Method and Device for Controlled Neural Stimulation

Abstract

A neurostimulation device has at least three stimulation electrodes configured to deliver an electrical stimulus to neural tissue. A control unit is configured to deliver a first stimulus phase in which a first stimulus electrode delivers a supra-threshold stimulus component, returned by at least two other of the stimulation electrodes. The control unit is further configured to deliver at least a second stimulus phase in which at least two of the stimulus electrodes deliver a sub-threshold stimulus component, returned by the first stimulus electrode.

Claims

1. A neurostimulation device comprising: at least three stimulation electrodes configured to deliver an electrical stimulus to neural tissue; and a control unit configured to deliver a first stimulus phase in which a first stimulus electrode delivers a supra-threshold stimulus component, returned by at least two other of the stimulation electrodes, the control unit further configured to deliver at least a second stimulus phase in which at least two of the stimulus electrodes deliver a sub-threshold stimulus component, returned by the first stimulus electrode.

2. The device of claim 1 wherein the electrical stimulus comprises at least three phases, delivered by at least three electrodes, and configured so that only one stimulus component from one electrode in one of the phases is a supra-threshold cathodic stimulus component.

3. The device of claim 2 wherein the supra-threshold cathodic stimulus component is delivered in a final phase of the stimulus.

4. The device of claim 1 configured to utilise four stimulus electrodes to deliver quadrupolar stimulation.

5. The device of claim 4 configured to deliver offset quadrupolar stimulation whereby the supra-threshold stimulus component is delivered by one electrode and returned by three electrodes.

6. The device of claim 1 further configured to adaptively select a number of stimulus electrodes in order to best minimise second cathode stimulation by switching between multipolar stimulation modes.

7. The device of claim 1 wherein a current borne by the first stimulus electrode is returned equally between the two or more other stimulus electrodes.

8. The device of claim 1 wherein a current borne by the first stimulus electrode is returned unequally between the two or more other stimulus electrodes.

9. The device of claim 8 wherein unequal currents carried by the two or more other stimulus electrodes are configured to take an inequality which minimises stimulus artefact at a recording electrode.

10. The device of claim 1, comprising a respective current source for each electrode configured to drive the desired current through that respective electrode in each phase of the stimulus.

11. The device of claim 1, comprising a respective current source between adjacent pairs of stimulus electrodes to effect differential drive of the desired current through each electrode.

12. The device of claim1, configured to selectively connect one or more of the stimulus electrodes directly to a supply rail to serve as a passive return electrode.

13. The device of claim 1, wherein the first stimulus electrode is interposed between the two or more other electrodes.

14. The device of claim 1, wherein the first stimulus electrode is positioned to one side of both or all of the return electrodes.

15. The device of claim 1, further comprising recording electrodes and measurement circuitry configured to obtain one or more recordings of a neural response evoked by the stimulus.

16. The device of claim 15 further configured to assess the one or more recordings of the neural response and to identify whether second cathode stimulation is occurring.

17. The device of claim 16 configured to apply stimuli of constant amplitude but having a distinct interphase gap(s), observe the ECAPs evoked by each such stimulus, and compare the morphology of each such observed ECAP, to detect indicia of second cathode stimulation.

18. The device of claim 16 configured to detect second cathode stimulation by applying stimuli of constant amplitude but having distinct stimulus phase sequencing, observe the ECAPs evoked by each such stimulus, and compare the morphology of each such observed ECAP, to detect indicia of second cathode stimulation.

19. The device of claim 16 further configured to use the outcome of second cathode stimulation to adjust a stimulus paradigm to seek prevention or reduction of second cathode stimulation when detected.

20. The device of claim 15, further configured to carry out automated stimulus amplitude determination by applying stimuli at sub-threshold and supra-threshold amplitudes and observing respective evoked ECAP responses, determining therefrom a stimulus amplitude threshold at which ECAP responses are first evoked, and setting the ongoing therapeutic stimulus amplitude as a multiple of the stimulus amplitude threshold.

21. The device of claim 20 configured for automated stimulus amplitude determination, by occasionally applying stimuli at sub-threshold and supra-threshold amplitudes and observing evoked ECAP responses, determining therefrom a stimulus amplitude threshold at which ECAP responses are first evoked, and setting the ongoing therapeutic stimulus amplitude as a multiple of the stimulus amplitude threshold.

22. The device of claim 21 wherein the ongoing therapeutic stimulus amplitude as set as a multiple within the range of 1.05 to 1.8 of the stimulus amplitude threshold.

23. The device of claim 22 configured to set the ongoing therapeutic stimulus amplitude as a multiple of 1.2 times the stimulus amplitude threshold.

24. A method of neurostimulation, the method comprising: delivering an electrical stimulus to neural tissue using at least three stimulation electrodes, the electrical stimulus comprising a first stimulus phase in which a first stimulus electrode delivers a supra-threshold stimulus component returned by at least two other of the stimulation electrodes, and the electrical stimulus further comprising at least a second stimulus phase in which at least two of the stimulus electrodes deliver a sub-threshold stimulus component, returned by the first stimulus electrode.

