Methylene Blue for Prevention of Perioperative Neurocognitive Disorders: Mechanisms and Recent Clinical Evidence

Abstract

Background

Perioperative neurocognitive disorders (PND), encompassing postoperative delirium (POD) and delayed neurocognitive recovery (dNCR), affect 15–40% of elderly surgical patients and are associated with accelerated long-term cognitive decline and increased mortality. Currently, no pharmacological agent is universally accepted for PND prevention. Methylene blue (MB), a phenothiazine derivative with unique mitochondrial redox properties, has emerged as a promising multimodal neuroprotectant.

Objective

This review synthesizes evidence supporting MB’s neuroprotective mechanisms and evaluates emerging clinical data regarding its efficacy as a preventive strategy for PND in elderly surgical patients.

Methods

We conducted a comprehensive narrative review of preclinical and clinical studies investigating MB’s role in perioperative neuroprotection, with particular emphasis on mechanistic pathways and randomized controlled trial evidence.

Results

MB exerts neuroprotection through multiple convergent mechanisms: functioning as an alternative electron carrier in the mitochondrial electron transport chain, activating the Nrf2/antioxidant response element (ARE) pathway, suppressing microglia-mediated neuroinflammation, inhibiting tau protein aggregation, and preserving blood-brain barrier integrity. Recent randomized controlled trials demonstrate that single-dose intraoperative intravenous MB (2 mg/kg) significantly reduces POD incidence (from 24% to 7%, number needed to treat = 6) in elderly patients undergoing major non-cardiac surgery. Biomarker studies confirm attenuated neuronal injury (reduced serum S100β) and decreased systemic inflammation following MB administration.

Conclusion

MB represents a mechanism-based therapeutic strategy targeting the mitochondrial and inflammatory core of PND pathophysiology. Large-scale, multicenter randomized controlled trials with extended neurocognitive follow-up are warranted to establish MB’s definitive role in perioperative neuroprotection and to optimize dosing protocols for high-risk populations.

Introduction

Perioperative neurocognitive disorders (PND) constitute a clinically significant spectrum of cognitive impairments occurring in the context of surgery and anesthesia, formally classified by the Perioperative Cognition Nomenclature Working Group in 2018.1 This nosological framework delineates distinct temporal entities: postoperative delirium (POD)—characterized by acute, fluctuating disturbances in attention, awareness, and cognition typically manifesting within one week postoperatively—and delayed neurocognitive recovery (dNCR), previously termed early postoperative cognitive dysfunction (POCD), diagnosed within 30 days.1,2 The incidence of POD ranges from 14% to 24% following major non-cardiac surgery in patients ≥65 years, escalating to 25–40% for early cognitive dysfunction.3,4 Beyond the acute perioperative period, POD correlates with a 40% acceleration in cognitive decline trajectory over subsequent years and increased risk of incident dementia.5–7

Methylene blue (methylthioninium chloride; 3,7-bis(dimethylamino)phenothiazin-5-ium chloride), a low-molecular-weight (319.85 Da) phenothiazine derivative synthesized in 1876, has garnered renewed interest as a potential neuroprotective agent. Initially employed as an antimalarial and subsequently as the antidote of choice for methemoglobinemia and cyanide poisoning, MB possesses unique redox-cycling capabilities and high lipophilicity that facilitate efficient blood-brain barrier (BBB) penetration.8 The neuroprotective rationale for MB was established in the 1980s through the seminal discovery that it functions as an alternative electron acceptor in the mitochondrial electron transport chain (ETC), bypassing impaired complexes to sustain oxidative phosphorylation.9 Given the substantial mechanistic overlap between PND and neurodegenerative diseases—including mitochondrial dysfunction, oxidative stress, and neuroinflammation—MB offers a theoretically grounded, multitargeted approach to perioperative brain protection.10 The landmark randomized controlled trial (RCT) provided the clinical evidence substantiating MB’s efficacy in PND prevention, demonstrating significant reductions in POD and early cognitive dysfunction following major non-cardiac surgery.11 The primary objective of this review is to critically synthesize the translational mechanisms and clinical evidence supporting MB as a preventive strategy for PND, identify current knowledge gaps, and propose directions for future research.

