Preoperative butyrylcholinesterase activity and risk of postoperative delirium: A meta-analysis

Abstract

BACKGROUND

Postoperative delirium (POD) is a common and serious complication in surgical patients, particularly older adults. Alterations in cholinergic function have been implicated in its pathophysiology.

AIM

To evaluate the association between preoperative serum cholinesterase (ChE) activity—specifically butyrylcholinesterase (BuChE) and acetylcholinesterase (AChE)—and the risk of POD in adult surgical patients in a meta-analysis.

METHODS

A systematic search was conducted in PubMed, EMBASE, and Web of Science up to March 28, 2025 for studies reporting preoperative serum BuChE or AChE activity in relation to subsequent POD incidence. Standardized mean differences (SMDs) and odds ratios (ORs) with 95% confidence intervals (CIs) were pooled using random-effects models. Subgroup and sensitivity analyses were performed based on follow-up duration and analytic models.

RESULTS

Thirteen studies (n = 2730 patients) were included. Patients who developed POD had significantly lower preoperative BuChE activity than those who did not (SMD = -0.28; 95%CI: -0.39 to -0.16; I² = 18%). Higher BuChE activity was associated with a reduced risk of POD (OR per 100 unit increment = 0.97; 95%CI: 0.95-0.99; I² = 0%). In contrast, pooled AChE activity did not differ significantly between POD and non-POD groups (SMD = -0.25; 95%CI: -0.53 to 0.03; P = 0.08; I² = 80%), and the ORs per 1 unit increment in AChE activity were not statistically significant (OR = 0.98; 95%CI: 0.95-1.01).

CONCLUSION

Lower preoperative serum BuChE activity is associated with an increased risk of POD in adults undergoing surgery. BuChE activity may serve as a potential preoperative biomarker for POD risk stratification.

Key Words: Acetylcholinesterase; Butyrylcholinesterase; Activity; Postoperative delirium; Meta-analysis

Core Tip: This meta-analysis synthesizes evidence from 13 studies involving 2730 surgical patients to assess the association between preoperative cholinesterase activity and postoperative delirium (POD). We found that lower preoperative butyrylcholinesterase (BuChE) activity was significantly associated with increased POD risk, whereas acetylcholinesterase activity showed no consistent association. These findings suggest that serum BuChE may serve as a simple, accessible biomarker for POD risk stratification. Our study provides the most comprehensive evidence to date linking peripheral cholinergic dysfunction to POD and highlights BuChE as a potential target for early identification and intervention.

INTRODUCTION

Postoperative delirium (POD) is an acute, fluctuating disturbance in attention, awareness, and cognition that frequently occurs following surgery, particularly in older adults[1,2]. It is reported that the incidence ranges from 10% to over 50%, depending on patient characteristics and the type of surgery, with particularly high rates observed in cardiac, orthopedic, and emergency procedures[3]. POD is associated with a range of adverse outcomes, including prolonged hospitalization, increased healthcare expenditures, higher risk of institutionalization, long-term cognitive decline, and elevated mortality[4,5]. Although established risk factors—such as advanced age, preexisting cognitive impairment, polypharmacy, comorbidities, and surgical type or duration—are well documented, their predictive utility remains limited[6,7]. As a result, there is increasing interest in identifying novel, modifiable, or readily accessible biomarkers that may enhance preoperative risk stratification and support preventive interventions[7].

Cholinesterases (ChE), including acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), are enzymes that hydrolyze acetylcholine, a key neurotransmitter in the central and peripheral nervous systems[8,9]. While AChE is primarily localized in neuronal synapses, BuChE is synthesized in the liver and widely distributed in plasma and glial cells[10,11]. BuChE and AChE play critical roles in maintaining cholinergic neurotransmission, which is essential for attention, arousal, and memory[12]. Dysregulation of the cholinergic system has long been implicated in the pathogenesis of delirium, and pharmacologic agents with anti-cholinergic properties are recognized contributors to cognitive dysfunction[13]. Hypocholinergic states may predispose patients to developing POD, whether due to reduced synthesis, increased degradation, or receptor dysfunction[13,14]. It has been hypothesized that reduced peripheral ChE activity could reflect or contribute to impaired central cholinergic tone, thus increasing vulnerability to postoperative neuroinflammatory and neurochemical insults[15].

