Delirium in the intensive care unit (ICU) is a prevalent and serious neurological complication among critically ill patients, with large multicenter studies reporting an incidence of 30% to 80%, particularly in those requiring mechanical ventilation. Delirium is not only associated with prolonged ICU and hospital stays but also with increased morbidity and mortality. Notably, up to 40% of survivors suffer from persistent cognitive dysfunction that may last for months or even years. Despite current standard pharmacological interventions-such as haloperidol and second-generation antipsychotics-failing to demonstrate significant clinical benefit in phase III trials, non-pharmacological strategies remain challenging to implement due to environmental and operational constraints within the ICU. This unmet clinical need underscores the imperative to develop novel, effective therapeutic approaches.

Emerging evidence suggests that ICU-acquired gut microbiota dysbiosis plays a pivotal role in the pathogenesis of delirium. Dysbiosis can compromise intestinal barrier integrity, promoting systemic inflammation and increasing susceptibility to various forms of delirium, including acute illness-related and postoperative types. These effects are likely mediated through the "gut-microbiota-brain axis," which may represent a central mechanism underlying neurocognitive dysfunction in critical illness.

Preclinical studies have demonstrated that fecal microbiota transplantation (FMT) can restore microbial balance and exert beneficial effects on neurological function. FMT has shown promise in ameliorating cognitive deficits in models of Alzheimer's disease, chronic cerebral hypoperfusion, traumatic brain injury, and chronic unpredictable mild stress (CUMS). Clinically, FMT has been associated with cognitive improvement in patients with dementia, recurrent Clostridioides difficile infection, and sepsis-associated encephalopathy. With expanding applications in both gastrointestinal and extraintestinal disorders, FMT has emerged as a transformative therapeutic modality, supported by robust short- and long-term safety and efficacy data.

This study aims to evaluate whether FMT can alleviate delirium severity, correct gut microbiota dysbiosis at 0, 72, and 120 hours post-enrollment, attenuate intestinal barrier dysfunction, reduce systemic inflammation and disease severity, shorten ICU length of stay, and lower rates of ICU mortality, in-hospital mortality, and 28-day all-cause mortality-ultimately positioning FMT as a potential breakthrough intervention for ICU delirium.

Inclusion Criteria:

• 1. Age ≥ 18 years old, no ethnic or gender restrictions;
• 2. CAM-ICU-positive delirium;
• 3. Expected ICU stay ≥ 5 days after inclusion in the study;
• 4. Signed written informed consent.

Exclusion Criteria:

• 1. Routine administration of antipsychotic medications: Conventional antipsychotic therapy refers to the daily ingestion or application of any sustained-release formulation (in any dosage form) of antipsychotic drugs with the ATC code N05A (in any form), including the following agents:

  1. Typical antipsychotics: Chlorprothixene, Flupentixol, Haloperidol, Levomepromazine, Loxapine, Melperone, Perphenazine, Pericyazine, Pimozide, Prochlorperazine, Zuclopenthixol, Pimavanserin, Sulpiride.
  2. Atypical antipsychotics: Amisulpride, Aripiprazole, Asenapine, Clozapine, Lurasidone, Olanzapine, Paliperidone, Quetiapine, Risperidone, Sertindole, Ziprasidone. However, the non-habitual administration of antipsychotic medications in the general ward prior to the patient's ICU admission (e.g., for the management of delirium) is deemed acceptable. Nevertheless, antipsychotic therapy should be discontinued upon the patient's ICU admission.

• 2. Severe systemic infection in the early stage of resuscitation, hemodynamic instability or insufficient tissue perfusion, and severe imbalance of water, electrolyte and acid-base.

• 3. Patients with a high risk of death within 5 days as judged by the clinician, or those whose treatment decisions are restricted.

• 4. Active major gastrointestinal bleeding, perforation, or other severe damage to the intestinal barrier.

• 5. Patients who cannot tolerate 50% of their caloric requirements through enteral nutrition due to severe diarrhea, significant fibrotic intestinal stenosis, severe gastrointestinal bleeding, or high-flow enteric fistula.

• 6. Planned or recent abdominal surgery (within 14 days).

• 7. Currently diagnosed with fulminant colitis or toxic megacolon.

• 8. Recently received high-risk immunosuppressive or cytotoxic drug treatment: such as rituximab, doxorubicin, or medium to high-dose corticosteroids (20 mg/d prednisone or higher) for more than 4 weeks.

• 9. Pregnant or lactating women.

• 10. Participated in other clinical trials as a subject within the past 3 months or at the time of enrollment.

• 11. Delirium assessment non applicable: this includes language barriers (patients with foreign language where delirium assessment cannot be confidently performed by the site staff), patients who are deaf, blind or aphasic. Comatose patients are not applicable for delirium assessment. Coma is defined by the following levels of consciousness: RASS -4 to -5. Further, RASS -3 may be considered as coma if this is the judgement of the treating physician. If a patient's coma is considered related to administration of sedative agents, an effort should be made to reduce or terminate the sedative treatment, according to the clinician's discretion.

• 12. Doubtful validity of informed consent: subjects with mental illness, intellectual disability, poor motivation, or other factors that limit the validity of informed consent for participation in this study.