Biomedical Engineering Reference
In-Depth Information
oriented, and they often result in substantial improvement, especially when started
at an early age.
During the past two decades, the study of autism's neuropathology has dra-
matically intensified. Most studies have reported alterations in some regions of
the brain in autistic individuals compared to typically developing ones. Increased
head size was the first observed characteristic in children with autism 60 years
ago in a study by Kanner [8]. Since then, several studies have reported enlarged
brain size and head circumference (HC) in autistic patients. Postmortem studies
have revealed evidence of increased brain weight, while bigger brain volume and
macrocephaly, defined as HC above the 97th percentile, were reported by several
clinical and neuroimaging studies [9-16]. In particular, Courchesne et al. [13, 17]
showed that while children with autism have ordinary size brains at birth, they
experience an acceleration of brain growth, resulting, between 2 and 4 years of
age, in increased brain volume relative to controls. By adolescence and adulthood,
differences in mean brain size between the two groups diminish largely as a result
of increased relative growth in the normal control group. Indeed, no differences
in brain volume between the two groups were reported in[12, 17, 18].
The rest of this section will briefly survey the neuropathological and neu-
roimaging literature on autism and will discuss the findings that have emerged.
Different studies have focused on different parts of the brain in order to identify
the structures that are involved in the abnormal neurodevelopment associated with
autism. Alterations of different brain regions have been reported by these studies
in autistic subjects relative to controls.
Limbic System
The limbic system, known as the center of emotions, includes many cortical
and subcortical brain structures. These structures are involved in emotion, and
emotional associations with the memory. Examples of these structures are the
amygdale, the hypothalamus, the orbito-frontal cortex, and the subiculum. Kemper
and Bauman [19] were the first to report an increase in cell packing and reduced cell
size in the hypocampus, amygdale, subiculum, and other structures as the result of
a comparative study between six autistic cases (9-29 years of age) and six age- and
sex-matched controls. Gurin et al. [20] have reported a thin corpus callosum (CC)
in a 16-year-old female with autism and severe mental illness. Microscopically,
however, no alterations were observed in the limbic structures.
A number of MRI-based studies have reported that the corpus callosum, which
is the largest commisure in the brain that allows neural communications between
the two hemispheres, has a reduced size in autistic subjects [21, 22]. However,
findings are inconsistent as to which segment of the CC is abnormal. Most studies
have reported reduction in the size of the body and the posterior subregions of
the CC in autistic patients [21, 23-25], whereas other studies have found that the
reduction was limited to the body and the anterior segment of the CC [20, 26].
