the long-term effects of hyperglycemia in the brain. A 2013 study by Marzelli et al. (26) demonstrated a decrease in gray matter volume in critical regions associated with cognitive capacities in children with type 1 diabetes who had a history of significant hyperglycemia. This finding aligns with newer findings in 2024, which detailed the extent of brain tissue damage and the disruption of neurovascular coupling, a process describing the relationship between various cell types and their surrounding vasculature (13). Additional research further supports these findings, observing changes in white matter volumes and thereby strongly suggesting alterations in the brain’s information-processing and transmission pathways (37,43). Lespagnol et al. (55) also demonstrated that advanced glycation end products (AGEs) exacerbate endothelial dysfunction, reducing vascular elasticity and narrowing cerebral vessels. These changes heighten the risk of ischemic damage in high-demand areas such as the hippocampus and prefrontal cortex, while chronic hyperglycemia continues to significantly reduce cerebral perfusion in these regions, further exacerbating previous neuronal damage (56). Moreover, prolonged hyperglycemic episodes disrupt the integrity of the blood-brain barrier, thereby allowing peripheral inflammatory molecules to increasingly infiltrate the neural environment (57). Notably, hyperglycemia also induces significant alterations in neurotransmitter dynamics. A 2023 study by Zhang et al. (37) found that hyperglycemia dysregulates the excitatory neurotransmitter glutamate, often resulting in excitotoxicity. This process, wherein excessive glutamate overstimulates neurons, leads to cellular dysfunction and death. Similarly, GABA-mediated inhibitory signaling is impaired, leading to neural imbalances that exacerbate cognitive dysfunction and mood disturbances (58). Mick and McCormick (59) further validated this understanding with their work on the role of GABAergic function in type 1 diabetes, and more recent research has linked GABAergic dysfunction in the prefrontal cortex to altered expression of inhibitory markers, which contributes to anxiety and depression-like behaviors in experimental models (60).

Over time, these acute and recurrent effects manifest as structural and functional changes within the brain. A 2006 study by Musen et al. (31) identified reduced gray matter density in the hippocampus and superior temporal gyrus of individuals with type 1 diabetes, linked to decreased neuronal volume, connectivity, and vulnerabilities from chronic hyperglycemia. Later studies further demonstrated that chronic hyperglycemia accelerates cortical thinning in the frontal and temporal lobes—regions critical for executive functions and memory (55). Moreover, these structural changes give further evidence to previously highlighted reductions in functional connectivity between key regions such as the hippocampus and prefrontal cortex (44). These findings also correlate with observed impairments in working memory and cognitive flexibility, providing a neural basis for the behavioral deficits linked to recurrent hyperglycemia. Emerging research suggests that these structural and functional impairments may also predispose individuals with type 1 diabetes to an increased risk of neurodegenerative diseases such as Alzheimer’s and vascular dementia (51). Chronic hyperglycemia contributes to systemic inflammation, oxidative stress, and the accumulation of AGEs, which can impair synaptic function and promote β-amyloid plaque formation, hallmarks of Alzheimer’s disease (61). Concurrently, insulin resistance in the central nervous system may disrupt insulin-signaling pathways essential for neuronal survival, synaptic plasticity, and memory consolidation. These metabolic disruptions impair cerebrovascular function and may contribute to microvascular damage, a key feature in vascular dementia. Various additional studies have suggested that such disruptions in connectivity and metabolic homeostasis significantly increase susceptibility to neurodegeneration, with chronic hyperglycemia and insulin resistance acting as overlapping pathological mechanisms (51,62,63).