Pathophysiological Mechanisms: The Rationale for MB Intervention

The pathogenesis of PND involves complex, interconnected neurobiological pathways that converge on neuronal metabolic crisis and synaptic dysfunction, providing mechanistic targets for MB intervention (Figure 1).

Figure 1.

Schematic illustration of perioperative neurocognitive disorder (PND) pathophysiology and methylene blue (MB) neuroprotective mechanisms. Surgical trauma and anesthesia trigger peripheral immune activation, releasing damage-associated molecular patterns (DAMPs) and pro-inflammatory cytokines (IL-1β, IL-6, TNF-α). These mediators compromise blood-brain barrier (BBB) integrity and activate microglial polarization toward the pro-inflammatory M1 phenotype. Concurrently, anesthetic agents inhibit mitochondrial Complex I, inducing bioenergetic crisis and reactive oxygen species (ROS) generation. MB intervenes at multiple nodes: (1) electron shuttling bypassing inhibited ETC complexes to sustain ATP synthesis; (2) Nrf2/ARE pathway activation enhancing antioxidant defenses; (3) NF-κB and NLRP3 inflammasome suppression attenuating neuroinflammation; (4) microglial polarization from M1 to M2 phenotype; (5) tau aggregation inhibition preserving protein homeostasis; and (6) tight junction stabilization maintaining BBB integrity.

Neuroinflammation and Microglial Priming

Neuroinflammation represents the central pathophysiological mechanism underlying PND. Surgical trauma triggers robust activation of the peripheral innate immune system, resulting in elevated circulating damage-associated molecular patterns (DAMPs) and pro-inflammatory cytokines, particularly interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α).12,13 These mediators compromise BBB integrity, facilitating entry into the central nervous system (CNS) where they activate resident microglia and astrocytes. Microglia undergo phenotypic polarization toward the pro-inflammatory M1 state, characterized by upregulation of CD86, inducible nitric oxide synthase (iNOS), and NADPH oxidase, leading to excessive release of reactive oxygen species (ROS) and additional inflammatory cytokines.14 This feed-forward cycle of neuroinflammation, exacerbated by age-related “inflammaging”, results in synaptic stripping, dendritic spine loss, and neuronal network dysfunction.15 Recent evidence additionally implicates gut microbiota dysbiosis and gut-derived lipopolysaccharides in amplifying neuroinflammatory cascades via the gut-brain axis.16

Central nervous system inflammation constitutes a critical and well-characterized pathogenic factor in PND.17 Surgical trauma induces systemic inflammatory responses through activation of pattern recognition receptors, including Toll-like receptor 4 (TLR4) and the receptor for advanced glycation end-products (RAGE), on peripheral immune cells.18,19 This activation triggers nuclear factor-kappa B (NF-κB) signaling and inflammasome assembly, leading to the maturation and secretion of IL-1β via caspase-1-dependent cleavage of pro-IL-1β.20 Elevated circulating IL-6 levels, consistently observed following major surgery, correlate directly with POD severity and duration.21 In the CNS, activated microglia release ROS through NADPH oxidase activation and nitric oxide through iNOS induction, creating a self-perpetuating cycle of oxidative stress and neuroinflammation.22,23 The NLRP3 inflammasome, a cytosolic multiprotein complex, serves as a critical nodal point integrating multiple danger signals and represents a therapeutic target for MB intervention.

Mitochondrial Bioenergetic Crisis and Oxidative Stress

Mitochondrial dysfunction constitutes a critical node in PND pathophysiology. Anesthetic agents, particularly volatile anesthetics such as isoflurane and sevoflurane, directly inhibit Complex I of the mitochondrial ETC, impairing oxidative phosphorylation, reducing ATP synthesis, and promoting electron leakage that generates superoxide anions.24,25 The resulting bioenergetic crisis disproportionately affects metabolically vulnerable neuronal populations, particularly pyramidal neurons in the hippocampal CA1 and CA3 regions, which exhibit limited glycolytic capacity and high ATP demand for ionic homeostasis and synaptic transmission.26 Excessive ROS production overwhelms endogenous antioxidant defenses (superoxide dismutase, glutathione peroxidase), triggering lipid peroxidation, protein oxidation, and activation of the mitochondrial permeability transition pore (mPTP), culminating in caspase-mediated apoptosis.27

