In this study, we identified the top 100 cited PND articles and analyzed their nature, content, and changes with time. The results of our bibliometric analysis indicated that among the articles, clinical trials accounted for over half; the papers focused mainly on mid-aged and older adult patients; the research theme was largely cardiac surgery and POD. As evident by citation frequency, the nature of high-impact studies has changed with time. The changes included higher citation rates, greater funding disclosures, more focus on basic research, and more open access publications; those trends indicate the possible future direction of research in this field.
Among the 100 most cited papers, the publication year ranged from 1955 to 2016. The number of published papers showed steady growth, reflecting the progressive development of research into PND. The greatest number of citations occurred in the 2000s, indicating that this research area gained considerable attention in that decade. Expansion in the number of PND studies may be associated with the multicenter, prospective cohort ISPOCD1 study published by Moller et al. in 1998 (4); which was the most cited paper in PND, and it had tremendous historical significance. Other groundbreaking works had a similar impact. One was the earliest paper to appear on our list when Bedford reported the occurrence of dementia in older adults following operations under general anesthesia in 1955 (2). It was a milestone paper in PND research, highlighting cerebral complications during the perioperative period. Another paper, dating from 2018, examined the nomenclature of cognitive changes associated with anesthesia and surgery. That paper recommended using the term “PND” as an overarching concept to describe preoperative or postoperative cognitive impairment; it led to PND becoming listed as a neurocognitive diagnosis in Diagnostic and Statistical Manual of Mental Disorders-5 (1).
In our analysis, we identified the United States as the leading country in total number of publications, followed by England. Among the top 10 journals with the most number of publications (58 published papers), seven were in the United States (46 published papers), and the other three journals were in England (12 published papers). The authors of the top 100 cited papers also showed a relatively concentrated pattern. The top 10 most highly cited authors of the 100 most cited papers had 58 publications, contributing more than half.
Among the top 100 cited articles, 54 were clinical trials; of those, 50% investigated mid-aged and older adult patients and 37% examined older adult patients. This finding may be related to aging of the global population and vulnerability of older adults to postoperative cognitive impairment. Interestingly, 7.4% of the articles focused on developmental children. The GAS study published in Lancet in 2016 (6) had 390 citations; it ranked seventh among the 10 most cited studies and had the highest ACR (97.5 citations/year). This result suggests that the influence of anesthesia on children’s neurological development is a key research area in PND. The pediatric anesthesia neurodevelopment assessment (PANDA) study was another highly cited paper, focusing on young children aged under 36 months and published in JAMA (7). It had the second-highest ACR (69.25 citations/year), but it was not included in the top 10 cited papers because of its publication year of 2016.
Among the 54 clinical trials, 28% investigated patients who underwent cardiac surgery; 26% examined orthopedic surgery and 22% chose non-cardiac surgery. This finding is consistent with the view that major surgery constitutes a risk factor for PND (8). Among the clinical trials, 59.3% focused on POD and 29.6% on POCD. That result may be related to differences in the trials with respect to onset period, incidence, and diagnostic criteria. POD is an acute event, comprising a set of fluctuating changes in attention, mental status, and level of consciousness; it reportedly occurs in 10–60% of older surgical patients, varying by surgical procedure (9). POCD is cognitive decline diagnosed up to 30 days after a procedure; its incidence is approximately 10–12% (10). The scales of Confusion Assessment Method (CAM) or the CAM adapted for the intensive care unit are mostly used to diagnose POD; POCD diagnosis requires more complex tests (11). With its relatively acute course, higher incidence, and simpler diagnosis, POD is more often the focus as the primary outcome in clinical trials.
When comparing newer with older articles, we found that newer papers had higher ACRs. This finding indicates that PND received considerable attention among researchers and that cognitive function drew increased interest in perioperative medicine. The greater number of funding disclosures reflects the support for this area of research, and greater open access publication increased the impact of findings. The stronger focus on basic science outcomes in newer articles indicates that the main area of PND research changed from clinical phenomena to underlying mechanisms, expecting to have better prevention or therapy for PND. This result also suggests future PND research directions.
Our bibliometric analysis has inherent limitations. Some newly published, high-quality articles were unable to gain sufficient citations to appear among the top 100 (12). Articles published in journals with higher impact factors may gain greater attention and thus have more citations (13). To evaluate any inherent bias, we analyzed the correlation between the number of citations of articles in our list and the impact factors of journals where the papers were published; we also examined the correlation between the number of citations and year of publication. We observed a positive correlation between the number of citations and the journals’ impact factors, which may be reflective of the inherent bias of high impact factors; we found no correlation between the number of citations and year of publication in the current study.