25. A computer program product comprising computer program code means to make a neurostimulator execute a procedure for neurostimulation, the computer program product comprising computer program code means for carrying out the method of claim 24.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] An example of the invention will now be described with reference to the accompanying drawings, in which:

[0031] FIG. 1 schematically illustrates an implanted spinal cord stimulator;

[0032] FIG. 2 is a block diagram of the implanted neurostimulator;

[0033] FIG. 3 is a schematic illustrating interaction of the implanted stimulator with a nerve;

[0034] FIG. 4A illustrates the profile of a prior art biphasic bipolar stimulus current over time,

[0035] FIGS. 4B and 4C illustrate delivery of each stimulus phase to a nerve, and FIG. 4D illustrates the respective electrode waveforms;

[0036] FIGS. 5A and 5B illustrate an embodiment of the present invention utilising biphasic tripolar stimulation;

[0037] FIG. 6 illustrates some ECAP morphology parameters improved by the embodiment of FIGS. 5A and 5B;

[0038] FIG. 7 illustrates an embodiment of the invention utilising a first arrangement of current sources to drive the tripolar stimulus electrode arrangement in accordance with one embodiment of the invention.

[0039] FIG. 8 illustrates another embodiment of the invention utilising a second arrangement of current sources to drive the tripolar stimulus electrode arrangement in accordance with another embodiment of the invention

[0040] FIG. 9 illustrates another embodiment of the invention utilising yet another arrangement of current sources to drive the tripolar stimulus electrode arrangement in accordance with yet another embodiment of the invention

[0041] FIGS. 10A and 10B illustrate misalignment of the electrodes with the axis of propagation of the action potential.

[0042] FIG. 11A illustrates the area of polarisation and the area of partial depolarisation of a nerve, caused by a first phase of a stimulus, and FIG. 11B illustrates the additional areas of polarisation resulting in a second stimulus phase;

[0043] FIG. 12 further illustrates the problem of non-linear ECAP growth caused by multiple stimulus sites and times in conventional biphasic bipolar stimulation

[0044] FIG. 13 illustrates a method to detect the existence of second cathode stimulation; and

[0045] FIG. 14 illustrates a further embodiment of the present invention which further provides for automated optimisation of stimulation sequences to remove second cathode effects;

[0046] FIG. 15 schematically illustrates a system for feedback controlled neurostimulation in accordance with an embodiment of the invention;

[0047] FIG. 16 illustrates a triphasic tripolar stimulus configuration in accordance with another embodiment of the invention;

[0048] FIGS. 17A and 17B illustrate the polarities of electrodes in a single phase of a quadrupolar stimulus configuration, and a pentapolar stimulus configuration, respectively, in accordance with further embodiments of the invention; and

[0049] FIG. 18 illustrates a triphasic quadrupolar stimulus configuration in accordance with yet another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] FIG. 1 schematically illustrates an implanted spinal cord stimulator 100. Stimulator 100 comprises an electronics module 110 implanted at a suitable location in the patient's lower abdominal area or posterior superior gluteal region, and an electrode assembly 150 implanted within the epidural space and connected to the module 110 by a suitable lead. Numerous aspects of operation of implanted neural device 100 are reconfigurable by an external control device 192. Moreover, implanted neural device 100 serves a data gathering role, with gathered data being communicated to external device 192 via any suitable transcutaneous communications channel 190.

[0051] FIG. 2 is a block diagram of the implanted neurostimulator 100. Module 110 contains a battery 112 and a telemetry module 114. In embodiments of the present invention, any suitable type of transcutaneous communication 190, such as infrared (IR), electromagnetic, capacitive and inductive transfer, may be used by telemetry module 114 to transfer power and/or data between an external device 192 and the electronics module 110. Module controller 116 has an associated memory 118 storing patient settings 120, control programs 122 and the like. Controller 116 controls a pulse generator 124 to generate stimuli in the form of current pulses in accordance with the patient settings and control programs 122. Electrode selection module 126 switches the generated pulses to the appropriate electrode(s) of electrode array 150, for delivery of the current pulse to the tissue surrounding the selected electrode(s). Measurement circuitry 128 is configured to capture measurements of neural responses sensed at sense electrode(s) of the electrode array as selected by electrode selection module 126.

[0052] FIG. 3 is a schematic illustrating interaction of the implanted stimulator 100 with a nerve 180, in this case the spinal cord however alternative embodiments may be positioned adjacent any desired neural tissue including a peripheral nerve, visceral nerve, parasympathetic nerve or a brain structure. Electrode selection module 126 selects a stimulation electrode 2 of electrode array 150 to deliver an electrical current pulse to surrounding tissue including nerve 180, and also selects two return electrodes 1 and 3 of the array 150 for stimulus current recovery to maintain a zero net charge transfer.