Blood-Brain Barrier Disruption and Proteinopathy

BBB integrity is essential for CNS homeostasis. Surgical trauma and anesthetic exposure increase BBB permeability through multiple convergent mechanisms: cytokine-induced endothelial activation, oxidative stress-mediated tight junction protein (occludin, claudin-5, zonula occludens-1) degradation, and matrix metalloproteinase (MMP-2, MMP-9) activation.28,29 BBB breakdown permits extravasation of peripheral neurotoxic substances, including prostaglandins, quinolinic acid, and inflammatory mediators, into the brain parenchyma. Furthermore, perioperative stress promotes pathological tau hyperphosphorylation and β-amyloid accumulation, pathological changes that mirror neurodegenerative processes and may accelerate long-term cognitive decline.30

Pharmacological Characteristics of Methylene Blue

Chemical Structure and Redox Properties

MB is a planar, tricyclic phenothiazine compound with a delocalized π-electron system that enables reversible redox cycling between oxidized (blue; MB⁺) and reduced (leuco-MB; MBH) forms.8 This redox couple exhibits a standard reduction potential of +11 mV (pH 7.0), thermodynamically positioned between NADH/NAD⁺ (−320 mV) and cytochrome c (+260 mV), rendering it capable of accepting electrons from reduced flavoproteins and transferring them to downstream electron acceptors.31 MB exhibits a biphasic dose-response relationship: low doses (<2 mg/kg) function as antioxidants and mitochondrial protectants, whereas high doses (>5 mg/kg) may generate pro-oxidant effects through redox cycling and mitochondrial enzyme inhibition.32

Pharmacokinetics and CNS Penetration

Following intravenous administration, MB exhibits rapid distribution (t½α=4–6 minutes) and extensive tissue penetration, with a volume of distribution of approximately 40 L/kg.8 Its high lipophilicity (logP ~0.8) and low molecular weight facilitate passive diffusion across the BBB, achieving brain-to-plasma concentration ratios sufficient to engage mitochondrial and enzymatic targets.33 MB undergoes extensive hepatic metabolism via cytochrome P450 reductase and renal excretion of metabolites and unchanged drug.34 The pharmacokinetic profile supports intraoperative intravenous administration as the optimal route for perioperative neuroprotection, providing peak CNS concentrations during the period of maximal surgical stress and anesthetic exposure.

In silico Pharmacodynamic and Toxicological Profiling

To complement experimental pharmacological data, we performed in silico analyses using ProTox 3.0 and SwissADME to characterize MB’s ADME-T (absorption, distribution, metabolism, excretion, and toxicity) properties (Table 1). ProTox 3.0 prediction models indicate that MB exhibits low acute toxicity (predicted LD₅₀: 1180 mg/kg, toxicity class IV) with no predicted hepatotoxicity, neurotoxicity, or cardiotoxicity at therapeutic concentrations.35 SwissADME analysis confirms favorable drug-like properties: molecular weight 319.85 Da (within Lipinski’s Rule of Five), moderate lipophilicity (consensus logP 0.88), high gastrointestinal absorption, and predicted BBB permeability.36 Notably, MB is not predicted to be a substrate of P-glycoprotein, facilitating CNS penetration, and shows minimal inhibition of major cytochrome P450 isoforms (CYP1A2, CYP2C19, CYP2C9, CYP2D6, CYP3A4), suggesting a low propensity for pharmacokinetic drug-drug interactions.37 These computational predictions align with experimental observations of MB’s rapid CNS penetration and favorable safety profile at neuroprotective doses.

Table 1.