Recent observational studies have examined the association between preoperative serum BuChE or AChE activity and the risk of POD[16-18]. However, findings have been inconsistent, potentially due to variations in study design, patient populations, surgical procedures, ChE measurement methods, and analytic approaches[16-18]. Additionally, no consensus exists regarding whether BuChE and AChE contribute equally to POD prediction or whether either enzyme may serve as a clinically meaningful biomarker. To address this knowledge gap, we conducted a systematic review and meta-analysis to evaluate the association between preoperative serum ChE activity—including both BuChE and AChE—and the risk of POD in adult surgical patients.

MATERIALS AND METHODS

This meta-analysis was conducted per the preferred reporting items for systematic review and meta-analysis 2020 guidelines[19,20] and the Cochrane Handbook for Systematic Reviews of Interventions[21], following established standards for study design, data extraction, statistical analysis, and interpretation. The protocol was prospectively registered in the International Prospective Register of Systematic Reviews under the identifier No. CRD420251034815.

Literature search

To identify eligible studies, we systematically searched PubMed, EMBASE, and Web of Science from database inception to March 28, 2025 using combinations of the following terms: (1) “cholinesterase”, “acetylcholinesterase”, “butyrylcholinesterase”, “pseudocholinesterase”, or “cholinesterase activity”; (2) “delirium”, “confusion”, “disorientation”, “cognitive”, or “cognition”; and (3) “postoperative”, “perioperative”, “preoperative”, “operative”, “operation”, “surgery”, “surgical”, “anesthesia”, or “anaesthesia”. The search was limited to human studies published in English as full-length articles in peer-reviewed journals. Reference lists of relevant original and review articles were manually screened to identify additional eligible studies.

Inclusion and exclusion criteria

The inclusion criteria for potential studies were defined according to the population, intervention, comparison, outcomes, and study framework.

P (patients): Adult patients (aged ≥ 18 years) undergoing surgical procedures under general, regional, or combined anesthesia across various clinical settings.

I (exposure): The exposure of interest in this review is preoperative blood ChE activity, specifically BuChE and/or AChE, measured using laboratory or point-of-care methods as specified in the original articles. Studies are included if they assessed serum BuChE and/or AChE activities prior to the initiation of surgery. Patients with low preoperative serum BuChE or AChE activities were considered the exposure group. The cut-offs for defining low serum BuChE or AChE activities were consistent with the values used in the original studies.

C (comparison): Patients without low serum BuChE or AChE activities before surgery.

O (outcome): The incidence of POD, compared between patients with and without low serum BuChE or AChE activities before surgeries.

S (study design): Observational studies with longitudinal follow-up, such as cohort studies (including prospective or retrospective cohorts), nested case-control studies, and post-hoc analyses of clinical trials.

Reviews, editorials, meta-analyses, preclinical studies, studies including pediatric patients, and studies that did not enroll patients undergoing surgeries, did not measuring preoperative serum BuChE or AChE activities, or failed to report the outcome of POD were excluded. If two or more studies with overlapping populations were found, the study with the largest sample size was enrolled for the meta-analysis.

Study quality evaluation and data extraction

Two authors independently performed the literature search, study selection, quality assessment, and data extraction. Discrepancies were resolved through discussion with the corresponding author. Study quality was assessed using the Newcastle-Ottawa Scale (NOS)[22], which evaluates study selection, control of confounding variables, and outcome measurement and analysis. NOS scores range from 1 to 9, with a score of 9 indicating the highest quality. Studies scoring ≥ 7 were considered to be of good quality. Data extracted for analysis included study characteristics (author, year, country, and design), participant details (patient and surgical characteristics, sample size, age, sex, and type of anesthesia), timing and methods of serum ChE activity measurement, methods and follow-up durations for POD detection, number of patients who developed POD in each study, reported outcomes, and variables that were matched or adjusted in the analysis of the association between preoperative serum ChE activity and POD risk.

