B. Psychiatric History
The brain consists of two cerebral hemispheres, the ventricles, the cerebellum, and the brain stem. The
Abnormalities of brain structure have been observed in many pediatric neuropsychiatrie illnesses, including autism, attention deficit/hyperactivity disorder (ADHD), childhood-onset schizophrenia, dyslexia, eating disorders, fetal alcohol syndrome, obsessive-compulsive disorder, Sydenham's chorea, and Tourette's syndrome. It is evident that a normative sample must be carefully screened to rule out these conditions. Likewise, affective disorders and substance abuse have been associated with structural anomalies in adults and should be considered as potential confounds in pediatric samples as well.
Beginning with Geschwind, several investigators have noted a relationship between handedness and structural symmetry measures of the brain. Handedness should not be viewed as strictly left or right but as a continuum, and it should be quantified as such. Patient and control groups must be matched for handedness since symmetry differences are often key features in discriminating groups such as ADHD, dyslexia, or Tourette's disorder.
The relationship between brain size and body size in humans is surprisingly weak. In contrast to the relative stability of brain weight after childhood, body weight varies widely among individuals and can vary substantially within individuals from time to time. Height is also a poor indicator of brain size, as can be implied by contrasting the notable increases in height from ages 4 to 18 years with the lack of corresponding increase in brain size. This general trend for the young to have disproportionately large head-to-height ratios compared to adults is widely observed throughout the mammalian species.
As indicated by autopsy and imaging studies, the male brain is approximately 10% larger than the female brain across all ages. Of course, gross structural size may not be sensitive to sexually dimorphic differences in connectivity between different neurons, known differences in receptor density, or more subtle differences in the size or connectivity of various nuclei. Given the multiple parameters determining brain size, a larger size should not be interpreted as imparting functional advantage or disadvantage.
A plot of total cerebral volume versus age for 146 healthy boys and girls is presented in Fig. 2. As can be seen, brain sizes are highly variable. This large variability of brain sizes means that a larger number of subjects, or following the same subjects over time, is necessary to discern how brain anatomy changes during adolescence. The relative stability of total brain size throughout childhood and adolescence belies the dynamic activity of the various subcomponents of the brain. In the following sections, the effects of age and gender on these different parts of the brain are examined.
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