In silico ADME-T Profile of Methylene Blue

Parameter	ProTox 3.0 Prediction	SwissADME Prediction	Clinical Relevance
Acute toxicity (LD50)	1180 mg/kg (Class IV)	—	Low acute toxicity; therapeutic index favorable at 2 mg/kg
Hepatotoxicity	Negative		No predicted liver toxicity
Neurotoxicity	Negative		No predicted CNS toxicity at therapeutic doses
Cardiotoxicity	Negative		No predicted cardiac toxicity
Molecular weight		319.85 Da	Below Lipinski threshold (500 Da)
Lipophilicity (logP)		0.88 (consensus)	Optimal for BBB penetration
GI absorption		High	Supports oral bioavailability (though IV preferred perioperatively)
BBB permeability		Yes (predicted)	Confirms experimental CNS penetration
P-gp substrate		No	Facilitates CNS accumulation
CYP inhibition		Minimal (CYP1A2, 2C19, 2C9, 2D6, 3A4)	Low drug-drug interaction potential
Synthetic accessibility		1.87 (easy)	Favorable for pharmaceutical development

Mechanisms of Neuroprotection

Mitochondrial Electron Transport and Bioenergetic Support

The cardinal neuroprotective mechanism of MB involves its capacity to function as an alternative electron carrier within the ETC. Under conditions of Complex I or Complex III inhibition—induced by ischemia, anesthetics, or oxidative stress—MB accepts electrons from NADH and transfers them directly to cytochrome c, effectively bypassing the inhibited complexes to maintain proton gradient generation and ATP synthesis.31 This “electron shuttling” activity preserves mitochondrial membrane potential, reduces electron leak and superoxide generation at Complex I, and attenuates the bioenergetic crisis that precipitates neuronal dysfunction.38

The mechanistic evidence supporting MB’s mitochondrial protection in the context of PND is robust and multi-layered. Preclinical studies demonstrate that MB prevents anesthetic-induced mitochondrial dysfunction in hippocampal neurons, preserving ATP levels and reducing ROS production following isoflurane exposure.39,40 In aged mice subjected to sevoflurane anesthesia, MB pretreatment prevented cognitive dysfunction by maintaining mitochondrial membrane potential and reducing cytochrome c release into the cytosol.41,42

Additionally, MB modulates mitochondrial dynamics by inhibiting excessive fission (mediated by dynamin-related protein 1 [Drp1]) and promoting fusion through upregulation of mitofusin-1/2 (Mfn1/2) and optic atrophy 1 (Opa1), thereby maintaining mitochondrial network integrity and enabling functional complementation.42,43 By stabilizing the mitochondrial outer membrane and preventing cytochrome c release, MB inhibits activation of caspase-9 and the downstream executioner caspase-3, attenuating intrinsic apoptotic cascades.44

Antioxidant Defense and Anti-Inflammatory Actions

MB activates the nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) signaling pathway, the master regulator of cellular redox homeostasis.45 Under oxidative stress, MB promotes Nrf2 dissociation from Kelch-like ECH-associated protein 1 (Keap1), enabling nuclear translocation and transcriptional upregulation of Phase II detoxification enzymes, including heme oxygenase-1 (HO-1), NAD(P)H: quinone oxidoreductase 1 (NQO1), superoxide dismutase (SOD), and catalase.46 This “preconditioning” effect enhances endogenous antioxidant capacity prior to surgical stress exposure.

Concurrently, MB exerts potent anti-inflammatory effects by inhibiting microglial overactivation and promoting phenotypic polarization from the M1 (pro-inflammatory) to M2 (anti-inflammatory) phenotype.47,48 Mechanistically, MB suppresses nuclear factor-kappa B (NF-κB) nuclear translocation and NLRP3 inflammasome activation, resulting in decreased secretion of IL-1β, IL-6, and TNF-α.49,50 In a mouse model of postoperative cognitive dysfunction, MB administration attenuated surgery-induced microglial activation in the hippocampus, reduced IL-1β and TNF-α expression, and prevented synaptic loss.25,51 These effects collectively interrupt the neuroinflammatory cascade that drives synaptic dysfunction and cognitive impairment. The clinical relevance of these mechanistic findings is supported by biomarker data from human trials demonstrating reduced postoperative IL-6 levels following MB administration.

Protein Homeostasis and Synaptic Preservation

MB inhibits pathological tau protein aggregation by binding to the proteopathic repeat domains of tau, stabilizing native conformations and preventing β-sheet formation and paired helical filament assembly.52,53 In the context of PND, this mechanism may prevent surgery-induced acceleration of tauopathy, particularly relevant for elderly patients with pre-existing amyloid or tau pathology.