[0053] Delivery of an appropriate stimulus to the nerve 180 evokes a neural response comprising a compound action potential which will propagate along the nerve 180 as illustrated, for therapeutic purposes which in the case of a spinal cord stimulator for chronic pain might be to create paraesthesia at a desired location. To this end the stimulus electrodes are used to deliver stimuli at any therapeutically suitable frequency, for example 30 Hz, although other frequencies may be used including as high as the kHz range, and/or stimuli may be delivered in a non-periodic manner such as in bursts, or sporadically, as appropriate for the patient. To fit the device, a clinician applies stimuli of various configurations which seek to produce a sensation that is experienced by the user as a paraesthesia. When a stimulus configuration is found which evokes paraesthesia, which is in a location and of a size which is congruent with the area of the user's body affected by pain, the clinician nominates that configuration for ongoing use.

[0054] The device 100 is further configured to sense the existence and intensity of compound action potentials (CAPs) propagating along nerve 180, whether such CAPs are evoked by the stimulus from electrodes 1-3, or otherwise evoked. To this end, any electrodes of the array 150 may be selected by the electrode selection module 126 to serve as measurement electrode 6 and measurement reference electrode 8. Signals sensed by the measurement electrodes 6 and 8 are passed to measurement circuitry 128, which for example may operate in accordance with the teachings of International Patent Application Publication No. WO2012155183 by the present applicant, the content of which is incorporated herein by reference. The output of circuitry 128 is used by controller 116 in a feedback arrangement to control the application of subsequent stimuli, and the controller 116 also stores the recording of the neural response or one or more parameters thereof such as ECAP amplitude to the Clinical Data storage 120.

[0055] To ensure patient safety it is necessary to ensure that stimuli are delivered in a charge balanced manner, and moreover that a bidirectional current is passed through each stimulus electrode to counteract electrochemical effects which unavoidably arise at the electrode-tissue interface. This imposes certain limitations on the allowable configurations of stimuli which may be used in any practical neurostimulation device. A conventional stimulus used in such devices is a biphasic bipolar stimulus as depicted in FIG. 4A-4D. FIG. 4A illustrates the profile of the stimulus current over time. FIGS. 4B and 4C illustrate delivery of each respective stimulus phase to a nerve. FIG. 4D illustrates that a biphasic bipolar stimulus unavoidably delivers at least two supra-threshold stimulus components.

[0056] In this conventional biphasic bipolar stimulus arrangement, a first stimulus phase delivers a current from electrode 402, and the current is returned by electrode 401. In a second phase an equal and opposite current is delivered, that is, a current is delivered from electrode 401 and returned by electrode 402. The stimulus is thus biphasic as it delivers two phases, and is bipolar as it utilises two electrodes. As shown in FIG. 4D, for effective therapy the stimulus amplitude must be set to a cathodic comfort level. The cathodic comfort level is usually of lesser magnitude than an anodic threshold level, meaning that only the cathodic stimulus component of each phase gives rise to neural recruitment.

[0057] However, due to the aforementioned safety constraints, the cathodic excitation of electrode 402 in the first phase (FIG. 4B) is equal to the cathodic excitation of electrode 401 in the second phase (FIG. 4C), but at times t.sub.1 apart. At a first time, in phase 1 (FIG. 4B), the cathodic excitation of electrode 402 evokes a neural response only on the Node(s) of Ranvier 422 which are triggered by electrode 402, depicted for illustrative purposes by excitation range 412. At a second time, t.sub.1 later than the first time, in the second phase (FIG. 4C) the cathodic excitation of electrode 401 evokes a neural response only on the Node(s) of Ranvier 421 which are triggered by electrode 401, depicted for illustrative purposes by excitation range 411. It is to be noted that excitation range 412 is adjacent electrode 402, and is not coterminous with excitation range 411 which is adjacent electrode 401. Thus, the stimulation site in the first phase is at least partly different, or even entirely different, to the stimulation site in the second phase of the stimulus.

[0058] While for typical values of t.sub.1 a given axon recruited by the first phase will typically be refractory during the second phase and will not be able to be recruited again, other axons (not shown in FIGS. 4A-4D) not recruited by the first phase remain available for excitation by the second phase in the manner described. The respective sub-population of fibres available for recruitment in the second phase is unknown or at least very poorly known, introducing considerable uncertainty about the respective amplitude of compound response which might arise in the second phase.

[0059] Accordingly, biphasic bipolar stimulation typically evokes a compound response made up of neural response components which arise at different times, and which arise at different locations, and which are of unequal amplitude with the extent of amplitude inequality being unknown. These neural response components overlap in, and jointly contribute to, electrical observations of the neural response as a whole, significantly impeding attempts to accurately understand the actual recruitment effect of each component of the stimulus.

[0060] In contrast to the conventional bipolar biphasic stimulation shown in FIGS. 4A-4D, the present invention proposes a tripolar (or greater) stimulus, configured in a particular manner to seek a paradigm in which only one component of the stimulus recruits any neural response. When achieved, this outcome means that the resulting compound action potential can be tied to a particular single stimulus site, and to a particular single stimulus time, for example for the purposes set out in the present Applicant's International Patent Publication No. WO 2016/161484, the content of which is incorporated herein by reference. Determinations of conduction velocity can also be improved, both because the stimulus site and time is known with improved accuracy and because the observed response is less time-spread making for sharper peaks and thus allowing improved resolution of the time of occurrence of such peaks when assessing time of arrival for conduction velocity determinations.

