Journal of Psychosomatic Res

The relationship between chronic fatigue and somatization syndrome:

A general population survey

Alexandra Martina,4, Trudie Chalderb, Winfried Rief a, Elmar Braehlerc

aSection for Clinical Psychology and Psychotherapy, Philipps University, Marburg, GermanybAcademic Department of Psychological Medicine, Weston Education Center, King’s College London, Strand, London, United Kingdom

cMedical Faculty, University of Leipzig, Leipzig, Germany

Received 26 June 2006; received in revised form 14 May 2007; accepted 15 May 2007

Abstract

Objective: The objective of this study was to assess the

prevalence of chronic fatigue (CF) and its association with

somatization syndrome [Somatization Syndrome Index (SSI) 4/6:

z4 somatoform symptoms in men, 6 in women] in the general

population. Methods: A representative sample of the German

population (N=2412) completed a fatigue questionnaire and a

screening instrument for current somatoform symptoms (Screening

for Somatoform Symptoms 7). Results: The prevalence rate of

CF was 6.1% (n=147). Females were affected significantly more

often as compared with males (7% vs. 5.1%). The mean number

of somatoform symptoms was higher in CF cases than in

control subjects without CF (11 vs. 2; Pb.001). Seventy-two

0022-3999/07/$ – see front matter D 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.jpsychores.2007.05.007

4 Corresponding author. Section for Clinical Psychology and Psycho-

therapy, Philipps University, Gutenbergstr. 18, D-35032 Marburg, Ger-

many. Tel.: +49 6421 282 3656; fax: +49 6421 2828904.

E-mail addresses: martin@staff.uni-marburg.de (A. Martin)8

sphatrc@iop.kcl.ac.uk (T. Chalder)8 rief@staff.uni-marburg.de (W. Rief)8

elmar.braehler@medizin.uni-leipzig.de (E. Braehler).

percent of the subjects with CF fulfilled the SSI4/6 criterion

for somatization syndrome. Quality of life (EUROHIS-QOL and

8-item Short-Form Health Survey) and well-being (5-item

WHO Well-Being Index) were markedly decreased in CF and

SSI4/6. The results of regression analyses suggest that fatigue

and somatization severity had a similar impact on quality of

life. Conclusions: The results suggest that CF is relevant in the

general population. Its substantial overlap with somatization

syndrome supports the hypothesis that the two syndromes

are only partially different manifestations of the same under-

lying processes.

D 2007 Elsevier Inc. All rights reserved.

Keywords: Chronic fatigue; Epidemiology; Prevalence; Somatoform disorder; Somatization syndrome

Introduction

Chronic fatigue (CF) is characterized by enduring

fatigue, tiredness, lack of energy, and weakness resulting

in severe impairment in daily functioning. Prevalence

rates of CF suggest that the syndrome is important

especially in clinical settings [1–3] but also in the general

population [4–9].

Chronic fatigue is not a specific illness, but it is occurring

with a wide range of somatic conditions or mental disorders.

There is still no commonly accepted definition of CF;

generally, an explicit assumption is not made about the

existence or nonexistence of medical conditions that could

explain the fatigue. Some recent evidence suggested that it

was not possible to identify an organic condition to explain

the fatigue in most subjects with CF [10]. In this study, we

assumed that CF is a syndrome defined as a set of symptoms

indicating the existence of a problem. Chronic fatigue

syndrome (CFS) is defined as unexplained fatigue lasting

for 6 or more months [refer to the U.S. Centers for Disease

Control and Prevention (CDC) [11] for further criteria]. It

has been suggested that the CDC’s definition of CFS is

too restrictive and that some classification criteria seem to

be arbitrary [12]. Some evidence suggest that fatiguing

earch 63 (2007) 147–156

Table 1

Description of the study sample collectively and separately for CF cases and control subjects

Study sample

(N=2412, 100%)

CF cases

(n=147, 6.1%)

Control subjects

(n=2265, 93.9%) Statistics

Sex [n (%)]

Male 1147 (47.5) 58 (39.5) 1087 (48.0) v2(1)=4.04

Female 1267 (52.5) 89 (60.5) 1178 (52.0)

Age [years, mean (S.D.)] 47.9 (18.2) 57.9 (17.4) 47.3 (19.0) t(2410)=�7.0444d=0.58 (95% CI=0.4–0.75)

Age group [years, n (%)]

b35 638 (26.5) 19 (12.9) 619 (27.3) v2(2)=45.444

35–64 1230 (51) 63 (42.9) 1167 (51.5)

N64 544 (22.5) 65 (44.2) 479 (21.1)

Family status [n (%)]

Single 590 (24.5) 21 (14.3) 569 (25.1) v2(2)=20.0444

Married/Living together 1243 (51.5) 70 (47.6) 1173 (51.8)

