Dexmedetomidine-esketamine and tDCS for Prevention of Neurocognitive Complications After Surgery

Inclusion Criteria: * Aged 65 to 90 years; * Preoperative Mini-Mental State Examination (MMSE) score \< 27 points, indicating possible cognitive impairment ranging from mild to moderate; * Scheduled to undergo elective non-cardiac, non-neurosurgical surgery under general anesthesia, with an expected surgical duration \> 1 hour; * Required patient-controlled intravenous analgesia (PCIA) after surgery. Exclusion Criteria: * Preoperative inability to communicate due to coma, severe dementia, end-stage disease, or language impairment; * History of schizophrenia, epilepsy, Parkinson's disease, brain trauma/surgery, or myasthenia gravis; * Presence of metal implants in the intracranial or cervical region (such as cochlear implants, aneurysm clips, deep brain stimulation electrodes), or skin damage or severe skin disease on the head; * Severe cardiac dysfunction (left ventricular ejection fraction \< 30%), comorbid with sick sinus syndrome, severe bradycardia (heart rate \< 50 bpm), or second-degree or higher atrioventricular block, or implantation of a cardiac pacemaker; * Uncontrolled hyperthyroidism or pheochromocytoma; * Severe liver dysfunction (Child-Pugh class C), severe renal dysfunction (requiring dialysis), or ASA classification ≥ IV; * Allergy to dexmedetomidine or esketamine; * Participation in other clinical studies within the past 3 months; * Other conditions that are deemed unsuitable for study participation.