Bipolar disorder is a chronic, severe, and complex psychiatric illness associated with decreased psychosocial functioning that can be further complicated by comorbid conditions, including post-traumatic stress disorder (PTSD) (Geddes and Miklowitz, 2013). Rates of PTSD comorbidity among individuals with bipolar disorder may exceed 50 % (Cerimele et al., 2017; Simon et al., 2004), possibly caused by increased exposure to traumatic experiences in the lives of bipolar patients (Quarantini et al., 2010).

The comorbidity of bipolar disorder and PTSD is likely multifactorial. For instance, childhood trauma may predispose individuals to develop bipolar disorder (Quidé et al., 2020). Genetic predisposition to bipolar disorder may result in childhood behavioral disturbances that lead to dysfunctional parental attitudes and emotional abuse (Aas et al., 2016). Additionally, higher rates of risk-taking behaviors, lower quality of life, and involuntary hospitalizations may result in higher rates of emotional and physical trauma (Katz et al., 2020). The co-occurrence of these disorders is particularly problematic for an individual's well-being and is associated with increased suicidality, higher rates of rapid cycling, higher symptom burden, lower quality of life, and lower rates of recovery maintenance (Cerimele et al., 2017).

Although the clinical significance of bipolar disorder and comorbid PTSD is clear, no treatment guidelines exist to address this clinical presentation. Unlike psychosocial treatments for bipolar disorder, which are adjunctive to pharmacotherapy and often provide only modest benefits for bipolar symptoms (Miklowitz et al., 2021), current PTSD treatments are highly effective (Lewis et al., 2020). Prolonged Exposure (PE) is a manualized treatment package for PTSD that consists of 9 to 12 ninety-minute sessions (Foa, 2011). The core components of PE are repeated and prolonged imaginal exposure to the index trauma and in-vivo homework assignments designed to promote habituation and reduce avoidance of trauma-related cues. PE is thought to work through fear extinction mechanisms that allow the patient to emotionally activate and process the traumatic memories in the absence of feared outcomes (Edna et al., 2007). PE is well-tolerated, and—relative to less structured trauma treatments—brief in duration and cost-effective (Le et al., 2014).

Nearly three decades of research demonstrate the efficacy of PE for PTSD, and thousands of clinicians worldwide have now been trained in its use (Powers et al., 2010). A recent meta-analysis of thirteen studies with a total sample size of 675 participants showed a large effect size for PE versus control on both primary and secondary outcome measures, both immediately at post-treatment and at longer-term follow-up. The average PE-treated patient demonstrating greater improvement on PTSD measures at post-treatment than 86 % of patients in control conditions (Powers et al., 2010). PE is clearly a highly effective treatment for PTSD, resulting in substantial treatment gains that are maintained over time.

Despite its robust empirical support, clinicians are often reluctant to use PE in patients with comorbid serious mental illness due to concerns that the treatment is contraindicated (van Minnen et al., 2012) for and even be harmful to patients in this population (e.g., by causing an increase in suicidal thinking and behaviors) (Ruzek et al., 2014). However, emerging research suggests that individuals with severe mental illnesses (Grubaugh et al., 2017) can safely and successfully complete PE treatment with comparable outcomes to patients with primary PTSD only (De Bont et al., 2016; Grubaugh et al., 2021) One study of PE in patients with severe PTSD and psychotic disorders found that the treatment was effective, safe, and feasible for use in this patient population, and the benefits were maintained for 12 months post-treatment (van den Berg et al., 2018). Grubaugh et al. (2016) conducted an open trial of PE for patients with SMIs, including bipolar disorder, and found clinically significant improvements in PTSD symptoms that were maintained at 6 months.

The objective of the current study was to evaluate the feasibility and preliminary efficacy of Prolonged Exposure treatment in a sample of patients diagnosed with both bipolar disorder and PTSD. We assessed feasibility by intervention completion and study retention and hypothesized that completion (Varker et al., 2021) rates would exceed 70 %, based on observed dropout rates in gold standard PTSD therapies. Our primary outcomes of interest for evaluating preliminary efficacy were PTSD symptoms and suicide risk. We hypothesized that both PTSD symptoms and suicidality would decrease following PE. Additional outcomes of interest were depression and mania symptoms, and state and trait anxiety. We hypothesized that depression symptoms would decrease following PE, (hypo)manic symptoms would remain stable, and state/trait anxiety would increase during active treatment but ultimately decrease.