Abstract
Most modern empirical studies on dissociation restrict this mental phenomenon to disruptions in memory, consciousness and identity [1]. As these disruptions involve manifestations of dissociation in psychological variables, we have proposed to name this phenomenon ‘psychological dissociation’ [2]. However, as previously described in 19th century French psychiatry, dissociation also manifests in disturbances of sensation, movement and other bodily functions [3–6]. We have proposed to call these disruptions ‘somatoform dissociation’ [2].
In 1859, Briquet [3] wrote that ‘hysteria is a general disease which modifies the whole organism’, and in 1887 Charcot [4] recognised that hysteria essentially involves disturbances of perception and control. In his view, those with hysteria somehow are unable to intentionally and consciously feel, see or hear what they are supposed to perceive, and they lack the usual control over bodily movements. Apart from these functional losses, Charcot observed that they also suffer from intrusion phenomena. According to Janet [5], hysteria, a conglomerate of disorders which are dominated by dissociative disorders, essentially involves dissociation. He maintained that dissociative symptoms are often induced by psychological trauma, severe illness or fatigue [7]. Subjects who are exposed to these events may fail to integrate their experiences and reactions, which are instead stored as dissociated ‘systems of ideas and functions’. These systems are totally or partially inaccessible to normal awareness, operate independently of voluntary control, and may include somatoform components of experience, reactions and functions [5–8].
Functional losses, which include kinesthetic, visual and auditory anaesthesia, analgesia and motor inhibitions, are incurred as a result of a lowering of the mental level and related narrowing of the field of consciousness. Janet [5] regarded these losses as permanent symptoms of hysteria. They include feelings of incompleteness and lapses of all the mental functions, which he termed ‘mental stigmata’. As mental stigmata all involve functional losses, these stigmata are currently referred to as negative dissociative symptoms [2–9]. Somatoform dissociation also entails intrusions, such as site-specific pain and changing preferences of taste and smell. These positive dissociative symptoms [2–9] relate to periodic reactivations of dissociated systems, which Janet termed ‘mental accidents’. The manifestation of any given symptom at a particular point in time depends on the dissociative state in which the patient remains. For example, in one state the patient may show negative symptoms, in another state s/he may display positive symptoms, and in a third state s/he may be free of such symptoms.
Following Janet, Breuer and Freud [10] maintained that hysteria involves dissociation, but, unlike Janet, they believed that the condition is exclusively induced by childhood sexual trauma [11]. However, Freud subsequently became convinced that sexual fantasy and forbidden wish fulfilment, not sexual trauma, are involved in the aetiology of hysteria. Simultaneously, he began to regard somatoform hysterical symptoms as the result of a process of conversion (i.e. the transformation of unacceptable mental contents into a somatic symptom). Other psychoanalytical orientated authors used the term ‘somatisation’ [12]. Since the beginning of this century several definitions of somatisation and conversion have been proposed, but all refer to a postulated transformation of psychological problems into somatoform symptoms [13].
Psychoanalytic views on hysterical somatoform symptoms have been criticised repeatedly on theoretical and clinical grounds [14–17]. In line with these criticisms, the International Classification of Diseases, 10th edition [18], refers to so-called conversion disorders in terms of ‘dissociative disorders of movement and sensation’. More generally, however, there continues to be confusion regarding the concepts and the classification of somatisation, conversion and dissociation. For example, in each version of the Diagnostic and Statistical Manual of Mental Disorders the conversion, somatisation and dissociative disorders have been categorised differently. In the DSM-IV [19], the symptoms of somatoform dissociation are categorised as symptoms of conversion, somatisation, sexual or pain disorders.
Current scientific interest in psychological manifestations of dissociation is reflected in the development of self-report questionnaires, such as the Dissociative Experiences Scale (DES) [20] and the Dissociation Questionnaire (DIS-Q) [21], as well as the development of structured clinical interviews, such as the Structured Clinical Interview for DSM-IV Dissociative Disorders (SCID-D) [22],[23]. The above questionnaires and interview schedules all include items that pertain to dissociative amnesia, depersonalisation, derealisation, identity confusion, and identity fragmentation.
