Meeting report: Second International Rett Syndrome Workshop and Symposium

Dawna Armstrong led off the summary session by reviewing the morphologic findings. Brain weights in Rett syndrome autopsy samples are 60% to 80% of expected values. Cerebral cortex has increased packing density, with decreased size of neurons and an apparently normal number of neurons, suggesting immature cortical development. Findings in the substantia nigra from Cheryl Kitt are similar. Dr Armstrong quantitated the dendritic tree in six cortical areas and found reduction in dendritic trees in all areas, with the most prominent being in the temporal lobe and the least prominent in the visual cortex. The reductions in dendritic branching do not differ with age, being the same from 4 to 35 years of age. Basal dendrites were more affected than apical dendrites, and neurons in layer 5 were more affected than those in layer 3. Dr Armstrong postulated that the increase in synaptic density from 28 weeks' gestation to birth occurred on schedule, but the secondary increase that occurs after 2 months, with a peak at 19 months, was disordered. Verne Caviness noted that these are the most informative data regarding Rett syndrome presented to date and can be summarized in three areas: (1) There is a morphologic disorder with a decrease in dendritic length, decrease in branching, and probably a decrease in the total number of synapses. He stressed that a morphologist requires a change of about 50% in the population to recognize a difference. (2) There is a dropout of certain histochemical variables, with a 93% reduction in the cholinergic system, and substantia nigra debris indicating cell death. (3) The phenomenon is generalized, with a 30% reduction volumetrically.(ABSTRACT TRUNCATED AT 250 WORDS)