Epidemiology of neurodevelopmental disorders in children J. Little Epidemiology Group, Department of Medicine and Therapeutics, University of Aberdeen, Aberdeen, UK Summary The epidemiology of mental and behavioural disorders is considered in comparison with spina bifida, chromosomal anomalies and brain tumours. Descriptive epidemiology is important not only in assessing the frequency of neurodevelopmental disorders, thereby aiding planning of service provision, but also because variations by geographical area, over time, and by personal characteristics provide clues regarding etiology.The value of the latter application is exemplified by research on spina bifida and other neural tube defects (NTDs).The descriptive epidemiology of mental and behavioural disorders has been less investigated.The descriptive epidemiology of NTDs suggested that diet might be of etiological importance. Analytical epidemiologic investigation proceeded by testing dietary hypotheses in case-control and cohort studies. Subsequently, folate supplementation was shown to reduce recurrence risk in a randomized controlled trial.The analytical epidemiology of other neurodevelopmental disorders is less well understood. Study design issues are discussed in relation to mental and behavioural disorders. & 2000 Harcourt Publishers Ltd INTRODUCTION A very broad range of disorders may be described as neurodevelopmental (Table 1). Some are obvious exter- nally at birth, for example spina bifida, or may be diagnosed soon after birth by a laboratory test, for example, chromosomal anomalies. Some become man- ifest later in childhood, and the diagnosis may take some time to establish, for example brain tumours. The mental and behavioural disorders represent `extremes' of con- tinuous variation in various aspects of learning ability and social function. It is particularly difficult to define these disorders because of differences in what is con- sidered to be `extreme' and because of co-morbidities. It is useful to consider the epidemiology of these disorders in childhood because of the need for pediatric, educational and related services in diagnosis and management. In the particular context of neoplasms, the predominant sites and types of cancer observed in childhood differ substantially from these observed in adult life. In this paper, aspects of the epidemiology of the mental and behavioural disorders will be considered in compar- ison to spina bifida, chromosomal anomalies and brain tumours. DESCRIPTIVE EPIDEMIOLOGY Descriptive epidemiology is the study of the distribution of disease or health status in populations, and typically investigates variation and frequency of disorders or aspects of health status by place, time and person. The number of cases of a disorder occurring in a population depends on the size of the population at risk and the distribution of factors associated with disease occurrence such as age and gender. Therefore, the measure of frequency is typically a rate, and sometimes age Ð or gender-specific rates are presented. In Table 2, data on the prevalence or incidence rates of a number of neurodevelopmental disorders in childhood are pre- sented. In addition to presenting information on the range of rates reported in the literature, an indication of the amount of information on which this range is based is given, together with a comment on geographical variation. Spina bifida is a congenital anomaly which is usually externally obvious at birth. A large number of studies of Prostaglandins, Leukotrienes and Essential Fatty Acids (2000) 63(1/2),11^20 & 2000 Harcourt Publishers Ltd doi:10.1054/plef.2000.0185, available online at http://www.idealibrary.com on Correspondence to : Professor J. Little, Epidemiology Group, Department of Medicine and Therapeutics, University of Aberdeen, Foresterhill House Annexe, Foresterhill, Aberdeen AB25 2ZD, UK.Tel.: +44 1224 554485; Fax: +44 1224 849153; E-mail: j.little@abdn.ac.uk Received 11February 2000 Accepted 17 March 2000 & 2000 Harcourt Publishers Ltd Prostaglandins, Leukotrienes and Essential FattyAcids (2000) 63(1/2), 11^20