Until recently, no such instrument existed for the measurement of somatoform dissociation (i.e. the partial or complete loss of the normal integration of somatoform components of experience, reactions, and functions). In order to systematically study somatoform dissociation, Nijenhuis, Spinhoven, Van Dyck, Van der Hart, and Vanderlinden [2],[24] developed the 20-item self-report Somatoform Dissociation Questionnaire (SDQ-20). These authors started from a list of 75 items that might reflect instances of the construct, according to clinical experience and expert judgement. By using standard statistical techniques, the items were selected which best discriminated between psychiatric patients with dissociative disorders and patients with other psychiatric disorders. All items of the SDQ-20 describe negative and positive dissociative symptoms. From the SDQ-20 was later derived the SDQ-5, which serves as a brief screening instrument for dissociative disorders [24],[25]. The psychometric characteristics of these scales proved very satisfactory. Somatoform dissociation is highly characteristic of dissociative disorders, not otherwise specified (DDNOS [19]) and dissociative identity disorder (DID [19]), and somatoform dissociation as measured by the SDQ-20 is highly correlated with psychological dissociation as measured by the DIS-Q [21].
Among various types of traumatic experiences, somatoform dissociation was best predicted by reported physical and sexual abuse occurring in an emotionally neglectful and abusive social context, as well as by early onset of reported intense, chronic and multiple traumatisation [26]. Another finding was that somatoform dissociative symptoms that resemble animal defensive reactions to major threat (e.g. analgesia and inhibited motor reactions) are very good predictors of dissociative disorders [27]. These findings concur with increasing evidence for a link between trauma and dissociation [28–30].
Some authors have argued that a large component of an individual's score on dissociation scales should not be attributed specifically to dissociative pathology, but to psychopathology in general [31]. Indeed, several studies (for a review see [32]) have shown that DES scores are substantially associated with general psychopathology, such as measured by the Symptom Checklist-90-R [33]. These findings suggest that the relative severity of somatoform dissociation in various psychiatric disorders may perhaps result from different degrees of general psychopathology. However, the validity of a separate construct of (somatoform) dissociation would be further supported if the SDQ-20 could differentiate among diagnostic categories with different levels of predicted dissociative pathology before and after statistically controlling for general psychopathology.
We hypothesised that patients belonging to various DSM-IV [19] diagnostic categories will report different degrees of somatoform dissociation. Previous findings [2],[24] suggested that somatoform dissociation is low in patients with anxiety disorders, depression, adjustment disorders and bipolar mood disorder [34].
Patients with eating disorders manifested above normal levels of dissociative symptoms as measured by the DIS-Q [35] and the DES [36]. A subgroup had dissociative experiences to a high degree [35],[37],[38]. Therefore, we hypothesised that at least a subgroup of eating disorder patients would obtain significantly increased SDQ-20 scores.
Strong correlations have been found between psychological dissociation and somatisation, in particular among subjects who report trauma [39–42]. Studying a large sample of traumatised subjects, Van der Kolk et al. [43] found that posttraumatic stress disorder (PTSD), psychological dissociation, somatisation, and affect dysregulation were highly interrelated. Since somatoform dissociative symptoms include pain (the SDQ-20 includes items assessing pelvic pain and pain while urinating) somatoform pain disorder may involve (somatoform) dissociative symptoms in at least some cases. Consistent with this hypothesis, Badura et al. [44] and Walker et al. [42] found associations among chronic pelvic pain, psychological dissociation, and somatisation. In summary, in support for the hypothesis somatoform dissociation would be more severe among a group of patients with somatisation disorder, conversion disorder, or somatoform pain disorder, than among patients with eating disorders.