[0061] FIGS. 5A and 5B illustrate an embodiment of the present invention effected by way of biphasic tripolar stimulation. In the first phase shown in FIG. 5A, stimulus electrodes 501 and 503 are cathodic and deliver current to the tissue, with all current returned by the central anodic stimulus electrode 502. For illustrative purposes, electrodes 501 and 503 deliver a nominal 0.5 units of current, while electrode 502 returns 1 unit of current. In the second phase shown in FIG. 5B, electrode 502 delivers 1 unit of current, and electrodes 501 and 503 each return 0.5 units of current. In accordance with the present invention, the current amplitude delivered is carefully selected to ensure that the cathodic stimulation threshold is in the range 0.5-1 units of current, and to ensure that the anodic stimulation threshold is greater than 1 unit of current. This arrangement thus provides for recruitment to be caused by a single cathode flanked by anodes on either side. Consequently, only one phase of this stimulus paradigm (the second phase), and only one electrode (electrode 502) ever gives rise to any neural recruitment, thus ensuring that the time and location of stimulation can be known with the greatest possible accuracy. For example this can allow the ECAP morphology to be assessed with confidence that a second stimulus site and time is not contributing to the ECAP.

[0062] In particular, the anodic current of electrode 502 in phase 1, and of electrodes 501 and 503 in phase 2, can be ignored at the stimulation levels of typical neurostimulation practice. This is because the typical stimulation levels required for effective therapy are between the electrophysiological threshold (the current at which it is first possible to detect ECAPs), and about two times that threshold current. While anodic recruitment is possible, the threshold current for action potential generation by the anode is significantly higher, with the threshold current for anodic excitation typically being about 5 to 8 times higher than for cathodic excitation. The present invention recognises that this presents a useful range of operation within which it is possible to maintain a single supra-threshold stimulus component. In embodiments using tripolar stimulation for this purpose, this ensures that the additional cathodes generated on the flanking electrodes (501, 503) in the additional phase is below the threshold for stimulation when the stimulation current is maintained below 2 times threshold. This particular configuration of the stimulus thus ensures that action potentials are generated from a single cathode in, i.e. by a single supra-threshold stimulus component of, the overall biphasic tripolar stimulus. We aim to measure the ECAP as close to the stimulus as possible, and as soon after the stimulus as possible, and so the stimulus is presented with an anode on the centre electrode of the tripole in the first phase, so that the cathodic stimulus component can be presented in the second and final phase of the stimulus allowing measurements to be commenced immediately thereafter.

[0063] ECAP morphology parameters which might for example be assessed include those shown in FIG. 6. A single action potential comprises a P1 peak indicating initial depolarization (cell capacitance) followed by an N1 valley or negative peak indicating depolarization (sodium channel activation) and finally a P2 peak indicating hyperpolarization (potassium channel activation). ECAP morphology parameters which might for example be assessed include characteristics such as P1-N1 peak-to-peak amplitude, N1-P2 peak-to-peak amplitude, P1 peak time, P1 peak amplitude, P1 half height peak width, P1-N1 zero crossing time, P1-N1 zero crossing slope, N1 peak time, N1 peak amplitude, N1 half height peak width, N1-P2 zero crossing time, N1-P2 zero crossing slope, P2 peak time, P2 peak amplitude, P2 half height peak width, and the like. As all of these parameters can respond non-linearly to stimulus amplitude if the stimulus recruits neurons at multiple different sites and at multiple different times, embodiments of the present invention may significantly improve the reliability of any or all such ECAP morphology parameters when used as feedback variables or as diagnostic indicators or the like.

[0064] FIG. 7 illustrates an embodiment of the invention utilising a first arrangement of current sources to drive the tripolar stimulus electrode arrangement in accordance with one embodiment of the invention. In this embodiment, two current sources 710 and 720 are used to effect unequal current sharing between electrodes 1 and 3. Electrode 2 is simply grounded to provide a return path for the entire current delivered by the current sources.

[0065] FIG. 8 illustrates another embodiment of the invention utilising a second arrangement of current sources to drive the tripolar stimulus electrode arrangement in accordance with another embodiment of the invention. In this embodiment, each of electrode 1, electrode 2 and electrode 3 are provided with a respective single ended current source 810, 820, 830. The current sources are configured to deliver respective individual single-ended drive current profiles denoted e1, e2 and e3.

[0066] FIG. 9 illustrates another embodiment of the invention utilising yet another arrangement of current sources to drive the tripolar stimulus electrode arrangement in accordance with yet another embodiment of the invention. This is a differential drive arrangement equivalent to the single ended drive arrangement of FIG. 8.

