To the Editor: We thank Drs. Esposito and Mellman for their interest in our recent study. They raise an important point concerning the potential underdiagnosis of acute stress disorder if persistent dissociation, rather than peritraumatic dissociation, is adopted as the diagnostic criterion. In response to this concern, it is important to consider the rationale underlying of the diagnosis of acute stress disorder. The introduction of acute stress disorder in DSM-IV was an attempt to differentiate individuals experiencing "normal" stress responses from those experiencing abnormal stress responses (1) and to identify individuals vulnerable to developing later PTSD (2). We would argue that in the context of injury survivors, the use of persistent relative to peritraumatic dissociation is more consistent with the rationale behind the acute stress disorder diagnosis.

Two clear issues are raised by the question of when to assess dissociation in acute stress disorder. First, we want to maximize the likelihood of identifying individuals who will develop PTSD. Second, we want to minimize the likelihood of pathologizing "normal" transient stress reactions. The problems associated with defining dissociation in the context of an acute stress disorder diagnosis have been well reviewed (3, 4). Of relevance to our discussion is the problem that while three or more dissociative symptoms are required "either while experiencing or after experiencing the distressing event" (criterion B), criterion G requires the symptoms of acute stress disorder to last "for a minimum of 2 days." Depending on how this apparent contradiction is interpreted, the diagnostic criteria seem to imply the use of either peritraumatic or persistent dissociation in the diagnosis of acute stress disorder. An increasing body of literature is questioning the usefulness of peritraumatic dissociation in the prediction of PTSD (5, 6). Marshall and Schnell (5) also highlight the problems associated with the reliability of retrospective reporting of peritraumatic dissociation. Very few studies, however, have compared the predictive ability of peritraumatic versus persistent dissociation. In one of the few published studies, Murray et al. (7) found that persistent dissociation was a stronger predictor of chronic PTSD than peritraumatic dissociation. They argue that although initial dissociation may put individuals at risk for PTSD, many are able to compensate in the posttrauma period. However, those who continue to dissociate are at a high risk of later PTSD.

In their recent review, McNally et al. (8) argued that one of the problems in studies of acute stress disorder to date is that the diagnosis may have been made too early after the traumatic event. They suggest that variability across studies in the ability of acute stress disorder to predict PTSD may be associated with the timing of the assessment. They argue that making a diagnosis too soon after the traumatic event will increase the likelihood that a transient stress reaction will be incorrectly classified as a case of acute stress disorder. This is particularly relevant for peritraumatic dissociation.

Our final comment relates to the difficulties of conducting psychiatric research in injured populations (9). In order to assess peritraumatic dissociation within our study in the manner Drs. Esposito and Mellman propose, we would have had to exclude (at a minimum) the participants with mild traumatic brain injury or substance intoxication at the time of the event. This would have excluded over 60% of our sample. We suggest that the usefulness of any diagnosis is determined by its clinical relevance. If acute stress disorder can only be used with a select few individuals within a given population, we would question its clinical utility. Putting aside the many other problems associated with the diagnosis of acute stress disorder, the adoption of persistent dissociation within this diagnosis allows improved clinical utility and, therefore, strengthens the relevance of this diagnosis.