Phrenic nerve maturation in the sudden infant death syndrome.

It has been suggested that delayed development in either the peripheral or central nervous system could underlie the sudden infant death syndrome (SIDS). We studied the phrenic nerve in an attempt to find if maturation in this nerve was delayed in SIDS, and to see if fiber size differences could explain the paucity of type I muscle fibers described in SIDS diaphragms. Samples of phrenic nerves were taken at post mortem from 11 SIDS and 10 control infants. Myelinated fibers were quantitated in 1-micron silver-stained resin sections using a semi-automatic image 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-microns diameter) phrenic nerve fibers 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 between cases and were not correlated with age. Maturation of the phrenic nerve, as judged by increasing size of myelinated fibers, is therefore similar in SIDS and control infants. This argues against a widespread development delay in SIDS. No changes in phrenic nerve fiber sizes were found to account for the finding of fewer type I fibers in SIDS diaphragms.