To the Editor: As experimental psychologists trained in research methodology, we read the article by Eric Vermetten, M.D., Ph.D., and colleagues (1) with mixed feelings. The authors employed magnetic resonance imaging (MRI) to measure the volumes of the hippocampus and amygdala in 15 female patients with dissociative identity disorder and 23 female nonpsychiatric women. Dr. Vermetten and colleagues concluded from their data that, relative to comparison subjects, patients with dissociative identity disorder had smaller hippocampal and amygdalar volumes. They admit that their study suffers from a number of shortcomings. Most important, the patients with dissociative identity disorder were significantly older than the comparison subjects. Although group differences in hippocampal and amygdalar volumes disappeared when the authors statistically controlled for age, they try to convince the reader that their results are nonetheless valid. They argue that there is generally no age-related reduction in hippocampal and amygdalar volumes in healthy women aged 20–50. This point is reiterated in an editorial by David Spiegel, M.D. (2, p. 566), who stated that the article by Dr. Vermetten and colleagues offers “clear evidence of smaller hippocampal and amygdalar volume among those with dissociative disorders” (2). Unfortunately for Dr. Vermetten and colleagues as well as Dr. Spiegel, there have been reports that this population also exhibits significant, albeit moderate, hippocampal volume decreases in a 5-year follow-up design (3). In relation to this, there is good evidence that suffering from a major psychiatric disorder may lead to accelerated aging in middle-aged people (4, 5). Therefore, MRIs of healthy, younger individuals do not constitute the ideal control condition for those of patients with dissociative identity disorder. It is with these considerations in mind that we fundamentally disagree with Dr. Spiegel when he claims that the study by Dr. Vermetten and colleagues offers “clear evidence.” It does not.
1.Vermetten E, Schmahl C, Lindner S, Loewenstein RJ, Bremner JD: Hippocampal and amygdalar volumes in dissociative identity disorder. Am J Psychiatry 2006; 163:630–6362.Spiegel D: Recognizing traumatic dissociation. Am J Psychiatry 2006; 163:566–5683.Raz N, Rodrigue KM, Head D, Kennedy KM, Acker JD: Differential aging of the medial temporal lobe: a study of a five-year change. Neurology 2004; 62:433–4384.Houx PJ, Vreeling FW, Jolles J: Rigorous health screening reduces age effect on memory scanning task. Brain Cogn 1991; 15:246–2605.Rabbitt PMA, McInnes L, Diggle P, Holland F, Bent N, Abson V, Pendleton N, Horan M: The University of Manchester longitudinal study of cognition in normal healthy age, 1983 through 2003. Aging Neuropsychol Cogn 2004; 11:245–279