[0067] Thus, use of three electrodes to effect tripolar stimulation in this manner in the present embodiment means that in the first phase of the biphasic stimulus the stimulus current is delivered from both the electrodes 1 and 3 (FIGS. 3, 7, 8, 9) which in this first phase are cathodic, and the entire return current is carried by electrode 2 which is anodic. And, in the second phase of the biphasic stimulus, the entire stimulus current is delivered from electrode 2 which in this phase is cathodic, while the return current is shared between the return electrodes 1 and 3 which in this phase are anodic. Thus, in both the first phase and the second phase of the stimulus, the sum of the current through electrode 1 (I.sub.1) and the current through electrode 3 (I.sub.3) is equal and opposite to the current through electrode 2 (I.sub.2). That is, I.sub.1+I.sub.3=−I.sub.2.

[0068] The present invention recognises that it is possible to ensure that only one of these stimulus components evokes a neural response, being the cathodic current on electrode 2. This is because it is possible to arrange the tripolar stimulus so that the cathodic currents on electrodes 1 and 3 in phase 1 never exceed about half the cathodic current delivered in phase 2 by electrode 2. It is therefore possible to maintain the cathodic current on electrodes 1 and 3 in the first phase below a stimulus threshold, while maintaining the cathodic current on electrode 2 in the second phase above the stimulus threshold. And, while the anodic current on electrode 2 in the first phase must for safety reasons typically be substantially equal (but opposite to) the cathodic current on electrode 2 in the second phase, the anodic stimulus threshold is typically greater than the cathodic stimulus threshold, so that the anodic current on electrode 2 in the first phase may be kept below an anodic stimulus threshold while maintaining the (equal but opposite) cathodic current on electrode 2 in the second phase above the cathodic stimulus threshold.

[0069] Thus, the biphasic tripolar stimulus may be configured to satisfy the requirements for charge balancing, and further to satisfy the requirement for all electrochemical effects at the electrode-tissue interface to be reversed by the use of alternating currents at each electrode, while nevertheless delivering a stimulus in which only one component of the stimulus evokes a neural response.

[0070] Further benefits and embodiments of the invention are also envisaged. In this regard, it is noted that one field of application for some embodiments of the invention is spinal cord stimulation therapy, which achieves its therapeutic benefit via stimulation of the dorsal columns of the spinal cord with electrodes that are placed in the epidural space. The dorsal column is arranged such that layers within the column radiating away from the midline carry nerve fibres which innervate a particular region (or dermatome) of the body. The goal of electrical spinal cord stimulation therapy is to recruit those fibres which correspond to the dermatomes where pain is present. Stimulation in adjacent areas where no pain is present can be uncomfortable for the recipient of the device, so much so that stimulation in unwanted areas can lead to recipients discontinuing their therapy.

[0071] There is considerable variation in the current required to achieve consistent recruitment of dorsal column fibres, and feedback loops have been established to maintain recruitment at a constant desired level. One technique relies on the use of the electrically evoked compound action potential as a measure of the spinal cord excitation, as described in the present Applicant's International Patent Publication No. WO 2012/155188, the content of which is incorporated herein by reference. When considering the use of the ECAP as an aid in establishing programming parameters which can be used to maintain consistent recruitment of the desired dermatomal areas to the extent possible, the present invention recognises that ECAPs evoked at multiple sites and/or at multiple different times by conventional biphasic bipolar stimuli can complicate the task of interpreting an observed ECAP and thus impede effective therapy.

[0072] A cathodic current has a higher recruitment efficiency than an anodic current and so it is desirable to employ cathodic currents, however monophasic pulses are unsafe and so the cathodic current must be accompanied shortly after with an anodic current to reverse the potential. This has the effect of reversing any electro chemistry which has taken place on the electrode surface. The duration and amplitude of this balancing pulse can be different to the duration and amplitude of the cathodic pulse (but of opposite sign) provided that the charge delivered during the two phases (duration * amplitude) is identical.

[0073] The simplest form of charge reversal is the use of biphasic stimuli pulse. The spread of the electric field from the stimulus is governed by the separation between the electrodes which for SCS devices are usually electrodes on the same lead. The stimulation occurs with either the cathode first or the anode as illustrated in FIG. 4. In the illustration the cathodic and anodic phases have the same pulse width but are opposite sign and this results in current flowing (from positive, in line with convention) to negative in the first phase and then the opposite in the second phase with the position of the anode and cathode swapped.

[0074] The net result of this is that for every single biphasic stimulus there arises two cathodes, one from the first phase and one from the second phase. In an ideal situation, with the cathode first, the stimulus recruits the fibres which then enter their refractory period and remain so during the presentation of the second phase. In this ideal situation no recruitment of any additional fibres occurs and the stimulus location is mapped to a single electrode.

