Epidemiology

A strong female predominance (about 75%) is consistently observed for both somatic symptom and functional neurological disorder.[5][12][13][14] A large proportion of patients (both male and female) also meet criteria for a co-existing psychiatric disorder.

Functional neurological disorder

  • Functional neurological disorder is common; incidence has been estimated at 4-12 per 100,000 in general US and European populations.[15]

  • Evidence from the US and the UK suggests that about 10% of patients referred to general neurology outpatient clinics have medically unexplained symptoms; that functional tremor, dystonia, myoclonus, and parkinsonism occurs in 2% to 3% of patients evaluated in specialty neurology clinics; that about 20% of patients with intractable epilepsy have functional seizures; that functional seizures have a US prevalence of 2-33 per 100,000; and that up to 18% of patients with epilepsy have co-existing functional seizures.[16][17][18][19]

  • Symptoms can develop at any age, with early to middle adulthood being most common. Although the disorder is generally thought to be rare in the paediatric age group, a national surveillance study from Australia reported an incidence of 2.3 to 4.2 per 100,000 in children <16 years of age.[20]

  • Evidence suggests that only about 4% of patients with a diagnosis of functional neurological disorder go on to develop non-functional neurological disease.[21]

Somatic symptom disorder

  • Prevalence is unclear as it was only introduced as a diagnosis in 2013; therefore, estimates are typically derived from research on somatoform disorders.

  • Estimates of prevalence in the general population range from around 4% to 17%; note that some of these figures are derived from self-report questionnaires, which may lead to an over-estimation of prevalence.[2][7][22][23][24]​​

  • 10% to 17% of primary care patients have multiple unexplained symptoms that are present for >2 years.[25][26]

  • The female-to-male ratio is approximately 10:1.[27]

  • Can occur in childhood, adolescence, or adulthood.[27]

  • Early symptoms may begin in childhood, with full criteria for somatic symptom disorder being met in adolescence.[28][29]

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