Despite the now widely accepted fact that biological factors are of crucial importance in the etiology of autism, so far the brain imaging studies have shown no consistent pattern, no consistent evidence of any type of lesion, and no single location of any lesion in subjects with autistic symptoms. This inconsistency in the results of various brain imaging studies has been attributed to the fact that people with autism represent a highly heterogeneous group in terms of underlying pathology. Therefore there is an ongoing effort to specify more homogenous subgroups among autistic individuals to enhance the accuracy of etiologic inquiry. This approach has been supported with the inclusion of the diagnosis 'Asperger's Disorder' in the Fourth Edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) of the American Psychiatric Association.
Associated medical conditions such as fragile-X syndrome, tuberous sclerosis, neurofibromatosis, and hypothyroidism are less common in Asperger's Disorder than in classical autism. Therefore it may be expected that there are fewer major structural brain abnormalities associated with Asperger's Disorder than with autism. To our knowledge, a very small number of structural brain abnormalities have been so far associated with Asperger's Disorder, which include left frontal macrogyria, bilateral opercular polymicrogyria, and left temporal lobe damage. On the other hand brain imaging techniques like positron emission tomography (PET), and single photon emission tomography (SPECT) which provide information about the functional status of brain may be more helpful in determining the brain dysfunction in individuals with Asperger's Disorder. Detailed neuropsychological testing may support these findings providing information about the performances of individual right or left hemispheric brain regions. The first SPECT study in a patient with Asperger's Disorder was published by the host of this page and his colleagues, and found left parietooccipital hypoperfusion. Continuation of research in Asperger's Disorder with various brain imaging techniques in coordination with neuropsychological evaluation in larger samples is clearly needed in this area.