Sudden infant death syndrome: increased synaptic density in the central reticular nucleus of the medulla

The medulla was sampled from nine cases of sudden infant death syndrome (SIDS) and from six age-matched control cases without neurological disease. Morphometric analyses were performed on serial Nissl sections through the left half of each medulla to determine the total volume of the hemimedulla and the numerical density of neurons (Nv, neurons per mm3) in the central reticular nucleus. Blocks of tissue from the right half were prepared for electron microscopy using the ethanolic phosphotungstic acid (EPTA) method to stain synaptic contacts. Stereological analyses were performed to determine the Nv of synapses in the central reticular nucleus. Total volume of the hemimedulla did not differ significantly between SIDS cases and controls. However, in the central reticular nucleus of SIDS cases, the Nv of neurons was significantly less than in controls (30%), while the mean profile area of reticular neurons was significantly greater (39%). Synaptogenesis in control cases was characterized by a gradual decrease in the Nv of synapses from approximately 150 million at 40 postconceptional weeks to 110 million at 84 weeks. In SIDS cases the Nv of synapses was significantly greater (38%). These results demonstrate a disorder of synaptogenesis in the central reticular nucleus of SIDS cases. In view of previous reports of a persistence of dendritic spines on reticular neurons in SIDS, the results are consistent with a failure to eliminate normally extraneous synapses during early development.