Is Attention-deficit Hyperactivity Disorder a Behavior Disorder?

What causes the variability in diagnostic and treatment outcomes?
Despite a growing body of evidence that ADHD is a frontal-striatal circuit disorder,⁶ diagnosis is often based on behavioral criteria, with some arguing that neuropsychological evaluation is irrelevant for diagnosis.⁷ After all, ADHD is considered a disruptive behavior disorder. Standard practice includes physical exam, informant history, behavior rating scales, and use of Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR) ADHD criteria.8 However, many children with different causes for their attention problems may meet ADHD diagnostic criteria. Although DSM-IV-TR specifies that children be diagnosed with ADHD inattentive type (IT), hyperactive impulsive type (HIT), or combined type (CT), it appears that the HIT is rare, the CT is common and likely due to poor self-regulation or inhibition, and the IT is due to other causes.^6,9 However, diagnostic purity is rare, and the use of cut-off criteria (i.e. six out of nine criteria required for diagnosis) or their subjective nature likely results in diagnostic heterogeneity, even within subtypes. For instance, a child who meets nine IT and no HIT symptoms would be diagnosed with ADHD-IT, as would a child with nine IT and five HIT symptoms. From a neuropsychological perspective, the first child s attention problem is probably not due to frontal striatal dysfunction, whereas the latter child likely has a milder variant of ADHD-CT. Both children might be placed into a heterogeneous IT group for subsequent research and treatment protocols, which obviously confounds study results. The situation is further complicated by the fact that ADHD is rarely diagnosed alone, with comorbidity rates quite high.¹⁰ Many children with ADHD have learning disabilities, externalizing oppositional or conduct disorders, or internalizing anxiety or depressive disorders. Although this may lead practitioners to focus on attention problems first and comorbidities second, what if the attention problems are secondary to the condition considered to be comorbid ? Although meta-analyses suggest that ADHD children with comorbid externalizing symptoms are indeed different from those with co-occurring internalizing problems,¹¹ the question remains whether either child s attention problems would be considered primary ADHD or secondary to the other disorder symptoms.² For instance, children with receptive language problems are often inattentive during classroom instruction, and they may well meet ADHD-IT criteria, but is their inattention distinct from, comorbid with, or the result of the language disorder? Although most ADHD cases likely have a genetic basis,¹² attention problems also result from disorders such as lead poisoning, sleep disturbance, or absence seizures. An astute clinician with sufficient time and effort will consider multiple alternative causes for a child s attention problems, but complex diagnostic protocols may not be feasible for primary-care physicians who may have limited training in child psychopathology. If there are so many causes of attention problems, with and without comorbid conditions, and brief interviews are conducted using subjective criteria for diagnostic and treatment purposes, it should not be surprising that there is considerable controversy regarding the causes of ADHD, the best way to diagnose the disorder, and how treatment effects should be determined.^{13 15}

The Search for Attention-deficit Hyperactivity Disorder Endophenotypes
Recent theoretical and empirical advances suggest that there are multiple subtypes or endophenotypes of ADHD,^{16 17} each with somewhat different clinical presentations that can baffle practitioners trying to arrive at an accurate diagnosis and treatment plan. Even a thorough medical evaluation, use of DSM-IV-TR criteria, and rating scale data may not be enough for accurate ADHD diagnosis, especially given the inconsistency among informant reports.¹⁸ For instance, one subtype likely displays attention problems due to limited overall arousal or cortical tone. Another subtype seems to have poor cognitive efficiency, processing speed, and flat or dysphoric affect. A third subtype displays obsessive, ruminative, anxious behavior that leads to perseverative, inflexible, and internally distracted behavior. A fourth subtype has spatial holistic processing weaknesses that co-occur with limited attention to self and the environment or neglect syndrome. The fifth subtype, those with true ADHD, show traditional problems with executive control of attention resources, impulsivity, and hyperactivity. Attention problems occur in all these subtypes, but only this last subtype has striatal pre-frontal circuit hypoactivity and shows a significant response to stimulants.^{2,19 22} This makes psychopharmacological sense because stimulants increase dopamine availability specifically in the striatum.^{23 24}