One aim of the study was to investigate whether the STMT could be used to predict the onset of dementia. In 162 patients, when the STMT score alone was used as the independent variable to predict dementia onset by ROC analysis, the AUC and OMQ values were 0.75 and 0.67, respectively. AUC values < 0.5, 0.5–0.7, and > 0.7 suggest low, moderate, and high reliability of the model; respectively. Similarly, OMQ values < 0.60, 0.60–0.67, and > 0.67 indicate that the model is moderately reliable, moderately reliable but with room for improvement, and highly reliable, respectively. Accordingly, our model for predicting dementia using only the STMT score is moderately reliable. Therefore, predicting the onset of dementia may be possible using only the STMT score.
Another aim of the present study was to examine which independent variables could be used in addition to the STMT score to improve its performance in predicting the onset of dementia. Therefore, the effects of the addition of age, sex, and elapsed years as independent variables were examined in terms of changes in AUC and OMQ values. In the cohort comprising all 162 patients, the effects of the addition of age, sex, and elapsed years one by one were not noticeable regarding AUC and OMQ values. When age, sex and elapsed years were all added simultaneously with STMT score, both AUC and OMQ values were the largest and the reliability of the model improved. Cognitive function tends to decline with age [13]. There are sex differences in dementia, with women developing Alzheimer’s disease at twice the rate of men [8, 12], which has been attributed to a decline in female hormonal function [9]. Additionally, elapsed years increase with aging. Accordingly, it seems reasonable to conclude that patients with lower STMT scores, older age, and female sex are more likely to develop dementia.
The study additionally examined which combination of the variables make the best-fitted predictive model for the development of dementia when the treatment statuses of the three diseases were considered as independent variables. The results showed that when the treatment statuses of hypertension, hyperlipidemia and diabetes, in addition to STMT score, age, sex and elapsed year, were all considered as independent variables, the AUC value was 0.79 and the OMQ value was 0.70, indicating that the prediction model was highly reliable.
Studies have reported the association between hypertension and risk of cognitive impairment, with untreated long-term hypertension leading to structural changes in the arterial wall that cause chronic cerebral hypoperfusion [14, 15]. Factors such as age at the onset of hypertension, chronicity, and antihypertensive medication play an important role in determining the risk of cognitive impairment [16, 17]. Meawhile, hypotension is also associated with the risk of developing dementia. The Swedish Kungsholmen Project reported that a diastolic blood pressure < 65 mmHg was associated with a relative risk of dementia of 1.5. Furthermore, in older people with a systolic blood pressure < 160 mmHg, a fall in systolic blood pressure of ≥ 15 mmHg is associated with a 3.1-fold increased risk of developing dementia [18].
Hyperlipidemia is a cardiovascular risk factor and strongly associated with atherosclerotic lesions and ischemic stroke [19]. Furthermore, type 2 diabetes is a known risk factor for atherosclerotic lesions and ischaemic stroke, with probabilities of 21% and 18–22% respectively [20]. In general, 20% of individuals are expected to develop dementia after a stroke, with the risk of dementia reported to increase by 3% annually [21]. A systematic review of Mendelian randomized studies reported that higher total cholesterol and low-density lipoprotein cholesterol levels were associated with a higher risk of Alzheimer’s disease. In contrast, higher high-density lipoprotein cholesterol levels have been reported to lead to a lower incidence of Alzheimer’s disease [22].
Several studies have established that hyperlipidemia increases the risk of cognitive impairment in patients with type 2 diabetes, and hyperglycemia is involved in the development of vascular lesions, leading to vascular dementia and Alzheimer’s disease [23]. Complications associated with diabetes, e.g. hypoglycemia, are risk factors for dementia. Severe hypoglycemic episodes in streptozotocin-induced diabetic rats were significantly associated with neuronal damage in both cortex and hippocampus [24]. Additionally, patients with hypoglycemia have a twofold higher risk of developing dementia compared with those without [14, 19, 25–27]. Increased frequency of hypoglycemic episodes has been reported to increase the risk of dementia progression [26].
A longitudinal study examining concurrent hypertension and diabetes reported that hypertension is a risk factor for cognitive decline only when it is accompanied with diabetes [28]. However, contrasting findings have been obtained from other studies that did not observe a similar relationship between hypertension and diabetes [29, 30]. Furthermore, some reports have examined the incidence of dementia in patients with both hypertension and diabetes and observed no increasing trend.
Despite some contradicting reports, the majority of literature supports the notion that the presence of the three diseases contributes to the development of dementia. This corroborates the findings of this study that people with lower STMT scores, older age, female sex, and the presence of the three diseases are more likely to develop dementia.
Early detection of MCI is important to prevent the onset of dementia. Regarding this, our study confirms that the STMT, a tool for the early detection of memory impairment, can be used as a tool for the prediction of dementia. Since the STMT is a simple tool that does not require specialized instruments, it may be used in daily clinical practice. MoCA is recognized for its high accuracy in identifying patients with MCI and is used in clinical practice [6]. However, MoCA-j is difficult to incorporate widely into routine clinical practice due to its high time burden. A simple test for predicting dementia using shape recognition has been proposed [31], which assesses more cognitive functions, such as constructive skills, in addition to memory functions. Thus, it may be difficult to obtain stable evaluation results. In addition, a computerized touchscreen-based test for the early diagnosis of dementia has been developed by Tsuboi et al. [32], which requires specialized tools such as touchscreens. In comparison, the STMT needs no special tools and only considers memory functions, which may lead to fewer discrepancies in the assessment results. In the STMT, the probability of developing dementia (P) is calculated for each individual by substituting the necessary values (the STMT score, sex, age, and treatment statuses of the three diseases) into the discriminant formula given in the Results section. A P > 74.7 predicts the development of dementia.