Discussion

The findings of our review highlight the significant neurological effects of glycemic variability, hypoglycemia, and hyperglycemia in individuals with type 1 diabetes. Key findings from various studies highlight the significant impact of glycemic extremes, both hypoglycemia and hyperglycemia, on brain structure and cognitive functions. Both acute and chronic glycemic fluctuations disrupt critical brain regions and are especially evident within the hippocampus, prefrontal cortex, and occipital lobes, impairing cognitive processes such as memory, attention, and executive functioning. Hypoglycemia creates an energy crisis in the brain, triggering oxidative stress and neuronal vulnerability, with cumulative damage impairing decision-making and memory retention (36). Conversely, hyperglycemia induces vascular compromise and neurotransmitter dysregulation, leading to structural and functional brain changes over time, such as decreased gray matter volume and impaired psychomotor performance (31). Hyperglycemia contributes to chronic endothelial dysfunction, oxidative stress, and impaired vasodilation, all of which compromise cerebral perfusion over time. In contrast, hypoglycemia may induce transient ischemic-like events by limiting glucose availability to the brain, thereby impairing neuronal energy metabolism and cerebral blood flow (64). These distinct mechanisms suggest that both glycemic extremes can independently damage cerebrovascular integrity, but through different physiological pathways. Key findings from Ferguson et al. (43) and Toprak et al. (44) further document the structural brain changes that occur during hypoglycemic episodes, suggesting that the brain prioritizes essential metabolic processes at the expense of higher cognitive functions. Marzelli et al. (26) extended these findings to hyperglycemia, showing that chronic high blood glucose levels disrupt metabolic pathways in critical brain areas, impairing functions such as decision-making, memory, and processing speed. The methodologies used across these studies, including functional MRI (fMRI) with blood-oxygen-level–dependent (BOLD) contrast (Figure 3) and DTI, provide detailed and validated insights into how glycemic extremes affect both structural and functional aspects of the brain. Figure 3 presents a conceptual schematic resembling an fMRI with BOLD contrast, depicting signal variations observed in a normal brain compared with a hypoglycemic brain, thereby highlighting both regional differences in oxygen utilization and neuronal activity under varying glucose conditions.

Figure 3.

Schematic of fMRI with BOLD contrast: axial view of the human brain, highlighting brain activity in both the hypoglycemic and control groups. Focus is placed on the activation areas marked in yellow and red, representing regions of increased brain activity during a task or stimulus. Created in BioRender (https://BioRender.com/p87c677).

One of the most promising interventions for mitigating the cognitive impacts of type 1 diabetes is the use of CGM. By providing real-time data and alerts, CGM enables patients to maintain tighter glycemic control and reduce both the frequency and severity of extreme glucose fluctuations (65). Emerging evidence suggests that CGM may not only improve metabolic outcomes, but also offer neuroprotective benefits, potentially mitigating long-term cognitive decline (66). To optimize these effects, CGM implementation should be paired with structured patient education that emphasizes consistent CGM use and proactive glycemic management. Further, advancements in diabetes technology, particularly CGM-integrated insulin pumps, automated insulin delivery systems, and hybrid closed-loop platforms, have improved real-time glucose regulation and minimized exposure to glycemic extremes.

Another critical clinical implication is the integration of routine neurocognitive assessments into diabetes care. Early detection of cognitive impairments, particularly in pediatric and adolescent patients, more adequately enables timely interventions in mitigating lifelong deficits caused by type 1 diabetes and glycemic variability (67). Neurocognitive assessments may be standardized to evaluate executive functioning, memory, and attention, with repeated annual measurements over time to monitor comorbidities.

The implications of these findings also extend beyond cognitive health. Chronic glycemic variability may influence endocrine and neuropsychiatric metabolic regulation pathways, contributing to hormonal imbalances and conditions such as anxiety and depression. Sustained hyperglycemia may increasingly disrupt neurotransmitter synthesis and receptor sensitivity, potentially leading to mood disturbances. Similarly, recurrent hypoglycemic episodes could dysregulate the hypothalamic-pituitary-adrenal axis, resulting in altered cortisol levels and stress responses. These mechanisms may exacerbate psychological stress, impaired sleep patterns, and reduced quality of life in people with type 1 diabetes (68). Future research should investigate these links by evaluating the effects of glycemic variability on neurotransmitter profiles, stress hormone regulation, and neural connectivity. Multidisciplinary care models involving endocrinologists, neurologists, and primary care physicians are crucial for addressing the cognitive and psychological impacts of type 1 diabetes comprehensively. Collaborative research should focus on evaluating the long-term cognitive benefits of CGM, exploring its role in reducing glycemic variability and preventing structural neurological changes. Additionally, studies examining the neuropsychiatric dimensions of type 1 diabetes, including attention-deficit hyperactivity disorder, anxiety, and depression, are essential for developing targeted interventions.

Conclusion

This review emphasizes the critical need for a reimagined and integrated approach to type 1 diabetes management that extends beyond traditional glucose metrics such as A1C. By incorporating CGM, routine neurocognitive assessments, and multidisciplinary care, health care providers may better address the overall well-being and improve the long-term health outcomes of their patients with type 1 diabetes.

Duality of Interest

No potential conflicts of interest relevant to this article were reported.

Author Contributions

D.A.D. conceptualized and designed the study, conducted the literature review, synthesized the findings, created the figures, and drafted the manuscript. F.F. provided oversight, critical feedback, and guidance throughout the research process and manuscript development. Both authors reviewed and approved the final version of the manuscript for submission. D.A.D. is the guarantor of this work and, as such, takes responsibility for the integrity of the content.