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Órgano Oficial del Instituto Nacional de Psiquiatría Ramón de la Fuente Muñiz

2005, Number 3

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Salud Mental 2005; 28 (3)

Anatomic and functional alterations in attention-defficit/hyperactivity disorder.

Almeida MLG

Language: Spanish
References: 57
Page: 1-12
PDF size: 106.48 Kb.


ABSTRACT

The most recent technologies of cerebral imaging provide new data about cerebral abnormalities in patients with diverse psychiatric disorders.
These methods include computed tomography (CT), magnetic resonance (MRI), electro magnetic (EMEKG), functional magnetic resonance (MRIf), positron emission tomography (PET), single photon emission computed tomography (SPECT), quantitative electroencephalography (QEEG), and evoked potentials (Eps), among others.
Illnesses such as anxiety disorders, depressive disorders, dementing disorders, obsessive compulsive disorder, schizophrenia, bipolar disorder, learning disorders, and attention-deficit/hyperactivity disorder (ADHD), are now considered the result of an interaction between environmental factors and abnormal function and structure of the brain.
Data obtained from studies in ADHD subjects indicate a biological basis for this disorder. The specific anatomical and functional alterations of the brain in these patients, has been possible thanks to neuro-imaging.
Satterfield & Dawson, in 1971, were pioneers proposing that the ADHD symptoms were related to a malfunction of the fronto-limbic circuits. They said that the normal fronto- cortical inhibitory control exerted over the limbic system is weak in patients with ADHD. This hypothesis was known as the “fronto limbic hypothesis of ADHD”; although, the results of the research derived from diverse disciplines such as neuropsychology, neuro-imaging, and neuro biochemistry, suggest that Satterfield & Dawson were partially right, in fact the neurobiology of ADHD would be more complex than they originally supposed.
Almost every research done with neuropsychological methods in ADHD patients points to the same results: a malfunction in the frontal cortex.
Damage in the orbital area of the human frontal cortex produces social disinhibition and impulsivity; also injury to dorsolateral prefrontal cortex (DLPFC), causes deficit in the ability of behavioral organization, planning, working memory and attention. The findings of neuropsychological research point toward dysfunction to the orbito frontal cortex and DLPFC in subjects with ADHD.
With the advance of the new neuroimaging techniques, the results delivered from the clinical and neuropsychological observations can be validated; when used in combination they can give us detailed information about the anatomic areas involved.
The early structural studies made in subjects with ADHD were done using CT. Unfortunately the poor resolution and the lack of quantitative measurement of the CT, besides of the small sample size of subjects using diverse methods of clinical diagnosis, and the difficulty to find healthy controls; made the results of these early studies with CT inconsistent.
The results reported by different authors were contradictory, since some of them reported no abnormality at all in subjects with ADHD, and some others found frontal and cerebellar atrophy in patients with ADHD.
The studies made with MRI improved the quality of the structural studies done in subjects with ADHD. At the same time, the improvement of the diagnostic tools in regard to validity and reliability, and the inclusion of adequate control groups; made the results obtained from these new studies to overcome the deficits of the studies that were made with CT.
The studies made with MRI found a decrease in the size of corpus callosum, basal ganglia, particularly in left globus pallidus, and cerebellum; and atrophy of the right frontal cortex and a change in the total cerebral volume.
On the other hand the studies made with techniques such as PET, SPECT and functional magnetic resonance, have found a decrease in cerebral blood flow (CBF) in the frontal lobes, striatum and cerebellum in subjects with ADHD.
The studies with SPECT made by several groups of researchers have shown increase of the CBF frontal cortex and the caudate nucleus, and decrease of the CBF of the occipital cortex; however, almost all these studies included small sample sizes and used inadequate control groups.
Other studies made with PET in adults with ADHD have shown a decrease of the metabolism of the frontal cortex; however, different groups of researchers have reported different findings, possibly due to methodological variations across the studies. Again the majority of these studies used small sample sizes.
The studies made with MRIf show that the fronto-stratial circuits work differently in subjects with ADHD when they are compared with controls. Again almost all the published studies included a small sample size.
In particular, the results from the studies done by Bush C. et al in 1999 using MRIf combined with a neuropsychological test named “Counting Stroop”, showed a lack of activation of the anterior cingulate gyrus in subjects with ADHD compared with controls.
The PET technology can study the brain chemistry in vivo. Some researchers have found a decrease of dopamine reuptake in the prefrontal and medial cortex and an increase in the mesencephalon in patients with ADHD. Some studies have found an increase of the dopamine transporter density in the striatum in subjects with ADHD.
In conclusion: the neuropsychological studies and the studies of neuroimaging, suggest that the fronto-stratial circuit in the right side plus the cerebellar influence are involved in the neurobiology of ADHD. The cortex-striatum-thalamic circuit selects, starts and executes cognitive and motor complex responses, and the cerebellar circuits provide the guide for these functions.
The hypothetical implications of the data contributed by the findings with neuroimaging techniques in ADAH are at this time only tentative. Reproduction of this data for independent researchers is needed. Besides, it is necessary to perform more detailed subdivisions of the brain areas implicated in ADHD.
The low statistical power due to the cohort small size included in almost all of these studies is an important problem because of the high anatomical variability of brain measurements.
Besides just a few of the published studies controlled the previous exposure to medications for ADHD. Finally, there are no studies in other circuits that play a role in the attention such as those formed by the thalamic pulvinar, the parietal inferior cortex, the primary sensory area, and the postero-inferior parietal cortex.
However, the research using methods of structural and functional imaging surely will be valuable in the future, in order to improve our understanding of the anatomical and functional physiopathology of ADHD and of the other psychiatric disorders.


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