As patients with DDNOS experience severe psychological dissociation [21], they would probably experience even higher degrees of somatoform dissociation than would subjects with DSM-IV somatoform disorders. Consistent with studies showing that DID is associated with severe psychological dissociation, as well as with our previous findings regarding severe somatoform dissociation in DID [2],[24],[25], it is hypothesised that DID will be associated with the highest SDQ-20 scores.
Because there may be differences in the relative severity of either somatoform or psychological dissociation within particular diagnostic groups, it is important to compare the relative capacities of the SDQ-20 and the DES to distinguish among these diagnostic groups. Putnam et al. [45] recently found that elevated dissociation in particular mental disorders (e.g. eating disorders) was due to a subgroup of highly dissociative subjects within the diagnostic category. Therefore, it also seemed important to assess the proportions of subjects within the various diagnostic groups with scores above the cutoff scores on psychological (DES) and somatoform dissociation (SDQ-5) screening instruments for dissociative disorders.
In summary, we hypothesised that: (1) the intercorrelation between somatoform dissociation and general psychopathology is considerable, but lower than the correlation between somatoform and psychological dissociation; (2) SDQ-20 scores and the number of cases obtaining SDQ-5 scores above the cutoff are increasingly higher, beginning with (i) non-dissociative, non-somatoform, and noneating disorders, then (ii) eating disorders, (iii) DSM-IV somatoform disorders, (iv) DDNOS, and (v) DID; and (3) somatoform dissociation also discriminates among these diagnostic categories after controlling for general psychopathology.
Methods
Subjects
Dissociative disorder patients (n = 44, mean age = 38.7 years, SD = 8.6 years, range = 21-59years, 42 women, two men) included patients with DID (n=23) and DDNOS (n = 21). All were diagnosed using the SCID-D by trained interviewers [22],[23].
Somatoform patients consisted of two groups, each recruited in a different centre specialised in the assessment and treatment of somatoform disorders (i.e. the Department of Psychosomatic Medicine, Willem Arntzhuis, Utrecht, the Netherlands, and the Outpatient Department of the General Psychiatric Hospital ‘De Grote Rivieren’, Dordrecht, the Netherlands). The 21 patients of the first centre included patients with conversion disorder (n = 5), pain disorder (n = 7), conversion and pain disorder (n = 5), somatisation disorder (n =2), and somatisation disorder and pain disorder (n = 2). The second group of somatoform disorders consisted of consecutive cases of DSM-IV conversion disorder (n = 26) who were treated at an inpatient psychiatric unit specialised in the treatment of this condition. The total group consisted of 41 women and six men, and the mean age was 39.4 (SD = 10.9 years, range = 19–68 years). The disorders were assessed by diagnosticians using DSM-IV criteria.
A group of consecutive eating disorder patients (n = 50, mean age = 22.8, SD = 8.4 years, range = 13–48 years, 49 women, one man) was recruited from a residential eating disorder unit. The group included patients with anorexia restrictive type (n = 25), anorexia mixed type (n = 11), bulimia (n = 7), and bulimia overweight (n = 7). Patients with bipolar mood disorder (n = 23, mean age = 45.3, SD = 6.3 years, range = 33-58 years, 11 women, 12 men) were recruited from a mood disorder clinic. All mood disorder patients were on a maintenance treatment of lithium, except one who received carbamazepine.
Finally, a group of consecutive psychiatric outpatients (n = 45, mean age = 34.6, SD = 10.1 years, range = 19–53 years, 27 women, 18 men) was selected, excluding dissociative disorders, somatoform disorders, eating disorders, and bipolar mood disorder. Diagnoses were given by clinicians according to DSM-IV criteria, and included Axis I anxiety disorders (n = 20), depressive disorders (n = 16), adjustment disorders (n = 10), alcohol abuse (n = 2), polysubstance dependence (n = 1), schizophrenia (n = 1), bereavement (n = 2) and problems of relationship (n = 3). Axis II diagnoses included borderline personality disorder (n = 3), avoidant personality disorder (n = 2), dependent personality disorder (n = 2), and personality disorder, NOS (n = 2). In the remainder of this paper, we will refer to this group as the ‘mixed psychiatric disorders’ group.
