Dose-dependent effect
At baseline and 1 year after the disaster, all participants completed the Composite International Diagnostic Interview, which assesses for the presence of PTSD and/or MDD. Participants also completed the List of Threatening Experiences, a 12-item questionnaire that measures major stressful life events.
Of 3,000 participants who initially agreed to take part in the trial, 1708 completed both the predisaster assessment in 2003 and the postdisaster assessment in 2011, 1 year after the earthquake and tsunami occurred. After excluding for a variety other criteria, 1,160 individuals were included in the final analysis.
“As it turns out, it was a very natural experiment,” said Dr. Buka. “We had a group of people whose past traumatic experiences we knew about, and then they were all subjected to this terrible earthquake, and then we were able to look forward into time and see who did and didn’t develop PTSD and MDD.”
When the study began in 2003, none of the 1,160 participants had a history of PTSD or MDD. After the 2010 earthquake, 9.1% of the survivors (n = 106) were diagnosed with PTSD, and 14.4% were diagnosed with MDD (n = 167).
Further analyses showed that prior disaster exposure was not a significant predictor of postdisaster PTSD. Nevertheless, for every unit increase in prior nondisaster stressors, the odds of developing postdisaster PTSD increased (odds ratio, 1.21; 95% confidence interval, 1.08-1.37; P = .001).
When categorizing predisaster stressors, the investigators found that individuals who had four or more predisaster stressors had a significantly greater chance of developing postdisaster PTSD than those with no predisaster stressors (OR, 2.77; 95% CI, 1.52 – 5.04).
Similar logistic regression analyses were performed for MDD, with comparable results. Although prior disaster exposure was not a significant predictor of postdisaster MDD, each one-unit increase in prior nondisaster stressors increased the odds of developing postdisaster MDD by 16% (OR, 1.16; 95% CI, 1.06-1.27; P = .001).
Categorization of these stressors revealed that experiencing any number of stressors significantly increased the odds of developing postdisaster MDD in a dose-response fashion.
In other words, every predisaster stressor – even a single one – increased an individual’s risk of developing postdisaster MDD, and each additional stressor further increased the risk.
Predisaster stressors
Interestingly, the study also showed that the risk of developing both PTSD and MDD was particularly high among those who had experienced multiple predisaster stressors, such as serious illness or injury, death of a loved one, divorce, unemployment, financial struggles, legal troubles, or the loss of a valuable possession.
These findings, the researchers note, demonstrate that a history of stressors increases what they called “stress sensitization,” which may make individuals more vulnerable to the negative effects of subsequent stressors rather than more resilient.
As such, individuals who have experienced several stressors over the course of a lifetime are at higher risk of developing a psychiatric disorder.
This was the case with PTSD, in which exposure to at least four previous manageable stressors was associated with greater odds of developing postdisaster PTSD. For MDD, on the other hand, there was a distinct dose-response relationship between the number of manageable predisaster stressors and the risk for postdisaster MDD.
The investigators explain that these findings are particularly relevant in light of the COVID-19 pandemic and the current focus on racial and economic inequality in the United States. “The findings highlight the sectors of the population that are at greatest risk,” Dr. Buka said. “And those are the ones who’ve had more challenging and traumatic lives and more hardship.
“So it certainly calls for greater concentration of psychiatric services in traditionally underserved areas, because those are also areas that have greater histories of trauma.”