Importance Reliable prevalence estimates are lacking for young-onset dementia (YOD), in which symptoms of dementia start before the age of 65 years. Such estimates are needed for policy makers to organize appropriate health care. Objective To determine the global prevalence of YOD. Data Sources The PubMed, Embase, CINAHL, and PsycInfo databases were systematically searched for population-based studies on the prevalence of YOD published between January 1, 1990, and March 31, 2020. Study Selection Studies containing data on the prevalence of dementia in individuals younger than 65 years were screened by 2 researchers for inclusion in a systematic review and meta-analysis. Data Extraction and Synthesis Prevalence estimates on 5-year age bands, from 30 to 34 years to 60 to 64 years, were extracted. Random-effects meta-analyses were conducted to pool prevalence estimates. Results were age standardized for the World Standard Population. Heterogeneity was assessed by subgroup analyses for sex, dementia subtype, study design, and economic status based on the World Bank classification and by meta-regression. Main Outcomes and Measures Prevalence estimates of YOD for 5-year age bands. Results A total of 95 unique studies were included in this systematic review, of which 74 with 2 760 379 unique patients were also included in 5-year age band meta-analyses. Studies were mostly conducted in Europe and in older groups in Asia, North America, and Oceania. Age-standardized prevalence estimates increased from 1.1 per 100 000 population in the group aged 30 to 34 years to 77.4 per 100 000 population in the group aged 60 to 64 years. This gives an overall global age-standardized prevalence of 119.0 per 100 000 population in the age range of 30 to 64 years, corresponding to 3.9 million people aged 30 to 64 years living with YOD in the world. Subgroup analyses showed prevalence between men and women to be similar (crude estimates for men, 216.5 per 100 000 population; for women, 293.1 per 100 000 population), whereas prevalence was lower in high-income countries (crude estimate, 663.9 per 100 000 population) compared with upper-middle-income (crude estimate, 1873.6 per 100 000 population) and lower-middle-income (crude estimate, 764.2 per 100 000 population) countries. Meta-regression showed that age range (P < .001), sample size (P < .001), and study methodology (P = .02) significantly influenced heterogeneity between studies. Conclusions and Relevance This systematic review and meta-analysis found an age-standardized prevalence of YOD of 119.0 per 100 000 population, although estimates of the prevalence in low-income countries and younger age ranges remain scarce. These results should help policy makers organize sufficient health care for this subgroup of individuals with dementia. Study Registration PROSPERO CRD42019119288.

中文翻译：

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年轻发病痴呆症的全球患病率：系统回顾和荟萃分析。

重要性 对于早发性痴呆 (YOD)，缺乏可靠的患病率估计，即痴呆症状在 65 岁之前开始。政策制定者需要此类估计来组织适当的医疗保健。目的 确定 YOD 的全球患病率。数据来源系统地检索了 PubMed、Embase、CINAHL 和 PsycInfo 数据库，以查找 1990 年 1 月 1 日至 2020 年 3 月 31 日期间发表的基于人群的关于 YOD 患病率的研究。研究选择研究包含有关个体痴呆症患病率的数据2 名研究人员对 65 岁以下的人群进行了筛选，以纳入系统评价和荟萃分析。数据提取和综合 提取了 5 岁年龄段（30 至 34 岁至 60 至 64 岁）的患病率估计值。进行随机效应荟萃分析以汇总患病率估计值。结果按照世界标准人口进行年龄标准化。根据世界银行分类和元回归，通过性别、痴呆亚型、研究设计和经济状况的亚组分析来评估异质性。主要成果和措施 5 岁年龄段 YOD 患病率估计。结果 本次系统评价共纳入 95 项独特研究，其中 74 项研究（2 760 379 名独特患者）也被纳入 5 岁年龄段荟萃分析。研究主要在欧洲以及亚洲、北美和大洋洲的老年群体中进行。年龄标准化患病率估计值从 30 至 34 岁群体中每 10 万人口 1.1 例增加到 60 岁至 64 岁群体中每 10 万人口 77.4 例。这使得 30 至 64 岁年龄范围内的全球年龄标准化患病率为每 10 万人 119.0，相当于世界上 390 万名 30 至 64 岁的人患有 YOD。亚组分析显示，男性和女性之间的患病率相似（粗略估计，男性为每 10 万人 216.5 例；女性为每 10 万人 293.1 例），而高收入国家的患病率较低（粗略估计，每 10 万人 663.9 例）人口）与中高收入国家（粗略估计，每 10 万人口 1873.6 人）和中低收入国家（粗略估计，每 10 万人 764.2 人）国家相比。荟萃回归显示，年龄范围 (P < .001)、样本量 (P < .001) 和研究方法 (P = .02) 显着影响研究之间的异质性。结论和相关性 这项系统回顾和荟萃分析发现，YOD 的年龄标准化患病率为每 10 万人 119.0，尽管对低收入国家和较年轻年龄段患病率的估计仍然很少。这些结果应该有助于政策制定者为这一痴呆症患者亚群组织足够的医疗保健。研究注册 PROSPERO CRD42019119288。