Eva M. Kingma

Intelligence is negatively associated with the number of functional somatic symptoms

Background: Functional somatic symptoms (FSS), that is, symptoms that cannot be conclusively explained by organic pathology, have a poorly understood aetiology. Intelligence was studied as a risk factor for FSS. It was hypothesised that intelligence is negatively associated with the number of FSS. To investigate the specific role of intelligence in FSS as opposed to medically explained symptoms (MES), the association of intelligence with FSS was compared with that of intelligence with MES. It was also hypothesised that lifestyle factors and socioeconomic factors mediate the relationship between intelligence and both FSS and MES, whereas psychological distress is especially important for FSS. Methods: All analyses were performed in a longitudinal study with two measurement waves in a general population cohort of 947 participants (age 33–79 years, 47.9% male). The Generalized Aptitude-Test Battery was used to derive an index for general intelligence, and the somatisation section of the Composite International Diagnostic Interview was used to measure the number of FSS and MES. Results: General intelligence was significantly associated with the number of FSS. The association of intelligence and FSS but not MES was mediated by work situation: participants of lower intelligence who reported more FSS were more often (unwanted) economically inactive. No evidence was found for a mediating role of psychological distress in the association of intelligence with FSS, even though distress was an important predictor of FSS. Conclusion: Intelligence is negatively associated with the number of FSS in the general population. Part of the association of intelligence with FSS is explained by a more unfavourable work situation for adults of lower intelligence.

The Association between Intelligence and Telomere Length: A Longitudinal Population Based Study

Low intelligence has been associated with poor health and mortality, but underlying mechanisms remain obscure. We hypothesized that low intelligence is associated with accelerated biological ageing as reflected by telomere length; we suggested potential mediation of this association by unhealthy behaviors and low socioeconomic position. The study was performed in a longitudinal population-based cohort study of 895 participants (46.8% males). Intelligence was measured with the Generalized Aptitude-Test Battery at mean age 52.8 years (33–79 years, SD = 11.3). Leukocyte telomere length was measured by PCR. Lifestyle and socioeconomic factors were assessed using written self-report measures. Linear regression analyses, adjusted for age, sex, and telomere length measured at the first assessment wave (T1), showed that low intelligence was associated with shorter leukocyte telomere length at approximately 2 years follow-up (beta = .081, t = 2.160, p = .031). Nearly 40% of this association was explained by an unhealthy lifestyle, while low socioeconomic position did not add any significant mediation. Low intelligence may be a risk factor for accelerated biological ageing, thereby providing an explanation for its association with poor health and mortality.

The power of longitudinal population-based studies for investigating the etiology of chronic widespread pain

In this issue of Pain, Gale et al. show that lower childhood intelligence is associated with increased risk of chronic widespread pain (CWP) later in life [2]. Although an association between chronic pain and cognitive ability has been shown before, the Gale et al. report is the first longitudinal study in a large general population cohort. Two important characteristics stand out in their study. First, its longitudinal design enabled the researchers to study causal direction as well as interesting explanatory mechanisms in the association between intelligence and pain. A previous crosssectional study interpreted this association as an effect of pain on cognitive function [8]. Gale et al., however, show that the reverse might also occur. Specifically, the authors found that lower intelligence predicted the development of CWP many years later. They suggest several interesting mechanisms to explain their findings, including inappropriate coping styles and suboptimal health literacy. These suggestions imply important roles for health seeking behavior and dealing with symptoms or symptom related information, and will inspire further research in this area. The second key feature of the Gale et al. study is that it used a population-based design, which contrasts with many previous etiological studies that compared patients to healthy controls. This approach is especially relevant because of the nature of CWP, which is the core symptom of fibromyalgia, one of the most prevalent functional somatic disorders (FSD). CWP was defined according to the American College of Rheumatology criteria for fibromyalgia. The definition is based on questions on symptom diversity and chronicity, which is preferable to identifying cases based on self-report of fibromyalgia. Not all persons fulfilling the criteria for an FSD present with a formal diagnosis [4,10]; one of the factors increasing the chance of an FSD diagnosis in persons with persistent functional somatic symptoms (FSS) was high intelligence [5]. This increased chance of an FSD diagnosis in highly intelligent people might mask the fact that the FSS on which the FSD diagnosis is based are less prevalent in this group [7]. The strategy used by Gale et al., which is based on the assessment of the core symptom CWP and not on the self-reported diagnosis of fibromyalgia, avoids this problem. This study is consistent with previous studies that associated lower intelligence with a large variety of somatic and psychiatric health problems as well as mortality. These health problems include various types of FSS in both adults [1,3,7] and adolescents [6]. The results suggest that lower intelligence may be a generic risk factor for morbidity and mortality that deserves further study,

Age- and sex-specific associations between adverse life events and functional bodily symptoms in the general population

