J. Holsheimer

Optimum electrode geometry for spinal cord stimulation: the narrow bipole and tripole

A computer model is used to calculate the optimum geometry of an epidural electrode, consisting of a longitudinal contact array, for spinal cord stimulation in the managmment of chronic, intractable pain. 3D models of the spinal area are used for the computation of stimulation induced fields, and a cable model of myelinated nerve fibre is used for the calculation of the threshold stimulus to excite large dorsal column and dorsal root fibres. The criteria for the geometry of the longitudinal contact array are: a low threshold for the stimulation of dorsal column fibres compared with dorsal root fibres; and a low stimulation voltage (and current). For both percutaneous and laminectomy electrodes, the contact length should be approximately 1.5 mm, and the optimum contact separation, as determined by the computer model, is 2–2.5 mm. The contacts for a laminectomy electrode should be approximately 4 mm wide. This electrode geometry is applicable to all spinal levels where the dorsal columns can be stimulated (C1-2 down to L1). The stimulating electrode should preferably be used as a tripole with one (central) cathode.

Effects of electrode geometry and combination on nerve fibre selectivity in spinal cord stimulation

The differential effects of the geometry of a rostrocaudal array of electrode contacts on dorsal column fibre and dorsal root fibre activation in spinal cord stimulation are analysed theoretically. 3-D models of the mid-cervical and mid-thoracic vertebral areas are used for the computation of stimulation induced field potentials, whereas a cable model of myelinated nerve fibre is used for the calculation of the excitation thresholds of large dorsal column and dorsal root fibres. The size and spacing of 2-D rectangular electrode contacts are varied while mono-, bi- and tripolar stimulation are applied. The model predicts that the highest preferential stimulation of dorsal root fibres is obtained in monopolar stimulation with a large cathode, whereas dorsal column fibre preference is highest in tripolar stimulation with small contacts and small contact spacings. Fibre type preference is most sensitive to variations of rostrocaudal contact size and least sensitive to variations of lateral contact size. Dorsal root fibre preference is increased and sensitivity to lead geometry is reduced as the distance from contacts to spinal cord is increased.

A model of the electrical behaviour of myelinated sensory nerve fibres based on human data

Calculation of the response of human myelinated sensory nerve fibres to spinal cord stimulation initiated the development of a fibre model based on electro-physiological and morphometric data for human sensory nerve fibres. The model encompasses a mathematical description of the kinetics of the nodal membrane, and a non-linear fibre geometry. Fine tuning of only a few, not well-established parameters was performed by fitting the shape of a propagating action potential and its diameter-dependent propagation velocity. The quantitative behaviour of this model corresponds better to experimentally determined human fibre properties than other mammalian, non-human models do. Typical characteristics, such as the shape of the action potential, the propagation velocity and the strength-duration behaviour show a good fit with experimental data. The introduced diameter-dependent parameters did not result in a noticeable diameter dependency of action potential duration and refractory period. The presented model provides an improved tool to analyse the electrical behaviour of human myelinated sensory nerve fibres.

Transverse tripolar spinal cord stimulation: theoretical performance of a dual channel system.

A new approach to spinal cord stimulation is presented, by which several serious problems of conventional methods can be solved. A transverse tripolar electrode with a dual-channel voltage stimulator is evaluated theoretically by means of a volume conductor model, combined with nerve fibre models. The simulations predict that a high degree of freedom in the control of activation of dorsal spinal pathways may be obtained with the described system. This implies an easier control of paraesthesia coverage of skin areas and the possibility to correct undesired paraesthesia patterns, caused by lead migration, tissue growth, or anatomical asymmetries, for example, without surgical intervention. It will also be possible to preferentially activate either dorsal column or dorsal root fibres, which has some important clinical advantages. Compared to conventional stimulation systems, the new system has a relatively high current drain.

How do geometric factors influence epidural spinal cord stimulation? a quantitative analysis by computer modeling

Effects of both anatomic and electrode geometry on the recruitment of rostrocaudal fibers in the spinal cord were investigated by computer simulation of epidural spinal cord stimulation. A three-dimensional model was used, representing the geometry and electrical conductivity of the spinal cord and surrounding tissues, in combination with a model representing the electrical properties of a myelinated nerve fiber. Recruitment contours in the dorsal columns were calculated at various spinal geometries as a function of electrode position, combination and area. Cathodal position appeared to be most significant. Recruitment areas resulting from different contact combinations of a mediodorsal array were almost identical. It was shown that perception threshold largely depends on both dorsal cerebrospinal fluid width and fiber size. The usual bipolar contact separation appeared to approximate the theoretically optimal value, resulting in maximum fiber recruitment at minimum stimulus.

