Disentangling PTSD and MDD: Examining temporal changes in symptoms during trauma-focused treatment

Major depressive disorder (MDD) frequently co-occurs with posttraumatic stress disorder (PTSD), yet little is known about the sequence of symptom changes in trauma-focused treatment for individuals with this comorbidity. This study used disaggregated cross-lagged multilevel models to examine the temporal relationships between PTSD and depression symptom change in a randomized clinical trial comparing cognitive processing therapy (CPT) and CPT enhanced with behavioral activation (BA+CPT). The sample included 78 U.S. active duty service members with comorbid PTSD and MDD. Symptoms were assessed weekly at each session. To reduce collinearity between PTSD and depression, models examined core PTSD symptoms (e.g., several intrusion and avoidance symptoms previously identified) and cognitive-affective and somatic depression symptom clusters. Core PTSD and depression symptom clusters demonstrated strong, bidirectional between-person effects (ds = 1.21 to 1.46), indicating that lower core PTSD scores relative to other participants were associated with subsequent lower scores in depression scores, and vice versa. For within-person effects, lagged effects of core PTSD symptoms significantly predicted subsequent reductions in cognitive-affective (d = 0.51) and somatic (d = 0.32) depression scores, indicating that when a participant's core PTSD symptoms decreased relative to their average score, both clusters of depression symptoms also decreased at the next session. However, the inverse relationships were not significant (ds = -0.03 to 0.10). Treatment condition did not moderate lagged relationships across all models. Among service members with comorbid PTSD and MDD, addressing PTSD symptoms may be more important for achieving downstream reductions in depression than vice versa.