Statistical analysis

To analyze the association between preoperative serum ChE activity and the risk of POD, we first compared the serum ChE activities (including BuChE and AChE). Because different methods and units were used to measure serum ChE activities among the included studies, the differences in serum ChE activities were summarized as a standardized mean difference (SMD) and corresponding 95% confidence interval (CI)[21]. In addition, the odds ratio (OR) and corresponding 95%CI for POD per 100 unit increment of BuChE and per 1 unit increment of AChE were also summarized. Data of ORs and standard errors were calculated based on the 95%CIs or P-values, followed by a logarithmic transformation to ensure stabilized variance and normalized distribution[21]. We used the Cochrane Q test and I² statistics[23] to assess heterogeneity, with I² < 25%, 25%-75%, and > 75% indicating mild, moderate, and substantial heterogeneity, respectively. A random-effects model was applied to integrate the results, accounting for study variability[21]. Via excluding individual studies sequentially, a sensitivity analysis was performed to evaluate the robustness of the findings. Predefined subgroup analyses were performed to evaluate the influences of study characteristics on the results, such as the follow-up duration and analytic models used for evaluating the association between serum ChE activity and POD. The medians of the continuous variables were selected as the cut-off values for defining subgroups. Publication bias was evaluated using funnel plots and visual inspection for asymmetry, supplemented by the Egger’s regression test[24]. Analyses were performed using RevMan (Version 5.1; Cochrane Collaboration, Oxford, United Kingdom) and Stata software (version 12.0; Stata Corporation, College Station, TX, United States).

RESULTS

Study inclusion

The study selection process is illustrated in Figure 1. A total of 1056 records were identified through database searches, and 224 duplicates were removed. Of the remaining 832 articles, 807 were excluded based on title and abstract screening, primarily due to irrelevance to the objective of the meta-analysis. Full-text reviews were conducted for the remaining 25 articles, of which 12 were excluded for reasons detailed in Figure 1. Thirteen studies met all the inclusion criteria and were included in the final meta-analysis[16-18,25-34] (Table 1).

Figure 1 Flowchart of database search and study inclusion. POD: Postoperative delirium.

Table 1 Characteristics of the included studies.

