Objective: Injuries to the upper and middle trunks of brachial plexus result in dysfunction of radial nerves in newborns but do not in adults. We hypothesized that the radial nerve had a lower proportion of myelinated nerve fibers (MNFs) from the lower trunk in newborns than in adults, and in newborns those MNFs were less developed than MNFs in the radial nerve from the middle and upper trunks.
Methods: We dissected bilateral brachial plexus of six newborn and six adult cadavers. The radial nerve and its fascicles were separated proximally to posterior divisions of the upper, middle and lower trunks, and fascicles of the radial nerve were harvested from three trunks to calculate respective percentage of MNFs accounting for the total number of MNFs in the radial nerve. We determined diameters of axons and g-ratios of MNFs in the radial nerve from three trunks.
Results: Compared with adults, the percentage of MNFs in the radial nerve from the lower trunk was lower (p < 0.05), from the middle trunk higher (p < 0.05) and from the upper trunk similar (p > 0.05) in newborns, though MNF counts from three trunks were higher in newborns, respectively (p < 0.01, all). In newborns, MNFs in the radial nerve from the lower trunk had smaller axonal diameters and higher g-ratios than those from the middle and upper trunks (p < 0.017, all), while in adults there were no such differences.
Conclusions: Lower proportion of MNFs in the radial nerve from the lower trunk in newborns than in adults, and in newborns immaturity of MNFs from the lower trunk relative to MNFs from the middle and upper trunks may be the major morphological basis of difference in clinical appearances of radial nerve palsy caused by injuries to C5-C6-C7 between newborns and adults.
Keywords: Brachial plexus injuries; adults; axon and myelin development; newborns; radial nerve; spinal nerve origin.