Abstract
Implanted stimulators of motor nerve roots have been used for functional restoration in paraplegia since many years.
While the high root density of the cauda equina limits the surgery
to one field, it also presents the problem of cross-talk stimulation: unwanted
activation of a root lying near an active set of electrodes.
The Lumbar Anterior Root Stimulator Implant (LARSI) did exhibit cross-talk, and we have hypothesized that this is because the anodes in all the slots were connected together, causing virtual cathodes outside the active slot.
In this paper, we report in vitro experiments suggesting that, with a better design, we can improve the cross-talk ratio (out-of-slot threshold/in-slot threshold) from <10 to >80.
1 Introduction
In the 70s, the Sacral Anterior Root Stimulator Implant
(SARSI) was developed, mainly for bladder-emptying, and also to improve bowel
function [1,2].
Another device using similar technologies (LARSI),
was produced in the 80s to bilaterally stimulate lumbar anterior roots for the
restoration of leg function. One of the patients, implanted in 1994, has shown
the possibility of FES-tricycling for exercise and
recreation [3]. We are now developing a
third kind of implant, the SLARSI (sacral and lumbar roots), which will combine
both functionalities.
With LARSI, complicated recruitment curves
were observed. We hypothesise this is
due to unexpected activation of other roots lying alongside the active slot in
the spinal canal [4].
electrodes Figure 1: Experimental
3-slots electrode book: Pt-Ir electrodes
only present in 2 slots.
1.1
Cross-talk assessment
Axons from a single root may branch to many muscles. This makes it difficult to distinguish muscle responses due to cross-talk activation from the normal activation of different muscle groups innervated by the same root. However, it is unlikely that axons from any root cross the midline; yet in LARSI patients, stimulation of many single root produced a contralateral muscle response. For all roots, we call the ratio of the contralateral threshold to the ipselateral threshold the cross-talk ratio. These were as small as 6.0, 4.3 and 2.1 for the three subjects.
1.2 Electrode connection in a slot
In all types
of implant, the roots are trapped in electrode books laid in the cauda equina. Each book contains three slots, in
each of which are one central cathode and two outer
anodes (figure 1).
In the SARSI, as the stimulation channels are electrically isolated, the ion current is largely confined to the slot. For the LARSI however, only the cathode current is switched, coming from all the anodes connected together (figure 2). The current provided by anodes from all slots then converges towards the ends of the active slot, creating a concentration effect (virtual cathodes) that might cause unwanted stimulation of roots lying nearby [5][6]. This we call cross-talk activation.
Figure 2: Experimental
arrangement for comparing the SARSI arrangement (a) with the LARSI arrangement
(b). In the actual LARSI, there are 12 slots, not 2.
2 Methods
2.1
In vitro experiment with xenopus sciatic nerve
The three questions were: (i) in a LARSI-like situation, is the cross-talk ratio (= adjacent-slot threshold / same-slot threshold) comparable to those observed in patients?; (ii) what is the improvement when using a purely tripolar configuration?; (iii) in the later case, what is the effect of imbalance between anodal currents?
Figure 3: Experimental
setup, Note: only one slot of the book is shown here
The explanted sciatic nerve is immersed in
amphibian Ringer solution (figure 3); one end resting, in the air, on a pair of
platinum hook electrodes connected to an amplifier followed by an oscilloscope
to display the nervous signals.
The immersed section lies in the central slot of an electrode book connected to a purpose-build stimulator producing charge-balanced square current pulses (see fig.1, book used during the experiment). We ensured that the lids of the books were glued so that no current could flow in or out of the slot, except at the ends. Also, when immersed, the external slot, not drawn on fig.3, was full of Ringer solution and free of any air bubbles that would otherwise affect the current flow. The ratio of current in the two external anodes can be adjusted as a fraction of the total current reaching the cathode (50-50 = true tripole, 100-0 and 0-100 = dipoles).
Three configurations are possible: current only to the electrodes in the central slot (direct stimulation), current only to the electrodes in the external slot (cross-talk stimulation), and current to both pairs of anodes while only the central cathode is connected (LARSI-like situation). We record activation thresholds: the pulse width is fixed and the amplitude increased until appearance of an action potential.