Ref.	Country	Design	Patients’ characteristics	No. of patients included	Mean age (years)	Men (%)	Anesthesia	Timing of ChE measuring	Method for measuring ChE	Follow-up duration	Method(s) for diagnosis of POD	No. of patients with POD	Outcome reported	Variables matched or adjusted
Zajonz et al[16], 2023	Germany	PC	Adults undergoing elective isolated on-pump CABG	93	65.6	87.1	General anesthesia	Preoperative	Bedside photometric testing	7 days after surgery	CAM-ICU and ICDSC	20	Difference of AChE and BuChE activity between patients with and without POD	Age, sex, EuroSCORE, HbA1c, diabetes, CPB duration, intraoperative variables
Gruendel et al[17], 2024	Germany	PC	Adults > 70 years undergoing scheduled (hip/knee) or emergency (hip/arm) orthopedic surgery	151	81.4	29.8	General or spinal anesthesia	Preoperative (within 24 hours before surgery)	Bedside photometric testing	7 days after surgery	CAM	38	Difference of AChE and BuChE activity between patients with and without POD	Age, sex, ASA, CCI, MMSE, Frailty Score, GDS, MNA, CDT
Schlake et al[18], 2024	Germany	PC	Adults undergoing elective cardiothoracic surgery or TAVI	237	74	63.4	General anesthesia	Preoperative	Photometric method	3 days after surgery	CAM or CAM-ICU	94	Difference of BuChE activity between patients with and without POD, and OR of BuChE activity for POD	Age, MoCA score at baseline type 2 diabetes, coronary heart disease, surgery type, red cell concentrate transfusion
Cerejeira et al[25], 2011	Portugal	PC	Older patients (≥ 60 years) undergoing elective THA	101	73	49.5	NR	At admission	Colorimetric assay	3 days after surgery	CAM criteria confirmed by DSM-IV-TR	37	Difference of AChE and BuChE activity between patients with and without POD	Age, sex, alcohol use, comorbidities, anti-cholinergic drug burden, and laboratory markers
Müller et al[26], 2019	Germany	PC	Adults undergoing elective inpatient surgery across various surgical disciplines	650	61.5	52.8	General anesthesia	Pre-op (≥ 1 day and just before induction)	Bedside photometric testing	3 days after surgery	Nu-DESC	179	Difference of AChE and BuChE activity between patients with and without POD, and OR of AChE and BuChE activity for POD	Age, sex, ASA PS, NYHA class, comorbidities, severity/duration of surgery and anesthesia, RASS scores, anti-cholinergic burden, transfusions, fluids, hemoglobin level
Adam et al[27], 2020	Germany	PC	Adults undergoing elective cardiac surgery	114	70.4	72.8	General anesthesia	Preoperative morning	Bedside photometric testing	2 days after surgery	CAM-ICU	31	Difference of AChE and BuChE activity between patients with and without POD, and OR of AChE and BuChE activity for POD	Age, sex, EuroSCORE, and anti-cholinergic use
Lin et al[28], 2020	China	PC	Older patients (≥ 65 years) undergoing elective TKA or THA	447	72.3	47.2	Combined spinal-epidural anesthesia	Preoperative (before anesthesia during spinal puncture)	Spectrophotometry	7 days after surgery	CAM criteria with severity rated by MDAS	51	Difference of AChE and BuChE activity between patients with and without POD	Age, sex, education, height, weight, BMI, ASA class
Chen et al[29], 2021	China	RC	Older patients (≥ 60 years) undergoing elective TKA or THA	116	85.6	22.4	General anesthesia	Preoperative	NR	3 days after surgery	CAM criteria	58	OR of BuChE activity for POD	Age, sex, smoking, alcohol drinking, comorbidities, history of mental illness/psychotropic drugs, transfusion, anesthesia, surgical approach, surgical grade, EF, LDH, Cystatin C, arrhythmia, and operation duration
Guenther et al[30], 2021	Germany	PC	Older patients (≥ 60 years) undergoing elective cardiac surgery	51	72	69.8	General anesthesia	Preoperative	Clinical chemistry analyzer	ICU stay	CAM-ICU	20	Difference of BuChE activity between patients with and without POD, and OR of BuChE activity for POD	Age, sex, EuroSCORE, Hb, platelets, and CPB time
Saha et al[31], 2021	Germany	PC	Adults undergoing elective cardiac surgery	107	66.9	80.4	General anesthesia	Preoperative	Clinical chemistry analyzer	5 days after surgery	CAM-ICU and ICDSC	34	Difference of BuChE activity between patients with and without POD	Age, sex, baseline MMSE, anti-cholinergic burden, and surgical characteristics
Takano et al[32], 2021	Japan	RC	Older patients (≥ 65 years) undergoing emergency major GI surgery	60	80.1	43.3	General anesthesia	Preoperative (within 48 hours before surgery)	Routine biochemical tests	30 days after surgery	Medical chart evidenced	7	OR of BuChE activity for POD	None
Bosancic et al[33], 2022	Germany	PC	Older patients (≥ 65 years) undergoing elective abdominal surgery	127	72.4	41	General, regional, or combined	Preoperative (morning of surgery)	Bedside photometric testing	7 days after surgery	CAM or CAM-ICU	52	Difference of AChE and BuChE activity between patients with and without POD, and OR of AChE and BuChE activity for POD	Age, sex, duration of surgery, and CCI
Chen et al[34], 2023	China	RC	Older patients (≥ 60 years) undergoing elective THA	476	78.4	33.8	General anesthesia	Preoperative	Routine biochemical tests	3 days after surgery	CAM	86	OR of BuChE activity for POD	None

ChE: Cholinesterase; POD: Postoperative delirium; PC: Prospective cohort; RC: Retrospective cohort; CABG: Coronary artery bypass grafting; CAM-ICU: Confusion Assessment Method for the Intensive Care Unit; ICDSC: Intensive Care Delirium Screening Checklist; AChE: Acetylcholinesterase; BuChE: Butyrylcholinesterase; EuroSCORE: European System for Cardiac Operative Risk Evaluation; CPB: Cardiopulmonary bypass; CAM: Confusion Assessment Method; ASA: American Society of Anesthesiologists Physical Status Classification; CCI: Charlson Comorbidity Index; MMSE: Mini-Mental State Examination; GDS: Geriatric Depression Scale; MNA: Mini Nutritional Assessment; CDT: Clock Drawing Test; TAVI: Transcatheter aortic valve implantation; OR: Odds ratio; MoCA: Montreal Cognitive Assessment; THA: Total hip arthroplast; NR: Not reported; DSM-IV-TR: Diagnostic and Statistical Manual-Fourth Edition, Text Revision; Nu-DESC: Nursing Delirium Screening Scale; PS:; NYHA: New York Heart Association Classification; RASS: Richmond Agitation-Sedation Scale; TKA: Total knee arthroplasty; MDAS: Memorial Delirium Assessment Scale; BMI: Body mass index; EF: Ejection fraction; LDH: Lactate dehydrogenase; Hb: Hemoglobin; GI: Glycaemic index.

