A review of epidemiological evidence for general anesthesia as a risk factor for Alzheimer's disease

The impact of surgery with general anesthesia on cognitive function and putamen volume: a cross-sectional study among older adults

Background

Previous studies have shown that surgery under general anesthesia may diminish cognitive function; however, the proposed mechanisms need further elucidation. The purpose of the current study was twofold: (1) to compare overall and domain-specific differences in cognitive function between the surgery under general anesthesia group and the control group, and (2) to investigate the possible mechanisms of surgery under general anesthesia affecting cognitive function, using T1-structural magnetic resonance imaging.

Methods

A total of 194 older adults were included in this study. Patients were divided into a surgery under general anesthesia group (n = 92) and a control group (n = 104). The two groups were matched for age, sex, and educational level. All participants underwent clinical evaluation, neuropsychological testing, blood biochemistry analysis, and T1 phase structural magnetic resonance imaging.

Results

We found that older adults with a history of surgery under general anesthesia had lower Montreal Cognitive Assessment (MoCA) scores and smaller right putamen volumes (p < 0.05). Linear regression analysis (mediation model) indicated that surgery under general anesthesia affected MoCA scores by diminishing the volume of the right putamen (B = 1.360, p = 0.030).

Conclusion

We found evidence that older adults who underwent surgery under general anesthesia had poorer cognitive function, which may have been caused by an apoptotic or otherwise toxic effect of anesthetic drugs on the volume of the right putamen.

taVNS Alleviates Sevoflurane-Induced Cognitive Dysfunction in Aged Rats Via Activating Basal Forebrain Cholinergic Neurons

Postoperative cognitive dysfunction (POCD) is a common complication of central nervous system after anesthesia or surgery. Sevoflurane, an inhalation anesthetic, may inhibit cholinergic pathway that induce neuronal death and neuroinflammation, ultimately leading to POCD. Transauricular vagus nerve stimulation (taVNS) has neuroprotective effects in POCD rats, but the mechanisms related to cholinergic system have not been revealed. Sprague-Dawley rats were anesthetized with sevoflurane to construct the POCD model. The immunotoxin 192-IgG-saporin (192-sap) selectively lesioned cholinergic neurons in the basal forebrain, which is the major source of cholinergic projections to hippocampus. After lesion, rats received 5 days of taVNS treatment (30 min per day) starting 24 h before anesthesia. Open field test and Morris water maze were used to test the cognitive function. In this study, rats exposed to sevoflurane exhibited cognitive impairment that was attenuated by taVNS. In addition, taVNS treatment activated cholinergic system in the basal forebrain and hippocampus, and downregulated the expression of apoptosis- and necroptosis-related proteins, such as cleaved Caspase-3 and p-MLKL, in the hippocampus. Meanwhile, the activation of Iba1⁺ microglial by sevoflurane was reduced by taVNS. 192-sap blocked the cholinergic system activation in the basal forebrain and hippocampus and inhibited taVNS-mediated neuroprotection and anti-inflammation effects in the hippocampus. Generally, our study indicated that taVNS might alleviate sevoflurane-induced hippocampal neuronal apoptosis, necroptosis and microglial activation though activating cholinergic system in the basal forebrain.

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Relationship between Surgery under General Anesthesia and the Development of Dementia: A Systematic Review and Meta-Analysis

Objective

To investigate the association between exposure to general anesthesia and the development of Alzheimer's disease (AD) and dementia by reviewing and integrating the evidence from epidemiological studies published to date.

Methods

We searched MEDLINE, EMBASE, and Google Scholar to identify all relevant articles up to April 2018 reporting the risk of AD/dementia following exposure to general anesthesia and finally updated in February 2020. We included patients older than 60 or 65 years who had not been diagnosed with dementia or AD before the study period. The overall pooled effect size (ES) was evaluated with a random-effect model. Subgroup analyses were conducted and possibility of publication bias was assessed.