Transverse tripolar stimulation of peripheral nerve: a modelling study of spatial selectivity

Various anode-cathode configurations in a nerve cuff are modelled to predict their spatial selectivity characteristics for functional nerve stimulation. A 3D volume conductor model of a monofascicular nerve is used for the computation of stimulation-induced field potentials, whereas a cable model of myelinated nerve fibre is used for the calculation of the excitation thresholds of fibres. As well as the usual configurations (monopole, bipole, longitudinal tripole, ‘steering’ anode), a transverse tripolar configuration (central cathode) is examined. It is found that the transverse tripole is the only configuration giving convex recruitment contours and therefore maximises activation selectivity for a small (cylindrical) bundle of fibres in the periphery of a monofascicular nerve trunk. As the electrode configuration is changed to achieve greater selectivity, the threshold current increases. Therefore threshold currents for fibre excitation with a transverse tripole are relatively high. Inverse recruitment is less extreme than for the other configurations. The influences of several geometrical parameters and model conductivities of the transverse tripole on selectivity and threshold current are analysed. In chronic implantation, when electrodes are encapsulated by a layer of fibrous tissue, threshold currents are low, whereas the shape of the recruitment contours in transverse tripolar stimulation does not change.

Modelling selective activation of small myelinated nerve fibres using a monopolar point electrode

The aim of this study is to investigate theoretically the possibility for activation of small myelinated nerve fibres without activating larger ones when stimulating a nerve fibre bundle using a monopolar point electrode. Therefore, the sensitivity of excitation and blocking threshold currents of nerve fibres to fibre diameter, electrode-fibre distance and pulse duration has been simulated by a computer model. A simple infinite, homogeneous volume conductor and a cathodal point source were used in combination with a model representing the electrical properties of a myelinated nerve fibre. The results show that selective activation of small myelinated fibres may be possible in a region at some distance from the electrode.

Corpus Cavernosum Electromyography with Revised Methodology: An Explorative Study in Patients with Erectile Dysfunction and Men with Reported Normal Erectile Function

INTRODUCTION A lack of standardization of the recording techniques of corpus cavernosum electromyography (CC-EMG) and objective criteria to characterize the recorded signals (CC-potentials) are the main difficulties hindering the clinical application of this method. These difficulties have been recently overcome by revising the recording and interpretation methodology of CC-EMG AIM: To assess if CC-EMG performed with the revised methodology is discriminative for well-defined clinical conditions in patients with erectile dysfunction (ED). METHODS Based on blinded clinical diagnosis, ED patients were catalogued into five subgroups: severe penile fibrosis, cavernous arterial insufficiency (CAI), cardio-vascular comorbidity (CVCM) without proven CAI, post-radical retropubic prostatectomy (RRP), and psychogenic ED. With four electrodes placed on the penile shaft bilaterally, CC-EMG was recorded monopolarly for 30 minutes during flaccidity. After evaluation of the recordings by visual inspection, CC-potentials were analyzed using cross- and autocorrelation techniques. The parameters evaluated were amplitude, duration, dominant frequency (DF), and maximum cross-correlation coefficient (Rmax) of CC-potentials recorded from proximal and distal parts of the CC. MAIN OUTCOME MEASURES Comparison of the values of parameters amplitude, duration, DF, and Rmax between patient and control groups. RESULTS A total of 119 patients with ED and 43 men with reported normal erectile function were studied. Thirteen out of 14 patients with severe penile fibrosis did not show any distinguishable CC-potential. Patients with CAI had significantly decreased amplitude compared with the potent controls, as well as the patients with CVCM but without proven CAI. Significantly decreased amplitude and Rmax were detected in ED patients following RRP compared with the controls. CONCLUSIONS Corpus cavernosum electromyography performed with the revised methodology is able to discriminate ED patients with conditions that are associated with cavernous smooth muscle degeneration and/or autonomic neuropathy from men with reported normal erectile function.