Early-onset dementia (EOD) is generally defined as dementia that occurs before 65 years of age as per several previous works of literature.12 As EOD develops at a relatively young age, economic and social activities are more restricted in EOD-affected individuals than in those with senile dementia, and inevitably burdens their families.3 In several previous studies, the prevalence of EOD is reportedly 7–45% among all dementia patients, and the most common type is known as Alzheimer’s disease.45 Recently, as the importance of early screening for dementia has been emphasized, the interest in EOD has also been increasing.6 Thus, it is emerging as an essential topic in healthcare policy.

EOD is considered a relatively heterogeneous disease entity compared to late-onset dementia (LOD) and, of course, has higher mortality than the general population.7 Moreover, EOD is usually more severe and presents with extensive pathologic changes such as greater brain parenchymal atrophy and white matter abnormalities.89 Therefore, EOD progresses relatively quickly, and the mortality risk is higher than that associated with LOD. Despite onset at an earlier age, the average survival time of EOD is less than ten years, similar to that of LOD.10

Socio-environmental determinants have been analyzed in several survival studies as modifiable factors that may be covered to some extent via efforts in healthcare policymaking.1112 The same is valid for dementia survival studies; Korhonen et al.13 reported that low household income was a strong predictor of dementia mortality in a population study based on Finnish data. A prospective cohort study conducted in the Netherlands also associated low household income with higher mortality risks in both male and female patients with dementia.14 Meanwhile, Chen et al.15 demonstrated a greater mortality risk in patients with dementia living in rural areas in a Chinese cohort study. However, these previous studies have primarily focused on typical, senile dementia. Thomas et al.16 conducted EOD-specific mortality studies with social determinants. They conducted a study in Scotland from 1974 to 1988 in patients with Alzheimer’s disease and vascular dementia whose disease onset had occurred before the age of 65 years. Their results revealed that socioeconomic deprivation and urban/rural scores were not significantly associated with survival in patients with EOD. However, recent EOD-related survival studies focusing on socio-environmental factors have been lacking until now.

As shown above, most dementia survival studies have concentrated on typical, senile dementia.4 Furthermore, EOD not only remains poorly understood. but is also insufficiently considered when distributing services and treatment resources related to dementia in terms of healthcare policies. However, patients with EOD must be treated as urgently due to a more aggressive course and relatively large deprivation of social functioning. Through a healthcare management system that not only detects EOD patients early, but also provides appropriate interventions, it is possible to slow the disease progression and maintain each patient’s ability of daily life.17

In this study, we assessed patients with EOD using the National Health Insurance Service (NHIS) database of Korea. We subsequently investigated the effect of socio-environmental factors on the long-term survival of patients with EOD. In particular, we focused on the effect of the imbalance of these factors by identifying the impact of lower household income and rural area residence on the long-term survival of patients with EOD.

This study establishes lower household income and rural residence as independent risk factors for increasing long-term mortality risk in patients with EOD in Korea. The nested models to exclude the interaction effect between the NHIP levels and residential areas also revealed the same results. Further, in the stratified models by NHIP levels, we confirmed that the residential areas significantly affect the long-term mortality of patients with EOD only in the second NHIP quartile group. Our study is significant because it investigated the effects of socio-environmental factors on long-term survival by focusing on the EOD patient group. As the first survival study of EOD using Korea’s NHIS database, another strength of this study is that it was possible to objectively obtain information on patients' SES levels and residential areas.

It should be noted that our definition of patients with EOD was somewhat different from those used in previous studies because information on symptom onset was unavailable due to the nature of the NHIS database. Since we enrolled patients diagnosed with dementia at the age of 40–65 years, they would have an earlier onset time than the EOD patient group generally defined in other studies. According to van Vliet et al.,18 it took approximately 4.4 years from symptom onset to diagnosis in patients with EOD; therefore, when interpreting our results, this time difference should be considered. Moreover, we inferred that the proportion of patients with a different phenotype from that of senile dementia would be higher than that observed in other studies due to the earlier onset.1920 Finally, symptom onset has a subjective aspect, but the timing of diagnostic confirmation from a physician is objective information. Therefore, we also demonstrate that the patient composition of our study adequately reflects the characteristics of EOD.

