This study has provided a large-scale and health-economic analysis of postoperative delirium after primary elective THA. From the year 2005 to 2008, the incidence of postoperative delirium was increasing annually from 0.96% to 1.28%. Then, the incidence of postoperative delirium decreased gradually to 0.66% in 2014 (Fig. 1). Interestingly, this trend had never been reported in previous studies. Although there was no change of the definition of delirium over this decade, according to ICD-9-CM, the diagnosis of delirium may vary among institutions [11]. One possible explanation for this trend may be that the number of THA performed was increased with aging of population, however, the lack of recognition and medical interventions led to a higher incidence of postoperative delirium. Then postoperative delirium received more and more attention and this trend was reversed after 2008. We identified an overall incidence of 0.90% after THA procedures, which is much lower compared with the previous studies ranging from 5% to 17% [1,7,8,9]. There are two possible reasons accounting for this obvious difference. First, previous literatures observed small-scale and selected senior patients, resulting in an over-reported incidence. Second, the diagnostic accuracy varied between institutions may also contributed to the difference [11,29].
In terms of demographics, patients suffered from postoperative delirium were significantly older than those without. This was highly consistent with previous studies which had identified advanced age as a common independent predictor of postoperative delirium [1,9,20,21,22,23,24,25].Interestingly, there was significant difference of race distribution between the two groups, indicating a racial difference in occurrence of postoperative delirium. Our study found that the Whites occupying a larger proportion in the postoperative delirium group. This is consistent with the previous report that the Whites undergoing general or orthopedic surgery were more likely to develop postoperative delirium [30].
The CCI score of patients with postoperative delirium was significantly higher. This is reasonable as higher CCI score means relatively worse healthy condition before surgery, and may increase postoperative complications including delirium. Postoperative delirium has been reported to increase hospitalization duration, medical cost, and mortality [1,7,9,11]. Similar findings were observed in our study. With the presence of postoperative delirium, the average LOS was 1 day longer and the total hospital charge was $9048 more per admission, which may be due to that these patients need additional nursing instructions and rehabilitation [12,31]. Further, postoperative delirium may be associated with other postoperative complications [32], including acute renal failure and pneumonia, which was consistent with the observation by Aziz et al [7].
A systematic review on postoperative delirium after total joint arthroplasty suggested that pre-screening and risk stratification is essential to improve outcomes [1]. Thus, in order to prevent postoperative delirium, it is critical to understand the risk factors before surgery. Logistic regression was applied and the results were consistent with previous publications [1,9,20,21,22,23,24,25].
Aziz et al. had performed a study of postoperative delirium using the same database from 2000 to 2009. They also found that delirium was associated with postoperative complications including seroma/hematoma, and wound infection, which was not observed in our study. The possible explanation may be that they studied both total and partial hip arthroplasty, while only primary elective total hip arthroplasty was included in our study. Meanwhile, data from different period (2009-2014 vs. 2000-2009) may also contribute to this inconsistency.
Several limitations exist in utilizing the NIS database. First, information of each patient is only recorded before discharge, meaning any complication that occurs after discharge will not be included in the NIS database. This limitation might contribute to the lower incidence of postoperative delirium as only early period medical records were analyzed. Second, only risk factors recorded in the NIS database could be analyzed. There are other known risk factors that were not available in the NIS database, such as a history of dementia, type of anaesthesia, commonly used perioperative medications (opioids, benzodiazepines, and ketamine), sedation during anesthesia recovery, vision impairment, functional impairment, and so on [22,25,33].