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SOMATIC PRESENTATIONS: EPIDEMIOLOGY |
From the Epidemiology and Health Psychology (R.L.), Institute of Psychology, University of Basel, Basel, Switzerland; Max Planck Institute of Psychiatry (R.L.), Munich, Germany; Clinical Psychology and Psychotherapy (G.M.), Institute of Psychology, University of Basel, Basel, Switzerland; and Academic Unit of Psychiatry (R.A.), University of Bristol, United Kingdom.
Address correspondence and reprint requests to Roselind Lieb, Institute of Psychology, Missionsstrasse 60-62, 4055 Basel, Switzerland. E-mail: roselind.lieb{at}unibas.ch
ABSTRACT
Objective: To review the available epidemiological evidence on associations between somatoform disorders with anxiety and depressive disorders.
Results: Clinical and population-based studies have found that the co-occurrence of some types of somatoform disorders (e.g., somatization disorder, somatic-symptom-index (SSI)4,6, and pain disorder) and anxiety and depressive disorders is common. These findings may suggest either a causal relationship between these disorders or that they share some common etiological factors. For other forms of somatoform disorders, empirical evidence about co-occurrence is even thinner or not available at all, especially from non-western settings.
Conclusion: Some implications of how these findings, or the absence of them, can help us understand better the etiology of somatoform disorders and improve the classification of mental disorders as a whole are discussed.
Key Words: epidemiology • somatoform disorders • emotional disorders • comorbidity
Abbreviations: DSM = Diagnostic and Statistical Manual of Mental Disorders; DIS = Diagnostic Interview Schedule; ECA = Epidemiological Catchment Area Program; FGD = functional gastrointestinal disorders; GAD = general anxiety disorder; OR = odds ratio; PTSD = posttraumatic stress disorder; SSI = Somatic Symptom Index.
INTRODUCTION
This article succinctly reviews the epidemiological evidence on the comorbidity between somatoform disorders and anxiety and depressive disorders. Much interest in the associations between somatoform and anxiety and depressive disorders has arisen from observations in clinical samples that a substantial proportion of patients with the Diagnostic and Statistical Manual of Mental Disorders (DSM) somatoform disorders also meet the criteria for DSM anxiety and/or depressive disorders (1–6). Less is known, however, about patterns of comorbidity in the general population.
Why is it important to study comorbidity in the general population? As Kessler (7) pointed out, patient samples do not reflect the natural patterns of comorbidity seen in the general population. One of the main reasons to explain this finding is that comorbidity is closely associated with treatment-seeking behavior (8). General population samples are less affected by illness behavior and thus data arising from these studies are less biased on patterns of comorbidity.
What Can We Learn From Population-Based Research?
Based on data from the Epidemiological Catchment Area Program (ECA) carried out in the 1980s in the United States (9), Swartz et al. (10) found that ECA respondents with a lifetime diagnosis of DSM-III somatization disorder, assessed with the Diagnostic Interview Schedule (DIS), had a much higher probability for having a lifetime history of DSM-III panic disorder (about 25 times higher), obsessive-compulsive disorder (about 12 times higher) and major depression (about 11 times higher) compared with those without this disorder. Because only somatization disorder was assessed within the ECA, no comorbidity patterns could be ascertained for other forms of DSM-III somatoform disorders, e.g., pain disorder, conversion disorder, or hypochondriasis. Escobar (11) suggested an empirically validated form of a subsyndromal diagnostic category, the Somatic Symptom Index (SSI)4,6, which included four somatoform symptoms for men and six for women. ECA respondents fulfilling the SSI4,6 criterion also reported higher lifetime rates of DSM-III major depression and dysthymia, compared with the general population. Lieb et al. (12) studied comorbidity between somatoform disorders and anxiety and depressive disorders on the basis of a representative community sample of 3021 adolescents and young adults aged 14 to 24 years (the Early Developmental Stages of Psychopathology study) (13,14). Using a standardized diagnostic interview (Munich version of the Composite Diagnostic Interviews (15–17), no respondent met the DSM-IV criteria for full-blown somatization disorder but there was an association between the subsyndromal SSI4,6 and anxiety and depressive disorders. In this sample of adolescents and young adults, individuals with a lifetime diagnosis of SSI4,6 were more likely to report a lifetime DSM-IV anxiety disorder (odds ratio (OR) = 3.6; 95% Confidence Interval (CI) = 2.0–6.4) as well as any lifetime DSM-IV depressive disorder (OR = 3.7; 95% CI = 2.0–6.7). Interestingly, obsessive-compulsive disorders (OR = 7.7; 95% CI = 1.9–30.2) and posttraumatic stress disorder (PTSD) (OR = 18.9; 95% CI = 8.3–42.9) also showed increased likelihood of comorbidity with SSI4,6. As can be deduced from the wide CIs for the latter two disorders, the accuracy of these estimates is affected by the low prevalence. It is also important to bear in mind that these associations were based on lifetime diagnoses reached through retrospective enquiry, which may affect the validity of the findings.