Separated/Divorced/Widowed 579 (24) 56 (23.1) 523 (38.1)

Education [n (%)]

Secondary school,b10 years 1127 (46.7) 86 (58.5) 1041 (46) v2(3)=12.144

Intermediate school leaving

certificate, b12 years

874 (36.3) 47 (32) 827 (36.5)

High school or higher 350 (14.5) 14 (9.5) 336 (14.8)

Still in school 61 (2.5) 0 (0.0) 61 (2.7)

Family income per month

[Euro, n (%)]

b750 105 (4.6) 13 (9.0) 92 (4.3) v2(3)=15.744

750–1249 515 (22.3) 44 (30.3) 471 (21.8)

1250–1999 901 (39.1) 53 (36.6) 848 (39.3)

z2000 784 (34) 35 (24.1) 749 (34.7)

Somatization syndrome [n (%)]

Fulfilled/Yes 392 (16.3) 106 (72.1) 286 (12.6) v2(1)=358.4444

95% CI=14.8%–17.7%

Somatoform symptom count

(SOMS-7) 0–53 [mean (S.D.)]

2.6 (5.1) 10.6 (8.0) 2.1 (4.4) t(151.9)=12.8444

d=1.35 (95% CI=1.2–1.5)

SOM [mean (S.D.)] 0.21 (0.31) 0.69 (0.27) 0.18 (0.45) t(152.7)=13.6444

d=1.37 (95% CI=1.2–1.6)

4 CF cases vs. control subjects, Pb.05.

44 CF cases vs. control subjects, Pb.01.

444 CF cases vs. control subjects, Pb.001.

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156148

illnesses can be at best viewed on a continuum with CF

at the one end and CFS at the more severe end of the

spectrum [13,14].

Aside from these, CFS as a discrete nosological entity

has been questioned [15,16]. A number of studies have

shown that CFS often co-occurs with other medically

unexplained bodily syndromes (e.g., fibromyalgia, irritable

bowel syndrome, and multiple chemical sensitivity) [17].

There is also a body of evidence showing the associations

among CF as well as CFS and somatization disorder (SD) as

well as subthreshold somatization syndromes [18,19].

Somatization disorder, according to the Diagnostic and

Statistical Manual of Mental Disorders, Fourth Edition and

the International Statistical Classification of Diseases, 10th

Revision (ICD-10), is not defined by a specific target

symptom but is characterized by a history of multiple

disabling somatic symptoms that cannot be fully explained

by any medical factor. Somatization disorder is rarely found

in the community, and the diagnostic criteria have also been

considered to be too restrictive. Escobar et al. [20]

suggested the concept of an abridged SD—the Somatization

Syndrome Index (SSI4/6)—and showed that this syndrome

was associated with similar risk factors, service use patterns,

and disabilities as found in the full SD. The condition is

characterized by four medically unexplained symptoms in

men and six in women without the SD’s restrictions about

symptom distribution across body sites and long timeline

criteria. In research, especially in epidemiological studies,

this somatization concept is widely accepted [21].

Wessely et al. [15] argued that in addition to a substantial

overlap between functional somatic syndromes and case

definitions, there are also similarities in patient character-

istics and treatment approaches. Because the similarities in

functional syndromes outweigh the differences, it has been

suggested that they are different manifestations of the same

biomedical and psychosocial processes [15,16].

Current knowledge about CF is mainly derived from

treated populations and may be biased (e.g., by the

differential help-seeking behavior linked with sex and

socioeconomic status) [22,23]. The co-occurrence rates of

various medically unexplained syndromes may also be an

overestimation, particularly if assessed in clinical settings

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156 149

[24]. To date, it is unclear as to how CF and somatization

syndrome overlap and as to what extent the two syndromes

have a differential effect on quality of life.

We therefore assessed CF in a representative general

population sample. We decided to investigate CF as a

broader category (without evaluation of possible organic

factors that might explain the fatigue) to preclude the

possible impact of too many restrictions made by the criteria

(such as in CFS). The first aim of the study was to assess the

point prevalence of CF and its sociodemographic character-

istics in the German general population. The second aim

was to examine the overlap of CF and somatization

syndrome SSI4/6 with respect to symptomatology. Third,

we aimed to examine the associations between CF and

somatization syndrome with various indicators of quality of

life because the latter factor can be considered to be an

important indicator of functioning.

