There are two main subtypes to the ADHD diagnosis - inattentive and combined. Treatment for ADHD, no matter what the subtype, is the same. Because hyperactivity is more noticeable and more easily diagnosed, those with the inattentive type can be missed, labeled as shy or "spacey." Until recently, much of the literature explaining ADHD focused on those with hyperactivity, skipping over those who have trouble paying attention but don't show signs of hyperactivity. Some theorize that ADHD combined type is a more severe type of ADHD, although those with inattentive type would disagree.
A recent study shows that there are, indeed, two separate types of ADHD. Scientists at the Center for Mind and Brain of the University of California, Davis, found specific physiological differences using an electroencephalogram (EEG) between teens with ADHD combined type and ADHD inattentive type. The study was published in the journal Biological Psychiatry.
Researchers looked at 40 children - 23 without ADHD and 17 who had either inattentive or combined type ADHD. The participants were between the ages of 12 and 17 years old. Each participant was asked to complete tasks on the computer, for example, they might see a series of arrows and be asked which direction the middle arrow pointed (<<<>>). They were given visual cues to help with the tasks.
Researchers noted that there were differences in the alpha and beta brainwaves in all three groups - those with ADHD combined type, those with ADHD inattentive type and those without ADHD. Specifically, those with inattentive type did not seem to process the visual cues. Those with combined-type ADHD had problems pressing a button (motor task performance.)
Different But Not More Severe
The researchers believe they have shown that there are two specific types of ADHD and that each has specific characteristics. According to Medical News Today, "The researches say that it seems to be a different type of ADHD, not simply one that displays additive effects."  In other words, ADHD combined type is not the same as inattentive type plus hyperactivity.
Understanding the differences in the subtypes could lead to more targeted treatments. Catherine Fassbender, one of the co-authors of the study states, "Our findings suggest targets for treatment should differ for the ADHD inattentive versus combined subtypes and that advanced analysis of brainwaves may provide a biomarker for testing treatment responses." 
Using EEG and Brainwave Tests for ADHD
Scientists have been working on other ways brainwave testing are helpful in treating and diagnosing ADHD. In July 2013, the FDA approved the NEBA system, which uses brainwaves to diagnose ADHD.
EEG biofeedback has also been used to treat ADHD. Some studies have shown an effectiveness rate of 60 to 70 percent and some patients were able to reduce their medication doses after using EEG feedback.
  "EEG Brainwave Tests Help Diagnose ADHD Symptoms," 2013, Oct. 15, Markus MacGill, Medical News Today