Attention Deficit/Hyperactivity Disorder as a Right Hemisphere Syndrome

Attention-deficit/hyperactivity disorder: a preliminary diffusion tensor imaging study

BACKGROUND

The purpose of this study was to explore whether there are white matter (WM) abnormalities in children with attention-deficit/hyperactivity disorder (ADHD) using diffusion tensor imaging. Based upon the literature, we predicted decreased fractional anisotropy (FA) findings in the frontal and cerebellar regions.

METHODS

Eighteen patients with ADHD and 15 age- and gender-matched healthy volunteers received DTI assessments. Fractional anisotropy maps of WM were compared between groups with a voxelwise analysis after intersubject registration to Talairach space.

RESULTS

Children with ADHD had decreased FA in areas that have been implicated in the pathophysiology of ADHD: right premotor, right striatal, right cerebral peduncle, left middle cerebellar peduncle, left cerebellum, and left parieto-occipital areas.

CONCLUSIONS

These preliminary data support the hypothesis that alterations in brain WM integrity in frontal and cerebellar regions occur in ADHD. The pattern of decreased FA might implicate the corticopontocerebellar circuit in the pathophysiology of ADHD.

Diagnostic issues for adolescents and adults with ADHD

Attention deficit hyperactivity disorder (ADHD) is a common childhood neuropsychiatric syndrome once thought to disappear with maturation. Current data indicate that ADHD remains "hidden" in many of the grown-ups who had it as children. Adult prevalence rates range from 1% to 6% of the population. Research suggests the core childhood symptoms of hyperactivity, inattention, and impulsivity shift with development, perhaps transforming into more overt difficulties in executive functions and affect regulation. ADHD is also usually nestled with other comorbid psychiatric conditions, especially in adolescents and adults, further complicating diagnosis and treatment. This article discusses how to recognize and diagnose ADHD in older patients. Key points include core symptoms present during childhood, appropriate family history in this strongly genetic condition, management of comorbidity, and the evolving role of diagnostic testing. Other medical causes for similar symptoms are considered.

Impaired spatial working memory in adults with attention-deficit/hyperactivity disorder: comparisons with performance in adults with borderline personality disorder and in control subjects

OBJECTIVE

This study investigated a previous claim that working memory may be 'particularly impaired' in adult attention-deficit/hyperactivity disorder (ADHD), compared with other psychiatric disorders which affect frontal lobe-mediated executive functions.

METHOD

Performance on spatial working memory (SWM) and two additional tasks were investigated for adult patients selected on the basis of DSM-IV ADHD (n = 19), adult patients selected on the basis of borderline personality disorder (BPD) (n = 19), and non-clinical control subjects (n = 19). Groups were matched for age, verbal IQ and gender.

RESULTS

Analysis of variance showed that the ADHD group had significant impairment of SWM performance relative to the non-clinical controls. Although there was a trend towards impairment in the BPD group relative to non-clinical controls, this did not reach significance.

CONCLUSION

The results are consistent with the claim that aspects of working memory are 'particularly impaired' in adult ADHD. Also, the BPD group had a longer deliberation time for one of the additional tasks, compared with the ADHD group, which indicated that the patient groups may have different patterns of neuropsychological impairments.

What the rodent prefrontal cortex can teach us about attention-deficit/hyperactivity disorder: the critical role of early developmental events on prefrontal function

The present review surveys a broad range of findings on the functions of the rodent prefrontal cortex (PFC) in the context of the known pathophysiology of attention-deficit/hyperactivity disorder (ADHD). An overview of clinical findings concludes that dysfunction of the right PFC plays a critical role in ADHD and that a number of early developmental factors conspire to increase the risk of the disorder. Rodent studies are described which go far in explaining how the core processes which are deficient in ADHD are mediated by the PFC and that the mesocortical dopamine (DA) system plays a central role in modulating these functions. These studies also demonstrate a surprising degree of cerebral lateralization of prefrontal function in the rat. Importantly, the PFC is highly vulnerable to a wide variety of early developmental insults, which parallel the known risk factors for ADHD. It is suggested that the regulation of physiological and behavioral arousal is a fundamental role of the PFC, upon which many "higher" prefrontal functions are dependent or at least influenced. These right hemispheric arousal systems, of which the mesocortical DA system is a component, are greatly affected by early adverse events, both peri- and postnatally. Abnormal development, particularly of the right PFC and its DAergic afferents, is suggested to contribute directly to the core deficits of ADHD through dysregulation of the right frontostriatal system.