Methods

Design and subjects

A cross-sectional survey of a random general population

sample of Germany was conducted based on face-to-face

contacts using questionnaires and with the assistance of an

independent demography consulting company (USUMA,

Berlin, Germany). The random selection was based on

multistage sampling with three stages (according to the

typical random selection procedure in national surveys in

Germany). First, 258 sample point regions were randomly

drawn from the last political election register, covering rural

and urban areas from all regions in Germany. The second

stage was a random selection of households using the

random route procedure (based on a starting address). The

third stage was a random selection of household respondents

with the Kish selection grid. The inclusion criterion was

being aged 14 years or older. Data collection took place

between September and October 2004.

The initial sample consisted of 4156 subjects, of whom

62.3% participated (n=2591). Reasons for dropout included

the following: three unsuccessful attempts to contact the

household or selected household member (12.6%); the

household or selected household member disagreed to

participate (23.6); and the household member was on a

holiday break or absent as a result of an illness (1.4%).

Some of the information could not be used because of

incomplete data collection during the interview (n=39,

0.9%) or self-report when completing the Fatigue Ques-

tionnaire (FQ; n=140, 5.5%). The final sample consisted of

2412 subjects (52.5% were females). The patients’ ages

ranged from 14 to 99 years (mean=47.9, S.D.=18.2);

approximately 52% were married (or living together with

a partner), 51% had 10 or more years of education (e.g.,

high school or college degree), and 27% were living in

households earning less than 1250 Euro per month (see

Table 1 for study sample characteristics). The sample had

the typical characteristics of the German population in terms

of sex, age, and level of education (see the 2004 population

report of the Federal Statistical Office of Germany, http://

www.destatis.de/themen/e/thm_bevoelk.htm: 51.4% were

females, 22% were older than 64 years, and 48.8% had

secondary school as their highest level).

All participants were visited personally, were informed

about the study procedures, and signed an informed consent

form (in minors, informed consent was obtained from the

parents). They were instructed that several rating scales

would follow, without informing them about the special

focus on CF and somatoform symptoms. Thereafter,

subjects completed the following self-rating scales:

– a demographic information sheet;

– the FQ to identify CF cases and noncases [25];

– a screening questionnaire on somatoform symptoms

[Screening for Somatoform Symptoms 7 (SOMS-7)]

[26] to assess somatization syndrome SSI4/6; and

– some rating scales to assess different aspects of

quality of life and well-being: EUROHIS-QOL [27],

8-item Short-Form Health Survey (SF-8) [28], and

5-item WHO Well-Being Index (WHO-5) [29].

Assessment instruments and case identification

The German version of the FQ presented by Chalder et al.

[25] was used. The FQ is a self-report questionnaire

inquiring about the various physical and mental fatigue

symptoms specifically related to CF. The FQ has been tested

in large community samples [8,22]. The internal consistency

of the German FQ was high (Cronbach’s a=.93), and the

factor structure of the original version was replicated [30].

The FQ consists of 11 items rated as less than usual, no

more than usual, more than usual, or much more than

usual—offering the option of binary (0 0 1 1) or Likert

(0 1 2 3) scoring. In the present study, we used the binary

scoring for CF case identification (score range=0–11)

according to Chalder et al. [25]: A score of 4 or higher

represents severe fatigue, whereas that of 4 or higher for 6 or

more months indicates caseness for CF. The FQ total score

(FQ-T) is the sum of the Likert-scored items (range=0–33)

and indicates severity of the fatigue symptoms.

The state version of the SOMS-7 [26,31] was used to

identify the number and severity of current somatoform

symptoms. It lists 53 bodily symptoms, covering all

somatoform symptoms mentioned in the Diagnostic and

Statistical Manual of Mental Disorders, Fourth Edition and

ICD-10 SD as well as somatoform autonomic dysfunction.

Subjects were asked to indicate the symptoms that were

present during the past 7 days and for which physicians had

not been able to find a clear organic cause. The intensity of

symptoms was Likert scaled from 0 (not at all) to 4 (very

strongly). The scale has high internal consistency (a=.92)and is valid in discriminating SD, somatization syndrome

Table 2

Distribution of prevalence rates of CF and somatization syndrome in the general population (N=2412)

CF cases (n=147, 6.1%) SSI4/6 cases (n=392, 16.3%)

n (%) Odds ratio (95% CI) n (%) Odds ratio (95% CI)

Sex

Male 58 (5.1) 1 191 (16.7) 1

Female 89 (7.0) 1.42 (1.01–1.99) 201 (15.9) 0.94 (0.76–1.17)

Age group (years)

b35 19 (3.0) 1 55 (8.6) 1

35–64 63 (5.1) 1.76 (1.04–2.97) 181 (14.7) 1.83 (1.33–2.5)

N64 65 (11.9) 2.10 (1.61–2.73) 156 (28.7) 2.07 (1.75–2.44)

Family status

Single 21 (3.6) 1 69 (11.7) 1

Married/Living together 70 (5.6) 1.62 (0.98–2.66) 188 (15.1) 1.34 (0.99–1.80)

