Environmental risk factors for attention-deficit hyperactivity disorder.
Attention-deficit hyperactivity disorder (ADHD) is the most common cognitive and behavioural disorder diagnosed among school children. It is characterized by deficient attention and problem solving, along with hyperactivity and difficulty withholding incorrect responses. This highly prevalent disorder is estimated to affect 5-10% of children and in many cases, persists into adulthood, leading to 4% prevalence among adults. Converging evidence from epidemiologic, neuropsychology, neuroimaging, genetic and treatment studies shows that ADHD is a valid medical disorder. The majority of studies performed to assess genetic risk factors in ADHD have supported a strong familial nature of this disorder. Family studies have identified a 2- to 8-fold increase in the risk for ADHD in parents and siblings of children with ADHD. Various twin and adoption studies have also highlighted the highly genetic nature of ADHD. In fact the mean heritability of ADHD was shown to be 0.77, which is comparable to other neuropsychiatric disorders such as schizophrenia or bipolar disorder. However, several biological and environmental factors have also been proposed as risk factors for ADHD, including food additives/diet, lead contamination, cigarette and alcohol exposure, maternal smoking during pregnancy, and low birth weight. Many recent studies have specifically examined the relationships between ADHD and these extraneous factors. This review describes some of these possible risk factors.


CONCLUSIONS
Although a substantial fraction of the aetiology of ADHD is due to genes, the studies reviewed in this article show that many environmental risk factors and potential gene-environment interactions also increase the risk for the disorder.
Banerjee
Tania Das
TD
Department of Neuroscience and Physiology, SUNY Upstate Medical University, Syracuse, New York 13210, USA.
Middleton
Frank
F
Faraone
Stephen V
SV
D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000209:etiology* / Q000235:genetics; D002648:Child; D004781:Environmental Exposure / Q000009:adverse effects* / Q000706:statistics & numerical data; D005260:Female; D005838:Genotype; D006801:Humans; D010641:Phenotype; D011247:Pregnancy; D011297:Prenatal Exposure Delayed Effects / Q000453:epidemiology; D011584:Psychology; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects / Q000453:epidemiology