Prevalence of intellectual disability in Finland

Intellectual disability (ID) is a developmental condition with problems in mental functions that are reflected in lower than average performance in tests for intelligence, indicating difficulties for the person to adapt in everyday life. The various needs for support of persons with ID necessitate well-organized services systems. To organize, run, and reform those systems reliable, up-to-date, and detailed epidemiological information is needed: What is the prevalence of ID? Where do the persons with ID live? What is their profile of needs of services? Administrative reports from different decades in Finland have indicated that the number of people with ID is around 0.6%. In Finnish population sample studies higher estimates have been reported. International studies have given very variable estimates depending on the population, design, and methods. There is a need to have a more precise estimate of the prevalence of ID in different age groups in Finland. The aim of this study was to estimate the prevalence of ID in the whole population in Finland, using multiple national health and social-care registers. METHODS This study consists of two separate register samples. In the first sample data were combined in 2000 from eight Finnish national registers, six of which concern benefits connected to long-term illnesses or disabilities allowed by the Social Insurance Institution of Finland (SII) (Child Disability Allowance, Disability Pension, Disability Allowance, Pensioners' Care Allowance, Funding of Rehabilitation, and Preferential Refunding of Long-term Medication), and two concern care provided by hospitals or social welfare (Hospital Discharge Register, and Care Register for Social Care). The list of inclusion diagnoses covered both intellectual disability and those aetiological diagnoses where ID is regularly present (e.g. Down's, Williams', Fragile-X and Angelman's syndromes, and progressive neurological diseases of Finnish heritage). Prevalence estimates were first calculated in four age groups (0–15, 16–39, 40–64, 65+), and thereafter in one-year age cohorts to obtain more understanding of how the prevalence varies with age/year of birth. The second data set was sampled from Hospital Discharge Registers covering the period 1996‒2013 to form an estimate of cumulative prevalence of ID at every age from birth until the maximal age possible (17 years for those born in 1996). The same inclusion list of diagnoses was used. RESULTS In the multiple register study, the average prevalence estimate for those of 16‒64 years of age was 0.81%. There was a decreasing trend with birth year in this age group, from 0.92% among 64-year-olds to 0.63% among 18-year-olds. Between the ages of 42–52 years the prevalence was exceptionally high, highest at 50 years (1.07%). In old age (65+ years) the average estimate was first 0.38%. At 66 years of age it was 0.49%, decreasing to an average of 0.30% at 80+ years of age. An abrupt drop in the prevalence rate was noticed at 65/66 years of age due to a blind spot in registers and this was corrected by computational means, which yielded a new estimate of 0.75% for those of 65+ years of age. The validity of the computational correction was evaluated using the register of Preferential Refunding of Long-term Medication. The Hospital Discharge Register sample yielded a cumulative prevalence of 1.19% at the age of 17. This marked a discontinuity compared with the multiple register study, where the prevalence estimate for the same age was only 0.67%. A total estimate for the population with ID in Finland in 2017 was 53,684 people, which is 0.97% DISCUSSION. These multiple register studies suggest a higher prevalence estimate of ID than in most prior register-based studies, but close to estimates found in longitudinal birth-cohort studies. The prevalence given in one-year age cohorts across the whole age span gave a qualitatively different picture and higher prevalence rates than after more robust grouping by age. The cumulative prevalence of ID increases steadily throughout developmental years. Cross-sectional studies at any age do not give a full picture. Inconsistencies in the age-specific prevalence distribution, together with other previous findings, hint at the possibility of so-called hidden disability, i.e. people with difficulties in coping, but not being recognized by the services. The prevalence distribution at all ages reflects in many ways the history of social and health care. There has been a great deal of progress in general, and especially perinatal health. Seasonal epidemics and economic recessions with their implications have affected the opposite direction. The century did also see the great wars of Finland. The emphasis in development of services has fluctuated between segregation and inclusion. Continuous monitoring of the epidemiology of ID by one-year age cohorts through the most useful registers seems to be both useful and practical. Different registers complement each other. Some reflect diagnostic activities, others, benefits or service delivery. However, for the purposes of service planning, the information in the registers that were used in this study was insufficient. More information would be needed of the actual needs for services, person-by-person. The question arises of whether a specific register of services for intellectual and other developmental disabilities would be helpful or does the information content in current registers develop to provide what is needed.