We can suggest that the primary reason for the lower long-term mortality risk in higher income and urban resident groups is the difference in medical access and healthcare availability. These influenced the difference in the period from symptom onset to that of diagnosis in individual patients; the higher the income accompanied by urban residence, the shorter the period between disease onset and diagnosis. Conversely, in the case of patients with low-income levels and rural area residence, the time from symptom onset to diagnosis might have been relatively long; therefore, we can infer that it might have been associated with a high long-term mortality risk due to the more prolonged prevalence of the disease. These differences in medical access are also indirectly identified via the rates of non-critical comorbidities such as hypertension, diabetes, and dyslipidemia.2122 In our results, the rates of these chronic diseases tended to be lower in the low-income group and rural residents. However, the high prevalence of these chronic conditions in the high-income and urban residence groups may, paradoxically, be associated with higher medical availability for these diseases.23

Meanwhile, most of the population in Korea lives in or around the city area with a high population density, and the country has relatively convenient and sufficient transportation facilities.24 We suggest this as one of reasons why the risk of chronic phase mortality of patients with EOD according to the residential area was relatively less significant compared to household income level. Meanwhile, in explaining the differences in outcomes of EOD by residential areas, the authors suggest that differences in the healthcare-seeking behavior of members according to regional differences as well as healthcare resource availability could affect the results of this study. When patients entered the cohort, the proportion of those in their 60s was the highest in the county; this difference in age composition might have led to a difference in healthcare-seeking behavior. In addition, we inferred that the differences in the patient’s invisible educational level depending on the residential area, which was not presented in this study, might have caused the difference in healthcare-seeking behavior.

Similarly, differences in income levels can also have a significant impact on the healthcare-seeking behavior.25 Though Korea provides universal health coverage under the NIHS system, some examination and treatment items are still not covered by the NHIS. Therefore, it was highly likely that patients with low-income levels did not properly receive examinations or treatments that NHIS did not cover in a timely manner. In addition, indirect costs such as transportation fees required to move to urban areas are inevitably felt as a significant burden to low-income people living in rural areas.26 Consequently, we could confirm through the results of this study that, in general, the risk of chronic mortality in patients with EOD was more affected by the household income level under the Korean NHIS system.

Another noteworthy point from our results was that the age at diagnosis with dementia tended to be lower in the medical-aid group. The fact that although the medical-aid group had a relatively younger age distribution, they had a high long-term mortality risk suggested that a low-income level was a strong predictor for long-term mortality risk in patients with EOD. From another perspective, the proportion of patients with atypical pathology distinct from senile dementia in the medical-aid group was high. Therefore, it can be interpreted that their disease course was relatively fatal.727

In South Korea, national-level efforts are ongoing to resolve such SES and regional disparities in patients with dementia. Since 2017, the national responsibility system for dementia has been introduced,28 and 256 Local Dementia Centers are in operation as of 2019.29 Local Dementia Centers are based in public health centers in each region and aim to provide screening and diagnostic tests for early diagnosis of dementia at low prices. This enables the improvement in the quality of managing patients with dementia and the minimization of the difference in access to medical institutions due to regional and income disparities. Future studies on the effectiveness of these policy-based efforts in patients with EOD are needed.

Meanwhile, studies have assessed the survival determinants targeting patients with EOD. A five-year longitudinal study conducted by Ueki et al.30 on patients with early-onset Alzheimer's disease found that male sex, early-onset disease, concurrent physical illness, and low mini-mental state examination score were related to mortality risk. Conversely, Thomas et al.16 reported that increased age at presentation was significantly related to long-term mortality in early-onset Alzheimer's disease and vascular dementia patients. In our study, male gender, older age at diagnosis, and several comorbidities, such as diabetes, ischemic heart disease, and chronic kidney disease, were identified as independent risk factors for long-term mortality in patients with EOD, consistent with previous studies.

Some limitations of this study as follows. The NHIS database did not provide detailed clinical information such as educational level, degree of initial cognitive impairment, psycho-behavioral symptoms, and daily life performance. Therefore, we could not provide information to estimate the disease severity at the time of diagnosis of EOD. Similarly, the dataset could not accurately indicate the time of symptom onset. Therefore, our inability to specify the period from symptom onset to diagnosis was a limitation. Finally, we did not present the cause of death.

In conclusion, lower household income levels and rural residence are associated with long-term mortality risk in patients with EOD in South Korea. The authors suggest that these socio-environmental factors influence healthcare facility availability and health-seeking behavior, leading to these results. An imbalance in socio-environmental factors can be overcome to some extent via policymaking efforts, and our study results provide a rational basis for establishing a healthcare policy for patients with EOD. For the efficient management of patients with EOD, nationwide policy-based support as well as additional related research are needed.