PHRENIC-NERVE MATURATION IN THE SUDDEN-INFANT-DEATH-SYNDROME

It has been suggested that delayed development in either the periphera l or central nervous system could underlie the sudden infant death syn drome (SIDS). We studied the phrenic nerve in an attempt to find if ma turation in this nerve was delayed in SIDS, and to see if fiber size d ifferences could explain the paucity of type I muscle fibers described in SIDS diaphragms. Samples of phrenic nerves were taken at post mort em from 11 SIDS and 10 control infants. Myelinated fibers were quantit ated in 1-mu m silver-stained resin sections using a semi-automatic im age analysis system. The numbers and sizes of all myelinated fibers in the right phrenic nerve were compared between SIDS and control groups . The proportion of small (less than 6-mu m diameter) phrenic nerve fi bers was similar in SIDS (53.1%, SD 12.8%) and control (53.6%, SD 14.0 %) cases. The proportion of small fibers decreased with increasing age at an equivalent rate in both SIDS and control groups. Total numbers of myelinated fibers were similar in the right phrenic nerves of SIDS (3429, SD 440) and control (3457, SD 507) infants, but varied widely b etween cases and were not correlated with age. Maturation of the phren ic nerve, as judged by increasing size of myelinated fibers, is theref ore similar in SIDS and control infants. This argues against a widespr ead developmental delay in SIDS. No changes in phrenic nerve fiber siz es were found to account for the finding of fewer type I fibers in SID S diaphragms.