[0075] However the present invention recognises that in practice this situation is impossible to achieve. In addition to the reasons discussed in the preceding, it is important to note the possibility of misalignment of the electrodes with the axis of propagation of the action potential, as illustrated in FIGS. 10A and 10B. Such misalignment can result from surgical misalignment of an electrode lead adjacent a nerve, and/or due to individual nerve fibres taking a somewhat wandering path within the nerve as is known to occur. In the event of such misalignment of the electrodes with the axis of propagation of the action potential, the first cathode can produce a partly depolarised membrane but fail to initiate an action potential, which the second cathode may then initiate if the second cathode is presented is within a short enough window. This geometric limit is illustrated in FIGS. 10A and 10B. The first phase of stimulation recruits a group of fibres and the second phase recruits some additional fibres not recruited by the first phase of stimulation. When the fibres recruited in the two phases correspond to different dermatomal areas then this would result in perception of stimulation in different locations and can lead to stimulation in undesired locations. Indeed, in the case of FIG. 10B such misalignment can even lead to dermatomes on the wrong side of the body being recruited, for example where stimulation on the right side is desired, such geometric misalignment can inappropriately increase the chance of stimulation incorrectly occurring on the left side of the body.

[0076] There is not only a geometric constraint to maintenance of recruitment to single location but there is also a timing constraint. The stimulus current not only depolarises a region of the nerve to produce action potentials but, due to the nature of the electric field, partly depolarises a more extended area around the area which has been generated action potentials. This is illustrated in FIG. 11A. Bipolar stimulation is commonly used during SCS and the 1st phase anode 1120 is often the adjacent electrode to the cathodic stimulus electrode 1110. As shown in FIG. 11A, the first phase cathodic stimulus delivered by electrode 1110 creates an area upon the nerve in which axons will depolarise and initiate action potentials. In an annular region around this area is a second area in which the excitation is sufficient to cause partial depolarisation of membranes to a degree which does not cause action potentials but which does lead to altered membrane conditions at a later time when the second phase is applied.

[0077] FIG. 11B illustrates the areas of the nerve excited by a second phase. Under these circumstances, even if the geometric alignment of the electrode array and the nerve is precise and the field from the 2nd cathode is applied only to fibres recruited by the first phase, the second phase also produces a second zone of partial depolarisation, and the overlap of this area with the area of partial depolarisation produced by the first phase would generate additional action potentials, thus giving rise to another undesirable mechanism by which the time and location of recruitment is confused due to the configuration of the applied stimulus.

[0078] The situation is further complicated when other confounding factors are considered. For instance the stimulation threshold for the two different locations for the first cathode 1110 and second cathode 1120 may be different from each other. In a situation where the threshold for activation is higher under the second cathode 1120 then the strength of stimulation in the undesired corresponding dermatomal location would be stronger. In the most extreme case where the threshold for the first cathode 1110 is highest of all the electrodes available, but it is the electrode best placed for use corresponding to the painful dermatome, stimulation of the painful area can never be achieved with biphasic stimuli without stimulation in unwanted areas by electrode 1120 and this leads to either non acceptance of the therapy or a diminution in performance.

[0079] The variation in recruitment which occurs with postural changes further confounds the goal of targeting the stimulation. It is desirable to produce a device which achieves closed loop control of the ECAP amplitude to provide recipients with additional benefit not the least of which is elimination of changes in therapy as a result of postural effects. Ideally the ECAP amplitude would maintain the same shape but grow in amplitude with increasing stimulation current with the form of k(I−T)×(t) where T is threshold, k is a gain term. That is, in this ideal case the shape of the ECAP scales linearly with the current. This would provide a key benefit of allowing simple detector designs one of which may consist of simply computing the dot product of the signal with an appropriately designed filter function as described in the present applicant's WO 2015/074121. In the event that any stimulus design produces recruitment in two locations and at two different times then any small change in threshold between the two locations would produce an ECAP shape which changes with increasing current, voiding the assumption of linear ECAP amplitude response and significantly complicating the task of assessing recruitment from detected ECAPs.

[0080] FIG. 12 further illustrates the problem of non-linear ECAP growth caused by multiple stimulus sites and times in conventional biphasic bipolar stimulation. Between threshold for the first cathode and threshold for the 2nd cathode a first ECAP shape A is present. Above the threshold for the 2nd cathode another ECAP shape B emerges. For certain combinations of distance between electrodes and fibre population conduction velocity the negative and positive peaks of the ECAPs can align with each other canceling each other. This in turn creates conditions where increasing stimulation current counterintuitively produces decreasing ECAP amplitude and this leads to feedback loop instability during closed loop control. It is thus highly desirable to detect and eliminate any 2nd cathode effects, particularly for closed loop controlled neuromodulation because it allows simplified detectors and improves loop stability.

[0081] Because the two modes of stimulation (first cathode, second cathode) can have different thresholds, and the ECAPs initiate at different locations along the dorsal column, then the ECAP morphology can change with current. When stimulus current is increased from a sub-threshold level to where the first threshold is reached, ECAP shape A emerges. When, with further increase in stimulus current, the second threshold is reached, the second ECAP B emerges. ECAP B has the same shape as ECAP A but is offset in time from the first. The sum of A & B has a shape that is different and this compound response is what can be observed by recording electrodes and accompanying measurement circuitry and an ECAP detector module. It is possible that the second ECAP causes the detector output to fall rather than rise, leading to loop instability or diminished performance.

