Posttraumatic stress disorder (PTSD) is the most frequently diagnosed mental health condition among veterans (Tanielian \& Jaycox, 2008), with estimated rates of the condition ranging from 5-20% (Hoge, Castro, et al., 2004; Hoge, Terhakopian, et al., 2007; Milliken, Auchterlonie, \& Hoge, 2007; Tanielian \& Jaycox, 2008). PTSD is not just a consequence of combat, however; many military personnel and veterans experience noncombat-related traumas such as sexual assault and domestic abuse, or have histories of early life trauma such as child abuse that can also contribute to PTSD. Regardless of the associated event, PTSD is associated with a host of functional problems and negative outcomes among military personnel including occupational and marital dissatisfaction, violence, alcohol and substance abuse, and suicide (Hoge et al., 2004; Jakupcak et al., 2007; Panagioti, Gooding, \& Tarrier, 2009).

Cognitive behavioral treatments are the most highly efficacious treatments for PTSD. Cognitive Processing Therapy (CPT) is one such treatment that has garnered a significant amount of empirical support, with a recent metaanalysis showing it was the most effective treatment for PTSD (Watts et al., 2013), typically yielding a 50% or larger reduction in PTSD symptoms from pre- to posttreatment. Despite CPT's effectiveness, a significant percentage of military veterans continue to experience moderate to severe symptoms afterwards, and nearly half continue to meet full criteria for the diagnosis (Harik, Grubbs, \& Schnurr, 2016). Novel strategies for improving treatment outcomes are therefore needed.

Accumulating evidence supports the effectiveness of stellate ganglion block (SGB) for the treatment of PTSD. SGB is a procedure in which an injection of a long-acting local anesthetic, using ultrasound or fluoroscopic guidance, is made in the right side of the neck around the cervical sympathetic chain that controls the "fight or flight" response (the sympathetic nervous system). The cervical sympathetic chain is a two-way conduit that connects the parts of the brain that control the fight or flight response to the rest of the body. By blocking or "turning off" the cervical sympathetic chain, it is believed that the parts of the brain that control the fight or flight response are allowed to completely reset, resulting in long-term relief of the associated anxiety symptoms. Originally developed for the treatment of pain conditions, multiple case series have shown that right-sided SGB is associated with significant reductions in PTSD symptoms after SGB (Alino et al., 2013; Hickey et al., 2012; Lipov et al., 2008; Mulvaney et al., 2010, 2014), especially negative mood and hyperarousal symptoms (Lynch et al., 2016). Results of a recent randomized controlled trial in which SGB was compared to sham treatment further support the procedure's efficacy (Olmsted et al., 2019). In that study, PTSD symptom reduction in the SGB group was twice as large as the sham group. Research also supports SGB's safety and acceptability among patients with combat-related PTSD (McLean, 2015).

To date, however, the effectiveness of SGB when delivered in combination with first-line PTSD treatments like CPT has not been examined. The present study is designed to examine this issue from two perspectives. First, we will examine if the administration of SGB during CPT leads to larger reductions in PTSD symptoms as compared to CPT alone. Second, we will examine if the administration of SGB after CPT leads to significant reductions in PTSD symptoms among veterans who continue to experience moderate to severe symptoms after completing the therapy. To achieve this the study entails a single-blind, randomized clinical trial with wait list control. All participants will receive cognitive processing therapy (CPT), an empirically-supported psychological (non-medication) treatment for PTSD. Participants will be randomly assigned to receive stellate ganglion block (SGB) treatment at one of two time points: (1) during CPT or (2) three months after completing CPT. Outcomes will be assessed repeatedly for six months postbaseline.