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Psychosomatics 49:152-162, March-April
doi: 10.1176/appi.psy.49.2.152
© 2008 Academy of Psychosomatic Medicine
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* Somatoform Disorders
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Overlap, Comorbidity, and Stability of Somatoform Disorders and the Use of Current Versus Lifetime Criteria

Kari A. Leiknes, M.D., M.H.A., Arnstein Finset, Ph.D., Torbjørn Moum, Ph.D., and Inger Sandanger, M.D., Ph.D.

Received June 9, 2006; revised August 25, 2006; accepted September 5, 2006. From the Institute of Basic Medical Sciences, Dept. of Behavioural Sciences in Medicine, Univ. of Oslo, Norway; The Akershus Univ. Hospital Faculty Division, University of Oslo, Norway. Send correspondence and reprint requests to Kari Ann Leiknes, M.D., M.H.A., Univ. of Oslo, POB 1111, Blindern, N-0317, Oslo, Norway. e-mail: k.a.leiknes{at}medisin.uio.no
© 2008 The Academy of Psychosomatic Medicine


  ABSTRACT

 
TOP
 ABSTRACT
INTRODUCTION
 METHOD
 RESULTS
 DISCUSSION
 REFERENCES
 
The authors examined disorder overlap, comorbidity, stability, and predictors of somatoform disorders (SDs) by "lifetime" and "current" symptom criteria in a general population sample of 421 respondents interviewed with the Composite International Diagnostic Interview in 1990 and 2001. Disorder overlap and comorbidity were considerable. "Current" SDs were four times more likely to occur among respondents with depression. Diagnostic stability was highest for "current" SDs (retrospective consistency: 42%). Young women were more prone to a stable (chronic) course over time. Previous depression and physical disease were risk factors for "current" but not for "lifetime" SDs; diagnostic criteria should therefore be based on current symptoms.


  INTRODUCTION

 
TOP
 ABSTRACT
 INTRODUCTION
METHOD
 RESULTS
 DISCUSSION
 REFERENCES
 
Somatoform disorders (SDs), as described in The Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM–IV)1 and the International Classification of Diseases, 10th Edition (ICD–10),2 are characterized by one or more medically unexplained symptoms (MUSs). The greater the number of MUSs, the more severe the condition is considered to be. For each individual patient, the nature and number of symptoms may change over time. Most diagnostic criteria will therefore include references to the number of clinically significant MUSs and the symptom timeline (time of onset, duration, and status as current or previously present). Because individual symptoms change over time, but SDs are considered to be stable and long-lasting conditions, core diagnostic criteria are built on the number of lifetime symptoms, rather than current symptoms. The justification for this practice is that counting only current symptoms might omit outpatients who often report MUSs but who fail to meet current occurrence criteria at a given point in time.

Although symptom variability over time has been demonstrated3,4 and the counting of diverse MUSs from symptom lists has been argued to be insufficient and not empirically based,5–7 the symptom-counting issue of SDs still remains a contentious one. A criticism of existing SD criteria has also been that they are either too broad and or too narrow.8 Since the introduction of the SD category in the DSM–III,9 a reduction in required MUS criteria for "somatization disorder (SDz)" has taken place in both DSM–IV and ICD–10. Numerous alternative diagnostic scales have also been put forth. An example is the Abridged Somatoform Disorder scale (SSI–4/6), which utilizes the Somatoform Symptom Index (SSI)10,11 symptom list and consists of four lifetime MUSs for men and six for women. Later, it was recommended that this symptom complex be reduced to the abridged three/five (men/women) (SSI–3/5) equivalent of DSM–IV.12 Other constructs have also been launched, such as the polysymptomatic somatoform,7 undifferentiated visceral somatoform,13 and multisomatoform disorder (MSD).14 The last consists of three current MUSs combined with one lifetime MUS. Recently, a further modification of this construct has been suggested, to "physical symptom disorder (PSD),"15 consisting of only one (or more) current MUSs. The ongoing reclassification and nosological debate about SDs,16–21 supports the grouping together of SDs and suggests the designation "psychosomatic distress syndromes" as the new collective and common term for SDs.22