Overview of study characteristics

Table 1 summarizes the key characteristics of the 13 studies included in this meta-analysis[16-18,25-34]. Overall, ten prospective cohort studies[16-18,25-28,30,31,33] and three retrospective studies[29,32,34], published between 2011 and 2024 and conducted in Germany, China, Portugal, and Japan, were involved. The study populations consisted of adults undergoing various types of surgery, including orthopedic, cardiac, gastrointestinal, and general procedures. In total, 2730 adult surgical patients were included, with individual study sample sizes ranging from 51 to 650 and mean ages spanning from 61.5 to 85.6 years. The proportion of male participants varied considerably across studies, ranging from 22.4% to 87.1%. All included studies measured ChE activity preoperatively, primarily using bedside photometric testing or biochemical assays. POD was diagnosed using validated instruments, including the confusion assessment method (CAM), confusion assessment method for the intensive care unit, nursing delirium screening scale (Nu-DESC), and Intensive Care Delirium Screening Checklist, within 2 to 30 days following surgery.

All the included studies assessed BuChE activity[16-18,25-34], and seven studies also measured AChE activity[16,17,25-28,33]. While two studies[32,34] reported only unadjusted group comparisons, others[16-18,25-31,33] provided adjusted ORs for the association between ChE activity and POD risk, accounting for covariates such as age, sex, comorbidities, surgical variables, and anti-cholinergic burden. The overall quality of the included studies was moderate to high, with NOS scores ranging from 7 to 9 (Table 2), indicating generally robust methodological quality.

Table 2 Study quality evaluation via the Newcastle-Ottawa Scale.

Ref.	Representativeness of the exposed cohort	Selection of the non-exposed cohort	Ascertainment of exposure	Outcome not present at baseline	Control for age and sex	Control for other confounding factors	Assessment of outcome	Enough long follow-up duration	Adequacy of follow-up of cohorts	Total1
Zajonz et al[16], 2023	1	1	1	1	1	1	1	1	1	9
Gruendel et al[17], 2024	1	1	1	1	1	1	1	0	1	8
Schlake et al[18], 2024	1	1	1	1	1	1	1	0	1	8
Cerejeira et al[25], 2011	1	1	1	1	1	1	1	0	1	8
Müller et al[26], 2019	1	1	1	1	1	1	1	0	1	8
Adam et al[27], 2020	1	1	1	1	1	1	1	0	1	8
Lin et al[28], 2020	1	1	1	1	1	1	1	1	1	9
Chen et al[29], 2021	0	1	1	1	1	1	1	0	1	7
Guenther et al[30], 2021	1	1	1	1	1	1	1	0	1	8
Saha et al[31], 2021	1	1	1	1	1	1	1	1	1	9
Takano et al[32], 2021	1	1	1	1	0	0	1	1	1	7
Bosancic et al[33], 2022	1	1	1	1	1	1	1	1	1	9
Chen et al[34], 2023	1	1	1	1	0	0	1	1	1	7

¹Each item scored as 1 if the criterion was fulfilled and 0 if not; total score represents the sum across domains (maximum = 9).