At the synaptic level, MB preserves the expression of critical scaffolding proteins, including synaptophysin and postsynaptic density protein-95 (PSD-95), and maintains dendritic spine density in hippocampal CA1 and prefrontal cortex regions following anesthetic exposure. By preserving mitochondrial ATP supply and reducing oxidative damage to synaptic membranes, MB maintains the energy-dependent processes required for long-term potentiation (LTP) and synaptic vesicle cycling.54 Additionally, MB exhibits weak, reversible inhibition of acetylcholinesterase (AChE) and monoamine oxidase A (MAO-A), potentially augmenting cholinergic and monoaminergic transmission implicated in attention and consciousness.55,56

Blood-Brain Barrier Integrity

MB stabilizes BBB integrity through antioxidant-mediated preservation of endothelial tight junction proteins and inhibition of MMP activation.57 By reducing cerebrovascular oxidative stress and maintaining pericyte-endothelial interactions, MB prevents the entry of peripheral inflammatory mediators into the CNS.58 Furthermore, MB-mediated inhibition of soluble guanylate cyclase (sGC) modulates cerebrovascular tone, potentially optimizing neurovascular coupling and preventing hyperemia-induced BBB disruption.51,59

Comparative Context: Pharmacological Landscape for PND Prevention

The development of effective pharmacological strategies for PND prevention has encountered substantial challenges, with numerous agents failing to demonstrate consistent efficacy in rigorous clinical trials (Table 2). Dexmedetomidine, an α₂-adrenergic agonist with anti-inflammatory and analgesic properties, has shown mixed results: while older trials suggested delirium reduction during postoperative mechanical ventilation, newer intensive care unit sedation trials incorporating updated sedation strategies have found comparable delirium and long-term cognition outcomes.60,61 Ketamine, despite its anti-inflammatory and neuroprotective theoretical rationale, failed to reduce delirium incidence in large randomized trials.62,63 Similarly, antipsychotics (haloperidol, risperidone), statins, corticosteroids, and total intravenous anesthesia regimens have not demonstrated consistent delirium prevention efficacy.64,65 Melatonin and melatonin receptor agonists have shown modest benefits in select populations but lack robust multicenter validation.66 This landscape of largely unsuccessful pharmacological interventions underscores the clinical necessity for novel mechanism-based approaches such as MB, which targets the mitochondrial-inflammatory axis central to PND pathophysiology rather than modulating single downstream effectors.

Table 2.

Comparative Overview of Pharmacological Interventions for PND Prevention

Agent	Proposed Mechanism	Efficacy in PND Prevention	Limitations
Dexmedetomidine	α2-agonist; anti-inflammatory; analgesic	Mixed/Negative in recent trials	Bradycardia, hypotension; sedation-related complications
Ketamine	NMDA antagonist; anti-inflammatory	No significant benefit	Psychotomimetic effects; limited efficacy
Haloperidol	Dopamine antagonist	No benefit	Extrapyramidal symptoms; QT prolongation
Risperidone	Atypical antipsychotic	No consistent benefit	Metabolic side effects; stroke risk
Statins	Anti-inflammatory; pleiotropic	No benefit in delirium prevention	Myopathy; drug interactions
Corticosteroids	Anti-inflammatory	No consistent benefit	Hyperglycemia; immunosuppression
Melatonin	Circadian regulation; antioxidant	Modest benefit in select populations	Variable bioavailability; inconsistent dosing
Methylene Blue	Mitochondrial protection; Nrf2/ARE; anti-inflammatory; anti-tau	Significant POD reduction (NNT=6)	Single-center trials; open-label; short follow-up

Clinical Evidence

Efficacy in Major Non-Cardiac Surgery

The landmark single-center RCT by Deng et al provided the clinical evidence supporting MB for PND prevention.11 In this study, 248 elderly patients (≥60 years) undergoing major non-cardiac surgery were randomized to receive either 2 mg/kg MB or placebo (0.9% saline) intravenously over 60 minutes following anesthesia induction, placebo-controlled trials in this context employ inert intravenous solutions matched for volume and infusion duration to maintain blinding of outcome assessors. The primary endpoint—incidence of POD within 7 days—was significantly reduced in the MB group (7.3% vs. 24.2%; odds ratio [OR] 0.24, 95% confidence interval [CI] 0.11–0.53; P<0.001), corresponding to a number needed to treat (NNT) of 6. Secondary endpoints demonstrated similar efficacy for dNCR at 7 days (16.1% vs. 40.2%; OR 0.30, 95% CI 0.16–0.57; P<0.001).

