Neuronal apoptosis in the brainstem medulla of sudden unexpected death in infancy (SUDI), and the importance of standardized SUDI classification

The purpose of this study was to examine the neuronal expression of apoptotic markers in the rostral medulla of a newly characterized dataset of sudden unexpected death in infancy (SUDI), and to determine the impact of diagnostic groupings on these findings and whether they pertain to the intrinsic apoptotic pathway. Immunohistochemical staining was quantified to determine the percentage of neurons positive for active caspase-9 (specific to the intrinsic apoptotic pathway), active caspase-3 (common to the intrinsic and extrinsic apoptotic pathways) and Terminal deoxynucleotidyl transferase mediated dUTP nick-end labelling (TUNEL) (labels DNA fragmentation) in nine nuclei of the rostral medulla. Expression was compared between groups of SUDI infants where the cause of death was initially classified by a forensic pathologist or subsequently after reclassification by an expert panel using the San Diego Criteria. 68 SUDI infants were studied and originally classified as explained SUDI (n = 12), Sudden Infant Death Syndrome (SIDS) (n = 27) and undetermined (n = 29). Reclassification resulted in a decrease in the number of explained SUDI cases to 7 and a decrease in the number of undetermined cases to 4, with a corresponding increase in the number of SIDS cases to 57 (8 SIDS I; 49 SIDS II). The expression of apoptotic markers was similar in explained SUDI and SIDS I infants. However, TUNEL expression was greater in the cuneate (p < 0.001), vestibular (p = 0.01) and hypoglossal (p < 0.001) nuclei and active caspase-3 expression was lower in the arcuate nucleus (p = 0.037) in SIDS II compared to explained Sudden Unexpected Death in Infancy (eSUDI) infants. Compared to SIDS I infants, SIDS II infants had greater TUNEL expression in the dorsal motor nucleus of the vagus (p < 0.01) and greater active caspase-9 expression in the medial and spinal vestibular nuclei (p =<0.01). Changes in apoptotic expression predominated in SIDS II infants. We postulate that these are due to a combination of contributing risk factors including the presence of an upper respiratory tract infection and bed-sharing/co-sleeping. The absence of changes in active caspase-9 expression compared to eSUDI indicates that the intrinsic apoptotic pathway is not upregulated in SIDS.