Attention deficit/hyperactivity disorder (ADHD) and Inattentive ADD (ADHD I) are not referred to as a "spectrum" disorder but I believe that just like there is an autism spectrum, there is also an ADHD spectrum and all the sub-types of ADHD fall on the spectrum.
In the early 1990s, the American Psychiatric Association (APA) defined autism as a spectrum disorder in the Diagnostic and Statistical Manual (DSM-IV). The APA used the term "spectrum" disorder because they had come to understand that autism presented in several ways and that there were people who were very mildly autistic and there were people that were severely autistic. The APA recognized that people on the autistic spectrum differed with regards to when they were first diagnosed, how severe their symptoms were, and what symptoms they presented with but that they all shared common characteristic behavioral and neurological symptoms.
As neuro-scientists look at our ADHD brains, I believe that they are coming to a similar conclusion. Instead of thinking of ADHD as a condition with three distinct sub-types, Inattentive ADHD, Hyperactive ADHD or Combined type ADHD; I believe that researchers will see ADHD as a condition with a neurological spectrum that affects different individuals differently. Their conclusion will be that people with ADHD differ with regards to when they are diagnosed, what symptoms they have and how severe their symptoms are but that fundamentally; they all have overactive or underactive brain activity in several specific brain areas. I also believe that studies will show that, in people with ADHD, these specific parts of their brain will be malfunctioning either slightly or severely and that there will be a ‘spectrum’ of malfunction.
A recent study looked at brain oxygen levels in a part of the brain that is known to be problematic in ADHD, the right pre-frontal cortex. This is an important brain area that controls attention. The researchers watched this part of the brain while children with ADHD performed a task. The children were asked to either press a button when given a certain cue or keep from pressing the button when given the cue. This is referred to as a "GO/NO-GO" task. The children with the greatest degree of attention deficit were found to have the least oxygenation activity in the right pre-frontal cortex. The scientist found a spectrum of attention behaviors or symptoms that correlated and was associated with the ADHD kid’s brain oxygen levels.
In another study where researchers performed a similar analysis using functional Magnetic Resonance Imaging (fMRI), researchers found that the ADHD kids with the most impulsive behavior had delays or minimal responses in areas of the brain that are responsible for response inhibition, the anterior cingulate, but the ADHD kids with better impulse control had brain scans that revealed that their anterior cingulates were working properly.
When the kids were asked to NOT press the button the impulsive ADHD kids pressed the button anyway, unable to inhibit this response. The ADHD kids who were not impulsive were able to control this particular area of the brain properly but their "Go" areas were slower to activate. The impulsive kids, not too surprisingly, had quick brain activity in their "GO" area, the area of the brain controlling action. When these kids were cued to press the button, their "GO" areas activated quickly.
These studies demonstrate the spectrum that I am referring to. I believe that as we come to learn more about the brain dynamics of ADHD and of other conditions that affect the brain such as depression, anxiety and post-traumatic stress disorder we will likely find that the symptoms and neurological changes seen in these disorders like on a spectrum in much the same way that autism spectrum disorder symptoms do.
Neurobehavioral and hemodynamic evaluation of Stroop and reverse Stroop interference in children withattention-deficit/hyperactivity disorder. Brain Dev. 2013 Feb 13. pii: S0387-7604(13)00022-3. doi: 10.1016/j.braindev.2013.01.005. Yasumura A, Kokubo N, Yamamoto H, Yasumura Y, Nakagawa E, Kaga M, Hiraki K, Inagaki M.
Too little, too late or too much, too early? Differential hemodynamics of response inhibition in high and low sensation seekers. Brain Res. 2012 Oct 24;1481:1-12. doi: 10.1016/j.brainres.2012.08.004. Epub 2012 Aug 8. Corbly CR, Liu X, Kelly TH, Lynam D, Joseph JE.