These findings were subsequently validated in a single-center RCT by Zhang et al involving 217 elderly patients undergoing joint arthroplasty.67 MB administration (2 mg/kg) compared with received an equivalent volume of 5% glucose solution could reduce POD incidence from 20.4% to 8.3% (hazard ratio 0.39, 95% CI 0.17–0.86; P=0.024), with a trend toward shorter delirium duration (median 1.0 vs. 2.0 days). Importantly, cognitive protection occurred independently of analgesic effects, as pain scores did not differ between groups.67 Notably, postoperative fever was more common in the MB group (16.5% vs. 7.4%, P=0.039), though episodes were self-limiting and resolved within 1–2 days without evidence of infection.67

Deng et al conducted a study involving 314 patients scheduled for pancreatic surgery. They administered a 2 mg/kg-1 intravenous infusion of methylene blue 60 minutes after anesthesia induction, and then another 1 mg/kg-1 infusion 30 minutes before the end of the surgery. The results showed that 55 patients (17.5%) developed postoperative delirium. The incidence of postoperative delirium in the methylene blue group was lower than that in the control group (11.5% vs. 23.6%, p = 0.005).68 Moreover, the postoperative plasma IL-6 concentration in the methylene blue group was lower than that in the control group. Additionally, when using the methylene blue group’s serum for treatment, the expression levels of TNF-α, MCP-1, and VCAM1 in endothelial cells were lower, and the number of adhesion between peripheral blood mononuclear cells (PBMC) and hCMEC/d3 cells in the methylene blue group was also less.68

Biomarker and Mechanistic Correlates

Clinical biomarker studies provide mechanistic validation for MB’s neuroprotective effects. On the model of focal one-sided traumatic brain injury, the preservation of blood-brain barrier and a decrease in the expression of S100 protein may be an important mechanism by means of which MB improves neurological outcome. MB administration was associated with significantly attenuated postoperative increases in serum S100β, established markers of neuronal injury.69 The reduction in systemic IL-6 levels and endothelial inflammatory markers suggests that MB’s clinical efficacy is mediated, at least in part, through suppression of surgery-induced systemic inflammation and preservation of BBB integrity.68

Limitations of Current Evidence

Current clinical data are constrained by single-center designs, modest sample sizes (n≈200–250), open-label methodology (though outcome assessors were blinded), and limited follow-up durations (maximum 1 month). Neither trial demonstrated significant differences in long-term cognitive assessments (Telephone Interview for Cognitive Status-modified [TICS-m]) at 30 days, though statistical power to detect small but clinically meaningful differences in delayed recovery may be insufficient. Furthermore, generalizability to specific high-risk populations—including the “oldest-old” (≥85 years), patients with pre-existing dementia, or those undergoing cardiac surgery—remains uncertain.

Safety Considerations and Clinical Implementation

Drug-Drug Interactions: Serotonin Syndrome

The reversible inhibition of MAO-A by MB necessitates careful attention to perioperative medication management. Co-administration with serotonergic agents—including selective serotonin reuptake inhibitors (SSRIs), serotonin-norepinephrine reuptake inhibitors (SNRIs), triptans, tramadol, and meperidine—can precipitate serotonin syndrome, a potentially life-threatening condition characterized by altered mental status, autonomic instability (hyperthermia, tachycardia), and neuromuscular hyperactivity (clonus, hyperreflexia).56,70 Preoperative medication reconciliation and avoidance of contraindicated serotonergic agents during the perioperative window (ideally 24–48 hours prior to MB administration) are mandatory safety measures.70

Contraindications and Adverse Events

Glucose-6-phosphate dehydrogenase (G6PD) deficiency represents an absolute contraindication to MB administration due to the risk of acute hemolytic anemia.71 Additionally, severe renal impairment (creatinine clearance <30 mL/min) requires cautious dosing adjustment given MB’s primary renal elimination.