Separated/Divorced/Widowed 56 (9.7) 1.70 (1.32–2.20) 135 (23.3) 1.51 (1.30–1.77)

Education

Secondary general school certificatea 86 (7.6) 1 229 (20.4) 1

Intermediate school leaving certificateb 47 (5.4) 0.69 (0.48–0.99) 121 (13.8) 0.63 (0.49–0.80)

High school or higher 14 (4.0) 0.71 (0.53–0.95) 36 (10.3) 0.67 (0.56–0.81)

Still in school 0 (0.0) – 6 (9.8) 0.75 (0.57–1.0)

Family income per month (Euro)

b750 13 (12.4) 1 30 (28.6) 1

750–1249 44 (8.5) 0.66 (0.34–1.28) 133 (25.9) 0.87 (0.55–1.39)

1250 –1999 53 (5.9) 0.67 (0.48–0.92) 132 (14.7) 0.66 (0.52–0.83)

z2000 35 (4.6) 0.69 (0.55–0.87) 75 (10.8) 0.67 (0.57–0.79)

a Secondary general school certificate or lower, b10 years.b Intermediate school leaving certificate, b12 years.

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156150

according to Escobar et al. [20], and other mental disorders

[26]. The SOMS-7 has shown a good association with

somatoform symptom characteristics assessed by structured

clinical diagnostic interviews (e.g., r=.68 for symptom

count/r=.70 for symptom severity) [26]. In the present

study, we used the following composite indices:

Somatoform symptom count, which represents the

number of symptoms of at least moderate disability,

excluding symptoms that might overlap with the FQ

items (namely, Item 30 on bexcessive tiredness upon

mild exertion,Q Item 35 on blocalized weakness,Q and

Item 46 on bloss of memoryQ). An SSI4/6 case definition

required a somatoform symptom count of 4 or higher in

men and that of 6 or higher in women.

Somatoform symptom severity (SOM), which is the mean

score of all responses (range=0–4), with higher scores

representing higher severity.

The 8-item EUROHIS-QOL index was developed as a

brief adaptation of the WHOQOL-100 and the WHOQOL-

BREF [27,32] to measure trans-sectoral quality of life,

covering psychological, physical, social, and environmental

aspects. The EUROHIS-QOL showed good cross-cultural

field study performance, good internal consistency, as well

as satisfactory convergent and discriminant validity and has

been recommended for use in public health research [32,33].

A universal one-factor structure with a good fit in structural

equation modeling analyses was identified [32]. The items

were Likert scored (1–5); the total score varies from 8 to 40,

with higher values indicating better quality of life.

The SF-8 [28] uses one question to measure each of the

eight SF-36 Health Survey domains of health-related quality

of life during the past 4 weeks [34,35]: general health (GH);

bodily pain (BP); physical functioning (PF); physical role

functioning (RP); vitality (VT); social functioning; mental

health (MH); and emotional role functioning (RE). The SF-8

provided results that are comparable with those of the SF-36

[36]. Subscale scores vary from 1 to 6 for GH and BP and

from 1 to 5 for PF, RP, VT, social functioning, MH, and RE,

with higher scores indicating better health.

The WHO-5 [29,37], a screening instrument for the

detection of depression in the general population, has good

internal and external validity [38]. The present study used

the WHO-5 Version II, which has five Likert-scored items

(0–5), with a possible sum score range of 0–25 and lower

values indicating less well-being during the past 2 weeks.

Statistical analyses

To assess differences between CF cases and noncases, we

compared mean scores of demographic and clinical varia-

bles with the use of t tests; v2 analysis was used for

categorical variables. We used two-factorial analyses of

variance (ANOVAs) with two between-subjects factors (CF

and SSI4/6) to assess their effects on dependent variables.

Effect sizes (Cohen’s d or partial g2) were computed. We

computed sequential multiple linear regressions to predict

aspects of quality of life (EUROHIS-QOL-8 and SF-8) and

well-being (WHO-5) by the dimensional scores of fatigue

and somatization, controlling for the effect of socioeco-

nomic variables. All tests were two tailed, with the a value

Fig. 1. Overlap of cases with CF and somatization syndrome.

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156 151

set at .05. All analyses were conducted with the SPSS

Version 12.0 (Chicago, IL, USA) statistical package.

