Morphometry of human superficial dorsal and dorsolateral column fibres: significance to spinal cord stimulation

In spinal cord stimulation (SCS) large diameter cutaneous (Abeta) fibres in the dorsal columns (DCs) are activated and have an inhibiting effect on the transmission of pain signals by Adelta and C fibres from the corresponding dermatome(s). The largest Abeta fibres can be activated up to a maximum depth of about 0.25 mm in the DCs. No data are available on the distribution of the large fibres in this superficial human DC layer at the common SCS levels Th10-11. Such data are indispensable to improve the predictive capability of a computer model of SCS. The whole myelinated fibre population in the superficial 300 mum of the dorsal column (DC0-300) at Th10-11 of two human subjects was morphometrically analysed. Some data was obtained from a third subject. The superficial dorsolateral column (DLC0-300) was included in this analysis because it was hypothesized that large dorsal spinocerebellar tract fibres could also be activated by SCS. Only very few fibres larger than 10.7 mum were found: a mean of 68 (0.5%) in DC0-300 and 114 (2%) in DLC0-300. Considering that the effect of SCS is primarily attributed to activation of these largest fibres, it is concluded that a surprisingly small average amount of 2.4 fibres per running 0.1 mm width and 6 fibres per segmental division of the DC is involved. Distinct mediolateral heterogeneity in fibre composition was found in both DC0-300 and DLC0-300. In the DC0-300, the mean diameter of fibres greater than or equal to7.1 mum increases significantly by 5% from medial to lateral. Density (i.e. number of fibres per 1000 mum(2)) and frequency (i.e. percentage of a fibre size group compared. to its parent population) of the large fibres increase significantly from medial to lateral in the DC0-300. For fibres greater than or equal to10.7 mum, these parameters increase by 200 and 269%, respectively. It is concluded that the difference in stimulation threshold of large Abeta fibres in the median and lateral DC can be mainly attributed to the absence and presence, respectively, of collaterals at the stimulation site. Marked differences were found between DC0-300 and DLC0-300. The largest DLC0-300 fibres (greater than or equal to10.7 mum) have a 320% higher frequency and a 473% higher density. Their mean diameter is, however, only 2% larger. The largest DLC0-300 fibres are not likely to be recruited by SCS, since they are not larger than their DC0-300 counterparts, they lack collaterals (which would reduce the threshold stimulus substantially) and they are more remote from the stimulation electrode.