First, I am not a diagnostician, and ADHD is a disorder I know very little about. So as always, get professional help when in doubt.
That said, in many ways the two are opposites—those with ADHD appear to have the genes, behavior, and symptom criteria for the diagnosis of someone tending to be impulsive and unable to be affected by consequences that are even bad for them. On the other end, HSPs tend to “pause to check” before acting, our depth of processing in action, and thus are highly conscientious. But again, there are three possibilities: It’s really HS and not ADD, it’s really ADHD and not HS, or it’s both.
Why It Might Be HS and Not ADHD:
A significant number of those with ADD could be misdiagnosed because they are chronically overstimulated in large, crowded classrooms, where teachers first notice a child being difficult and different for some reason. This overstimulation may be present at home, too, with TV, siblings, and so forth, plus after school activities that other kids do so the HSC is enrolled too. These environments might seem tolerable for most kids, but it probably is not for HSCs. They can never escape the overstimulation. Obviously knowing which is going on is important because treatment would be quite different.
The situation is complicated by there being two kinds of ADD—hyperactive, more common in boys and men, and merely distractable, more common in girls and women. Both could be misdiagnosed when the underlying cause is chronic overarousal due to sensitivity. And maybe it is more socially acceptable for a sensitive boy to be restless and bouncy when overaroused, rather than withdrawn, quiet, or daydreaming in the corner, as the sensitive, overstimulated girls may do with a little less stigma.
In adults, I wonder if being a high sensation seeker could be mistaken for ADHD, and we certainly know HSPs can also be HSSs.
To me the important test is whether the person has good concentration in a quiet place. Most of those with ADHD do not, and may concentrate better with some noise. Most HSPs have excellent concentration in a quiet place and when not overaroused. So the most important treatment for them is reducing stimulation and other sources of arousal and stress.
Some might say that the best test is to give the medications that are used to treat ADHD (ironically, these are stimulants), and if a person improves, that is probably the diagnosis. I am not sure about that, not being an expert in the field.
Why It Might BE ADHD and not HS:
Especially with children, diagnosis can be difficult, but a child who feels bad about, for example, not turning in homework or remembering important details could become shy. Feeling different would go both with ADHD and HS. Both parents and adults with the disorder might want to think of the symptoms as having to do with a normal trait—that the above situation is true, and the distractibility is only due to overstimulation of an HSP.
As I said, in this case ADHD and HS seem almost mutually exclusive, but I would not go so far as to say that you cannot have both. I have met HSPs who have told me that they do, and the workings of the brain and of genetics are still too uncertain except that things are always more complicated than we thought.
As always, if you think you or your child might have this disorder, you should definitely seek a professional opinion before deciding it is “just sensitivity.” See the first FAQ for help with this.