A major challenge to future SD diagnosis is the substantial overlap between existing individual functional syndromes, with similarities outweighing the differences between them.23,24 Another challenge is the comorbidity issue25–27 and, as pointed out earlier, a high risk of comorbid anxiety and depression.28,29 The importance of probing for or ruling out concomitant physical disease23,27 is yet another issue. In order not to miss any coexistent physical disease, when dealing with chronic physical symptoms, it has been recommended that clinicians examine for physical disease first, before evaluating the patient for anxiety and/or depression.13

In spite of this, SDs are considered by DSM–IV and ICD–10 to be stable and long-lasting conditions, even though the stability of the whole SD category over a 31/2-year period has been reported to be as low as 48%.30 Predictors of a stable course were female gender, previous substance abuse, and anxiety disorder.30 The importance of diagnostic utility (usefulness), meaning the essence of what the diagnosis conveys in course, outcome, and etiology, has been advocated in psychiatric diagnoses.31 Also, it has been suggested that one regard syndromes as discrete only if they have been shown to have distinct natural boundaries or "zones of rarity."3 Diagnostic changes purporting to improve clinical utility in future revisions of DSM have been recommended.32 Advancing classification toward coherent medical etiological explanations33 and basing all future criteria of SDs on empirically-based research7 is also urged. Even though serotonergic amino acids have been found as biological correlates of multiple unexplained symptoms,34 psychopathology and etiological mechanisms are, nonetheless, still not well understood. Because of the paucity of longitudinal follow-up studies,35 existing knowledge concerning the discreteness, course, and prognosis of SDs is still sparse.4,29,30,35–37

Against this background, this article presents three central diagnostic issues: 1) category discreteness and "zones of rarity;" that is, the degree of overlap between SD subtypes and co-occurrence with anxiety and depression; 2) stability of SDs over time; and 3) differences in SD diagnoses based on "current" versus "lifetime" symptom criteria. More specifically, we pose the following research questions: 1) To what extent do disorders of the SD category overlap at a given time-point? What is the comorbidity of SDs with anxiety and depression? 2) What is the stability of SDs over time? What is the likelihood that those diagnosed with an SD ("lifetime" or "current") in 1990 would receive the same diagnosis in 2001? What proportion of respondents with SDs, including the MSD and SSI–3/5 at baseline, retained the same or received another SD diagnosis at follow-up? 3) What baseline sociodemographic and morbidity variables (age, gender, anxiety, depression, physical morbidity, psychiatric diagnoses) predict having a) any "lifetime" SD; and b) any "current" SD at follow-up?


  METHOD

 
TOP
 ABSTRACT
 INTRODUCTION
 METHOD
RESULTS
 DISCUSSION
 REFERENCES
 
Sample
Participants in the Norwegian general population (Oslo, Norway-Lofoten cohort),38 age 18 or older, who had been interviewed in 1989/1990 (hereafter referred to as baseline or 1990) with the Composite International Diagnostic Interview (CIDI), Version 1.0,39 were reinterviewed 10–11 years later, in 2000/2001 (hereafter referred to as follow-up or 2001) with the computerized M–CIDI, Version 1.2.40 The baseline population consisted of a random sample drawn from Statistics Norway (Norway’s central institution for official statistics), out of which 2,727 persons responding to the Hopkins Symptoms Checklist 25-item scale questionnaire (HSCL–25)41 were further selected for the CIDI interview.42,43 All those with HSCL scores ≥1.55 were selected for CIDI interview, in addition to a random sample of those with HSCL–25 scores ≤1.55. Details of the initial sampling have been described previously.3,38,42–44

In all, 605 respondents were interviewed with the CIDI Somatoform Section at baseline, and 421 persons (242 women and 179 men; 57.5% and 42.5%, respectively) were re-interviewed in 2001 (response rate: 69.6%). Only data pertaining to follow-up respondents interviewed at both time-points (N=421) are reported in this article. The CIDI-trained interviewers at both time-points were nurses, social workers, and medical and psychology students, as well as the psychiatrists participating in the research project. All interviewers were trained according to the CIDI training program, by the study’s leading psychiatrist. Good interrater reliability has previously been demonstrated for the CIDI,45 and, similarly, no interviewer outliers were found in our data.