As assessed with Chi-squared analysis, there were differences among various diagnostic groups with respect to living apart or with a partner (χ2 = 16.9, df = 4, p < 0.002). More eating disorder patients were living apart, while more bipolar mood disorder patients were living with a partner. There was an imbalance of the gender distribution between groups as men were underrepresented among somatoform disorders patients, and were practically absent in the dissociative disorders and eating disorders groups (Fisher's Exact tests). A one-way analysis of variance (ANOVA) revealed group age differences (F = 32.93, df = 4,206, p < 0.0001). According to Tukey's Honestly Significant Difference (HSD) tests, eating disorder patients were younger, and bipolar disorder patients were older than patients from the other diagnostic categories. Finally, the group with somatoform disorders included more individuals who had only received primary school education (χ2 = 36.3, df = 16, p = 0.002) than individuals in the other diagnostic categories.
Instruments
The Structured Clinical Interview for DSM-IV Dissociative Disorders (SCID-D) [22],[23] is a diagnostic instrument developed for the assessment of DSM-IV dissociative disorders. It assesses five dissociative symptom areas (amnesia, depersonalisation, derealisation, identity confusion, and identity fragmentation). Good to excellent reliability and validity have been reported both in the US and in the Netherlands [22],[23],[46].
The Somatoform Dissociation Questionnaire (SDQ-20) [2],[24] is a 20-item, self-report questionnaire measuring somatoform dissociation. According to Mokken scale analysis [47],[48] the items are strongly scalable on a unidimensional latent scale, the reliability of the instrument is high (Cronbach's α = 0.95), and the scores are not dependent on gender or age. The high intercorrelations with the DIS-Q total and subscale scores support the convergent validity of the SDQ-20. Higher scores of patients with dissociative disorders in comparison with patients with other DSM-IV diagnoses demonstrate criterion-related validity. The construct validity was supported by the finding that the degree of somatoform dissociation in DID and DDNOS was correlated with reported trauma, in particular sexual and physical abuse [26].
The SDQ-5 is a five-item dissociative disorders screening instrument which was derived from the SDQ-20 [24],[25]. The scores range from 5 to 25, and the optimal cut-off point in the screening for dissociative disorders is > 8. The SDQ-5 has high sensitivity and specificity, excellent negative predictive value, and satisfactory positive predictive value in predicting cases of dissociative disorders. There were no indications that the SDQ-5 yielded different scores when administered as a separate scale, as opposed to when embedded in the SDQ-20 [24].
The Dissociative Experiences Scale (DES) [20] is a 28-item, self-report questionnaire that evaluates psychological dissociation. The scores range from 0 to 100. The DES has adequate test-retest reliability, good internal consistency, and good clinical validity [20],[49–51]. Dissociative Experiences Scale scores of ≥ 30 in a North American sample [49], and, respectively, ≥ 25 in a Dutch sample [52] were found to yield optimal sensitivity and specificity in screening for dissociative disorders.
The Symptom Checklist-90-R [33] is a self-report rating scale with good psychometric properties which measures general psychopathology.
Procedure
All patients completed the SDQ-20 and the SCL-90-R. The patients with bipolar mood disorder, somatoform disorders, and dissociative disorders also completed the DES, except for the conversion disorder patients from the second inpatient unit (where the DIS-Q was used). In all cases, informed consent was obtained after presentation of a brochure providing information about the study.
Data analyses
The associations among the SDQ-20, the SCL-90-R, and the DES were calculated using Pearson product-moment correlations. Analyses of variance (ANOVA) were performed to examine whether the average sum scores on the SDQ-20 and the DES differed among diagnostic groups. Differences among these groups were assessed post hoc with Tukey's HSD tests. ANCOVA, entering the SCL-90-R as a covariate, was applied to assess whether the SDQ-20 and the DES differentiated among diagnostic groups after statistically correcting for general psychopathology. Next, the estimated marginal means on the SDQ-20 and DES of the various diagnostic groups were compared pairwise, and post hoc Bonferroni correction was applied.
