A systematic study was made of the cardiac conduction system in babies dying suddenly and unexpectedly and in whom careful necropsy had provided no explanation. Forty such hearts were compared with 16 hearts from babies of the same age range, but who had died of known causes, and with the hearts from five stillbirths at term and 1 five month fetus. An unusual histologic process involved the A-V node and His bundle of 56 of the 62 hearts, including all the hearts in the sudden unexplained deaths as well as all the deaths of known cause; the only hearts not showing this change were those of the stillbirths and the fetus. Adult human hearts do not show this change. The histologic process consisted of a resorptive degeneration involving the left portion of the His bundle and the left margin of the A-V node. There was no associated inflammation or significant hemorrhage, but only focal degeneration of conduction tissue cells which were being removed by macrophages and replaced with collagen by young fibroblasts. This molding and shaping process apparently begins at about the time of birth and may be initiated in part by the normal postnatal hemodynamic changes leading to increased physical stress on the central fibrous body and the septal attachments of the atrioventricular valves. However, there must be additional controlling factors (e.g., chemical reactions at the interface between cells of conduction tissue and fibroblasts). The molding process (which distinctly includes cell death) proceeds focally rather than diffusely. It probably has periodic acceleration of activity and must ordinarily be completed within the first year or two of life since it is not normally found in the hearts of older subjects.

When one is dealing with critically important structures such as the A-V node and His bundle, which are only millimeters in maximal dimensions, then the presence of any histologic lesion becomes functionally significant. One can neither ignore small lesions nor attach significance only in direct relation to their size. Because the degenerating cells may be anticipated to behave electrophysiologically in a different fashion from normal cells, it is suggested that focal malfunction of the A-V node and His bundle may develop periodically. Presence of the focal resorptive degeneration in all the postnatal hearts, both of “controls” and of sudden unexplained deaths, suggests this is a ubiquitous postnatal process. It is postulated that lethal arrhythmias or conduction disturbances due in large part to the consequences of developmental histologic changes in these critical regions of the heart may be the final common pathway in crib deaths. Many otherwise innocuous or less serious problems in babies may act as triggering mechanisms. Today, cardiac arrhythmias and conduction disturbances are rarely lethal when properly treated, particularly when the general myocardium is as conspicuously normal as it was in the present study, but such treatment requires anticipation of the event and prompt diagnosis when it occurs. With these considerations, crib death may be a potentially reversible phenomenon.