Measures
In order to ensure exact computing of diagnoses at both time-points, SDs analyzed in this article were computed from the interview raw data, applying the DSM–IV and ICD–10 symptom criteria (as in Table 1), based on CIDI’s identification and probing of clinically significant MUSs. The specific MUSs examined at both time-points and included in the diagnostic computation are shown in Appendix 1. MSD criteria were based on the CIDI symptom list (see Appendix 1) and probing of clinically significant current MUSs present in the last month. More specifically, CIDI probes for the recency of clinically significant MUSs in a pooled fashion, divided into three sections: 1) any 1 of 10 previous pain symptoms; 2) any 1 of previous gastrointestinal or pseudoneurological symptoms; and 3) any 1 other symptom, as indicated in Appendix 1. Since the CIDI is based on extant ICD–10 and DSM–IV "lifetime" symptomatology criteria, detailed knowledge about how recent each individual symptom has been experienced is not necessary in the diagnostic algorithms of these SDs diagnoses. Because overlap (comorbidity) of SD subtypes was to be assessed, exclusion criteria for the SDs were not used in the diagnostic computation. SD groups, based on "lifetime" MUSs (of at least 6 months’ duration), were 1) somatization disorder (SDz); 2) hypochondriasis (HC); 3) undifferentiated somatoform disorder (SDud); and 4) chronic pain disorders associated with psychological factors (persistent somatoform pain disorder [PD]), and those based on "current" MUSs (of less than 6 months’ duration) were 5) multisomatoform (MSD); and 6) somatoform disorder, not otherwise specified (SDnos) (shown in Table 1). Because of the differences in classification between ICD–10 and DSM–IV, conversion disorder was not included in the analyses of this article. ICD–10 anxiety and depressive disorders prevalent in the last 12 months were analyzed by the CIDI’s computerized diagnostic algorithm program, and ICD–10 scores and the ICD–10 codes included in the subsequent data analyses are given in the legend of Table 3.


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TABLE 1. ICD–10 and DSM–IV Diagnostic Criteria by Number of Clinically Significant MUSs, Onset and Duration for Subtypes, and Groups of SDs Examined in the Study




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TABLE 3. Overlap Between SD Subtypes and Groups, Percent Overlap, and Prevalence of Diagnoses (Not Weighted) in 1990 and 2001 (N=421)



Statistical Analyses
The degree of statistical association was tested using both the tetrachoric correlation coefficient (r*) and Cohen’s kappa ({kappa}) for dichotomous variables and diagnostic groups. Cohen’s {kappa} was chosen in order to approximate the intraclass correlation. However, {kappa} is influenced by trait prevalence (distribution or base rates); therefore the tetrachoric correlation (r*), which assumes that the underlying diagnostic trait is continuous and normally distributed, was chosen. The tetrachoric coefficient thus allows for comparisons of the level of overlap (strength of association) with and between diagnoses/disorder subtypes, which are not confounded by their respective base rates.

Cross-tabulation of diagnoses and diagnostic groups in 1990 with the same diagnostic variables in 2001 was undertaken in order to measure stability of diagnoses over time by prospective and retrospective consistency. Prospective consistency represents the percent of respondents with diagnoses in 1990 having the same diagnoses again in 2001, and retrospective consistency represents the percent of respondents with diagnoses in 2001 having had the same diagnoses in 1990. Also, the likelihood of having the same diagnoses again in 2001 was estimated by calculating the odds ratio (OR). Statistical significance was tested by using a 95% confidence interval (CI) and p values. The level of significance was set at p<0.05 and will be reported as either p<0.05, p<0.01, p<0.001, or not significant (NS).

Two logistic-regression analyses were undertaken, using 1) any "lifetime" SD diagnosis in 2001; and 2) any "current" SDs in 2001 as dependent variables (see Table 1 for groups). The same predictor variables from 1990 were entered on both occasions, except for "lifetime" 1990 SDs in Analysis 1, and "current" 1990 SDs in Analysis 2). Predictor variables were all dichotomous, except for age and physical disease, which were continuous. Physical illness was defined as a count of the number of physical diseases reported by the respondent, according to a 15-item, precoded physical disease list (see note to Table 5). Dichotomous variables were gender, anxiety, depression, and "lifetime" SDs or "current" SDs, respectively. Statistical interactions between all pairs of independent variables in the multiple-regression analyses were tested separately, one pair at a time.


