Posts from — December 2009

The November issue of the medical journal Pediatrics published research from Dr. Robert Kahn et al regarding the relationship between tobacco smoke, lead concentrations, and ADHD.

Kahn, a physician and researcher at Cincinnati Children’s Hospital Medical Center, Ohio, found that two risk factors: 1) exposure to tobacco in the womb and 2) exposure to lead in childhood significantly increased the likelihood of ADHD developing  in children.

The researchers used data from the National Health and Nutrition Examination Survey. Tobacco exposure in the womb was measured by reports of cigarette use during pregnancy, and childhood lead exposure was assessed by blood levels. Of the 2588 cases they reviewed, the researchers determined that children aged 8 – 15 who were exposed to tobacco smoke in the womb were 2.4 times more likely to have ADHD. Children with lead blood levels in the top third of the population had a 2.3-fold increased likelihood of ADHD diagnosis.

Lead researcher, Tanya E. Froehlich, MD, cited that the combination from both lead and tobacco smoke created a synergistic effect, an even greater effect than smoke or lead alone. Children who were exposed to  both tobacco smoke in the womb and higher lead levels had a more than eightfold increased chance of having ADHD compared to children who weren’t exposed to either.

The study does have limitations; the researchers analyzed data on smoking that was derived from the mothers’ answers on a questionnaire. The data did not include the number of cigarettes smoked. And while the researchers found a link between tobacco, lead and ADHD, they did not prove that these factors actually caused the disorder. This is similar to previously published research on prenatal tobacco smoke and lead levels.

Curiously, smoking tobacco is twice as popular in the adult ADHD population compared to the non-ADHD adult population.  Columbia University researchers established a study to determine if smoking ameliorated ADHD symptoms in adults back in 2006.  If tobacco smoke truly increases the risk of developing ADHD, the popularity of smoking among ADHD adults may create a cycle of producing more ADHD children if smoking is done prenatally.

While a strong genetic link is still the likely cause of ADHD, environment still plays a significant role in brain development. The researchers assert that perhaps up to 35 per cent of cases of ADHD in youngsters aged between 8 and 15 could be reduced by getting rid of both prenatal exposure to tobacco and childhood exposure to lead.

Attention-deficit hyperactivity disorder (ADHD) is conservatively estimated to affect anywhere between 5-7% of the world’s school-children. Is it possible that ADHD is the result of a brain that just develops slower than the brain of a child that doesn’t have ADHD?

Research has shown that the ADHD brain may develop differently than one without ADHD. However, no conclusive pathology (diseased or damaged location in the brain) exists; we don’t know what causes it.

What we do know: As a child’s brain develops, his experiences and environment help shape the connections in the brain. This development is also connected to genetic endowment. Thus, the brain develops dependent upon a complex interplay between nature (genetic endowment) and nurture (experience/environment). These connections form networks which help us process language, calculate math, feel, see, smell, think, and all else the brain is responsible to perform.

The part of the brain that is crucial to a child’s development is the cerebral cortex, the brain’s outer layer. The brain’s most complex functions like attention, consciousness, memory, and language are believed to be regulated in the cerebral cortex. As a child develops, gains experiences, and is subjected to his/her environment, the connections between the neurons (nerve cells in the brain) increase causing the cortex to thicken. The brain acts much like a muscle during adolescence; it’s a use it or lose it proposition. Connections which are frequently used are strengthened while unused connections are pruned away.

Researchers Philip Shaw, Judith Rapaport and others from the National Institute of Mental Health have proposed that ADHD may be the result of  lagging brain development resulting in an average 3 year delay. This theory is supported by earlier studies which found that children with ADHD have similar brain activity to slightly younger children without the condition. 

Shaw and Rapaport used MRI (magnetic resonance imaging) to measure the brains of 447 children of different ages. They frequently noticed that the volume of the brain in the prefrontal cortex was thinner in ADHD children than other children of the same age. The cortex developed correctly over time, but the ADHD brain’s  development lagged behind about 3 years before it reached maturity. Among other things, the prefrontal cortex has the responsibility of governing attention, short-term memory,  and controlling inappropriate thoughts and actions. The researchers theorized that ADHD is a lack of control over these tasks, so it was logical to suggest that ADHD is a matter of developmental delay.

Significantly, Shaw and Rapaport found that the primary motor cortex developed faster in ADHD children. As its name implies, the motor cortex helps to plan and control movements. Shaw theorized that this might explain the restlessness, fidgeting and uncontrolled hyperactivity found in ADHD children.

This research raises more questions than it answers. Currently, the cause of the delay is unknown.  If ADHD is just a developmental delay, why do approximately 70% to 80% of children carry their ADHD traits into adulthood? This fact does not rule out Shaw’s conclusions, just that developmental delay may only be present in a minority of children labeled ADHD. Other children, the 70% to 80% previously mentioned, do carry their brain differences into adulthood.

Other research points to a set of genes responsible for the ADHD trait. If either genes and/or developmental delay are the cause, then what is one to do? The brain is an incredibly flexible organ. It is shaped by a variety of factors which means that parents, teachers, and other professionals can influence outcomes. Finding a program that addresses the needs of an ADHD child, helps shape behavior, and optimizes their potential is still the best practice.