The brains of kids with ADHD are different.
ADHD is a neurobehavioral disorder with two key components: impulsivity/hyperactivity and inattention/ distractibility. Some children are predominantly impulsive and hyperactive; others are mostly easily distracted. Most are a combination.
"These children have a Ferrari brain with bicycle brakes," explains Edward M. Hallowell, M.D., director of the Hallowell Centers for Cognitive and Emotional Health in Sudbury, Massachusetts, and New York City and author of several books on ADHD. "Their mind is racing, but they can't slow down when they need to."
Scans show that when a child has ADHD, the part of her brain that helps control behavior, maintain focus, and prioritize what she needs to do and then do it is slightly smaller. What's more, neurotransmitters--chemicals such as serotonin, dopamine, and norepinephrine that send messages from one part of the brain to another--get used up more quickly. So if a teacher gives a set of five instructions, the child may only be able to get through the first two steps before losing track of what she should do next.
Symptoms range from mild to severe and may be noticeable as early as age 2 or 3. But in most cases, they're not apparent until a child starts school and needs to pay attention and sit quietly. Bright kids who have mild symptoms may squeak by for years until the academic demands for planning, organization, and time management become too overwhelming.