Association between preoperative BuChE activity and POD

The pooled results of ten studies[16-18,25-28,30,31,33] showed that compared to patients who did not develop POD, those with POD had a significantly lower serum BuChE activity before surgery (SMD: -0.28, 95%CI: -0.39 to -0.16, P < 0.001; Figure 2A) with mild heterogeneity (I² = 18%). Sensitivity analysis excluding one study at a time showed consistent results (SMD: -0.24 to -0.32, all P < 0.05). Further subgroup analysis showed similar results in studies with follow-up duration < and ≥ 7 days (SMD: -0.24 vs -0.32, P for subgroup difference = 0.53; Figure 2B). In addition, the meta-analysis involving eight studies[18,26,27,29,30,32-34] showed that a 100 unit increment of preoperative serum BuChE activity was associated with a reduced risk of POD (OR = 0.97, 95%CI: 0.95-0.99, P = 0.01; Figure 2C), with no significant heterogeneity (I² = 0%). Sensitivity analysis by excluding one study at a time did not significantly change the results (OR all = 0.97, P < 0.05). Further subgroup analyses did not suggest significantly different results in studies with follow-up duration < and ≥ 7 days (OR = 0.97 vs 0.98, P for subgroup difference = 0.73; Figure 2D), or between studies with univariate and multivariate analyses (OR = 0.96 vs 0.97, P for subgroup difference = 0.67; Figure 2E).

Figure 2 Forest plots for the meta-analysis. A and B: Forest plots for the meta-analysis of difference of preoperative serum butyrylcholinesterase activity between adult patients with and without the development of postoperative delirium (POD). A: Overall meta-analysis; B: Subgroup analysis according to follow-up duration; C-E: Forest plots for the meta-analysis of association between preoperative serum butyrylcholinesterase activity and risk of POD; C: Overall meta-analysis for the odds ratio for POD per 100 unit-increment of preoperative serum butyrylcholinesterase activity; D: Subgroup analysis according to follow-up duration; E: Subgroup analysis according to the analytic models; F-H: Forest plots for the meta-analyses of association between preoperative serum acetylcholinesterase (AChE) activity and risk of POD; F: Meta-analysis of difference of preoperative AChE activity; G: Subgroup analysis of difference of preoperative AChE activity according to follow-up duration; H: Overall meta-analysis for the odds ratio for POD per 1 unit-increment of preoperative serum AChE activity. POD: Postoperative delirium; CI: Confidence interval; AChE: Acetylcholinesterase.

Association between preoperative AChE activity and POD

The pooled results of seven studies[16,17,25-28,33] showed that patients with POD were not associated with a significantly lower level of serum AChE activity before surgery as compared to those without (SMD: -0.25, 95%CI: -0.53 to 0.03, P = 0.08; I² = 80%; Figure 2F). Sensitivity analysis showed that the results become significant after excluding the study by Müller et al[26] in 2019 (SMD: -0.34, 95%CI: -0.55 to -0.12, P = 0.002; I² = 48%), whereas excluding other studies, one at a time, did not significantly change the results (SMD: -0.20 to -0.32, all P > 0.05). Further meta-analysis did not suggest a significantly different result between studies with follow-up durations < and ≥ 7 days (SMD: -0.27 vs -0.25, P for subgroup difference = 0.96; Figure 2G). Finally, a meta-analysis including three studies[26,27,33] did not suggest a significant association between serum AChE activity and POD (OR per 1 increment of AChE activity: 0.98, 95%CI: 0.95-1.01, P = 0.17; I² = 35%; Figure 2H). Further sensitivity analyses excluding one study at a time showed similar results (OR = 0.97 to 0.99, all P > 0.05).

Publication bias

Funnel plots for the meta-analyses examining the association between preoperative serum BuChE and AChE activity and the risk of POD are presented in Figure 3. Visual inspection of the plots indicated symmetry, suggesting a low risk of publication bias. Egger’s regression tests further supported this, indicating no significant evidence of publication bias for the meta-analyses of BuChE difference, OR for BuChE, and AChE difference (P = 0.64, 0.71, and 0.38, respectively). The Egger’s test was not performed for the meta-analysis of AChE-related ORs due to the inclusion of only three studies.

Figure 3 Funnel plots for estimating potential publication biases underlying the meta-analyses. A: Funnel plots for the meta-analysis of difference in serum butyrylcholinesterase activity; B: Funnel plots for the meta-analysis of odds ratio for serum butyrylcholinesterase activity; C: Funnel plots for the meta-analysis of difference in serum acetylcholinesterase activity; D: Funnel plots for the meta-analysis of odds ratio for serum acetylcholinesterase activity. SE: Standard error; SMD: Standardized mean difference; OR: Odds ratio.

