Bipolar disorder (BD) is a chronic disabling disorder affecting approximately 2 % of the global population (Carvalho et al., 2020; Clemente et al., 2015). BD patients usually present with mood swings (including manic, hypomanic, and depressive), psychotic symptoms, cognitive impairment, declined social and occupational function, and attention deficiency, all of which may aggravate health outcomes and even lead to mortality (Global Burden of Disease Study, 2015; McIntyre et al., 2020; Samame et al., 2014). The remaining life expectancy of BD patients has decreased by 10–20 years (McIntyre et al., 2020).

A meta-analysis including 31 studies assessed the standardized mortality ratio (SMR) in BD patients (Hayes et al., 2015), reporting an SMR of 2.05 (95 % confidence interval, CI:1.89–2.23) for all-cause mortality. For cause-specific mortality, the highest SMR was suicide (14.44; 12.43–16.78), followed by unnatural causes (7.42; 6.43–8.55), violent death (3.68; 2.77–4.90), and natural causes (1.64; 1.47–1.83). Several nationwide studies have also examined mortality in BD, including Denmark (Kessing et al., 2021; Laursen et al., 2013), Sweden (Crump et al., 2013; Laursen et al., 2013), Finland (Laursen et al., 2013), and Hong Kong (Chan et al., 2021). These studies reported similar results showing a 2–3 fold increased risk of all-cause mortality in BD patients compared to that of controls. However, these studies only adjusted for age, sex, and other sociodemographic factors without considering physical comorbidities. Physical comorbidities such as diabetes or cardiovascular disease not only increase the risk of natural causes of mortality (De Hert et al., 2009) but are also associated with a higher risk of suicide in BD patients (Chen et al., 2021).

Comorbid anxiety or obsessive-compulsive disorder increases the risk of suicide in BD patients compared with that of non-comorbid patients (Amerio, 2019; Spoorthy et al., 2019); however, the role of neurodevelopmental disorders, such as attention-deficiency hyperactivity disorder (ADHD) and autism spectrum disorder (ASD), remains under-investigated. ADHD is a common psychiatric comorbidity in BD patients (10 %–17 %) (Brancati et al., 2021; Schiweck et al., 2021), while the prevalence of comorbid ASD remains uncertain (Axelson et al., 2006; Joshi et al., 2013). According to a meta-analysis, ADHD patients have a 2.13-fold higher all-cause mortality rate than controls, while ASD have 2.37-fold (Catala-Lopez et al., 2022). Another study reported that BD patients had a more severe disease course than controls, which was associated with a higher risk of deaths (Schoepf and Heun, 2014).

In the current study, we used a longitudinal population database to investigate cause-specific mortality in BD patients. We also examined its association with physical comorbidities, neurodevelopmental disorders, and the severity of BD.