Results

A total of 23 studies with 412253 patients were included in our analysis. A statistically significant positive association between exposure to general anesthesia and the occurrence of AD was detected in the overall analysis (pooled ES = 1.11, 95%confidence interval = 1.07–1.15), but with substantial heterogeneity (p_χ² < 0.001, I² = 79.4). Although the overall analysis revealed a significant association, the results of the subgroup analyses were inconsistent, and the possibility of publication bias was detected.

Conclusion

s. This meta-analysis demonstrated a significant positive association between general anesthesia and AD. However, considering other results, our meta-analysis must be interpreted with caution. Particularly, it should be considered that it was nearly impossible to discriminate the influence of general anesthesia from the effect of surgery itself on the development of AD. Further, large-scale studies devised to reduce the risk of bias are needed to elucidate the evidence of association between general anesthesia and AD. Trial registration. PROSPERO International prospective register of systematic reviews CRD42017073790.

Current Perspectives on Postoperative Cognitive Dysfunction in the Ageing Population

Postoperative cognitive dysfunction (POCD) is defined as a prolonged cognitive function impairment that occurs within weeks to months of a surgical procedure. It is especially prevalent in the elderly population, leading to increased morbidity and mortality. As anaesthetic and surgical care continues to improve and become increasingly safer, a significantly greater number of older patients have elective surgical procedures today, yet this comes with an increased POCD risk as they go through the perioperative phases. Although the pathophysiology behind the development of POCD is still under investigation, current causative mechanisms include the mode of anaesthesia administered, anaesthetic used, cerebral hypoperfusion, hyperventilation and neuroinflammation. These findings lend an insight into the importance of being cognisant of the higher likelihood of POCD in at-risk patients, including the elderly, and taking precautions to include preoperative and postoperative cognitive testing, careful monitoring during anaesthesia, blood pressure control and early treatment of postoperative complications as they arise. In this review, we provide an update on the current understanding of the pathophysiology leading to POCD, identifying risk factors, prevention and treatment strategies, with a specific focus on the elderly population.

Exposure to General Anesthesia and Risk of Dementia: A Nationwide Population-Based Cohort Study

BACKGROUND

There is a growing concern that general anesthesia could increase the risk of dementia. However, the relationship between anesthesia and subsequent dementia is still undetermined.

OBJECTIVE

To determine whether the risk of dementia increases after exposure to general anesthesia.

METHODS

A population-based prospective cohort study analyzing the Korean National Health Insurance Service-National Sample Cohort database was conducted of all persons aged over 50 years (n = 219,423) from 1 January 2003 and 31 December 2013.

RESULTS

44,956 in the general anesthesia group and 174,469 in the control group were followed for 12 years. The risk of dementia associated with previous exposure to general anesthesia was increased after adjusting for all covariates such as gender, age, health care visit frequency, and co-morbidities (Hazard ratio = 1.285, 95% confidence interval = 1.262-1.384, time-varying Cox hazard model). In addition, the number of anesthetic agents administered, the number of exposures to general anesthesia, the cumulative exposure time, and the organ category involved in surgery were associated with risk of dementia.

CONCLUSION

In light of the increasing societal burden of dementia, careful surveillance for dementia and prevention guidelines for patients after general anesthesia are needed.

Focused transcranial ultrasound for treatment of neurodegenerative dementia

Introduction

Preclinical studies support investigation of focused ultrasound for breakdown of cerebral pathologies in neurodegenerative conditions including Alzheimer's disease (AD) and Parkinson's disease (PD).

Methods

A focused transcranial Doppler device with probes (2 MHz, 520 mW/cm²) affixed bilaterally was used to target the hippocampus (AD) or substantia nigra (PD) with functional magnetic resonance imaging navigation for enhanced plaque removal. A total of 22 patients (n = 11 AD, n = 11 PD) underwent 8 consecutive, weekly, 1-hour treatments wherein sleep was encouraged naturally or pharmacologically. Cognitive and motor functioning assessment was carried out using standardized evaluations at baseline and conclusion.

Results

Of all, 62.5% of patients had one or more improved cognitive scores without data incongruence, 87% had stable or improved fine motor scores, and 87.5% had stable or improved gross motor scores. No adverse events were reported.

