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Cause of ADHD: Is it
biological?
 There
is no single cause of ADHD, making it very likely that it's relatively
heterogeneous. However, a variety of studies using neurochemical
markers, family-genetic analysis, patterns of comorbidity and family
studies have began to distinguish between some of the larger, more
homogenous groups in their search for the cause of ADHD.
Cause of ADHD:
Biological Etiology
Although the specific
cause of ADHD has been relatively elusive, recent advances in research
methods and technology have helped us to find some of the
neurobiological correlates with ADHD. This research into the cause of
ADHD, is focus on neural circuits centered in the prefrontal cortex
and striatum, as well as on the brain stem catecholamine systems that
innervate this circuitry. Information from neural imaging studies
suggest that impairments in these prefrontal-striatal regions may play
a key role in determining the cause of ADHD.
Studies using magnetic
resonance imaging (MRI) have identified subtle abnormal changes in the
prefrontal cortex and basal ganglia unique to ADHD. Findings of a
smaller right prefrontal cortex, in addition to a smaller caudate
nucleus and globus pallidus, in children with ADHD suggest the
possibility that there may be some association with fewer prefrontal
corticostriatal fibers and less pallidal feedback to prefrontal
regions of the brain. Also, the reduced area in the corresponding
anterior genu region of the corpus callosum may indicate the presence
of fewer interhemispheric fibers in prefrontal regions. Abnormalities
have also been found in ADHD regions that project to the prefrontal
cortex, including the parietal-occipital regions, and the
cerebellum. These findings raise the possibility that brain
abnormalities in ADHD extend beyond the prefrontal cortex and striatum
to the posterior and subcortical regions that innervate these frontal
circuits.
Some of these
neuroimaging studies have provided some additional evidence of
prefrontal-striatal impairment in ADHD. Research with single photon
emission computerized tomography (SPECT) and
positron emission tomography (PET) have reported that there may be
lower basal activity in the prefrontal cortex and striatum of children
and adults, but not adolescents with ADHD. More recent studies of the
cause of ADHD employing functional MRI, have tentatively identified
altered prefrontal-striatal activation with deficits in an
individual’s control of inhibition. Reduced striatal activation during
response inhibition tasks have been consistently reported in children
and adolescents with this disease. However, prefrontal activation
during the same tasks was enhanced in children with ADHD, but reduced
in adolescents with this disorder.
The prefrontal cortex
and striatum are believed to be very sensitive to catecholaminergic
input from the brain stem nuclei, including noradrenaline (NA) from
the locus coeruleus, and dopamine (DA) from the ventral tegmental area
and substantia nigra. The fact that almost all medications which work
in individuals with ADHD affect the levels of noradrenaline and
dopamine transmission, suggests that perturbations of these systems
may play a significant role in the cause of ADHD. However, a large
group of studies of the catecholamine function in ADHD have continued
to yield highly inconsistent findings. Only more recent studies that
used central indices of catecholamine function that examined more
homogenous subgroups of children with ADHD, have provided evidence of
dopamine and noradrenaline dysfunction which is believed to possibly
be a cause of ADHD.
Cause of ADHD:
Imaging Results
Recent developments in
attempting to find the cause of ADHD, involve the use of the PET and
SPECT imaging in combination with dopamine-selective radiotracers to
examine localized dopamine function in vivo. These studies have
revealed preliminary evidence of increased striatal dopamine
transporter binding in adults with ADHD, and altered dopamine
synthesis in the prefrontal cortex and right midbrain of children and
adults with ADHD. This data may indicate some localized dopamine
deficits in the nigrostriatal and mesocortical fiber systems in the
cause of ADHD.
Some
information from DSM-IV-TR Mental Disorders Diagnosis, Etiology &
Treatment
Additional Information and webpage by
Paul Susic
MA
Licensed Psychologist Ph.D. Candidate
(Health and Geriatric Psychologist)
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