There have been widely different prevalence rates reported in both the United States and the United Kingdom for the hyperkinetic disorder, attention deficit disorder with hyperactivity (ADDH) and attention deficit hyperactivity disorder (ADHD). For example the draft ICD-10 (WHO 1988) criteria in the United Kingdom were based on a study by Taylor, Sandberg, Thorley et al (1991), which found a prevalence of 1.7 per cent for hyperkinetic disorder in boys. Hyperkinetic disorder was defined as a score of 1.5 or greater on the hyperactivity-inattentiveness criteria of the Conners Teacher Rating Scale (TRS), together with a score of 1.0 or greater on the hyperactivity scale of the Parent Account of Children’s Symptoms (PACS).
In the United States, ADHD, as defined by the third DSM edition (DSM-III) (American Psychiatric Association 1980), which required observations of symptoms by a parent or teacher, had a prevalence of 6 per cent. ADHD, as defined by the revised third edition, DSM-III-R (American Psychiatric Association 1987) which also allowed a diagnosis at home or at school, was thought likely to have a higher prevalence rate.
The diagnostic criteria of the most recent classifications for ADHD in DSM-IV (1994), and the research diagnostic criteria of ICD-10 (1993), are almost identical. Both require observation of symptoms in two or more settings (home/school/clinic) and should provide similar prevalence rates in the United Kingdom and the United States (estimated by DSM-IV to be 3-5 per cent). The multiple diagnoses possible in DSM-IV in the presence of co-morbid conditions, compared to the compound diagnoses of ICD-10, may affect prevalence rates. Even small differences in diagnostic procedures can affect rates, which in turn have a powerful effect on the predictive value of diagnostic tests. The advent of DSM-IV ‘predominantly inattentive type’ of attention deficit disorder (ADD) could, for example, increase trait prevalence rates.
One reason for the above United States/United Kingdom differences in prevalence rates is that prevalence estimates are arrived at using at least two different methods (Glow 1980; Quay 1979), using either a categorical or a trait (or empirical) approach to diagnosis.
Australian studies have shown prevalence rates ranging between 2.3 per cent and 6 per cent (Glow 1980) depending on the methodology used. Current epidemiological studies will need to take account of newest DSM edition, DSM-IV (American Psychiatric Association 1994).
Age and gender difference
In general, boys are rated higher on disruptive behaviour scales (Werry and Hawthorne 1976). Also, incidences of hyperactive problems are usually reported as being 49 times as common in boys than in girls depending on the setting (community or clinic) (DSM-IV 1994). Age differences in DSM-IV subtypes of ADHD have been postulated by Barkley (1995a), but data are not currently available.
Key points – epidemiological data
Epidemiological data remain imprecise and are influenced greatly by the method of ascertainment. Also, as DSM-IV and ICD-10 have only recently been published, epidemiological data based on these classification systems are not currently available. Most studies show a considerably higher incidence of ADHD in males than in females.