To explore whether odor enrichment can reduce the risk of delayed neurocognitive recovery as an intervention strategy.

Postoperative neurocognitive disorder, one of the most common postoperative complications in older adults, includes acute postoperative delirium, delayed neurocognitive recovery (within the 30 days recovery period, dNCR), and postoperative neurocognitive dysfunction (from expected recovery 30 days to 12 months). Previous studies showed that the incidence of dNCR ranged from 18% to 40% and is mainly characterized by cognitive impairment, which can develop into long-term cognitive impairment, even elevating the risk of Alzheimer's Disease (AD) related dementia and premature mortality. These symptoms can have significant adverse effects on quality of life and may increase the risk of other physical and psychological disorders, potentially impacting social stability. These statistics underscore the importance of early detection and timely intervention in mitigating the long-term effects of dNCR. Despite the significance of dNCR, effective treatment options remain elusive.

Based on these insights, previous studies have explored the impact of olfactory enrichment in animal models. Zhang et al. demonstrated that surgery and anesthesia could cause olfactory impairment, which might contribute to dNCR, while suggesting that odor enrichment, a non-pharmacological intervention, could reduce postoperative cognitive impairment in animal models. Olfactory enrichment, also known as olfactory training, is a safe and affordable treatment and may have preventative or supportive effects on olfactory function or cognitive function. Evidence supports its effectiveness in treating olfactory function from various causes and in reducing cognitive impairment.

Despite these findings, there is limited research on whether odor enrichment can serve as a mitigating strategy for dNCR among human populations. To address this gap, we conducted a clinical study to investigate whether odor enrichment could mitigate dNCR in geriatric patients and to explore the relationship between olfactory function and dNCR.

Inclusion Criteria:

1. were aged at least 65 years old;
2. were expected a postoperative hospital stay of at least 7 days;
3. had American Society of Anesthesiologist (ASA) class I to II;
4. were native Mandarin speakers;
5. had been referred for a total knee or hip replacement under general anesthesia.

Exclusion Criteria:

1. were prior diagnosed of neurologic diseases by the International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10);
2. had a history of mental disorders diagnosed using the Diagnostic and Statistical Manual of Mental Disorders (DSM-5);
3. had a history of illness or surgery with nasal or sinus;
4. caught cold within one week;
5. impaired vision or auditory function which may affect the assessments;
6. were unwilling to comply with the protocol or procedures;
7. did not pass the Mini-mental State Examination (illiterates get less than 18 points, primary school graduates get less than 20 points, secondary school graduates or higher get less than 24 points) .