Estimation of fiber diameters in the spinal dorsal columns from clinical data

Abstract

Lack of human morphometric data regarding the largest nerve fibers in the dorsal columns (DC's) of the spinal cord has lead to the estimation of the diameters of these fibers from clinical data retrieved from patients with a new spinal cord stimulation (SCS) system. These patients indicated the perception threshold of stimulation induced paresthesia in various body segments, while the stimulation amplitude was increased. The fiber diameters were calculated with a computer model, developed to calculate the effects of SCS on spinal nerve fibers. This computer model consists of two parts: 1) a three-dimensional (3-D) volume conductor model of a spinal cord segment in which the potential distribution due to electrical stimulation is calculated and 2) an electrical equivalent cable model of myelinated nerve fiber, which uses the calculated potential field to determine the threshold stimulus needed for activation. It is shown that the largest fibers in the medial DC's are significantly smaller than the largest fibers in the lateral parts. This finding is in accordance with the fiber distribution in cat, derived from the corresponding propagation velocities. Moreover, it is shown that the mediolateral increase in fiber diameter is mainly confined to the lateral parts of the DC's. Implementation of this mediolateral fiber diameter distribution of the DC's in the computer model enables the prediction of the recruitment order of dermatomal paresthesias following increasing electrical stimulation amplitude.

Neurite orientation dispersion and density imaging of the healthy cervical spinal cord in vivo

“…We speculate that such relatively isotropic contributions to the DW signal in WM may partly arise from the water pool within the biggest axons. A tiny but nonnegligible percentage of axons is known to be characterised by large diameters (Feirabend et al, 2002;Häggqvist, 1936;Wesselink et al, 1998) with the tails of the fibre diameter distribution at cervical level extending up to 15 μm and beyond (Makino et al, 1996). These axons, even if not numerous, can make a substantial contribution to the DW signal, given their large size and therefore relatively large volume fraction .…”

Modeling Effects of Scar on Patterns of Dorsal Column Stimulation

“…The mean fiber diameter is approximately 5.0 μm superficially and 4.4 μm in the deep dorsal columns . Diameters of dorsal column fibers vary as they ascend in the cord after leaving the dorsal root entry zone as measured by conduction velocity changes (25–70% in the low thoracic regions as compared with high cervical regions) . The model here examined fiber lengths essentially spanning two to three spinal levels to minimize the effects of variable fiber diameters.…”

“…Computational modeling of the effects of spinal cord stimulation (SCS) has been demonstrated to be beneficial in gaining a better understanding of the mechanism of SCS function and in predicting electrode configurations that have been clinically proven to better focus the stimulation benefits while reducing stimulation adverse effects . Much of this work has come from the efforts of Holsheimer and colleagues . Other parameters such as frequency and pulse width also may modify stimulation effects on the dorsal column fibers .…”

Spinal cord stimulation: Indications, mechanism of action, and efficacy

“…Because of recent computer modeling evidence from the University of Twente, Enschede, The Netherlands [15][16][17], and because of recent reports in the literature, it is becoming clear that those patients with low back pain and bilateral leg pain, as in the patient with chronic arachnoiditis or epidural fibrosis secondary to multiple or single back operations (failed back surgery syndrome), can have not only their lower extremity pain but also their low back pain, relieved with dual quadropolar or octopolar lead arrays [18]. Again the choice of these systems is left to the surgeon.…”