Abstract
In this study, the neuroanatomy of the swine lumbar spinal cord, particularly the spatial orientation of dorsal roots was correlated to the anatomical landmarks of the lumbar spine and to the magnitude of motor evoked potentials during epidural electrical stimulation (EES). We found that the proximity of the stimulating electrode to the dorsal roots entry zone across spinal segments was a critical factor to evoke higher peak-to-peak motor responses. Positioning the electrode close to the dorsal roots produced a significantly higher impact on motor evoked responses than rostro-caudal shift of electrode from segment to segment. Based on anatomical measurements of the lumbar spine and spinal cord, significant differences were found between L1-L4 to L5-L6 segments in terms of spinal cord gross anatomy, dorsal roots and spine landmarks. Linear regression analysis between intersegmental landmarks was performed and L2 intervertebral spinous process length was selected as the anatomical reference in order to correlate vertebral landmarks and the spinal cord structures. These findings present for the first time, the influence of spinal cord anatomy on the effects of epidural stimulation and the role of specific orientation of electrodes on the dorsal surface of the dura mater in relation to the dorsal roots. These results are critical to consider as spinal cord neuromodulation strategies continue to evolve and novel spinal interfaces translate into clinical practice.
Highlights
IntroductionEpidural electrical stimulation (EES) of the spinal cord has emerged as a promising therapy for enabling motor function (Gerasimenko et al, 2001; Minassian et al, 2004; Lavrov et al, 2008, 2015a,b; Harkema et al, 2011; Gad et al, 2013; Hachmann et al, 2013; Angeli et al, 2014; Grahn et al, 2017), respiratory muscle activation (Kowalski et al, 2013) and bladder control (Gad et al, 2016) following spinal cord injury (SCI)
Because no statistical differences were found between the three measured transverse diameters at dorsal root entry with respect to rostral, middle or caudal reference points, we averaged them into a single diameter measurement per segment
Evaluation of functional neuroanatomy of the swine spinal cord was performed based on electrical stimulation (EES) evoked motor responses
Summary
Epidural electrical stimulation (EES) of the spinal cord has emerged as a promising therapy for enabling motor function (Gerasimenko et al, 2001; Minassian et al, 2004; Lavrov et al, 2008, 2015a,b; Harkema et al, 2011; Gad et al, 2013; Hachmann et al, 2013; Angeli et al, 2014; Grahn et al, 2017), respiratory muscle activation (Kowalski et al, 2013) and bladder control (Gad et al, 2016) following spinal cord injury (SCI). Several known factors can influence the effect of EES, such as spatial orientation of dorsal spinal cord structures (Holsheimer and Struijk, 1991), electrical properties of intraspinal elements (Barolat, 1998), nerve fibers activated (Holsheimer, 2002), presence of ipsi- and/or contralateral afferents (Lavrov et al, 2008) and the Functional Neuroanatomy of the Spinal Cord timing of stimulation pulses in relation to the intended motor activity (Moraud et al, 2016; Shah et al, 2016; Wenger et al, 2016). In vivo experiments in rodent models have shown that high-intensity EES leads to activation of ventral spinal neural structures that in turn produce an early response (ER) with latencies of 3–5 ms at recording sites with active muscles. The difference in timing of these two responses is likely due to an intraspinal synaptic relays from dorsal root structures to ventral horn and ventral roots (Gerasimenko et al, 2006; Lavrov et al, 2006, 2008; Courtine et al, 2009)
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