Assessment and intervention practices for children with ADHD and other frontal-striatal circuit disorders - Hale et. al. - 2010 - Article

ADHD is widely recognized to be a frontal-subcortical circuit disorder, with the affected brain regions as possible contributors to both cognitive as well as behavioral symptoms. Frontal-subcortical circuit disorders most often lead to attention impairments, which implicates that differential diagnosis only based on behavioral criteria can be very difficult.

Cognitive and neuropsychological functions

The main area involved in ADHD is the prefrontal cortex and its associated structures, which are responsible for executive functions such as planning, problem solving and evaluating. Also, structures responsible for transferring information from one hemisphere to the other, like the corpus callosum, are found to be affected in children with ADHD. Also, research has shown that cerebellar regions like the right prefrontal are reduced in volume. This suggests that ADHD not only affects the frontal lobes, but a number of interrelated midline circuits and tracts extending from the subcortical cerebellar to the cortical prefrontal regions. This axis is likely the seat of most known psychopathologies, like ADHD. Therefore, symptoms like impulsivity, hyperactivity and inattention are probably just one facet of the disorder, with multiple other systems and area’s likely to be affected.

Impulsivity due to an impaired inhibitory control system results in maladaptive externalizing behaviors like fearlessness and carelessness. This should be regulated by the frontal lobes, like in typically developing children. Executive attention as well as behavioral regulation highly depends on inhibitory functions. Therefore, the theory of ADHD by Barkley states that deficits in multiple cortical and subcortical regions can be found in children with poor inhibitory control. A distinction can be made between top-down cortical inhibitory problem considering interference control and behavioral inhibition, and bottom-up subcortical problems which are related to (the lack of) response to reinforcement and punishment. It seems that the right prefrontal cortex is more involved in representation and maintenance of adaptive responding and suppressing irrelevant events, while the basal ganglia is more involved in suppression of response. All in all, inhibitory functions consist of multiple implicated areas.

It seems that children with ADHD not so much have a primary attention deficit, but experience difficulty with the executive control of attention. This can be explained by the specific abnormalities linked with attention dysfunction, which are also involved in the inhibitory process. The deficits in these brain area’s result in a disorder of attention control . Since dysfunctions in the frontal brain areas can be associated with multiple disorders, heterogeneity among children referred for attention problems must be considered.

A defect in the regulation of dopamine can lead to dysfunctional frontal-subcortical circuits, resulting in hyperactive behavior. Also, children with ADHD often experience impaired motor precision and consistency, which frequently results in comorbid Developmental Coordination Disorder. Since individuals with ADHD show impairments in both areas, it’s important to determine whether a motor deficit is the result of a cortical motor (e.g. basal ganglia), subcortical cerebellar problem or both. Furthermore, children with ADHD have more sensory processing deficits than controls. This could be due to impaired thalamatic functioning in individuals with ADHD or poor efficiency of connectivity of white matter connections between the anterior and posterior regions. Also, problems with transferring information because of a deficit in the corpus callosum like mentioned earlier could lead to problems with sensory-motor integration.

Executive functioning

Although deficits in executive functioning is probably the most consistent neuropsychological finding among children with ADHD, there is also still some controversy. We consider ADHD as best characterized by executive deficits in vigilance, response inhibition, planning and working memory. It’s important to mention that these impairments couldn’t be explained by intelligence, comorbidities or achievements. Because of its similarities with executive function, fluid reasoning can be considered as a cognitive test. Both require the abilitiy to solve novel problems, benefit from feedback and adaptive responding. As the complexity of problem solving increases, so does dorsolateral prefrontal functioning. Children with ADHD and social problems show impairments in their fluid reasoning and visual organization/praxis measures with high executive demands. This suggests that fluid reasoning is in part a measure of right dorsolateral prefrontal functioning, in which individuals with ADHD show deficits. A strong predictor of fluid abilities is working memory, which is also often impaired in individuals with ADHD. Especially spatial working memory appears to be impaired. Executive functions that include working memory have been found to be related to hyperactive-impulsive symptoms.

Children with ADHD exhibit deficits in processing speed and efficiency, possibly resulting from a lack of energy supply to rapidly firing neurons. This appears to be specific for the inattentive ADHD subtype. Because of all those deficits discussed, and its impact on encoding, storing, and retrieving information, it’s not surprising that children with ADHD often experience learning difficulties. Academic deficits have long been considered the final common pathway for children with ADHD. Specific Learning Disabilities (SLD) therefore is an often found comorbid disorder. Difficulties with attending to the novel information presented is what makes new learning especially hard for children with ADHD. Long-term memory storage on the other hand doesn't seem to be impaired. The difficulty with new information may be due to a working-memory overload because of the inability to filter out irrelevant information. Researchers found meaning to facilitate learning. When children ought something important, they more easily remember it. Therefore emotional salience increases episodic memory in children with ADHD. They also found that nucleus accumbens deficiencies may lead to deficient motivation or reinforcement during new learning situations.

