This 15-year follow-up study of adolescents with depression and healthy controls, which were screened from a Swedish population, demonstrates a strong relationship between the presence of somatic symptoms in adolescence and adverse mental health outcomes in adulthood. The relationship was most pronounced when somatic symptoms were concurrent with depression, but the relationship also existed in the controls that had no experience of previous depression. The result that somatic symptoms predicted adverse health outcomes is in line with other prospective studies [24
Most previous studies have focused on depressive and anxiety disorders as outcome measures, often in relation to treatment and over a shorter time period.
In our study, using structured interviews, the relationship between mental health and somatic symptoms was evident also for more severe mental disorders (e.g., psychotic and bipolar disorders as well as suicidal behavior).
The present study demonstrates that number of somatic symptoms reported by adolescents with depression is closely related to the severity of adult psychiatric diagnoses in a stepwise manner. This finding shows that somatic symptoms in adolescent depression do not reflect transient problems. Mental health problems remain with increasing severity for each somatic symptom experienced during adolescence. The quarter of the adolescents that suffered from more than four somatic symptoms had a particularly poor outcome with high rates of severe mental disorders (e. g., suicidal behavior, recurrent and chronic depression, bipolar disorders, psychotic disorders, and panic disorder). The strong prediction of poor mental health indicted by somatic symptoms is of great importance in health care. Patients with somatic symptoms without medical explanation are often considered to be problematic and to have health anxiety. Currently, preferred clinical management is aimed at minimizing the use of health care and avoiding iatrogenic illness [28
]. This study cannot deny that health anxiety can play an important role in somatic symptoms. However, considering the poor prognosis for mental health, the need for patient health care should not be underestimated.
In the healthy adolescents with no lifetime experience of depression by age 16, somatic symptoms predicted depression and other mental disorders in adulthood. Somatic symptoms thus preceded depression in this group, consistent with results in some previous population-based prospective studies [14
]. From baseline, rarely did depressed adolescents without somatic symptoms develop somatoform disorders (5.7%), and not significantly more often than controls without somatic symptoms (4.2%). These results do not indicate a bidirectional relationship of somatic symptoms and depression.
The finding that a few somatic symptoms in healthy adolescents predicted mental health disorders in adulthood indicates that somatic symptoms either reflect vulnerability for mood disorders or constitute a subclinical mood disorder, rather than being caused by depression.
When somatic symptoms were compared with depressive symptoms (DSM-IV criteria) in a regression analysis, depressive symptoms did not better predict depression and anxiety. In fact, abdominal pain and perspiration without exertion better predicted adult depression than all the investigated depression criteria. This is a surprising finding given that depression in adolescence strongly predicts depression in adulthood [23
]. The fact that depression criteria did not better predict depression and anxiety than concurrent somatic symptoms in depressed adolescents suggests that depression and concurrent somatic symptoms share a common pathway for mental disorders. This also indicates that cognitive and affective depression criteria and somatic symptoms may be different expressions of a common disorder.
In further regression analysis, family adversities, adolescent behavioral problems, stressful relationships, and long-term depression were added. Long-term adolescent depression is thought to have a toxic effect on the brain and in previous studies has been strongly associated with poor adult mental health [23
]. In this analysis, abdominal pain in adolescence was a strong predictor of adult depression and anxiety and a predictor equally as good as an adolescent depressive episode most of the time of at least one year.
Another population study also found that children with abdominal pain are at increased risk for adult mental disorders [31
]. The strong predictive power of abdominal pain shows that not only number but also certain individual somatic symptoms are important. The unexpectedly strong influence of abdominal pain is not easily explained, and further research is needed. The predictive link between abdominal pain, perspiration without exertion, and future mood disorders poses a question for discussion. Which criteria would preferably be included in the diagnosis of depression in the next diagnostic manual?
Suicidal behavior in adulthood was common in adolescents with multiple (>5) somatic symptoms. This supports some earlier studies that found a relationship between somatic symptoms and suicidal thoughts [33
]. The finding that somatic symptoms in adolescence predict future suicidal attempts even several years in the future has, to our knowledge, not been described previously. The group of depressed adolescents without somatic symptoms, which had a better prognosis for mental disorders, still had more suicidal behavior as well as alcohol abuse. This subgroup might be unique and represent a severe risk of suicidal behavior in a way we cannot explain.
Different studies taken together indicate a possible link between somatic symptoms, depression, and suicidal behavior. This link could be attributed to low-grade inflammation. The inflammation in depression, which is characterized by increased levels of cytokines like interleukin-1 and 6 (IL-1,
IL-6), and tumor necrosis factor (TNF), may cause the occurrence of somatic symptoms of depression [35
]. The recent finding of pathologically high levels of cytokines in the brain (IL-6) of suicide attempters links somatic symptoms with suicidal behavior and severity of depression [37
]. Pro-inflammatory cytokines like IL-6 enhance the catabolism of L-tryptophan, which lowers the levels of serotonin in the central nervous system [38
]. It is known that low levels of L-tryptophan, a precursor of serotonin in cerebrospinal fluid (CSF), can predict future suicide in suicide attempters [39
]. One of several possible hypotheses is that the number of somatic symptoms in depression reflects increasing levels of IL-6 in the brain, which correlates with suicidal behavior caused by low levels of intracerebral serotonin.
Patients with somatic symptoms are typically categorized into different somatoform disorders. These disorders have been the subject of criticism by both professionals and patients. The diagnoses can cause confusion and offer little information about treatment or clinical guidance [28
]. To offer new opportunities for research and treatment, it has been suggested that somatoform disorders be abandoned and instead somatic symptoms be included in DSM-V axis III as functional somatic symptoms [28
]. Results from our study are in line with this. The present study and our previous cross-sectional study suggest that a depression diagnosis would benefit from including number of somatic symptoms as a marker of the severity of current depression [14
] and as a prognostic marker of future mental disorders and suicidal behavior.
The clinical implication of this study is that adequate treatment guidelines are needed for patients with somatic symptoms. The prognosis for mental disorders is as poor for adolescents with depression and several somatic symptoms as for those with long-term adolescent depression. Even in healthy adolescents without a lifetime history of depression, somatic symptoms predict future mental disorder.