Speed of performance of children with developmental right hemisphere syndrome and with attention-deficit hyperactivity disorder

Slowness is a common complaint in children with attention-deficit hyperactivity disorder (ADHD) and with developmental right hemisphere syndrome. However, it was our clinical impression that slowness in developmental right hemisphere syndrome was more prominent than in ADHD. Our objective was to assess slowness as operationalized by speed of performance in children with developmental right hemisphere syndrome, children with ADHD, and controls. The research sample comprised 19 children in each group, matched for age, gender, socioeconomic status, IQ, and handedness. The subjects were administered a reaction time battery assessing speed of performance. Overall, the average performance differed among the three study groups (F(2,53) = 2.40, P < .01). Children with developmental right hemisphere syndrome were slower than their peers with ADHD (t(35) = 1.99, P < .05) and slower than controls (t(35) = 4.55, P < .001). Children with ADHD performed more slowly than controls, although for the majority of tasks, this was nonsignificant. We conclude that slowness is an integral and consistent component of developmental right hemisphere syndrome and cannot be attributed only to the ADHD symptomatology.

Animal models of attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) involves clinically heterogeneous dysfunctions of sustained attention, with behavioral overactivity and impulsivity, of juvenile onset. Experimental models, in addition to mimicking syndromal features, should resemble the clinical condition in pathophysiology, and predict potential new treatments. One of the most extensively evaluated animal models of ADHD is the spontaneously hypertensive rat. Other models include additional genetic variants (dopamine transporter gene knock-out mouse, coloboma mouse, Naples hyperexcitable rat, acallosal mouse, hyposexual rat, and population-extreme rodents), neonatal lesioning of dopamine neurons with 6-hydroxydopamine, and exposure to other neurotoxins or hippocampal irradiation. None is fully comparable to clinical ADHD. The pathophysiology involved varies, including both deficient and excessive dopaminergic functioning, and probable involvement of other monoamine neurotransmitters. Improved models as well as further testing of their ability to predict treatment responses are required.

Quantitative Signal Intensity Measures on Magnetic Resonance Imaging in Attention-Deficit Hyperactivity Disorder

OBJECTIVE

The aim of this study was to investigate possible abnormalities of cerebral myelination in subjects with attention-deficit hyperactivity disorder (ADHD).

BACKGROUND

Anatomic and functional neuroimaging studies of subjects with ADHD demonstrated a right frontostriatal deficit and abnormal cerebral asymmetries. Some also reported white matter abnormalities, such as smaller white matter volumes in the right anterior-superior frontal region, and the smaller bilateral retrocallosal region. Smaller volumes in specific areas of the corpus callosum have also been reported. We hypothesized that white matter signal intensities may also show differences indicating abnormal cerebral myelination.

METHOD

We analyzed T2-weighted magnetic resonance images of 11 adolescents with ADHD and 20 controls. Regions of interest were set in both the white and gray matter in frontal and parieto-occipital associative regions.

RESULTS

The ADHD group showed a higher signal intensity ratio, probably reflecting a higher degree of myelination. Significant interhemispheric differences emerged only in the posterior region in the ADHD group.

CONCLUSIONS

The higher degree of myelination in the right frontal region of ADHD may be due to a compensatory mechanism for the right frontostriatal dysfunction.

Smaller prefrontal and premotor volumes in boys with attention-deficit/hyperactivity disorder

BACKGROUND

Anatomic magnetic resonance imaging (MRI) studies of attention-deficit/hyperactivity disorder (ADHD) have been limited by use of callosal rather than sulcal/gyral landmarks in defining cerebral lobes and functionally relevant sublobar regions (e.g., prefrontal cortex). We present an investigation of cerebral volumes in ADHD using a Talairach-based approach that uses cortical landmarks to define functionally relevant regions.

METHODS

Volumes were compared between groups of 12 boys with ADHD and 12 age- and gender-matched control subjects, using a series of multiple analyses of variance.

RESULTS

Boys with ADHD had (on average) 8.3% smaller total cerebral volumes. Significant reductions in lobar volumes were seen only for the frontal lobes. Within the frontal lobes, a reduction was seen in both gray and white matter volumes, with some evidence suggesting lateralization of these findings: reduction in frontal white matter volume was specific to the left hemisphere; there was a bilateral reduction in frontal gray matter volume but more so in the right hemisphere. Subparcellation of the frontal lobe revealed smaller prefrontal, premotor, and deep white matter volumes.

CONCLUSIONS

Findings suggest that ADHD is associated with decreased frontal lobe gray and white matter volumes. More than one subdivision of the frontal lobes appears to be reduced in volume, suggesting that the clinical picture of ADHD encompasses dysfunctions attributable to anomalous development of both premotor and prefrontal cortices.