Up to now, few studies have examined the comorbidity between DSM-IV pain disorder and depressive and anxiety disorders in representative population samples. Based on the Mental Health Supplement of the German National Health Interview and Examination Survey (18), which involves a community-based sample of >4000 participants aged 18 to 64 years, Fröhlich et al. (19) reported that 33% of the men and 37% of the women with a 12-month diagnosis of DSM-IV pain disorder met the criteria for at least one of the assessed DSM-IV anxiety disorders. There was a strong association between pain and anxiety disorders for all specific anxiety diagnoses and in both genders. Among males with a 12-month history of DSM-IV pain disorder compared with those without this diagnosis, the increased likelihood ranged from OR = 13.0 (95% CI = 5.1–33.2) for DSM-IV general anxiety disorder (GAD) to OR = 5.5 (95% CI = 3.2–8.7) for DSM-IV specific phobia. Among females, these increased probabilities ranged from OR = 4.9 (95% CI = CI 2.5–9.4) for GAD to OR = 2.1 (95% CI = 1.5–3.2) for specific phobias. Similar results were found for major depression (OR = 4.3; 95% CI = 2.4–7.6) and dysthymia (OR = 9.8; 95% CI = 5.4–17.5) among males and females (major depression: OR = 2.4; 95% CI = 1.6–3.4 and dyshymia: OR = 3.9; 95% CI = 2.5–6.0). These findings support the view that comorbidity between pain disorder and anxiety/depressive disorder seems to be the rule rather than the exception.
Lieb et al. found slightly different results for younger age in the previously described community-based study (12). Among 14- to 24-year-old persons, a lifetime diagnosis of DSM-IV pain disorder was significantly associated with a lifetime diagnosis of DSM-IV panic disorder (OR = 4.5; 95% CI = 3.2–22.2) as well as a lifetime DSM-IV PTSD (OR = 4.5; 95% CI = 1.2–16.7). Among depressive disorders, DSM-IV major depression but not DSM-IV dysthymia were strongly associated with pain disorder (OR = 4.3; 95% CI = 2.3–8.1). It is worth noting that, other than using a younger sample, these findings relate to lifetime diagnoses whereas Fröhlich et al. utilized 12-month diagnoses.
We are not aware of any other published population-based study on the association of other forms of DSM-somatoform disorders (e.g., hypochondriasis, conversion disorder, and body dysmorphic disorder) with anxiety and depressive disorders. In most surveys, these disorders were not assessed, whereas in others the prevalence rates of the full-blown diagnoses were too low to assess comorbidity (20–23). The lack of evidence is therefore a major obstacle to properly address this question. Ascertaining somatization disorders can be complex and this may have gone against the inclusion of these disorders in some large surveys undertaken throughout the world.
Why is it Important to Evaluate the Associations Between Somatoform Disorders and Anxiety and Depressive Disorders?
The limited but consistent population-based knowledge on the associations between somatoform disorders and anxiety and depressive disorders reported so far has concerned the lifetime or 12-month comorbidity. We have shown, on the basis of a few cross-sectional population-based surveys in which DSM-somatoform disorders were assessed using standardized diagnostic interviews, that individuals who met the diagnostic criteria for some somatoform disorders (subclinical form SSI4,6 and pain disorder) showed high comorbidity rates with depressive and anxiety disorders.