Results

Prevalence rates of CF in the general population

Severe fatigue (FQ symptom score of 4 or higher) was

reported by a total of 342 subjects [14.2%, 95% confidence

interval (CI)=12.8–15.6]. Of these, 193 subjects reported the

onset of their fatigue to be less than 6 months ago (shorter

fatigue at b3 months: n=146, 6%, 95% CI=5.1–7.0;

prolonged fatigue at 3–6 months: n=46, 1.9%, 95%

CI=1.4–2.6%). One hundred forty-seven subjects reported

that their fatigue lasted for at least 6 months and were

diagnosed as having CF. Thus, the prevalence rate of CF

Fig. 2. Pattern of somatoform symptoms in subjects with CF (n=147), subjects wi

(no CF, n=2265). The graph shows the symptoms that were reported by at least

cases was 6.1% (95% CI=5.1–7.1) in the German general

population (N=2412). Most CF cases reported that they

were tired at least half of the time (n=114/147, 80.3%).

Table 2 presents the point prevalence rates of CF for

socioeconomic subgroups and shows that females were

affected significantly more as compared with males (7% vs.

5.1%). The prevalence of CF increased with age: only 3% of

the subjects younger than 35 years reported CF, in contrast

to almost 12% of the population aged 65 years or older. The

prevalence rates differed with family status, with the highest

rates in the group of divorced or widowed subjects.

Prevalence rates were two times higher in the group with

a low family income than those in the groups with a

monthly income of at least 1250 Euro.

Socioeconomic characteristics of persons with CF as

compared with control subjects

Chronic fatigue cases were compared with the subjects

who did not meet the CF criteria [control group (CG)] on

socioeconomic and clinical characteristics (Table 1). The

two groups differed significantly with respect to all

variables. The CF group subjects were more likely to be

female as well as older and less likely to be single. The CF

group also had lower educational degrees and lower income

as compared with the CG.

Chronic fatigue and somatoform symptoms

The point prevalence rate of somatization syndrome SSI4/

6 was 16.3% in the general population sample (Table 1). The

distribution of somatization syndrome across the socio-

th somatization syndrome (SSI4/6, n=392), and control subjects without CF

30% of the subjects with CF.

Table 3

Quality of life and well-being by CF and SSI4/6 caseness (results of ANOVAs)

With CF/With

SSI4/6 (n=106)

With CF/Without

SSI4/6 (n=41)

Without CF/With

SSI4/6 (n=286)

Without CF/Without

SSI4/6 (n=1977)

Test statistics

Factor 1: CF Factor 2: SSI4/6 CF�SSIMean S.D. Mean S.D. Mean S.D. Mean S.D. F Partial g2 F Partial g2 F Partial g2

EUROHIS-QOLa 23.0 4.7 26.9 4.4 26.9 4.5 31.9 4.0 124.4444 .049 121.3444 .048 2.3 .001

SF-8 scalesb

GH 2.7 0.9 3.3 0.9 3.5 0.8 4.4 0.9 132.1444 .053 86.1444 .035 3.0 .001

BP 3.2 1.4 3.7 1.3 3.8 1.1 5.2 1.0 98.7444 .040 87.3444 .036 20.3444 .009

PF 2.7 1.0 3.2 0.9 3.3 0.9 4.4 0.9 101.8444 .041 89.0444 .036 7.544 .003

RP 2.9 1.0 3.4 0.9 3.5 0.9 4.5 0.8 122.8444 .049 92.6444 .038 8.544 .004

VT 2.3 0.7 2.7 0.6 3.1 0.7 3.9 0.8 159.8444 .063 63.9444 .026 5.74 .002

Social functioning 3.1 1.1 3.6 1.1 3.7 1.0 4.6 0.8 98.6444 .040 75.3444 .031 3.9 .002

MH 3.0 1.2 3.3 1.1 3.7 1.0 4.6 0.7 187.6444 .074 69.1444 .028 12.2444 .005

RE 2.9 1.2 3.9 1.0 3.8 1.0 4.7 0.7 129.2444 .052 160.8444 .064 0.5 .000

WHO-5 well-beingc 8.9 5.3 11.9 4.7 14.4 5.0 18.6 4.2 208.9444 .080 70.2444 .028 1.9 .001

a df for F=1, 2404.b df for F=1, 2363.c df for F=1, 2399.

4 Overall comparison, Pb.05.

44 Overall comparison, Pb.01.

444 Overall comparison, Pb.001.

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156152

economic subgroups was highly comparable with the dis-

tribution pattern of CF (see Table 2 for the odds ratios). The

only difference appeared to be the sex distribution.

More than two thirds of the CF subjects fulfilled the

SSI4/6 criterion (n=106/147, 72.1%), reflecting a high

comorbidity with multiple somatoform symptoms. Only

12.6% of the control subjects fulfilled the SSI4/6 criterion;

the group difference is highly significant [v2(1)=358.4,

bivariate association Phi=0.39). The Venn diagram in Fig. 1

shows the relationship of CF and SSI4/6 caseness.