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TABLE 5. Stability of SD Subtypes and Groups of SDs, Anxiety, and Depression By Prospective and Retrospective Consistency (N=421)



The statistical software Scientific Software International (SSI; LISREL 8.7) program was used to estimate the tetrachoric correlation coefficient46 for binary ratings. For all other statistical data analyses, the SPSS program (SPSS, Inc., Chicago, IL), Version 12.0 for Windows was used.

Ethical Considerations
The Oslo, Norway-Lofoten follow-up study was approved by the Norwegian Data Inspectorate and the Norwegian Social Science Data Services and was conducted in accordance with the guidelines of the Helsinki, Finland, II Declaration.


  RESULTS

 
TOP
 ABSTRACT
 INTRODUCTION
 METHOD
 RESULTS
DISCUSSION
 REFERENCES
 
Mean age, in years (standard deviation) of follow-up respondents was 42.9 (14.4) in 1990 and 53.9 (14.3) in 2001. The mean number of physical diseases suffered, out of 15 (see Table 2) was 2.2 (1.7) in 1990 and 2.1 (1.7) in 2001.


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TABLE 2. 1990 Predictors of 1) Any "Lifetime" and 2) Any "Current" SD in 2001 (N=421), Logistic-Regression Analyses, Zero-Order (Bivariate) and Controlled OR and 95% CI



The degree of overlap between SD subtypes, as measure by tetrachoric correlations (r*), was generally very high (Table 3). For some categories, such as chronic pain disorder (PD) and somatoform, undifferentiated (SDud), the overlap was 100% (r*=1.0) in both 1990 and 2001. In fact, clear distinctions between subtypes were very hard to make, except for a slight distinction revealed when SDs were grouped into "current" and "lifetime" categories (as specified in Table 1).

The comorbidity of "lifetime" and "current" SDs with depression and anxiety, at both individual time-points, are given in Table 4. The association between SDs and depression ranged from OR: 2.9 to 5.1 (r*=0.31 to 0.46) and is slightly stronger than that between SDs and anxiety (OR: 2.0 to 2.5, and r*=0.25 to 0.32), with comorbidity between depression/anxiety and "current" SDs being somewhat more pronounced than for "lifetime" SDs.


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TABLE 4. Overlap Between Groups of SDs, Anxiety, and Depressive Disorders, At Each Time-Point, 1990 and 2001, Respectively (N=421)



The stability of diagnosis was measured by prospective and retrospective consistency, as presented in Table 5. For the SD subtypes, prospective consistency varied from 8.5% to 53.4%, and retrospective consistency from 12.5% to 43.0% (Table 5). The SSI–3/5 represented the lowest range in both cases, and the consistency for SSI–3/5 by {kappa} was not statistically significant. Although measurement of agreement by {kappa} for the remaining subtypes of SDs (SDud, PD, HC, SDnos, and MSD) were statistically significant (p<0.05), the best {kappa} values found were only around 0.2, representing just "fair" (not good) agreement. The highest {kappa} was found for the MSD subtype, {kappa}=0.211 (p<0.001; Table 5). Prospective and retrospective consistency were generally higher (up to 53.4% and 43.0%, respectively) for the one-symptom criteria SD subtypes (SDud, PD, HC, SDnos), as compared with the "≥3" SD subtypes (SSI–3/5, MSD; up to 32.9 and 34.8%, respectively). Interestingly, both prospective and retrospective consistency for the MSD "current" subtype was higher than in the SSI–3/5 "lifetime" subtype (for MSD: 32.9%–34.8%, and for SSI–3/5: 8.5%–12.5%). Prospective consistency for the "lifetime" SD group was 53.3%, the highest, but measurement of agreement by {kappa} showed only 0.146 (p<0.01), as compared with 0.214 (p<0.001) for "current" SDs. In fact, retrospective consistency was 42.1% for "current" SDs, higher than for "lifetime" SDs (33.1%).

