Incidence & Risk Factors of Postoperative Delirium After Spinal Surgery in Older Patients

Although postoperative delirium is a common complication in older patients, few papers have described risk factors after of spinal surgery. The purpose of this study was to analyze various perioperative risk factors for delirium after spinal surgery in older patients. This study was performed on retrospective data collection with prospective design. We analyzed 138 patients over 65 years of age who underwent spinal surgery. Preoperative factors were cognitive function (Mini-Mental State Examination-Korean (MMSE-K) and the Korean version of the Delirium Rating Scale-Revised-98 (K-DRS 98)), age, sex, type of admission, American Society of Anesthesiologist classification, metabolic equivalents, laboratory findings, visual analog scale, and Oswestry Disability Index. Intraoperative factors were operation time, blood loss, and type of procedure. Postoperative factors were blood transfusion and type of postoperative pain control. Postoperative delirium developed in 25 patients (18.16%). Patients were divided into two groups: Group with delirium (group A) and group without delirium (group B). MMSE-K scores in Group A were significantly lower than in Group B (p < 0.001). K-DRS 98 scores were significantly higher in Group A than Group B (p < 0.001). The operation time was longer in Group A than Group B (p = 0.059). On multivariate regression analysis, the odds ratio of K-DRS 98 was 2.43 (p = 0.010). After correction for the interaction between age and MMSE-K, patients younger than 73 years old had a significantly lower incidence of delirium with higher MMSE-K score (p = 0.0014). Older age, low level of preoperative cognitive function, long duration of surgery, and transfusion were important risk factors of postoperative delirium after spinal surgery. It is important to recognize perioperative risk factors and manage appropriately.

Statistical analysis. The patients were divided into two groups: Group with delirium (group A) and group without delirium (group B). Student's t-test was used for statistical analysis of the difference in mean values between the two groups (age, VAS score, ODI, pre-Hb, MMSE-K, K-DRS 98, operation time, blood loss). The chi-square test for independence was used to compare sex, METs, type of procedure, and type of pain control. Fisher's exact test was also used to compare the type of admission, ASA classification, and blood transfusion. To analyze risk factors of postoperative delirium, univariate and multivariate logistic regression analyses were used. Univariate logistic regression analysis was performed using explanatory variables of age, sex, pre-Hb, MMSE-K, K-DRS 98, operation time, blood loss, VAS score, ODI, and blood transfusion. Multivariate logistic regression analysis was performed with only factors that were considered to contribute to the risk, comprising age and sex, MMSE-K, K-DRS 98, operation time, and blood transfusion. The level of significance was set at p < 0.05. All statistical analyses were performed using SPSS ver. 20.0 (SPSS Inc., Chicago, IL, USA).
intraoperative risk factors. There was no statistical difference in intraoperative factors (Table 1). Operation time of group A was longer than that of group B (185.8 ± 106.8 vs. 147.7 ± 83.3 minutes, p = 0.052), but the difference was not statistically significant. The amount of blood loss was not significantly different between the two groups (179.2 vs. 187.1 ml, p = 0.825). Type of procedure was also not significantly different (cervical: 4 vs. 28, lumbar decompression: 19 vs. 67, lumbar fusion: 2 vs. 18, p = 0.289). postoperative risk factors. Postoperatively, blood transfusion and type of pain control were not significantly different in their respective categories (p = 0.254, 0.504, respectively) ( Table 1). Ten patients in group A (40%) and 29 patients in group B (26.6%) had a transfusion in the intraoperative or postoperative phase. Twenty patients in group A (80%) and 83 patients in group B (73.5%) were given opioids in the postoperative phase.
Logistic regression analysis. On univariate logistic regression analysis, preoperative MMSE-K and K-DRS 98 scores were significantly related to postoperative delirium; specifically, lower MMSE-K score (odds ratio: 0.34, p < 0.001) and higher K-DRS 98 score (odds ratio: 2.34, p < 0.001) increased the risk of delirium (Table 2). Intra-and post-operative blood transfusion also increased the risk of postoperative delirium (odds ratio: 2.46, p = 0.327/odds ratio:1.66, p = 0.921). The risk of delirium was also higher in older patients, but not significantly so (odds ratio: 1.01, p = 0.798).
On multivariate logistic regression analysis, there was a decreased risk with age (odds ratio: 0.80, p = 0.93). To investigate the interactions between age and other variables, multivariate logistic regression analysis was repeated using age and each variable. There was a significant interaction between age and MMSE-K score (p = 0.0014) ( Table 3). When other variables are corrected, including the interaction of age and MMSE-K, the higher K-DRS-98 score significantly increased the risk of the incidence of delirium. Patients younger than 73 years old had a www.nature.com/scientificreports www.nature.com/scientificreports/ significantly lower incidence of delirium with higher MMSE-K score, one-point increase in MMSE-K score was found to reduce the risk of delirium by 87%. This effect was greater in patients under 73 years old than patients older than 73 years.