The SDQ-5 scores were derived through summation of the relevant SDQ-20 item scores, and subsequently the proportion of cases per diagnostic category that obtained above cut-off scores (≥ 8) was assessed. The proportion of cases according to the recommended DES cut-off score (≥ 25) in the screening for dissociative disorders as found in a Dutch psychiatric sample [52] was also calculated.
Statistical analyses were performed with SPSS-PC 7.5 [53].
Methods
The SDQ-20 was strongly intercorrelated with the DES (r = 0.85, p < 0.0001). The intercorrelations of the SDQ-20 with the SCL-90-R total and subscale scores were more moderate, but still of considerable strength (0.30 ≤ r ≤ 0.55; all p < 0.0001). The associations with the SCL-90-R somatisation, agoraphobia, anxiety subscales and the SCL-90-R total scores obtained the highest values (all r ≥ 0.50).
According to ANOVA, the SDQ-20 differentiated among diagnostic groups (F = 57.80, df=5, 203, p < 0.0001; Table 1). Tukey's HSD indicated that somatoform dissociation was significantly higher in DID than in DDNOS. Somatoform dissociation was also significantly higher in both of these dissociative disorders than in the other diagnostic categories. Finally, patients with somatoform disorders obtained significantly higher SDQ-20 scores than patients with mixed psychiatric (without dissociative or somato-form) disorders, or bipolar mood disorder (Table 1).
Differences among several diagnostic categories on the Somatoform Dissociation Questionnaire (SDQ-20) and the Dissociative Experiences Scale (DES)
ANCOVA showed that the estimated marginal mean SDQ-20 scores of various diagnostic categories still discriminated among these diagnostic categories when these scores were adjusted for the SCL-90-R total scores as a covariate (F = 52.49, df = 5, 193, p < 0.0001; Table 2). Pairwise comparison revealed that the adjusted SDQ-20 scores significantly discriminated among the diagnostic groups as did the unadjusted SDQ-20 scores, except that the adjusted scores also discriminated somatoform disorders and eating disorders (Table 2). By applying a post hoc Bonferroni correction, the differences between the mixed group, and eating disorders, as well as bipolar mood disorder, failed to remain statistically significant.
Differences on the Somatoform Dissociation Questionnaire (SDQ-20) among several diagnostic categories after adjustment for general psychopathology using adjusted estimated marginal means
The DES scores were significantly different among patients with bipolar mood disorder, somatoform disorders, DDNOS, and DID (ANOVA, F = 42.71, df = 3, 84, p < 0.0001) (Table 1). As Tukey's HSD indicated, the DES scores of the DID patients significantly exceeded those of the DDNOS patients, and the patients with dissociative disorders had significantly higher scores than did the patients with somatoform disorders or bipolar mood disorder. However, the difference between somatoform disorders and bipolar mood disorder was not significant. ANCOVA demonstrated that the estimated marginal mean DES scores discriminated between these diagnostic categories when the scores were adjusted for the SCL-90-R total scores as a covariate (F = 19.36, df = 3, 81, p < 0.0001) (Table 3). Finally, Bonferroni corrected pairwise comparisons showed that the indicated differences among groups remained statistically significant after adjustment of the DES scores for the influence of the SCL-90-R scores (Table 3).
Differences on the Dissociative Experiences Scale (DES) among several diagnostic categories after adjustment for general psychopathology using adjusted estimated marginal means
The proportions of patients from the various diagnostic categories with above cut-off scores on the SDQ-5 and the DES are displayed in Table 4. The diagnostic categories encompassed, in the hypothesised order, increasing proportions of cases with above cut-off scores. None of the patients with bipolar mood disorder, and only one patient of the mixed psychiatric group, had scores above the SDQ-5 cut-off. In contrast, 40% of the cases with eating disorder and 64.6% of the patients with somatoform disorder passed the SDQ-5 cut-off, as did most cases of DDNOS and all but one cases of DID. However, only 23.8% of the cases with somatoform disorders had scores which exceeded the DES cut-off. The DES and SDQ-5 screening results with bipolar mood disorder and dissociative disorders yielded comparable results.