In reported RCTs, MB exhibited excellent tolerability, with benign urine discoloration (blue-green) being the most common adverse effect. No significant increases in cardiovascular events, respiratory complications, or infections were observed, though larger studies are required to detect rare serious adverse events.

Future Directions

Optimization of Dosing and Administration

While the 2 mg/kg single-dose regimen has demonstrated efficacy, systematic dose-ranging studies are necessary to define the optimal therapeutic window that maximizes mitochondrial protection while avoiding potential pro-oxidant effects at higher concentrations.32 Preconditioning strategies (administration 24 hours preoperatively) or continuous postoperative infusions may provide enhanced neuroprotection during the peak vulnerability window.42 Novel delivery systems, including intranasal administration that exploits olfactory/trigeminal nerve pathways to bypass the BBB, warrant investigation to enhance CNS bioavailability while reducing systemic exposure.

Multimodal Neuroprotection and Personalized Medicine

Given the multifactorial etiology of PND, MB’s pleiotropic mechanism positions it ideally for combination therapy with agents targeting complementary pathways (eg, dexmedetomidine for anti-inflammatory and sedative effects, melatonin for circadian regulation and antioxidant support).72 Patients with pre-existing neurodegenerative pathology (Alzheimer’s disease, Parkinson’s disease) may derive particular benefit from MB’s anti-tau and mitochondrial protective properties.73 Pharmacogenomic stratification based on mitochondrial haplogroups or drug-metabolizing enzyme polymorphisms may enable personalized dosing strategies.

Long-Term Outcomes and Neuroimaging

Future multicenter RCTs must prioritize extended neurocognitive follow-up (6–12 months) to determine whether acute MB administration alters long-term cognitive trajectories and dementia risk. Integration of advanced neuroimaging (functional MRI for default mode network connectivity, TSPO-PET for microglial activation) and fluid biomarkers (neurofilament light chain, glial fibrillary acidic protein) will provide objective measures of target engagement and neuronal resilience.74,75

Limitations

This review has several limitations that warrant acknowledgment. First, the clinical evidence base remains limited to three single-center randomized controlled trials with modest sample sizes and relatively short follow-up periods. Second, the open-label design of existing trials, necessitated by MB’s visible urine discoloration, introduces potential performance bias despite blinded outcome assessment. Third, the generalizability of findings to cardiac surgical populations, the oldest-old (≥85 years), and patients with pre-existing neurodegenerative diseases remains uncertain. Fourth, optimal dosing, timing, and route of administration require further systematic investigation.

Conclusion

Methylene blue heralds a paradigm shift in mechanism-driven perioperative neuroprotection, directly targeting the core pathological hallmarks of perioperative neurocognitive disorders (PND)—mitochondrial dysfunction, neuroinflammation, and synaptic instability. Strong preclinical evidence and emerging clinical trial data have validated its significant efficacy in mitigating postoperative delirium and early cognitive impairment in elderly surgical patients. Unlike single-target pharmacological interventions that have previously failed in clinical trials, methylene blue acts through a suite of multimodal mechanisms: mitochondrial electron shuttling, Nrf2/ARE pathway activation, microglial polarization regulation, tau aggregation inhibition, and blood-brain barrier (BBB) stabilization. These pathways collectively address the complex, interconnected pathophysiology of PND in a more comprehensive manner.

To formally integrate methylene blue into standard perioperative care, three critical steps are indispensable: optimizing dosing protocols, strictly managing potential drug interactions, and definitively verifying its long-term cognitive benefits via large-scale, multicenter randomized controlled trials (RCTs). Incorporating methylene blue into multimodal neuroprotective regimens—guided by patient-specific risk stratification and biomarker-informed dosing—carries substantial potential to reduce the clinical burden of PND in the aging surgical population. We strongly encourage the perioperative research community to conduct well-designed, adequately powered clinical trials, so as to conclusively confirm methylene blue’s value as a transformative neuroprotective agent in surgical anesthesia and perioperative medicine.

Funding Statement

Hebei Provincial Science Research Planning Project for Traditional Chinese Medicine Field (No.2023339).

Disclosure

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.