Importance Traumatic brain injury (TBI) is common in both veteran and civilian populations. Prior studies have linked moderate and severe TBI with increased dementia risk, but the association between dementia and mild TBI, particularly mild TBI without loss of consciousness (LOC), remains unclear.

Objective To examine the association between TBI severity, LOC, and dementia diagnosis in veterans.

Design, Setting, and Participants This cohort study of all patients diagnosed with a TBI in the Veterans Health Administration health care system from October 1, 2001, to September 30, 2014, and a propensity-matched comparison group. Patients with dementia at baseline were excluded. Researchers identified TBIs through the Comprehensive TBI Evaluation database, which is restricted to Iraq and Afghanistan veterans, and the National Patient Care Database, which includes veterans of all eras. The severity of each TBI was based on the most severe injury recorded and classified as mild without LOC, mild with LOC, mild with LOC status unknown, or moderate or severe using Department of Defense or Defense and Veterans Brain Injury Center criteria. International Classification of Diseases, Ninth Revision codes were used to identify dementia diagnoses during follow-up and medical and psychiatric comorbidities in the 2 years prior to the index date.

Main Outcomes and Measures Dementia diagnosis in veterans who had experienced TBI with or without LOC and control participants without TBI exposure.

Results The study included 178 779 patients diagnosed with a TBI in the Veterans Health Administration health care system and 178 779 patients in a propensity-matched comparison group. Veterans had a mean (SD) age of nearly 49.5 (18.2) years at baseline; 33 250 (9.3%) were women, and 259 136 (72.5%) were non-Hispanic white individuals. Differences between veterans with and without TBI were small. A total of 4698 veterans (2.6%) without TBI developed dementia compared with 10 835 (6.1%) of those with TBI. After adjustment for demographics and medical and psychiatric comobidities, adjusted hazard ratios for dementia were 2.36 (95% CI, 2.10-2.66) for mild TBI without LOC, 2.51 (95% CI, 2.29-2.76) for mild TBI with LOC, 3.19 (95% CI, 3.05-3.33) for mild TBI with LOC status unknown, and 3.77 (95% CI, 3.63-3.91) for moderate to severe TBI.

Conclusions and Relevance In this cohort study of more than 350 000 veterans, even mild TBI without LOC was associated with more than a 2-fold increase in the risk of dementia diagnosis. Studies of strategies to determine mechanisms, prevention, and treatment of TBI-related dementia in veterans are urgently needed.

重要性 颅脑外伤（TBI）在退伍军人和平民人口中都很常见。先前的研究已将中度和重度TBI与增加的痴呆风险相关联，但是，痴呆与轻度TBI，尤其是轻度TBI而无意识丧失（LOC）之间的关联仍不清楚。

目的 探讨退伍军人的TBI严重程度，LOC与痴呆诊断之间的关系。

设计，背景和参与者 这项队列研究从2001年10月1日至2014年9月30日在退伍军人健康管理局的医疗保健系统中诊断为TBI的所有患者，以及一个倾向匹配的比较组。基线时患有痴呆的患者被排除在外。研究人员通过仅限于伊拉克和阿富汗退伍军人的“全面TBI评估”数据库以及包括各个时代的退伍军人的“全国患者护理数据库”，确定了TBI。每个TBI的严重程度均基于记录的最严重伤害，并按照国防部或国防部和退伍军人脑损伤中心的标准分为轻度无LOC，轻度LOC，轻度LOC状态未知或中度或重度。国际疾病分类，第九次修订该代码用于识别在索引日期之前的两年中在随访期间以及痴呆症中的诊断以及医学和精神病合并症。

主要结果和措施 经历过TBI或无LOC的退伍军人和没有TBI暴露的对照参与者的痴呆症诊断。

结果 该研究包括在退伍军人健康管理局医疗体系中诊断为TBI的178 779例患者和在倾向匹配的对照组中诊断为178 779例的患者。基线时，退伍军人的平均（SD）年龄接近49.5（18.2）岁；33 250（9.3％）为女性，259 136（72.5％）为非西班牙裔白人。有和没有TBI的退伍军人之间的差异很小。没有TBI的总计4698名退伍军人（2.6％）患有痴呆，而TBI的退伍军人为10 835（6.1％）。在调整了人口统计学特征以及医学和精神病学合并症后，轻度无TBI的TBI痴呆的调整后风险比为2.36（95％CI，2.10-2.66），轻度TBI合并LOC的痴呆的风险比为2.51（95％CI，2.29-2.76），3.19（ LOC状态未知的轻度TBI为95％CI，3.05-3.33），中度至重度TBI为3.77（95％CI，3.63-3.91）。

结论与相关性 在这项对超过35万名退伍军人的队列研究中，即使是轻度的无LOC的TBI也会使痴呆症的诊断风险增加2倍以上。迫切需要研究确定退伍军人中TBI相关痴呆的机制，预防和治疗的策略。