The number of concurrent somatoform symptoms (11 in

CF group vs. 2 in the CG) and the mean SOM were

increased in the CF group; the effect sizes (d=1.35–1.37)

suggest large effects.

The prevalence of single somatoform symptoms was

significantly higher in the CF group than in the CG for 47 of

53 bodily symptoms [v2(1)=5.3–270.4] (not significant forloss of consciousness as well as 3 symptoms related to

menstruation, unpleasant sensations in/around the genitalia,

and pain during intercourse). The most frequent symptoms

were headache (48%) and pain in the back (69%), joints

(66%), and arms or legs (63%). Chronic fatigue subjects

also frequently reported cardiovascular complaints (e.g.,

heart palpitations in 30%, pressure in the heart region in

25%, sweating in 25%, and breathlessness without exertion

in 32%), gastrointestinal complaints (e.g., food intolerance

in 30%, abdominal pain in 30%, loss of appetite in 30%, dry

mouth in 35%, frequent urination in 35%, and bloating in

22%), sexual symptoms (e.g., sexual indifference in 44%

and erectile/ejaculatory dysfunction in 35%), and pseudo-

neurological symptoms (e.g., impaired coordination/balance

1 Data on the prevalence rates for each symptom can be obtained from

the authors.

in 29%, lump in throat in 13%, and unpleasant numbness/

tingling sensations in 22%).

Fig. 2 shows the current somatoform symptoms that were

present in at least 30% of the CF cases as well as the

prevalence of these symptoms in the CG and the SSI4/6

group. The symptom frequency and symptom pattern were

highly comparable between the CF and somatization

syndrome groups, with a bivariate association r of .97

(Pb.01). The symptom patterns in subjects fulfilling the

criteria for both syndromes (i.e., CF and SSI4/6) were again

very similar between the CF and SSI4/6 groups.1

The symptom frequency was much higher in the CF

group as compared with the CG. However, the symptom

distribution pattern was comparable; the bivariate associa-

Fig. 3. Effects of the between-subjects factors CF and somatization

syndrome on quality of life (EUROHIS-QOL; mean and standard deviation

values are given by group). (See text for further explanation.)

Table 4

Sequential multiple regression to predict quality of life (EUROHIS-QOL)

Model Predictor variable b (Model 4)a DR2b R2

1 Sex �.005 – –

Age �.021 – –

Family status �.05944 – –

Education .028 – –

Income .2014 – .1454

2 FQ-T �.2684 .1644 .3104

3 SOM �.3644 .0824 .3924

4 FQ-T�SOMS-7 .037 .001 .3934

2V SOM �.3644 .1954 .3414

3V FQ-T �.2684 .0514 .3924

Sample population=2301. For further explanation, see text.a Standardized b weights of the final regression model.b Incremental.

4 Pb.001

** Family status is significant at Pb.01.

2 Data not reported but can be requested from the authors.

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156 153

tion r of the symptom patterns in the CF group and the CG

was .93 (Pb.01). Therefore, the patterns of bodily symp-

toms did not appear to be qualitatively different among the

CF group, SSI4/6 group, and CG and differed only in terms

of general symptom probability.

Quality of life and well-being

We analyzed the effects of CF and SSI4/6 caseness on

aspects of subjective quality of life and well-being

(ANOVAs with two between-subjects factors; see Table 3

for the mean scores by each factor value and test

statistics). The trans-sectoral quality of life (EUROHIS-

QOL) was markedly decreased in the CF and SSI4/6

groups and was the lowest for those subjects with CF and

SSI4/6. Mean scores did not differ between the CF-only

and SSI4/6-only conditions. The dotted lines in Fig. 3 are

almost parallel, illustrating that the two between-subjects

factors had an independent and additive effect on quality

of life, whereas the interaction of CF and SSI4/6 was

not significant.

A similar pattern of results was obtained for the SF-8

dimensions with the tool’s various aspects of health-related

quality of life. First, a multivariate ANOVA was computed

to control for inflation of Type I error. The main effects of

the two group factors were highly significant, suggesting the

health-related quality of life to be decreased in CF [�=0.89,F(8, 2356)=34.9, Pb.001] and SSI4/6 [�=0.92, F(8,

2356)=14.4, Pb.001]. There was an overall significant

CF�SSI4/6 caseness interaction for the SF-8 [�=0.98,F(8, 2356)=5.6, Pb.001]. Univariate ANOVAs showed that

the main effects for CF and SSI4/6 were significant across

all eight SF-8 dimensions; that is, health-related quality of

life was lower for those who fulfilled CF or SSI4/6 criteria,

very comparable in both conditions, and lowest in the case

of both syndromes. The interaction effects were significant

in five of the eight variables; however, the effect sizes were

very small for the interactions (partial g2=.000–.009).