What does this mean and why is it important to look into the associations between somatoform and anxiety and depressive disorders? The occurrence of more than one disorder in the same individual may have clinical implications because treatment strategies may be different in patients affected by a single disorder rather than several disorders. Not only the comorbidity between different mental disorders but also the comorbidity with medical conditions can have an impact on treatment (24). Besides this practical clinical implication, the investigation of patterns of comorbidity may also provide valuable insight to understand etiological factors and ultimately assist improving the way we classify mental disorders. Ehlert et al. (25), for example, showed that patients with functional gastrointestinal disorders (FGD) (i.e., irritable bowel syndrome or nonulcer dyspepsia) but no mood alterations presented lower cortisol levels, whereas those with comorbid depressive mood had higher cortisol levels, suggesting some psychobiological substrate that may could also help to identify subgroups of FGD patients. A better understanding of the underlying neurobiological underpinnings of frequently co-occurring disorders may also help to determine whether these are independent entities or not (26).
As suggested by Kessler (27) and Faraone et al. (28), several possibilities may explain some observed patterns of comorbidity between somatoform disorders and anxiety and depressive disorders:
Temporal Patterns of Comorbidity
Population-based longitudinal cohorts that allow evaluating the temporal relationships between somatoform disorders and anxiety and depressive are lacking. Thus, there is no empirical evidence to answer the question whether primary somatoform disorders predict subsequent anxiety/depressive disorders or vice versa. Lieb et al. (12), using retrospectively collected data, assessed the temporal priority of the first onset of somatoform disorders and depressive and anxiety disorders. As for the comorbidity with depressive disorders, their results suggested that DSM-IV pain disorder and SSI4,6 were usually reported as being the temporally primary conditions. More than three quarters (78%) of the respondents with a concomitant lifetime depression and pain disorder reported that the pain disorder had an earlier onset than the depressive disorder. Among those respondents with lifetime SSI4,6 and depression, 62% reported that the somatoform condition occurred before the onset of the comorbid depressive disorder. For anxiety disorders, the pattern was indistinguishable. Almost half of all individuals with pain or SSI4,6 somatization disorder reported that anxiety preceded these disorders and the other half the opposite. Across all analyses, the simultaneous (in the same year) onset of somatoform, depressive and anxiety disorders was rare. Fröhlich et al. (19) found similar results when using retrospective age of onset information. For instance, DSM-IV pain disorder was reported as the primary condition in 75% of those cases with comorbid depressive disorder. According to Fröhlich et al. (19), the observed temporal pattern seems to be in line with findings from other epidemiological studies (29). Although this may be a rather crude and biased way of establishing a temporal relationship between these conditions, at least it suggests there may be some temporal relationship that would need to be tested using better methodological designs. Equally, we are aware that even if we were able to demonstrate the presence of a temporal relationship, this would not necessarily represent an etiological, causal relationship.
In one of the few prospective longitudinal studies that included the assessment of somatoform disorders, Lieb et al. (30) investigated whether primary DSM-IV anxiety or depressive disorders predicted incident DSM-IV somatoform conditions during the 4-year follow-up of a young adult cohort. The results showed that anxiety and depressive disorders predicted the first onset of secondary somatoform conditions. In this study, anxiety and depressive disorder could be shown to increase the risk for somatoform conditions in this young sample. These findings would argue for a different temporal direction than the above-mentioned. According to these findings, primary anxiety and depressive disorders would seem to be associated with secondary somatoform disorders. It is important to bear in mind that these findings arose from a more robust methodological design to test temporal relationships but are limited in view of the young sample involved.
These findings hereby summarized show that the empirical evidence accumulated is far too sparse and limited to reach any firm conclusions on the patterns of associations between somatoform disorders and anxiety and depressive disorders. Existing results are inconsistent, suggesting temporal relationships in both directions, e.g., from primary somatoform conditions to secondary mood disorders and vice versa. More population-based longitudinal research is needed to shed light on the temporal patterns of comorbidity between somatoform conditions and anxiety and depressive disorders. Such studies may also help to explore the meaning of any observed temporal association between somatoform and anxiety and depressive disorders, for instance, whether these are causal relationships or simply associations underlying risk factors in the co-occurrence of these conditions.
CONCLUSIONS
NOTES
Received for publication February 8, 2007; revision received June 14, 2007.
This article is being co-published by Psychosomatic Medicine and the American Psychiatric Association.
DOI:10.1097/PSY.0b013e31815b0103
REFERENCES
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