2.2 Current imbalance
In the new implant (SLARSI), the two anodes in each slots are connected to a single wire. Thus we have no control on how the current is shared between the anodes. Various factors will influence its division: the books are not perfect, the central cathode may not lie precisely in the middle of the anodes (resistance imbalance) and the surface area of all electrodes may differ (capacitance and resistance imbalance); furthermore, the connective tissue growth after implantation may not be symmetrical (further resistance imbalance).
To evaluate the influence of the resistance imbalance on current division, special electrode books were produced. The cathodes of those books present a known offset with respect to the centre of the slot. They are immersed in saline, three concentrations cover the range of likely tissue resistivities. Constant current pulses are sent to the pair of anodes connected together and the current in each branch is measured.
3 Results
3.1 Cross-talk experiment
The activation threshold varies from one nerve to another, as well as between sets of measurements on the same nerve due to the amount of connective tissue at electrode level, the time since explantation and other factors. It is therefore difficult to compare direct and cross-talk activation thresholds. However, given that both direct and cross-talk data are collected with little delay, the ratios of cross-talk to direct threshold should not be affected by those changes, hence fig.4 presents the results as threshold ratios, for both SLARSI and LARSI-like situation. The SLARSI curves are interrupted for a near symmetrical anodal current division (40-60 to 60-40) because the limits of the stimulator were reached without causing the onset of an action potential. It looks safe to say that the peak cross-talk ratio for a SLARSI arrangement (i.e. tripole) is >80.
3.2 Current imbalance
This work is still in progress and the first results are presented in
fig.5. The dashed lines give the maximum
offset expected in a normal book, taken as half an electrode width, it
corresponds to an imbalance of ±3%.
Further experiments will look at the influence of capacitance variation,
mainly due to the difference in the surface area of the anodes.
Figure 4: cross-talk ratio
as a function of the current fraction
4 Discussion and Conclusions
4.1 Discussion
1. Using xenopus sciatic nerves in vitro, we have shown that (i) the cross-talk ratio in a LARSI situation can be as low as 10 , and we expect it would be lower if there were more slots, as in the implants, where values as low as 2.1 were observed; and (ii) the cross-talk ratio in the SLARSI situation (i.e. with electrically independent slots) for an imbalance of ±10% will be above 80.
2. Tests with the “offset books” show that we can expect the current
imbalance due to misalignment not to exceed ±3%.
4.2 Conclusion
There remains some uncertainty, because the experiments used frog peripheral nerves in place of human intradural roots, and because we have no information about the possible resistive asymmetry of the connective tissue in the slots. However, this suggests that, with the SLARSI, the cross-talk ratio should be increased to at least 80, which is a great improvement. We think that this will allow strong bladder contractions by stimulation of the parasympathetic fibres without exciting muscle motor fibres outside the active slot [7].
Figure 5: current imbalance
in books with known cathode offset
|
Figure 5: current imbalance
in books with known cathode offset |
References
[1]
Brindley G.S. “The first 500 patients with sacral anterior root stimulator
implants: general description”. Paraplegia, vol. 33 pp 5 – 9, 1995.
[2]
Creasey G.H. “Electrical stimulation of sacral roots for micturition after spinal cord injury”. Urologic Clinics
of
[3]
Perkins T.A., Donaldson N.,
Harper V.J., Tromans A.T., Norton J.A., Rushton D.N., Wood D.E. “Standing, stepping and cycling for
a T9 paraplegic with a Lumbo-sacral Anterior Root
Stimulator Implant (LARSI)”. INS 4th International Congress, IFESS,
[4]
Donaldson N., Rushton D.N., Perkins T.A., Wood D.E., Norton J., Krabbendam A.J. “Recruitment by motor nerve root stimulators:
significance for implant design”. Med.
[5] Mortimer J.T. “Neural Prosthesis: Fundamental Studies”. Prentice Hall Biophysics and Bioengineering Series, pp 74-77, 1990.
[6] Roth B.J. “Mechanisms for Electrical Stimulation of Excitable Tissue”. Crit Reviews in Biomed Eng, vol. 22 (3/4) pp 255 – 263, 1994.
[7] Brindley G.S., Polkey C.E., Rushton D.N. “Sacral anterior root stimulators for bladder control in paraplegia”. Paraplegia, vol. 20(6) pp 365-381, 1982.
Acknowledgements
The work presented here is the result of a collaboration between the Implanted Devices Group of Prof.
Donaldson, UCL,
We acknowledge the financial support of the European Commission for the NeuralPRO network (FP5 Program, Research and Training Network).