The analysis of the WHO-5 scores also showed a

significant reduction of well-being in CF and somatization

syndrome. Again, the effects of CF and SSI4/6 were

additional, and the interaction of the two factors was

not significant.

The previous analyses compared the effects of CF and

somatization syndrome based on the case categories. We

conducted additional sequential multiple regressions using

the dimensional predictors FQ-T and SOM as well as

controlling for the impact of possibly influential socio-

demographic variables. Table 4 presents details of the

regression analysis predicting quality of life (EUROHIS-

QOL) as a criterion variable. Sociodemographic variables

were entered as the first predictors’ block, together explain-

ing 14.5% of the variance. The second block included the

FQ-T, resulting in 16.4% of incremental explained variance.

Somatoform symptom severity was entered as the third

block, adding 8.2% of the variance. The interaction of the

two latter variables was entered next, but this did not

contribute significantly to the total explained variance of

almost 40%. We then reanalyzed the multiple regression by

changing the order of the second and third blocks (i.e.,

entering SOM first and FQ-T only at the next block). Again,

both variables changed R2 significantly, but this time, with

the incremental R2 of SOM being .195 and the R2 of FQ-T

being .05 (Psb.001).

According to this procedure, multiple regression analyses

were also computed for the indicators of health-related

quality of life and well-being. Overall, the same pattern of

results was obtained.2 In summary, some of the sociodemo-

graphic variables, particularly family income and family

status, affected quality of life significantly. In addition,

fatigue severity and SOM significantly predicted quality of

life, namely in an additive way.

Discussion

Our study confirms high prevalence rates for CF and

somatization syndrome. Subjects were asked to report the

somatic symptoms for which physicians had not been able

to find a clear organic cause. The pattern of these self-

reported symptoms in CF cases was similar to the

distribution pattern in the general population, albeit symp-

tom prevalence rates were on a much higher level in CF.

Chronic fatigue and somatization syndrome were associated

with an almost comparable reduction in various aspects of

quality of life.

Prevalence of CF

The point prevalence of CF was high, with 6.1% of the

German population reporting significant fatigue lasting for

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156154

at least 6 months. To compare the prevalence estimation

with figures of previous studies, one needs to keep in mind

that the prevalence of self-reported fatigue varies widely,

depending on how it is defined [5]. The point prevalence

rates in general population samples have been consistently

higher for CF, ranging from 4.3% to 18.3% [7–9,39],

than for CFS-like syndromes, ranging from 1.8% to 3.6%

[4–6,9], and lowest for CFS, ranging from 0.037% to 0.42%

[22,40,41]. Reported prevalence rates also depend on how

they were assessed. Studies based on questionnaire assess-

ments have yielded higher prevalence rates as compared

with studies based on interviews [9]. Two former studies

used the same instrument (i.e., the FQ) to investigate CF in

community populations. Both studies showed a higher

prevalence of CF in Norway (11.4%, sample age

range=19–80 years) [7] and the United Kingdom (18.3%,

sample age range=18–45 years) [8]. Because these studies

did not include adolescents, we reanalyzed our data for

subjects aged 18 years or older, resulting in a CF prevalence

rate of 6.3% (147/2330 subjects). Considering only subjects

aged between 19 and 80 years (as in Reference [7]), we

again obtained a point prevalence rate of 6.1% (138 CF

cases among 2251 subjects). A direct comparison of this

study with the results of the former studies is nonetheless

difficult because of differences in methodology (e.g., survey

methods and sample characteristics).

Sociodemographic characteristics

We found the highest CF prevalence rates among

females, persons of older age, those who are widowed or

divorced, and those with a lower family income. There is

almost consistent evidence that women are more often

affected by CF as compared with men, both in clinical and

population-based samples [4,5,7–9]. Some previous studies

found a similar age effect, with more severe fatigue in older

age groups [7], and very low prevalence rates in people

younger than 18 years [9]. Based on the present study, it

cannot be ruled out that the increasing probability of

reporting CF in older age groups might be related to other

diseases. However, it remains unclear whether higher

comorbidity of medical conditions and CF in the elderly

population is based on a causal relationship or just a

coincidence. Our results further suggest an inverse relation-

ship of CF with socioeconomic status. This is consistent

with previous findings of an association with lower income

[9] and higher CFS levels in people with lower levels of

education and occupational status [7,22]. All together, these

findings do not support the assumption that CF or CFS is a

syndrome of primarily middle-class patients.