[0082] These non-linear effects do not occur when recruitment only occurs at a single stimulus site at a single time, as occurs in accordance with embodiments of the present invention. Under such conditions an ECAP with constant shape emerges and grows linearly with stimulus amplitude as is desired to ease the difficult task of ECAP detection.

[0083] To optimise a stimulation paradigm which is better targeted to the desired dermatome, some embodiments of the invention further take steps to actively identify whether second cathode stimulation is occurring, and to take steps to prevent second cathode stimulation when detected. To detect the occurrence of second cathode stimulation is not possible on the basis of patient feedback as the strength of perception grows as the current is increased in both desired and undesired areas and in a uniform manner, so that the patient cannot discern any useful distinctions to report on. However, these embodiments of the invention recognise that identification of second cathode affects can be achieved from measurements of the electrically evoked compound action potential.

[0084] The recorded potential is sum of potential produced by both the first and second cathode, as depicted in FIG. 12. The ECAP which is actually measured by recording electrodes some distance away from the stimulus electrodes is thus the sum of the responses 1210 and 1220 from both cathodes which are initiated at different times in the stimulus cycle. While the recording obtained from the recording electrodes can only ever reflect the compound response 1230 made up of components 1210 and 1220 together, the present invention recognises that there are a number of ways to detect if a second cathode stimulation is occurring.

[0085] A first manner in which to detect second cathode stimulation is to inspect the recorded response 1230 to identify whether there exists two observable N1 peaks, which may occur when the two cathodes are separated by a suitable distance. However, this approach is not able to identify a large range of scenarios in which second cathode stimulation occurs but does not manifest in two observable N1 peaks.

[0086] Accordingly, an alternative method to detect the existence of second cathode stimulation involves varying pulse parameters which do not affect recruitment but which will bring about a change in the observed compound action potential should any second cathode stimulation be taking place. One example, shown in FIG. 13, involves varying the interphase gap. A first stimulus 1310 is applied, having a first interphase gap between the two phases of the stimulus. A first recording 1340 of the resulting ECAP is obtained. A second stimulus 1360 is applied at another time, having a second interphase gap different to the first interphase gap. Both the first interphase gap and the second interphase gap should be maintained within the absolute refractory period, which for example can be measured in accordance with the teachings of the present Applicant's International Patent Publication No. WO 2012/155189, the content of which is incorporated herein by reference. As shown in FIG. 13A & FIG. 13B, because each phase of the stimuli 1310 and 1360 are the same, the respective response components 1320 and 1370 are the same, and the respective response components 1330 and 1380 are the same. However, the time spacing between the respective response components differs, so that the compound responses 1340 and 1390 are different and can thus be inspected for indicia of second cathode stimulation. For example, comparing ECAP parameters such as one or more peak amplitudes, one or more peak to peak amplitudes, and/or one or more peak widths, allows the different morphology of responses 1340 and 1390 to be detected. A degree by which such metrics (or a combination thereof) differ can be taken as being indicative of a degree to which second cathode stimulation is occurring, which in turn could for example be used to control a feedback loop gain or to assist PID loop control to avoid overshoot to provide control over how rapidly the system responds to detected second cathode stimulation for example.

[0087] There are many alternative schemes for detecting the presence of second cathode stimulation in accordance with embodiments of the present invention. For instance, one such embodiment may utilise a set of stimulus electrodes, and deliver a first stimulus from one subset of the electrodes and measure a first neural response evoked by the first stimulus. A second stimulus may then be delivered from a different subset of the electrodes and a second neural response evoked by the second stimulus may be measured. Provided the positions of the stimulus electrodes relative to each other remains the same, and assuming the thresholds for cathodic stimulation are the same, then a comparison of the first and second neural responses would yield an indication of whether second cathode stimulation is present.

[0088] FIG. 14 illustrates a further embodiment of the present invention which further provides for automated optimisation of stimulation sequences to remove second cathode effects. This detection method provides a means to optimise the stimulation parameters to avoid the above noted undesirable effects of second cathode stimulation. It is to be noted that even when exploiting tripolar stimulation as taught for example in FIG. 5, it remains possible that the threshold for cathodic stimulation for electrodes 501 and/or 503 could feasibly be significantly lower than the cathodic stimulation threshold of electrode 502 and that the 0.5 units of cathodic excitation delivered from electrodes501 and 503 in phase 1 could in such instances give rise to second or even third cathode stimulation. Accordingly, even when utilising such a tripolar stimulation configuration, it remains desirable to monitor for any instances of second cathode stimulation. Accordingly, in the embodiment of FIG. 14 a process is implemented which automatically adjusts the tripolar stimulus based on the detection of any second cathode effects, to thereby iteratively adjust one or more stimulus parameters. For instance in the tripolar stimulus pattern, the threshold for stimulation can be increased (i.e. recruitment by electrodes 501 and 503 can be decreased and preferably eliminated) by extending the pulse width of the respective anodic phase and reducing the current amplitude, which is therapeutically acceptable provided that (1) the total time for both phases is less than the refractory period, and (2) the same charge is delivered in both phases. Within these constraints it is possible to tune the stimulus to prevent any form of second cathode stimulation.

