title: Trajectories of a set of ten functional somatic symptoms from adolescence to middle age.
abstract: BACKGROUND
  Functional somatic symptoms (FSS), or symptoms without a clear medical explanation are a considerable challenge for health care systems. There is no general consensus as to which symptoms should be regarded functional. Few longitudinal studies on the development of FSS exist and these have mainly been based on the assumption that the factorial structure of a FSS scores variable remains invariant over time. When the analysis covers longer periods of the life course, this may be challenged. The aim of our study was to investigate how ten functional somatic symptoms (FSS) evolve when individuals are ageing.
  
  
  METHODS
  The data of the Northern Swedish Cohort (n= 1001) from questionnaire surveys at ages 16, 18, 21, 30 and 42, were analysed. Participation rates remained very high over the five surveys. The list of symptoms included backache, breathlessness, dizziness, fatigue, headache or migraine, nausea, overstrain, palpitations, sleeplessness and stomach ache. We used multivariate trajectory analysis (TA) with logistic broken-stick regression models to describe sub-groups in the data. In multivariate TA the joint development of the set of item variables can be investigated. There is no need to construct a special FSS summary score variable.
  
  
  RESULTS
  Four well separated trajectories were identified. In two groups, healing symptoms (25.4% of the sample) and low symptom load (32.2% of the sample), the symptom level stayed relatively low in adulthood. In the third group of high symptom load (17.2%) the probability of having symptoms was high for all FSS variables. In the fourth group of increasing symptoms (25.3%) the level of symptoms was first intermediate, but increased markedly with age.
  
  
  CONCLUSIONS
  Instead of a single FSS score we were able to assign each individual to one of four trajectories described jointly by 10 separate symptoms. The profile of development, but not the probability level, was rather similar over the symptoms within the trajectories, with few exceptions. The results provide better understanding of the longitudinal development of the symptoms from the adolescence to the middle age.
authors.affiliation: Faculty of Natural Sciences, University of Tampere, 33014 Tampere, Finland.
authors.forename: Tapio
authors.initials: T
authors.lastname: Nummi
authors.affiliation: School of Health Sciences, University of Tampere, 33014 Tampere, Finland.
authors.forename: Pekka
authors.initials: P
authors.lastname: Virtanen
authors.affiliation: Finnish Institute of Occupational Health, Centre of Expertise for Health and Work Ability, Helsinki, Finland.
authors.forename: Päivi
authors.initials: P
authors.lastname: Leino-Arjas
authors.affiliation: Department of Public Health and Caring Sciences, Uppsala University, Uppsala, Sweden.
authors.forename: Anne
authors.initials: A
authors.lastname: Hammarström
keywords: Finite mixtures; Logistic regression; Longitudinal data; Multivariate trajectory analysis; Pain