Epidemiology

Medically unexplained physical symptoms are extremely common in adults. They are more frequently seen in females, in persons from lower socioeconomic backgrounds, and in certain ethnic groups. These symptoms are also fairly common in childhood and adolescence; a recent study showed that 10% of children surveyed had unexplained headaches, 9% had unexplained abdominal pain, and 4% had unexplained pain in the extremities.8

The Epidemiological Catchment Area Survey of Mental Disorders in the United States reported that fewer than 0.1% of respondents met strict DSM-III criteria for SD, and about 5% met criteria for abridged SD, a less restrictive construct of somatization.9 In Germany, about 20% of a community sample met criteria for undifferentiated SD, but very few (about 1%) met criteria for the more specific somatoform disorders.10

The prevalence of unexplained physical symptoms is much higher in primary care and medical specialty settings. A World Health Organization (WHO) study at 14 primary care sites in Asia, Europe, and the Americas showed that the rates for abridged SD differed widely across countries. They were lowest in Nigeria (7.6%), Italy (8.9%), and the United States (9.8%) and highest in Germany (25.5%), Brazil (32%), and Chile (36.8%).11

 

In a North American study, patients with SD reported spending an average of 8.8 days sick in bed per month, compared with half a day for those without the disorder. More than 80% of patients with SD stopped working because of “poor health” and their per capita yearly health costs were 9 times higher than average.12 A study of patients with SD who attended a university outpatient service in London showed that 61% were receiving disability benefits, 64% had been treated for spurious physical disorders, 60% had had surgeries with no pathology found, and 16% were using either wheelchairs or crutches without any evidence of organic disorder.13

Unexplained physical symptoms tend to have a chronic, protracted course. For example, in the South London Somatization Study, most patients had a chronic unremitting course. About 80% of these persons still qualified for a somatoform diagnosis 4 years later.14 A follow-up in the WHO study showed that, 1 year later, somatization persisted in about half of the patients.11

The causes of SD are multifactorial. Although no consistent biological (brain) markers have been documented, genetic factors may play a role. Findings from studies of adopted females with SD suggest excessive alcoholism or violent behavior in the biological fathers.15 More recent studies have shown that there is familial clustering of functional syndromes such as fibromyalgia.16 A large number of studies have shown a frequent association between unexplained physical symptoms and early traumatic experiences; medical illness with long hospitalizations; serious medical illness of a parent; natural disasters; or psychological factors such as sensitization processes, specific cognitive styles, or benefits derived from the sick role (secondary gain).17-23

The most common psychiatric conditions that coexist with unexplained physical symptoms are major depression, anxiety, substance use, personality disorders, and posttraumatic stress disorders.24 Averaging data from several studies, about two-thirds of patients with unexplained symptoms also met criteria for at least one of these psychiatric disorders and a large portion of the remaining one-third, who did not fully meet the criteria, had significant symptoms, mainly depression or anxiety.25

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