Which neuronal elements are activated directly by spinal cord stimulation

The purpose of this paper is to discuss which nerve fibers in the various quadrants of the spinal cord are immediately activated under normal conditions of spinal cord stimulation, ie, at voltages within the therapeutic range. The conclusions are based on both empirical and computer modeling data, The recruitment of dorsal column (DC) fibers is most likely restricted to Abeta fibers with a diameter greater than or equal to 10.7 mum in a 0.20-0.25 mm layer under the pia mater and fibers of 9.4-10.7 mum in an even smaller outer layer when a conventional SCS lead is used. In a 0.25-mm outer layer of the T11 segment the number of Abeta fibers greater than or equal to 10.7 mum, as estimated in a recent morphometric study, is about 56 in each DC. Because a DC at T11 innervates 12 dermatomes, a maximum of 4-5 fibers (greater than or equal to 10.7 mum) may be recruited in each dermatome near the discomfort threshold. The dermatome activated just below the discomfort threshold is likely to be stimulated by just a single fiber, suggesting that paresthesia and pain relief may be effected in a dermatome by the stimulation of a single large Abeta fiber. The depth of stimulation in the DCs, and thereby the number of recruited Abeta fibers, may be increased 2-3 fold when stimulation is applied by an optimized electrode configuration (a narrow bi/ tripole or a transverse tripole). Assuming that the largest Abeta fibers in a dorsal root have a diameter of 15 mum, the smallest ones recruited at discomfort threshold would be 12 mum. The latter are presumably of proprioceptive origin and responsible for segmental reflexes and uncomfortable sensations, Furthermore, it is shown to be unlikely that, apart from dorsal roots and a thin outer layer of the DCs, any other spinal structures are recruited when stimulation is applied in the dorsal epidural space. Finally, anodal excitation and anodal propagation block are unlikely to occur with SCS.