Percentages of cases of various diagnostic categories scoring below and above the cut-offs on the Somatoform Dissociation Questionnaire (SDQ-20) and Dissociative Experiences Scale (DES)
As far more female patients than male patients were included in the diagnostic categories DID, DDNOS, and somatoform disorders, the results could have been biased by this imbalance. However, including only women in the analyses did not affect the results.
In a meta-analytic validation study on the DES, Van IJzendoorn and Schuengel [32] remarked that this instrument has many strengths and some weaknesses. One weakness is that DID patients who were unaware of their diagnostic status obtained lower DES scores than DID patients who were knowledgeable of their condition. As the measurement of somatoform dissociation could also be subject to this bias, we compared the SDQ-20 scores of DDNOS and DID patients when the scales were administered by a diagnostic consultant before the diagnosis had been made and shared, or by the treating therapist in a postdiagnostic stage. Interestingly, the patients who were not aware of their diagnosis (DDNOS, M = 47.6, SD = 12.0; DID, M = 60.0, SD = 14.86) tended to obtain higher scores than dissociative patients who were aware of their psychiatric status (DDNOS, M = 35.6, SD = 7.9; DID, M = 53.0, SD = 12.73).
Since the bipolar mood disorder patients were generally older than the patients from the other groups, and patients with somatoform disorders generally had received less education, we checked for differences in the SDQ-20, DES, and SCL-90-R scores according to age and level of education, but found that they did not influence the present results.
Discussion
Somatoform dissociation was strongly associated with psychological dissociation as measured by the DES. These findings support the convergent validity of the SDQ-20. Other recent studies also found strong correlations between SDQ-20 and DES scores, both among Turkish and North-American psychiatric patients [54],[55], and among a non-psychiatric Dutch population [56].
In view of the striking comorbidity displayed by patients with dissociative disorders [57–59], the considerable correlations between somatoform dissociation and general psychopathology were expected. However, we also predicted that this association would be weaker than the association between somatoform and psychological dissociation. This first hypothesis was supported by the present data. Interestingly, the association of the SDQ-20 with the SCL-90-R somatisation subscale was of the same magnitude as the correlation with the SCL-90-R total scale, and the anxiety and agoraphobia subscales. These findings suggest that the SDQ-20 and the somatisation subscale of the SCL-90-R assess related, but distinct constructs. It follows that somatoform dissociation cannot be equated with a tendency to report physical complaints.
The SDQ-20 differentiated among most diagnostic groups, and in all cases but one, the differences were as hypothesised. The associations between somatoform dissociation and SCL-90-R total and subscale scores could raise a concern about whether the SDQ-20 would actually be a measure of general psychopathology. However, statistically controlling for the influence of the SCL-90-R total score did not affect the ability of the SDQ-20 to differentiate among the diagnostic groups. Somatoform dissociation, whether unadjusted or adjusted for general psychopathology, was significantly raised in a step-wise manner beginning with (i) somatoform disorders, then (ii) DDNOS, and finally (iii) DID. These results strongly support the discriminant validity of the SDQ-20, and provide evidence that somatoform dissociation stands apart from general psychopathology.
The mixed psychiatric group and patients with bipolar mood disorder obtained low SDQ-20 scores. This result is in concordance with their low DES scores, the low DES scores of patients with anxiety disorders and affective disorders as assessed in other studies (for a review see [31]), as well as the low DES scores of bipolar mood disorder patients [33]. Only one patient of the mixed psychiatric group, and none of the bipolar mood disorder patients passed the SDQ-5 cut-off point. We conclude that anxiety disorders, depression, and adjustment disorders are not associated with dissociative pathology, and that the present data strongly contradict Merskey's [60] assertion that DID would be misdiagnosed bipolar mood disorder.