[0089] In still further embodiments of the invention, further parameters can be revised to seek the elimination of second cathode effects, including for instance increasing the number of anodes, increasing the number of stimulus electrodes from three to five so as to shift from tripolar stimulation to pentapolar stimulation, or using four stimulus electrodes with three return electrodes in an offset-quadrupolar arrangement, adjusting the sharing of current between electrodes to be unequal in cases where this improves the elimination of second cathode stimulation, increasing the number of stimulus phases to provide greater ability to keep sub-threshold components further below a respective stimulation threshold, and so on.

[0090] FIG. 15 schematically illustrates a system for feedback controlled neurostimulation in accordance with an embodiment of the invention utilising the above-described principles. In particular, control block 1510 is configured to control a current sharing ratio defined by parameter A, and is also configured to control multipolar mode selection and multiphasic mode selection in accordance with the principles set forth in the preceding.

[0091] FIG. 16 illustrates a triphasic tripolar stimulus configuration in accordance with another embodiment of the invention. In this embodiment, the second phase delivered on electrode E2, denoted 1624, is configured to be the only cathodic stimulus component which elicits a neural response. This is because stimulus component 1624 is the only component which exceeds a stimulus threshold, namely cathodic stimulus threshold 1650. Stimulus component 1624 is set to an amplitude which elicits the desired therapeutic effect, referred to as the cathodic comfort level 1662.

[0092] All other components of the stimulus then can be derived from the amplitude of stimulus component 1624. Specifically, if we define the amplitude of stimulus component 1624 as X, then anodic stimulus components 1614 and 1634 delivered by electrodes E1 and E3 respectively should take an amplitude of 0.5 X to effect 100% current return in the second phase. Anodic stimulus components 1614 and 1634 can take alternative amplitudes (such as 0.7 and 0.3, respectively) provided the net anodic amplitude adds to X to effect 100% current return in the second phase.

[0093] Similarly, anodic stimulus components 1622 and 1626 delivered by electrode E2 in the first and third phases respectively should each take an amplitude of 0.5 X to effect current balancing of electrode E2 across the three phases. Anodic stimulus components 1622 and 1626 can take alternative amplitudes (such as 0.7 and 0.3, respectively) provided the net anodic amplitude of these components adds to X in order to effect current balancing of electrode E2 across the three phases.

[0094] Finally, cathodic stimulus components 1612 and 1632 delivered by electrodes E1 and E3 respectively in the first phase should each take a value of 0.25 X, or should at least sum to 0.5 X, to effect 100% current return in the first phase. Similarly, cathodic stimulus components 1616 and 1636 delivered by electrodes E1 and E3 respectively in the third phase should each take a value of 0.25 X, or these components should at least sum to 0.5 X, to effect 100% current return in the third phase. A further constraint is that current balancing of electrode E1 should be effected, and current balancing of electrode E3 should be effected, across the three phases, as can be simply achieved.

[0095] The embodiment of FIG. 16 thus provides for the cathodic stimulus component 1624 to be four times larger than any other cathodic stimulus component (1612, 1616, 1632, 1636), thus permitting significant variation of the amplitude of component 1624 for therapeutic purposes while retaining all other cathodic stimulus components below threshold. It is further noted that no anodic stimulus component delivered by any electrode exceeds the respective anodic stimulus threshold (1641, 1642, 1643). Thus, the stimulus configuration of FIG. 16 provides for single cathode neural excitation as desired.

[0096] In the example of FIG. 16 the anodic threshold is indicated as being double the magnitude of the cathodic threshold however it is to be appreciated that this relationship might differ in other embodiments within the scope of the present invention.

[0097] FIG. 17A illustrates the polarities of electrodes in a single phase of a quadrupolar stimulus configuration in accordance with another embodiment of the invention, and FIG. 17B illustrates the polarities of electrodes in a single phase of a pentapolar stimulus configuration in accordance with a further embodiment of the invention. In accordance with the principles described in detail in relation to FIG. 16, these multipolar configurations provide for current return to be shared between a larger number of return electrodes thus introducing a greater latitude to configure a biphasic or multiphasic stimulus in a manner which ensures that only one stimulus component gives rise to neural recruitment.

[0098] For example, FIG. 18 illustrates a triphasic quadrupolar stimulus configuration in accordance with yet another embodiment of the invention. This configuration permits the cathodic stimulus components delivered by electrodes E1, E3 and E4 in the first and third phases to have an amplitude which is only one sixth the amplitude of the supra-threshold stimulus component delivered by electrode E2 in the second phase. This provides additional operational freedom in selecting the amplitude of the single supra-threshold stimulus component, while maintaining all other stimulus components below threshold. The pentapolar configuration of FIG. 17B may be exploited to provide a one eighth amplitude in triphasic, while increasing the number of phases may additionally or alternatively be exploited to provide further operational freedom in selecting the amplitude of the single supra-threshold stimulus component.

[0099] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not limiting or restrictive.