Change over time to other SD subtypes (percentage within baseline N) ranged from 1.9% (SDud) to 53.2% (SSI–3/5), as illustrated in Figure 1. Likewise, the change to no-SD in 2001, ranged from 33.3% (HC) to 45.6% (PD; Figure 1). The very low base rate for HC (N=6) must, however, be taken into account. Of those individuals who had an SD at baseline, 54% to 67% fulfilled the criteria for any SD at follow-up, and, for those individuals without SDs at baseline, 37% to 41% had an SD at follow-up (Figure 1). The percentage of respondents still having the same diagnosis at follow-up was highest for "lifetime" SDs in the younger age-group (18–49 years), at 65.1% for young women, versus only 33.3% for younger men (Figure 2). Likewise, for "current" SDs, the percentage of younger women still having the same diagnosis at follow-up was 48.5%, as compared with 23.5% for younger men (Figure 2).


Figure 1
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FIGURE 1.  Percentage Within Baseline N of Somatoform Disorder (SD) Subtype Cases (Yes) and Non-Cases (No), Either the Same, Changed to Other, or No SDs at 11-Year Follow-Up (N=421)

MSD: multisomatoform disorder; SDnos: somatoform disorder, not otherwise specified; SSI–3/5: (abridged) Somatoform Symptom Index; PD: pain disorder; HC: hypochondriasis; SDud: undifferentiated somatoform disorder.



Figure 2
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FIGURE 2.  Percentage of Younger (18–49 years) and Older (Over Age 50) Men and Women Respondents Still in the Same Diagnostic Group at the 11-Year Follow-Up (N=421)

SD: somatoform disorder.


Female gender and younger age emerged as strong markers for having both "lifetime" and "current" SDs in 2001 (Table 2). In the two regression analyses, differences appeared among the other independent variables entered. In the first analyses, when "lifetime" SDs was used as the dependent variable, only anxiety (p<0.05) in 1990 remained significant in the controlled multivariate analyses. In the second analyses, when "current" SD was used as dependent variable, depression (p<0.001), physical illness (p<0.05), and having "current" SDs (p<0.05) in 1990 remained significant in the controlled multivariate model. In the controlled model, the likelihood (OR) of having "current" SDs in 2001 if depressed in 1990 was 3.5 higher than if not depressed. Likewise, if afflicted with physical disease in 1990, the OR for having any "current" SD, per physical disease, was 1.3 (p<0.01). There was no statistical interaction between pairs of independent variables in the multiple-regression analyses, when tested separately, one pair at a time.


  DISCUSSION

 
TOP
 ABSTRACT
 INTRODUCTION
 METHOD
 RESULTS
 DISCUSSION
REFERENCES
 
The main findings of this study are the following: 1) There was a substantial overlap of SD subtypes; 2) There was poor stability over time, but the highest stability was seen for SDs when grouped as "current," as compared with "lifetime;" 3) There was considerable comorbidity, with depression at baseline being a stronger predictor of "current" SDs at follow-up than was having "current" SDs at baseline.

Overlap between the individual SD subtypes is so extensive that similarities, as pointed out previously,23,24 outweigh the differences between them. This gives support to the prospect of grouping SDs into fewer diagnostic entities, as suggested in the PSD measure.15 Although overlap between the individual SD subtypes is so extensive that differentiation, or finding boundaries, that is, "zones of rarity,"31 seems impossible, it is possible to differentiate between the "current" and "lifetime" groups, when considering the observed differences in comorbidity patterns with anxiety and depression, and the differences in predictors in the two multivariate analyses.

Generally, stability of SD subtypes measured by prospective consistency was higher than that of retrospective consistency. A main finding in this research is the higher stability of the "current" SDs than "lifetime" SDs grouping. Retrospective consistency for "current" SDs was nearly the same (42%) as the previously overall 48% stability for the whole SDs group on the basis of lifetime symptoms, as reported by Lieb and colleagues.30 Even so, this stability of SDs is low, reflecting not so much the fact that the CIDI instrument is inconsistent in detecting these disorders from baseline to follow-up,3,4 but that the course of these disorders is remittent rather than chronic in nature over time. This view can be contrasted with that applied by the DSM and ICD criteria,1,2 where the attitude is taken that, once an SD has occurred, then it is debilitating for a lifetime. The stronger consistency of "current" SDs than "lifetime" SDs clearly demonstrates a superior utility31 of current symptom criteria for SD diagnoses.

