and veterans who did not. The major differences appear to be related to the level of combat stress. Of male theater veterans exposed to high war-zone stress, 43% met the criteria for a specific psychiatric disorder other than alcohol abuse or dependence. That was significantly higher than rates found for either low or moderate war-zone stress in male theater veterans (21.1%), era veterans (25.6%), or civilians (18.1%). Because the use of self-reported war-zone stress-exposure information was found to correspond well with the information found in the military records, such as receipt of combat medals, this is one of the few early studies that differentiated between the types of stress encountered in a war zone. Among male veterans with high war-zone stress exposure, 63.1% had at least one lifetime psychiatric disorder and 29.8% had a current diagnosis. Compared with the 783 male veterans with low war-zone stress, the 406 with high war-zone stress had more GAD (10.8% vs 24.4%, p < 0.05), dysthymia (1.4% vs 12.3%, p < 0.001), major depression disorder (MDD) (3.1% vs 11.1%, p < 0.01), obsessive-compulsive disorder (0.5% vs 5.5%, p < 0.01), and antisocial personality disorder (7.1% vs 16.6%, p < 0.01). Female veterans had fewer psychiatric disorders associated with high war-zone stress, although 22.3% had lifetime major depression and 9.9% had lifetime dysthymia, and these rates were significantly higher than in female veterans exposed to low war-zone stress.

Two primary studies were included that used data from the Vietnam Era Twin Registry study, a longitudinal twin study. Koenen et al. (2003b) used the Combat Exposure Index and the DIS-III-R to measure PTSD and other psychiatric outcomes in monozygotic twin pairs. The cohort consists of male-male twin pairs born in 1937-1957 in which both members of the pair served in the military during the Vietnam era (Eisen et al. 1991). Zygosity was determined by a questionnaire and blood-group typing methods that achieved 95% accuracy. Thirty-seven male twin pairs were identified in which one twin was a Vietnam-theater veteran with combat-related PTSD and a score of at least 7 on the Combat Exposure Index, and 76 male twin pairs were identified in which one twin was a Vietnam-theater veteran without lifetime PTSD and score of at least 7 on the Combat Exposure Index; all co-twins were not deployed to Vietnam. Deployed twins with PTSD were more likely than twins without PTSD to have any mood disorder (OR 8.89, 90% CI 3.66-21.59), MDD (OR 6.32, 90% CI 2.57-15.55), dysthymia (OR 3.80, 90% CI 1.09-13.28), any anxiety disorder (OR 8.63, 90% CI 2.20-33.83), or panic disorder (OR 4.48, 90% CI 1.04-19.41). Twins with PTSD were also more likely than their co-twins to have any mood disorder (OR 4.00, p < 0.05) or MDD (OR 2.50, p < 0.05) but not other comorbid disorders. Combat-exposed twins that had diagnoses of any of the comorbid psychiatric disorder above also reported more PTSD symptoms in all three symptom clusters (see Chapter 5), including symptoms that had no diagnostic overlap with the comorbid disorders, than did veterans without these disorders.

In another study of the same Vietnam Era Twin Registry cohort, Koenen et al. (2003a) used the entire sample of all 1874 monozygotic twin pairs from the Vietnam Era Twin Registry on whom complete DIS-III-R diagnostic information was available. The study focused on five lifetime diagnoses: MDD, alcohol dependence, drug dependence, cannabis dependence, and tobacco dependence. (Information on substance dependence from the study is discussed in the next section of this chapter.) Measures used were a four-level index of combat exposure (Janes et al. 1991) and a structured psychiatric interview administered to the Vietnam Era Twin Registry twins by telephone with DIS-III-R. Participants were given a diagnosis of combat PTSD (C-PTSD) if they reported combat as a traumatic event and met DSM-III-R criteria for PTSD in relation to the event. Conditional logistic regression was used to account for the paired structure of the data and to calculate the matched pairs’ ORs and 95% CIs. Premilitary trauma history, age

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