OBJECTIVE To test age- and sex-specific associations between adverse life events and functional bodily symptoms (FBS) in the general population. METHODS In a population-based cohort, 964 participants (mean age 55 years SD 11, 48% male) completed two measurements waves of the present study. Lifetime exposure to 12 adverse life events was assessed through a modified version of the List of Threatening Experiences. Stress-sensitive personality was assessed with the 12-item neuroticism scale of the Eysenck Personality Questionnaire-Revised. Socio-economic status was retrieved from questionnaires. Participants completed the somatization section of the Composite International Diagnostic Interview to survey the presence of 42 FBS in the previous year. RESULTS Regression analyses, adjusted for age, revealed that lifetime scores of adverse life events were significantly associated with FBS in the previous year, an association that was nearly identical for females (beta=0.18, t=4.07, p<0.01) and males (beta=0.19, t=4.24, p<0.01). This association remained statistically significant when stress-sensitive personality and socio-economic status were added to the model. Associations between adverse life events during childhood and FBS were statistically significant in females (beta=0.13, t=2.90, p=0.04) but not in males (beta=0.06, t=1.24, p=0.22), whereas there was a stronger association with adverse life events during adulthood in males (beta=0.20, t=4.37, p<0.01) compared to females (beta=0.15, t=3.38, p=0.01). Life events in the previous year were not associated with FBS in the previous year. CONCLUSION Adverse life events during lifetime were associated with FBS in the previous year. This association was dependent on age and sex but largely independent of having a stress-sensitive personality or low socio-economic status. Future studies could adopt a life course perspective to study the role of adverse life events in FBS.

The prospective association between childhood cognitive ability and somatic symptoms and syndromes in adulthood: the 1958 British birth cohort

Background Cognitive ability is negatively associated with functional somatic symptoms (FSS) in childhood. Lower childhood cognitive ability might also predict FSS and functional somatic syndromes in adulthood. However, it is unknown whether this association would be modified by subjective and objective measures of parental academic expectations. Methods 14 068 participants from the 1958 British birth cohort, whose cognitive ability was assessed at 11 years. Outcomes were somatic symptoms at 23, 33 and 42 years. Self-reported irritable bowel syndrome (IBS), chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) and operationally defined CFS-like illness were measured at 42 years. Results Lower cognitive ability at age 11 years was associated with somatic symptoms at ages 23, 33 and 42 years. Adjusting for sex, childhood internalising problems, previous somatic symptoms and concurrent psychological symptoms, childhood cognitive ability remained negatively associated with somatic symptoms at age 23 years (β=−0.060, 95% CI −0.081 to −0.039, p<0.01), 33 years (β = −0.031, 95% CI −0.050 to −0.011, p<0.01), but not with somatic symptoms at 42 years. Overall, we found no clear association between lower childhood cognitive ability and CFS/ME, CFS-like illness and IBS. Associations between cognitive ability and somatic symptoms at 23 years were moderated by low parental social class, but not by subjective indicators of parental academic expectations. Conclusions Lower childhood cognitive ability predicted somatic symptoms, but not CFS/ME, CFS-like illness and IBS in adulthood. While earlier research indicated an important role for high parental academic expectations in the development of early-life FSS, these expectations do not seem relevant for somatic symptoms or functional somatic syndromes in later adulthood.

Fatigue, not self-rated motor symptom severity, affects quality of life in functional motor disorders

While fatigue is found to be an impairing symptom in functional motor disorders (FMD) in clinical practice, scientific evidence is lacking. We investigated fatigue severity and subtypes in FMD compared to organic neurological disease. Furthermore, the role of fatigue within FMD and its impact on quality of life and self-rated health were investigated. Data from 181 patients participating in the self-help on the internet for functional motor disorders, randomised Trial were included. Data from 217 neurological controls with neuromuscular disorders (NMD) originated from a historical cohort. Fatigue was measured using the checklist individual strength (CIS). Motor symptom severity, depression and anxiety were correlated to fatigue. For multivariable regression analyses, physical functioning and pain were additionally taken into account. Severe fatigue was, respectively, present in 78 and 53% of FMD and NMD patients (p < 0.001). FMD patients scored higher than NMD patients on all fatigue subdomains (p < 0.001). In the FMD group, fatigue subdomains were correlated to depression, anxiety and partly to motor symptom severity. Quality of life was negatively associated with fatigue [OR 0.93 (0.90–0.96), p < 0.001] and depression [OR 0.87 (0.81–0.93), p < 0.001], but not self-rated motor symptom severity. Self-rated health was negatively associated with fatigue [OR 0.92 (0.88–0.96), p < 0.001] and pain [OR 0.98 (0.97–0.99), p < 0.001]. Fatigue was found to be a prevalent problem in FMD, more so than in organic neurological disease. It significantly affected quality of life and self-rated health, while other factors such as motor symptom severity did not. Fatigue should be taken into account in clinical practice and treatment trials.