Comorbidity with depression was considerable. Our finding of a fourfold-to-fivefold greater likelihood of having "current" SDs with depression is somewhat higher than the earlier reported threefold increased risk of coexistent anxiety/depression.28 Considering also the association with depression over time, and the fact that depression in 1990 emerged as a strong predictor for "current" SD in 2001, the prognostic relevance of this comorbidity is yet again underlined.29

A further reinforcement of the "current" and "lifetime" SD diagnostic differentiation in having "zones of rarity"31 is reflected in the differential strength of predictors. Depression, physical illness, and the presence of any "current" SDs in 1990 were significant (p<0.05) predictors for any "current" SDs in 2001; whereas only anxiety in 1990 was a significant predictor for any "lifetime" SDs in 2001. Gender differences in the reporting of MUS and somatization have previously been well established,3,47,48 with women tending to be more stable or prone to chronicity than men.30 Female gender and younger age are found as a significant predictors of both "current" and "lifetime" SDs, similar to the finding of increased prevalence of SDs among young female internal-medicine inpatients.26 Confounding variables, observer bias, or other variable interactions, and recall variability of symptoms over time,3,4 including the use of symptom lists containing more symptoms for women than men, could all be factors influencing these gender and age differences.

The future status of SDs as either discrete mental or physical disorders is undergoing critical debate.15–23 In the light of this research, we can lend support to moving forward in the direction of "current" criteria, whatever the future nosological placement or terminology of the SDs might be.15,21,22 Also, a provision should be made for chronicity and for special attention to be focused on young women, who are at risk for having debilitating symptoms over a long time.

It is clear that the "current" SDs concept, as confined by the MSD criteria14 and further upheld in the PSD,15 is a step toward more reliable diagnostic entities. Although one might fear a broadening of the somatoform diagnostic gateway from basing disorder core criteria on one or more current MUSs,15 rather than lifetime MUSs, this does not seem to be the case at all. In fact, diagnostic precision is gained by the change from "lifetime" to "current" symptom criteria. The fear that diagnoses based only on current criteria might leave out a number of patients who often report MUSs but who fail to meet current occurrence criteria at a given point in time is not supported by our data. Although the wide "at least one or more symptom" count is retained in the PSD,15 lowering the precision, we keep the advantage of currency. Also, provision is made for severe, debilitating, single-symptom conditions often seen by medical specialties other than psychiatry.13

The strengths of this study are the sample from the general population, the prospective longitudinal design over a long period of time (11 years), and the high response rate (70%). Only data pertaining to the same persons interviewed at both time-points are presented in this article. There are very few long-term studies in the field of SDs, and, to our knowledge, the topic concerning "lifetime" and "current" criteria in describing the course and prognosis of present ICD–10 and DSM–IV SDs is unique. However, the whole follow-up sample is not very large (N=421), and a limitation does pertain to the low base rates of some SDs subtypes, such as HC. Problems of SDs are many, and a next important issue of diagnostic concern is that of attribution of symptoms.

In conclusion, there is a substantial overlap between SDs, a considerable comorbidity with anxiety and depression, and poor stability over time, but a greater consistency with "current" SDs than with "lifetime" SDs. Future diagnostic revisions should therefore combine existing SD subtypes and base the criteria on current symptoms.


  ACKNOWLEDGMENTS

 
The authors thank all the generous lay interviewers in 1990 and 2001, and the researchers T. Sørensen, J. Nygård, W. Platou, and O. Klungsøyr, who collaborated in the Oslo, Norway-Lofoten study.

This article has been inspired by the stimulating roundtable discussions of the CISSD (Conceptual Issues in Somatoform and Similar Disorders) Project, which met in Oxford, England, in March 2006. The general-population Oslo, Norway-Lofoten study and follow-up research has been made possible by grant funding from the Research Council of Norway, The Norwegian Council for Mental Health, Norwegian Women’s Public Health Association, Henrik Homans Minde Legacy, Dr. Trygve Gythfeldt and Wife’s Foundation, Josef and Haldis Andresen Legacy, Maja and Jonn Nilsen Legacy, Per Risteigen Legacy, and the Solveig and Johan P. Sommer’s Foundation.


  REFERENCES

 
TOP
 ABSTRACT
 INTRODUCTION
 METHOD
 RESULTS
 DISCUSSION
 REFERENCES
 

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