New technologies in neurobiological, genetic, and cognitive research are adding to our understanding of ADHD. Researchers currently believe that between 3 and 6 percent of children exhibit symptoms, with three times as many boys as girls being diagnosed. ADHD is now known to continue into adulthood. Rosemary Tannock, Research Institute of The Hospital for Sick Children, Toronto, Canada, reports that it has become increasingly clear that ADHD involves genetic, biological and environmental factors.
At this time, results from studies in these different fields cannot be integrated into a coherent whole. Neuroimaging techniques, including studies of brains in action, indicate that there may be subtle anatomical and structural differences in the brains of children with ADHD. ADHD tends to run in families and people with ADHD also have higher rates of conduct disorder, oppositional disorder, and anxiety and affective disorders. Those diagnosed with ADHD are also more likely to exhibit learning disabilities.
Studies of twins indicate that there is a genetic component to ADHD. It is seen more frequently in biological relatives than in adoptive relatives. Tannock states that it may best be viewed as “the extreme end of a behavior continuum that varies genetically throughout the population rather than as a discrete disorder.” Inattention, hyperactivity and impulsiveness probably share a common genetic component.
Links with other disorders
Results of studies indicate that ADHD and reading disabilities are probably genetically independent — that is, they are not linked biologically and are therefore not inherited together. There is some evidence, however, that hyperactivity and spelling disorders may be due to common genetic factors.
Two studies investigated the genetic and environmental influences in the overlap of ADHD with conduct and oppositional disorders. Results are difficult to assess because information, particularly that obtained from mothers, may be biased. However, high levels of several behavioral disorders co-occurring with ADHD indicate that many genes rather than a single one are probably involved. In addition, it appears likely that environmental factors such as family dynamics and parenting styles can either exacerbate or reduce symptoms of ADHD.
Although significant progress has been made in ADHD research in the 1990s, these findings must be considered preliminary. One underlying problem in trying to identify the origin of ADHD is the considerable difficulty in defining the disorder. The current diagnostic definition outlines the onset, duration, severity, and impairment without describing precise details about how these are manifested.
Neuroimaging studies provide preliminary evidence of a link between the observed cognitive impairments and subtle differences in brain anatomy. However, to date, there are no known biological markers for ADHD or its symptoms. “If markers do exist they are likely to be subtle, small-scale, distributed in multiple locations and systems and hence difficult to measure,” Tannock writes.
Future studies are needed that combine cognitive, family-genetic and neuroimaging approaches. In order to integrate cognitive/behavioral methods with functional neuroimaging methods, the relations between behavior and brain structure or function must take into account the normal variations in development. Researchers need to address such questions as: Are certain types of ADHD more genetically based than others? How do specific genes interact with specific environmental factors in the development of low and average attention skills and impulsiveness? What are the environmental factors that add to the risk for ADHD?
“Attention Deficit Hyperactivity Disorder: Advances in Cognitive, Neurobiological, and Genetic Research”, Journal of Child Psychology and Psychiatry,Volume 39, Number 1, January 1998, pp. 65-99.
Published in ERN April 1998 Volume 11 Number 4