DISCUSSION

This meta-analysis systematically evaluated the association between preoperative serum ChE activity and the risk of POD in adult surgical patients. The pooled analysis demonstrated that individuals who developed POD had significantly lower preoperative BuChE activity than those who did not. Additionally, each 100-unit increase in BuChE activity was associated with a statistically significant reduction in POD risk. These findings remained robust in sensitivity analyses and were consistent across subgroups defined by follow-up duration and the use of univariate vs multivariate statistical models.

In contrast, the role of AChE activity in POD risk remains uncertain. Although the pooled analysis of seven studies suggested a trend toward lower preoperative AChE activity in patients with POD, the result did not reach statistical significance (P = 0.08), and heterogeneity was substantial (I² = 80%). Notably, sensitivity analysis revealed that excluding the study by Müller et al[26] rendered the association statistically significant. This study was the largest cohort and included diverse surgical populations, unlike many others that focused on specific procedures. Moreover, it employed the Nu-DESC screening tool[26], which is highly sensitive and may capture milder or subsyndromal delirium, potentially attenuating group differences[35]. These factors likely explain why the Müller et al’s study exerted a disproportionate influence on the pooled estimate and contributed to heterogeneity[26]. Moreover, differences in the timing and accuracy of AChE measurement, as well as population-specific factors, could also contribute to variability in results.

The biological mechanisms linking low preoperative BuChE activity to an increased risk of POD are biologically plausible and supported by neurochemical and inflammatory pathways[36]. BuChE, synthesized in the liver and present in plasma and glial cells, plays a compensatory role in hydrolyzing acetylcholine, particularly when AChE is deficient or overwhelmed[37]. A reduction in BuChE activity may reflect impaired cholinergic reserve and reduced capacity to buffer central neurotransmitter imbalance during the perioperative period[38]. The cholinergic system modulates arousal, attention, and cognition and is also involved in the “cholinergic anti-inflammatory pathway” that limits systemic inflammation via vagal nerve signaling[39,40]. Therefore, low BuChE activity may indicate vulnerability to neuroinflammation, oxidative stress, and neurotransmitter dysregulation—hallmarks of POD pathophysiology[41-43]. Additionally, BuChE is considered a negative acute-phase reactant, and its suppression may signal subclinical systemic illness or frailty[44], both of which are known risk factors for delirium[45].

Beyond genetic variation, drug-induced epigenetic regulation of ChE enzymes may also contribute to POD. Rump et al[46] reported that midazolam increased BuChE expression through reduced BCHE gene methylation and histone modifications, leading to significant disruption of ChE homeostasis. These results suggest that perioperative medications can influence delirium risk not only via direct neuropharmacologic effects but also through epigenetic modulation of cholinergic pathways[46]. Although such mechanisms were not evaluated in the included studies, they highlight the need for future research that integrates epigenetic, genetic, and clinical risk factors to better understand and predict POD.

Our subgroup analyses further confirmed the consistency of findings across varying durations of POD follow-up and analytic approaches (univariate vs multivariate models), reinforcing the robustness of the observed association between BuChE activity and POD. In contrast, substantial heterogeneity in the AChE analyses, combined with the limited number of studies reporting AChE-related outcomes, constrains the interpretability and generalizability of these findings. The unclear association between AChE activity and POD highlights the need for more targeted studies to evaluate its prognostic significance and potential mechanistic role.

This meta-analysis has several notable strengths. It represents the first comprehensive evidence of the relationship between preoperative serum ChE activity and POD risk in adult surgical patients. The analysis encompassed over 2700 patients across diverse surgical populations and clinical settings. A rigorous methodology was employed, adhering to preferred reporting items for systematic review and meta-analysis and Cochrane guidelines, including predefined subgroup and sensitivity analyses to evaluate the influence of study-level characteristics and the robustness of the results. Furthermore, the included studies demonstrated generally high methodological quality, as reflected in their NOS scores.