Discussion

The safety of focused transcranial Doppler and possible enhancement in patient functioning were suggested by outcome data.

Transcriptomic analysis reveals the molecular mechanism of Alzheimer-related neuropathology induced by sevoflurane in mice

Anesthetics could induce cognitive dysfunctions, such as Alzheimer's disease in humans or mice. However, the precise molecular mechanism is unclear. Sevoflurane is a common anesthetic widely used in clinical practice. Here, we demonstrated the induction of cognitive dysfunction induced by Sev in mice to corroborate the signaling pathway and the differentially expressed genes (DEGs) followed by analyzing their functions. The cognitive function of mice was measured by the Morris water maze test. Transcriptomic data were annotated with Illumina HiSeq. 2000. Further, the changes in related proteins or genes were analyzed by western blotting and real-time quantitative polymerase chain reaction. Our results showed that Sev could cause a decline in cognitive competence in mice. The transcriptomic data indicated that adding up to 566 genes were upregulated and 1073 genes were downregulated. The genes of Plin4, Lcn2, Lrg1, Foxf1, and Ctla2a were significantly upregulated, while the genes of Arc, Npas4, Egr2, Hes5, and Cdh9 were downregulated dramatically. The Gene Ontology term with the highest enrichment of DEGs are involved in the regulation of cellular and macromolecule metabolism and cation and nucleic acid binding, respectively. The Kyoto encyclopedia of genes and genomes analysis indicated that the mitogen-activated protein kinases (MAPK) pathway was one of the most important metabolic pathways. In addition, the metabolic pathways related to cognitive function, such as the nervous system and neurodegenerative disease showed significant changes. Furthermore, we found that p38, c-Jun N-terminal kinase, and extracellular signal-regulated kinase of the MAPK signaling pathway played important roles in this process. In conclusion, these results provide the first important clues for identifying the DEGs and signaling pathways in the hippocampus due to a Sev-induced cognitive deficiency in mice.

The anaesthetic xenon partially restores an amyloid beta-induced impairment in murine hippocampal synaptic plasticity

BACKGROUND

It is controversially discussed whether general anaesthesia increases the risk of Alzheimer's disease (AD) or accelerates its progression. One important factor in AD pathogenesis is the accumulation of soluble amyloid beta (Aβ) oligomers which affect N-methyl-d-aspartate (NMDA) receptor function and abolish hippocampal long-term potentiation (LTP). NMDA receptor antagonists, at concentrations allowing physiological activation, can prevent Aβ-induced deficits in LTP. The anaesthetics xenon and S-ketamine both act as NMDA receptor antagonists and have been reported to be neuroprotective. In this study, we investigated the effects of subanaesthetic concentrations of these drugs on LTP deficits induced by different Aβ oligomers and compared them to the effects of radiprodil, a NMDA subunit 2B (GluN2B)-selective antagonist.

METHODS

We applied different Aβ oligomers to murine brain slices and recorded excitatory postsynaptic field potentials before and after high-frequency stimulation in the CA1 region of hippocampus. Radiprodil, xenon and S-ketamine were added and recordings evoked from a second input were measured.

RESULTS

Xenon and radiprodil, applied at low concentrations, partially restored the LTP deficit induced by pre-incubated Aβ_1-42. S-ketamine showed no effect. None of the drugs tested were able to ameliorate Aβ_1-40-induced LTP-deficits.

CONCLUSIONS

Xenon administered at subanaesthetic concentrations partially restored Aβ_1-42-induced impairment of LTP, presumably via its weak NMDA receptor antagonism. The effects were in a similar range than those obtained with the NMDA-GluN2B antagonist radiprodil. Our results point to protective properties of xenon in the context of pathological distorted synaptic physiology which might be a meaningful alternative for anaesthesia in AD patients.