Since the frontally mediated executive functions govern all other aspects of cognition, it’s not surprising that children with ADHD also experience visual-spatial-holistic right hemisphere and auditory-verbal-crystallized left hemisphere dysfunctions. These impairments can also influence the ability to process information. Especially visual working memory seems to be impaired, but luckily this can be improved with medication treatment. The problems with visual-spatial-holistic processing may be due to its strong relationship with executive function, which is often impaired in children with ADHD. Furthermore, attention deficits overlap with auditory processing and language disorders, which can interfere with the learning, behavioral and social outcomes of affected children. Poor internalization of language in ADHD is in part responsible for poor behavior regulation and impulse control. Children who do not process language efficiently will appear inattentive in the classroom, and could be diagnosed with comorbid ADHD. But do those children really have ADHD? Early research did not screen for comorbid SLD, but auditory processing and language deficits are more often associated with SLD than ADHD. It could be that language deficits in ADHD are more related to the executive and expressive aspects of language.

Comorbidities

Children with ADHD often show impulsive, aggressive and noncompliant behavior which results in conflicts with their peers and family. They show impaired emotional and behavioral inhibition. These behaviors most often occur with children who have comorbid disorders like Conduct Disorder(CD) or Oppositional Defiant Disorder (ODD). The latter is the most common comorbid condition, with an occurrence of 40 to 60% of children with ADHD, mostly males.

Impaired executive functioning can result in low self-esteem, poor concentration and irritability. Therefore, children with ADHD may develop a depression. Rates of comorbidity range from 10 to 50%, and children with the inattentive type of ADHD are more likely to develop an internalizing disorder. Another highly common comorbid disorder is Bipolar Disorder (BPD) . This comorbidity increases the risk of neurocognitive deficits and poor treatment response because mania are often mistaken for ADHD in children, but require a different form of medication.

Anxiety disorders can be primarily linked to children who suffer from the inattentive subtype of ADHD. A more severe form of anxiety is Obsessive-Compulsive Disorder which also shows similarities with ADHD and therefore is a comorbid disorder. Many children with Tourette Syndrome also suffer from ADHD. This is probably because the involuntary tics wax and wane, with attentional problems found to precede or follow the onset of tics.

Function measures

Some executive function measures are more effective than others in the diagnosis of ADHD and for conducting school neuropsychological evaluations. Most tests measure response inhibition, interference control, working memory vigilance and planning, but are generally more sensitive to ADHD than they are specific. This is probably due to the fact that several other disorders also show impaired performance on these and other executive measures. School neuropsychologists should use a variety of direct assessment tools for ADHD diagnosis and intervention purposes, because no single assessment tool is sufficient enough. Indirect measures of ADHD symptoms, such as objective behavior rating scales are also important for a comprehensive ADHD evaluation, but not sufficient for a differential diagnosis of ADHD.

Because comorbidity is more the rule than the exception among psychiatric disorders including ADHD, interrelationships can only be clarified by examination of underlying deficits. It’s possible that children who meet behavioral criteria for ADHD actually suffer from their comorbid disorder as the primary disorder, with secondary attentional problems. Therefore it’s important to determine to what extent psychiatric disorders have similar or different etiologies, if these differences can lead to more accurate diagnostic techniques and whether these differences can lead to more effective treatment practices and outcomes.

Although research into these matters provide sometimes conflicting findings, the overall conclusion is that most (if not all) psychopathologies have a basis in the frontal-subcortical circuits, and that their dysfunctions lead to observable behavior and learning problems. There are at least five interdependent frontal-subcortical circuits with dozens of related executive functions. Therefore impairments in this area can lead to many different psychiatric disorders. So far there is no clear explanation for all these differences, but different gene environments are suggested as one of the reasons. Another important issue is how we define disorders. Because behavioral diagnostic methods lead to heterogeneous samples of children diagnosed with psychiatric disorders, this is often more meaningful for diagnosis than neuropsychological measures. However, to explain differences within the group of children with a certain disorder, neuropsychological assessment can be very useful. More information about differences within a disorder can lead to more targeted treatment plans and interventions, because those are currently quite ‘global’.