However, several limitations should be acknowledged. First, the number of included studies, particularly those investigating AChE, was limited, reducing the statistical power to detect minor effects and to explore sources of heterogeneity. Second, there was substantial variability across studies regarding patient demographics, surgical types, anesthesia techniques, timing and methods of ChE measurement, and criteria used for POD diagnosis. In particular, anesthesia approach (general, regional, or combined) may itself affect POD risk by altering anesthetic drug exposure, cerebral physiology, and perioperative inflammatory responses. Although several studies adjusted for anesthesia type, residual confounding cannot be excluded, and this factor should be considered when interpreting our results. Diagnostic heterogeneity is another consideration, as different tools were employed across studies, including CAM, confusion assessment method for the intensive care unit, Nu-DESC, and Intensive Care Delirium Screening Checklist, sometimes within the same study. While all are validated instruments, they vary in sensitivity and specificity, with some (e.g., Nu-DESC) more likely to detect milder or subsyndromal cases. Such variation may partly account for inconsistencies across studies and could have influenced the pooled estimates. Future research using standardized delirium assessment protocols would enhance comparability.

Third, although most studies adjusted for key confounders such as age, sex, and comorbidities, other relevant factors—such as nutritional status, perioperative anti-cholinergic medication use, baseline cognitive function, and inflammation markers—may not have been consistently controlled but can influence the risk of POD[47,48]. Another consideration is the presence of genetic variants of BuChE, which are more prevalent in certain endogamous populations and may result in markedly reduced or absent enzyme activity[49]. Such variants are clinically important for anesthetic safety, as they predispose to prolonged apnea following succinylcholine administration[49]. However, none of the studies included in this meta-analysis evaluated BuChE genotypes or reported variant carriers. Consequently, the potential relationship between BuChE genetic variants and POD remains unexplored and warrants future investigation. Moreover, the observational design of all included studies limits causal inference, and the presence of reverse causation or unmeasured confounding cannot be excluded. Lastly, the lack of standardized cut-off values for defining “low” BuChE or AChE activity limits clinical interpretability and application.

Despite these limitations, the findings of this study have several important clinical implications. According to the updated European Society of Anaesthesiology and Intensive Care Medicine guideline[50], POD arises when anesthesia- and surgery-related factors interact with a patient’s underlying vulnerabilities, highlighting the need for multidimensional preoperative evaluation. In this context, serum BuChE testing should be viewed as a complementary biomarker that, when combined with assessments of physical, cognitive, mental, and social status, may enhance risk stratification and support individualized perioperative care. However, current European guidelines on POD do not mention BuChE in preoperative risk assessment, reflecting the paucity and inconsistency of prior evidence[50]. In one of the included studies, Müller et al[26] concluded that the clinical implications of BuChE remained unclear. Indeed, not all studies have reported positive associations. For example, postoperative assessments in cardiac surgery populations failed to discriminate POD status, possibly due to perioperative factors such as cardiopulmonary bypass and transfusion effects[51]. More recently, Schlake et al[18] reported that BuChE activity was not independently associated with POD, instead reflecting patients’ baseline morbidity and surgical burden. These discrepancies underscore that BuChE should not be regarded as a standalone predictor but rather as one element within a multidimensional risk assessment. Despite these divergent findings, our meta-analysis integrating 13 studies demonstrates a consistent overall association between lower preoperative BuChE activity and higher POD risk, suggesting potential clinical value when interpreted alongside other vulnerability markers. Early recognition of at-risk individuals could facilitate targeted prevention strategies, including cognitive screening, optimization of perioperative medications, and delirium monitoring protocols. However, before routine clinical implementation, further prospective studies are needed to establish optimal cut-off values, validate predictive performance, and determine whether interventions guided by BuChE activity improve patient outcomes. Future research should aim to elucidate the mechanistic links between peripheral ChE activity and central cholinergic dysfunction, incorporating neuroimaging, cerebrospinal fluid biomarkers, and inflammatory markers. Standardized protocols for ChE measurement and delirium assessment will improve comparability across studies. Additionally, randomized trials investigating whether perioperative modulation of cholinergic tone—such as through ChE inhibitors or reduction of anti-cholinergic burden—can reduce POD incidence in patients with low BuChE activity may offer valuable therapeutic insights.

CONCLUSION

This meta-analysis demonstrates that lower preoperative serum BuChE activity is significantly associated with an increased risk of POD in adult surgical patients. In contrast, the role of AChE remains uncertain due to limited evidence and inconsistent findings. Preoperative BuChE measurement may represent a promising tool for delirium risk stratification and perioperative care planning; however, further high-quality research is required to validate its clinical utility.