Overview and Current Status of Alzheimer's Disease in Bangladesh

Alzheimer’s disease (AD) is a complex neurological disorder with economic, social, and medical burdens which is acknowledged as leading cause of dementia marked by the accumulation and aggregation of amyloid-β peptide and phosphorylated tau (p-tau) protein and concomitant dementia, neuron loss and brain atrophy. AD is the most prevalent neurodegenerative brain disorder with sporadic etiology, except for a small fraction of cases with familial inheritance where familial forms of AD are correlated to mutations in three functionally related genes: the amyloid-β protein precursor and presenilins 1 and 2, two key γ-secretase components. The common clinical features of AD are memory impairment that interrupts daily life, difficulty in accomplishing usual tasks, confusion with time or place, trouble understanding visual images and spatial relationships. Age is the most significant risk factor for AD, whereas other risk factors correlated with AD are hypercholesterolemia, hypertension, atherosclerosis, coronary heart disease, smoking, obesity, and diabetes. Despite decades of research, there is no satisfying therapy which will terminate the advancement of AD by acting on the origin of the disease process, whereas currently available therapeutics only provide symptomatic relief but fail to attain a definite cure and prevention. This review also represents the current status of AD in Bangladesh.

Anesthesia, brain changes, and behavior: Insights from neural systems biology

Long-term consequences of anesthetic exposure in humans are not well understood. It is possible that alterations in brain function occur beyond the initial anesthetic administration. Research in children and adults has reported cognitive and/or behavioral changes after surgery and general anesthesia that may be short lived in some patients, while in others, such changes may persist. The changes observed in humans are corroborated by a large body of evidence from animal studies that support a role for alterations in neuronal survival (neuroapoptosis) or structure (altered dendritic and glial morphology) and later behavioral deficits at older age after exposure to various anesthetic agents during fetal or early life. The potential of anesthetics to induce long-term alterations in brain function, particularly in vulnerable populations, warrants investigation. In this review, we critically evaluate the available preclinical and clinical data on the developing and aging brain, and in known vulnerable populations to provide insights into potential changes that may affect the general population of patients in a more, subtle manner. In addition this review summarizes underlying processes of how general anesthetics produce changes in the brain at the cellular and systems level and the current understanding underlying mechanisms of anesthetics agents on brain systems. Finally, we present how neuroimaging techniques currently emerge as promising approaches to evaluate and define changes in brain function resulting from anesthesia, both in the short and the long-term.

Lasting impact of general anaesthesia on the brain: mechanisms and relevance

General anaesthesia is usually considered to safely induce a reversible brain state allowing the performance of surgery under optimal conditions. An increasing number of clinical and experimental observations, however, suggest that anaesthetic drugs, especially when they are administered at the extremes of age, can trigger long-term morphological and functional alterations in the brain. Here, we review available mechanistic data linking general-anaesthesia exposure to impaired cognitive performance in both young and mature nervous systems. We also provide a critical appraisal of the translational value of animal models and highlight the important challenges that need to be addressed to strengthen the link between laboratory work and clinical investigations in the field of anaesthesia-neurotoxicity research.

Factors Influencing Clinical Correlates of Chronic Traumatic Encephalopathy (CTE): a Review

Chronic traumatic encephalopathy (CTE) is a neuropathologically defined disease reportedly linked to a history of repetitive brain trauma. As such, retired collision sport athletes are likely at heightened risk for developing CTE. Researchers have described distinct pathological features of CTE as well a wide range of clinical symptom presentations, recently termed traumatic encephalopathy syndrome (TES). These clinical symptoms are highly variable, non-specific to individuals described as having CTE pathology in case reports, and are often associated with many other factors. This review describes the cognitive, emotional, and behavioral changes associated with 1) developmental and demographic factors, 2) neurodevelopmental disorders, 3) normal aging, 4) adjusting to retirement, 5) drug and alcohol abuse, 6) surgeries and anesthesia, and 7) sleep difficulties, as well as the relationship between these factors and risk for developing dementia-related neurodegenerative disease. We discuss why some professional athletes may be particularly susceptible to many of these effects and the importance of choosing appropriate controls groups when designing research protocols. We conclude that these factors should be considered as modifiers predominantly of the clinical outcomes associated with repetitive brain trauma within a broader biopsychosocial framework when interpreting and attributing symptom development, though also note potential effects on neuropathological outcomes. Importantly, this could have significant treatment implications for improving quality of life.