Consistent with our prediction, somatoform dissociation was elevated in a subgroup of patients with eating disorders. About one-third surpassed the SDQ-5 cut-off in the screening for dissociative disorders. This result concurs with earlier findings that a subgroup of patients with eating disorders experienced substantial psychological dissociation [35],[38].
Patients with somatoform disorders reported considerable somatoform dissociation. As many as two-thirds of them scored above the SDQ-5 cut-off in the screening for dissociative disorders. This finding suggests that these patients would possibly have a dissociative disorder as a comorbid diagnosis, although in the present study presence of dissociative disorder according to DSM-IV criteria was not assessed. In the ICD-10 [18], so-called conversion disorders, but not pain and somatisation disorders, are categorised and labelled as dissociative disorders. The present data suggest that somatoform dissociation may be typical for a substantial subgroup of patients with somatoform disorder.
Somatoform dissociation was high in DDNOS and extreme in DID, which confirms our previous findings [2],[24]. DDNOS and DID were also associated with, respectively, high and extreme levels of psychological dissociation. This result is consistent with the finding that psychological dissociation is highly characteristic of dissociative disorders, and that it increases with the complexity of the dissociative disorder [21–23],[61],[62].
Psychological dissociation as measured by the DES differentiated among bipolar mood disorder, DDNOS, and DID, both when the scores were unadjusted and adjusted for the influence of general psychopathology. However, the DES did not distinguish somatoform disorder from bipolar mood disorder. About two-thirds of the present DSM-IV somatoform disorders patients experienced significant somatoform dissociation, but psychological dissociation was only characteristic of a quarter of them.
Some limitations of this study demand discussion. The eating disorder and the somatoform disorder patients were admitted to inpatient units specialised in the treatment of these conditions. Assuming that as a rule, inpatients would display more psychopathology than outpatients, the scores of the eating disorder and somatoform disorders patients may have been raised for this reason, and, thus, may have influenced the comparisons. In contrast, the inpatient status of the eating disorder and somatoform disorder patients made it more difficult for the SDQ-20 to differentiate these inpatients from the dissociative disorder outpatients, especially those with DDNOS. A future comparison between consecutive psychiatric outpatients without dissociative disorders or somatoform disorders and somatoform disorder outpatients would nevertheless be important. As the group of patients with somatoform disorder included various diagnostic subcategories (conversion disorder, somatoform pain disorder, and somatisation disorder), future work should study somatoform dissociation among homogeneous subgroups of DSM-IV somatoform disorders. Another limitation of this study is that two subgroups (dissociative disorders and eating disorders) almost entirely consisted of women. However, including only women in the analyses did not affect the results. The patients with bipolar mood disorder were older than the patients of some other diagnostic categories, but these differences did not affect the results either.
The DID and DDNOS patients who were aware of their diagnostic status did not report more somatoform dissociative phenomena than dissociative disorder patients who were not aware of their diagnosis. The trend was even in the other direction, which could perhaps reflect a positive treatment effect.
Conclusions
The present data support the SDQ-20′s convergent, criterion-related, and discriminant validity. The evidence indicates that this instrument measures a unique construct, somatoform dissociation, which is strongly associated with, but not identical to psychological dissociation, and which differs from general psychopathology. The findings confirm our previous conclusion that dissociative disorders are highly characterised by somatoform dissociation. They also suggest that somatoform dissociation is a core feature in many patients with somatoform disorders, and an important symptom cluster in a subgroup of patients with eating disorders.
Footnotes
Acknowledgements
This study was supported by a grant of the Stichting Dienstbetoon Gezondheidszorg, Soesterberg, Netherlands, number 11.92. The authors kindly thank Dr Paul Brown and Kathy Steele, R.N., M.N., C.S., for their comments on a previous version of the article.