Relationship of CF and somatization syndrome

The results show a substantial overlap of CF and

somatization syndrome; almost three quarters of CF cases

also met SSI4/6 criteria (72%). This cannot be explained by

similarities of the syndrome criteria because we defined

SSI4/6 based on somatoform symptoms only, which do not

include the FQ items that we used to define CF. The CF

group reported many additional somatoform symptoms of at

least moderate intensity, and the comparison with the CG

revealed large effect sizes. The distribution pattern of the

symptoms in CF resembled the pattern in the SSI4/6 group

(r=.97). The latter condition was defined by the number of

medically unexplained symptoms, not special target symp-

toms. Similarities were also evident for the distribution

pattern in control subjects without CF (r=.93). This

relationship indicated that the very frequent symptoms in

CF were also the symptoms with the highest frequencies in

the general population, although absolute base rates

differed. Overall, these results suggest that the somatic

symptom pattern in CF is not specific.

For at least 30% of the CF cases, 14 somatoform

symptoms were prevalent. Highest frequencies were

reported for back pain, pain in arms or legs, joint

pain, and headache; these pain complaints are also

among the CDC criteria for CFS. However, symptoms

related to sexual, cardiovascular, and gastrointestinal

functions were among the most frequently reported

symptoms in CF. Recent studies have found comparable

results in CFS, showing a greater prevalence of some

atypical symptoms as compared with some of the typical

CFS symptoms [9,42,43].

In addition, we compared the impact of CF and SSI4/6

on measures of quality of life. Both clinical syndromes

were associated with a reduction in cross-sectoral and

health-related quality of life, as well as lower reports of

well-being. The effect sizes regarding the two syndromes

were shown to be substantial, whereas the interaction of

the two syndromes added little or no variance. Thus,

the effect is mainly additive, showing that subjects with

CF and multiple somatoform symptoms reported the

poorest quality of life and well-being. The results of

regression analyses suggest that fatigue and somatization

severity had a similar impact on quality-of-life outcomes,

even when controlling for the impact of sociodemo-

graphic variables.

Our results confirm previous findings that highlight the

need to consider the number and severity of somatoform

symptoms in subjects presenting with CF. Katon and Russo

[44] showed that the extent of impairment in daily activities

and the tendency to amplify symptoms increased linearly

with the number of lifetime physical symptoms that a

patient with CF experienced.

Limitations

Our findings are limited by difficulties associated with

studying fatigue and somatoform complaints in a represen-

tative general population survey. Both clinical syndromes

assessed were based on self-reports. The design of the

survey precluded an independent medical assessment to rule

A. Martin et al. / Journal of Psychosomatic Research 63 (2007) 147–156 155

out organic causation. The study’s classification of soma-

tization syndrome was based on the subjects’ recall of the

somatic symptoms for which a physician was unable to

identify a clear organic causation. Ideally, a physical origin

of the symptoms by medical investigations would be ruled

out to classify them as somatoform symptoms, although the

interrater reliability of doctors’ ratings about the medical

causality of symptoms is low [45]. At least, the SOMS is a

questionnaire with well-established psychometric proper-

ties, and its indices were shown to be closely associated

with data of diagnostic interviews [26].

The definition of CF cases used in this study was not

identical to other definitions used in the literature and

was not limited to medically unexplained CF. It is not

possible to generalize the results obtained to the group of

CFS patients. However, we intended not to be too restrictive

in our case definition, and some evidence suggest that the

difference between CFS and CF is more quantitative than

qualitative [13,14].

The cross-sectional nature of the study limited our ability

to determine whether the sociodemographic factors (e.g.,

lower family income) were actual risk factors, correlates, or

a result of CF presence and severity. Overall, the response

rate (62.3%) was satisfactory and similar to other GH

surveys [46]. However, this still leaves 37.7% of people for

whom data can only be estimated.

Conclusions and practical implications

The results suggest that CF is a relevant syndrome in the

general population. Its substantial overlap with somatoform

syndrome supports the hypothesis that the two syndromes

are only partially different manifestations of the same

underlying processes. These processes could be cognitive–

perceptual, psychophysiological reactivity, and/or related to

illness behavior [47].

Previous results showed that specialists tend to focus on

symptoms according to their area of specialization and tend

to ignore symptoms of other body sites [48,49]. As a result,

coexisting conditions may be overlooked and patients may

try to get assistance from several specialty clinics, with a

potential cumulative effect on health care costs, resulting in

the risks of overinvestigation and iatrogenesis [50]. The

presence of fatigue and that of somatic symptoms not only

impair quality of life and functional abilities but also affect

treatment response (multiple symptom syndromes may be

more difficult to treat as compared with single symptom

conditions) [51]. It is important that clinicians of all

specialties inquire about additional medically unexplained

bodily symptoms beyond a patient’s initial presenting

complaint. Our data show that this is particularly important

if patients present with fatigue. Furthermore, clinicians need

to offer a comprehensive treatment approach with the aim of

providing general coping strategies, rather than offering

interventions focused on a specific symptom.

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