Discussion
In our study, the overall prevalence of postoperative delirium after spinal surgery was 18.16%. We found that older age, low level of preoperative cognitive function (lower MMSE-K, higher K-DRS 98), long duration of surgery, and transfusion were important risk factors of postoperative delirium after spinal surgery. Both modifiable and non-modifiable risk factors contributed to risk for postoperative delirium after spinal surgery. However, duration of surgery and transfusion were the only two modifiable risk factors, and all other risk factors were non-modifiable.
The incidence of postoperative delirium after spinal surgery was from 0.49% to 21% in previous reports 7,10,19-21 . In our study, the incidence of postoperative delirium was 18.16% and comparable with other studies. Kobayashi et al. reported the incidence of postoperative delirium to be 0.49% after lumbar spinal surgeries and significantly more occurence in older patients 20  There are several reports of risk factors for postoperative delirium, and the risk is multifactorial 22 . Predisposing factors include older age, cognitive impairment, alcohol/drug abuse and dependence, psychiatric comorbidity, sensory impairment, and dehydration/malnutrition. In addition, functional dependence is one of most important predisposing factors; use of stretchers or wheelchairs at admission was significantly higher in patients with delirium than those without delirium. This may indicate that patients with postoperative delirium had poorer preoperative physical condition. Patients who undergo spinal surgery complain about the impaired activities of daily life due to pain in the trunk and extremities. Therefore, patients undergoing spinal surgery tend to be prone to postoperative delirium.   Age is a major factor affecting the onset of delirium 23 . Our study indicated that age older than 70 years were associated with an increased risk of delirium. The higher incidence of postoperative delirium among the older patients may be associated with increased comorbidities, decreased physical activity, decreased brain volume, and decreased cerebral neurotransmitters production 24 . The narrowing of vessels due to vascular disease which is associated with aging decreases oxygen supply to the brain and promote the development of postoperative cerebral dysfunction 24 .
If postoperative delirium occurs, the length of hospital stay is prolonged, and the likelihood of medical complications is increased 25,26 . It is reported that the postoperative outcomes of delirious patients at discharge are worse than those of non-delirious patients because of difficulties in early ambulation and rehabilitation exercises. Although delirium can be caused by a single factor, the incidence of delirium increases as the number of risk factors increases 27,28 . It is important to prevent delirium through a multifaceted approach to reduce risk factors. Inouye reported that delirium-related risk factors could be improved in older patients to reduce the incidence and duration of delirium 17 .
The risk factors described above should be assessed at hospital admission. There are non-modifiable and modifiable risk factors. In our study, age and preoperative cognitive function were non-modifiable factors, and operation time and transfusion were modifiable factors. Active interventions were needed to correct risk factors before and after surgery 17,27,28 . We should try to shorten the duration of surgery and reduce rate of transfusion, as these are modifiable factors. Even with non-modifiable factors, recognizing, preparing, and predicting the risk factors could help preventing postoperative delirium and providing appropriate treatment. Encouraging postoperative ambulation and preventing other possible medical complications after surgery are thought to reduce delirium. For patients at high risk of postoperative delirium, supportive postoperative care is needed. In patients with postoperative delirium, well-organized wards, emotional support from family members, appropriate stimuli, and use of fewer medications can be helpful in the treatment of delirium and can reduce the duration of delirium 17,27 .
This study has several limitations. First, it was a retrospective study based on a data review that could not evaluate of severity and details of delirium. Second, diagnosis of delirium was based on consultation with the Department of Psychiatry, so details of the delirium were not available. Third, the risk for delirium is multifactorial, and there are many other potential risk factors that we did not assess. Other risk factors that may affect postoperative delirium and how they interact need to be explored. Understanding modifiable and non-modifiable risk factors can improve prevention and management of postoperative delirium. The risk of postoperative delirium can be reduced with careful attention to perioperative risk factors 29 . conclusions Older age and low preoperative cognitive function were the most important risk factors of postoperative delirium after spine surgery. In addition, longer duration of surgery and transfusion can affect postoperative delirium. Surgeons should consider these risk factors in patients at high risk for postoperative delirium.