But there are revolutions brewing.
The intersection of high resolution neuro-imaging and genetic analysis is dramatically changing our understanding of disorders sometimes known as 'mental retardation':
A developmental and genetic classification for malformations of cortical development -- Barkovich et al. 65 (12): 1873 -- NeurologyThis very specialized article is not for the layperson; it's really written for the pediatric neurologist. Beneath the jargon, however, are some tectonic shifts in the definition of developmental disorders. It's a migration from external appearances (pre-1990 and still the world in which most of us live) beyond imaging studies (1990-2001, cutting edge yet) to the intersection of genes and environment (2005 and beyond).
Increasing recognition of malformations of cortical development and continuing improvements in imaging techniques, molecular biologic techniques, and knowledge of mechanisms of brain development have resulted in continual improvement of the understanding of these disorders. The authors propose a revised classification based on the stage of development (cell proliferation, neuronal migration, cortical organization) at which cortical development was first affected. The categories are based on known developmental steps, known pathologic features, known genetics (when possible), and, when necessary, neuroimaging features. In those cases in which the precise developmental and genetic features are uncertain, classification is based on known relationships among the genetics, pathologic features, and neuroimaging features. The major change since the prior classification has been a shift to using genotype, rather than phenotype, as the basis for classifying disorders wherever the genotype–phenotype relationship is adequately understood. Other substantial changes include more detailed classification of congenital microcephalies, particularly those in which the genes have been mapped or identified, and revised classification of congenital muscular dystrophies and polymicrogyrias. Information on genetic testing is also included. This classification allows a better conceptual understanding of the disorders, and the use of neuroimaging characteristics allows it to be applied to all patients without necessitating brain biopsy, as in pathology-based classifications.
The word "autism" doesn't appear in the paper, but that's only a matter of time. We are about to change the way we think of disorders of the developing mind and brain, and many of our labels are going to go away. In the new world we'll begin to make sensible statements about prognosis and to design studies of therapy that can be truly meaningful. This is good.
PS. See also: Asperger's a vague diagnostic concept.