Anesthesia Exposure and Risk of Dementia and Alzheimer's Disease: A Prospective Study

OBJECTIVES

To evaluate the associations between anesthesia and dementia or Alzheimer's disease (AD) risk using prospectively collected data.

DESIGN

Cohort study.

PARTICIPANTS

Community-dwelling members of the Adult Changes in Thought cohort aged 65 and older and free of dementia at baseline (N = 3,988).

MEASUREMENTS

Participants self-reported all prior surgical procedures with general or neuraxial (spinal or epidural) anesthesia at baseline and reported new procedures every 2 years. People undergoing high-risk surgery with general anesthesia, other surgery with general anesthesia, and other surgery with neuraxial anesthesia exposures were compared with those with no surgery and no anesthesia. Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for dementia and AD associated with time-varying lifetime and recent (past 5 years) anesthesia exposures.

RESULTS

At baseline, 254 (6%) people reported never having anesthesia; 248 (6%) had had one or more high-risk surgeries with general anesthesia, 3,363 (84%) had had one or more other surgeries with general anesthesia, and 123 (3%) had had one or more surgeries with neuraxial anesthesia. High-risk surgery with general anesthesia was not associated with greater risk of dementia (HR = 0.86, 95% CI = 0.58-1.28) or AD (HR = 0.95, 95% CI = 0.61-1.49) than no history of anesthesia. People with any history of other surgery with general anesthesia had a lower risk of dementia (HR = 0.63, 95% CI = 0.46-0.85) and AD (HR = 0.65, 95% CI = 0.46-0.93) than people with no history of anesthesia. There was no association between recent anesthesia exposure and dementia or AD.

CONCLUSION

Anesthesia exposure was not associated with of dementia or AD in older adults.

Neuroprotective Effects Against POCD by Photobiomodulation: Evidence from Assembly/Disassembly of the Cytoskeleton

Postoperative cognitive dysfunction (POCD) is a decline in memory following anaesthesia and surgery in elderly patients. While often reversible, it consumes medical resources, compromises patient well-being, and possibly accelerates progression into Alzheimer's disease. Anesthetics have been implicated in POCD, as has neuroinflammation, as indicated by cytokine inflammatory markers. Photobiomodulation (PBM) is an effective treatment for a number of conditions, including inflammation. PBM also has a direct effect on microtubule disassembly in neurons with the formation of small, reversible varicosities, which cause neural blockade and alleviation of pain symptoms. This mimics endogenously formed varicosities that are neuroprotective against damage, toxins, and the formation of larger, destructive varicosities and focal swellings. It is proposed that PBM may be effective as a preconditioning treatment against POCD; similar to the PBM treatment, protective and abscopal effects that have been demonstrated in experimental models of macular degeneration, neurological, and cardiac conditions.

Hereditary vulnerabilities to post-operative cognitive dysfunction and dementia

In view of multiple prospective investigations reporting an incidence of 10% or greater in elderly patients after cardiac and non-cardiac procedures, it is surprising that no families, twins or even individual cases have been reported with persistent post-operative cognitive dysfunction (POCD) or post-operative dementia (POD) that is otherwise unexplained. As POCD and POD research has shifted in recent years from surgical and anesthetic variables to predictors of intrinsic, patient-specific susceptibility, a number of markers based on DNA sequence variation have been investigated. Nevertheless, no heritable, genomic indices of persistent POCD or post-operative dementia lasting 3 months or longer after surgery have been identified to date. The present manuscript surveys challenges confronting the search for markers of heritable vulnerability to POCD and POD, and proposes steps forward to be taken now, including the addition of surgical and anesthetic descriptors to ongoing longitudinal dementia protocols and randomized clinical trials (RCTs) comprising serial psychometric testing, and a fresh focus on phenotypes and genotypes shared between outliers with "extreme" POCD and POD traits.