U.S. patent application number 16/752209 was filed with the patent office on 2020-05-21 for implantable electrode positioning.
This patent application is currently assigned to Saluda Medical Pty Ltd. The applicant listed for this patent is Saluda Medical Pty Ltd. Invention is credited to Milan Obradovic, John Louis Parker.
Application Number | 20200155240 16/752209 |
Document ID | / |
Family ID | 56106304 |
Filed Date | 2020-05-21 |
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United States Patent
Application |
20200155240 |
Kind Code |
A1 |
Parker; John Louis ; et
al. |
May 21, 2020 |
Implantable Electrode Positioning
Abstract
A method of surgically positioning an electrode array at a
desired implantation location relative to a nerve. A temporary
probe electrode is temporarily positioned adjacent to the nerve and
at a location which is caudorostrally separate to the desired
implantation location of the electrode array. The implanted
position of the probe electrode is temporarily fixed relative to
the nerve. During implantation of the electrode array, electrical
stimuli are applied from one of the temporarily fixed probe
electrode and the electrode array, to evoke compound action
potentials on the nerve. Compound action potentials evoked by the
stimuli are sensed from at least one electrode of the other of the
temporarily fixed probe electrode and the electrode array. From the
sensed compound action potentials a position of the electrode array
relative to the nerve is determined.
Inventors: |
Parker; John Louis;
(Artarmon, AU) ; Obradovic; Milan; (Artarmon,
AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Saluda Medical Pty Ltd |
Artarmon |
|
AU |
|
|
Assignee: |
Saluda Medical Pty Ltd
Artarmon
AU
|
Family ID: |
56106304 |
Appl. No.: |
16/752209 |
Filed: |
January 24, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15535014 |
Jun 9, 2017 |
10588698 |
|
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PCT/AU2015/050753 |
Nov 30, 2015 |
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16752209 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 2562/046 20130101;
A61B 2034/2053 20160201; A61B 5/4893 20130101; A61N 1/372 20130101;
A61N 1/36185 20130101; A61N 1/36071 20130101; A61B 2505/05
20130101; A61B 5/686 20130101; A61B 5/04001 20130101; A61N 1/3614
20170801; A61B 5/407 20130101; A61B 34/20 20160201; A61N 1/0553
20130101 |
International
Class: |
A61B 34/20 20060101
A61B034/20; A61B 5/00 20060101 A61B005/00; A61B 5/04 20060101
A61B005/04; A61N 1/05 20060101 A61N001/05; A61N 1/372 20060101
A61N001/372; A61N 1/36 20060101 A61N001/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 2014 |
AU |
2014905030 |
Claims
1. A method of surgically positioning an electrode array at a
desired implantation location relative to a nerve, the method
comprising: implanting a temporary probe electrode adjacent to the
nerve and at a location which is caudorostrally separate to the
desired implantation location of the electrode array; temporarily
fixing the implanted position of the probe electrode relative to
the nerve; during implantation of the electrode array, applying
electrical stimuli from one of the temporarily fixed probe
electrode and the electrode array, to evoke compound action
potentials on the nerve; sensing, from at least one electrode of
the other of the temporarily fixed probe electrode and the
electrode array, the compound action potentials evoked by the
stimuli; and determining from the sensed compound action potentials
a position of the electrode array relative to the nerve.
2. The method of claim 1 wherein the probe electrode is surgically
introduced via the same incision as the electrode array.
3. The method of claim 1 wherein the desired positioning of the
electrode array is relative mediolaterally to a physiologic midline
of the nerve.
4. The method of claim 3 wherein the probe electrode is configured
to simultaneously stimulate an even distribution of fibres
mediolaterally across the nerve.
5. The method of claim 3 wherein identification of the physiologic
midline of the nerve, and positioning of the electrode array
relative to the identified midline, is achieved by providing two
laterally spaced apart sense electrodes on the electrode array, and
monitoring a relative strength of the compound action potential
sensed by each of the sense electrodes.
6. The method of claim 1 wherein a radial spacing of the electrode
array from the nerve is determined.
7. The method of claim 6 wherein the probe electrode comprises
first and second stimulus electrodes each at distinct radii away
from the nerve, used to deliver stimuli of equal intensity, at
different times.
8. The method of claim 1 wherein the probe electrode is fixed by
being temporarily anchored upon a vertebra, within the epidural
space.
9. The method of claim 1 wherein the probe electrode is a
peripheral nerve stimulator delivering transcutaneous stimuli, to
evoke CAPs on peripheral nerve(s) at a location of interest.
10. The method of claim 9 wherein an array location at which sense
electrodes sense a maximal collision of CAPs evoked by spinal cord
stimulus electrodes with CAPs evoked by the peripheral nerve
stimulator is taken to be an optimal caudorostral position of the
spinal stimulus electrodes relative to the location of
interest.
11. A system for positioning an electrode array at a desired
implantation location relative to a nerve, the system comprising: a
temporary probe electrode configured to be implanted adjacent to
the nerve at a location which is caudorostrally separate to the
desired implantation location of the electrode array, and
configured to be temporarily fixed relative to the nerve while
implanted; an electrode array configured to be implanted adjacent
to the nerve at the desired implantation location, and comprising
at least one electrode configured to evoke or sense compound action
potentials; and a controller configured to: cause electrical
stimuli to be applied from one of the temporarily fixed probe
electrode and the electrode array to evoke compound action
potentials on the nerve during implantation of the electrode array;
sense from at least one electrode of the other of the temporarily
fixed probe electrode and the electrode array the compound action
potentials evoked by the stimuli; and determine from the sensed
compound action potentials a position of the electrode array
relative to the nerve.
12. A non-transitory computer readable medium for surgically
positioning an electrode array at a desired implantation location
relative to a nerve, comprising instructions which, when executed
by one or more processors, causes performance of the following:
computer program code means for, during implantation of the
electrode array, applying electrical stimuli from one of the
electrode array and a probe electrode which is temporarily fixed
adjacent to the nerve at a location which is caudorostrally
separate to the desired implantation location of the electrode
array, to evoke compound action potentials on the nerve; computer
program code means for sensing, from at least one electrode of the
other of the electrode array and the probe electrode, the compound
action potentials evoked by the stimuli; and computer program code
means for determining from the sensed compound action potentials a
position of the electrode array relative to the nerve.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is continuation of U.S. application Ser.
No. 15/535,014, filed Jun. 9, 2017, which is a national stage of
Application No. PCT/AU 2015/050753, filed Nov. 30, 2015, which
application claims the benefit of Australian Provisional Patent
Application No. 2014905030 filed Dec. 11, 2014, the disclosures of
which are incorporated herein by reference in their entireties.
TECHNICAL FIELD
[0002] The present invention relates to monitoring compound action
potentials during surgery to assist with implantable electrode
placement.
BACKGROUND OF THE INVENTION
[0003] A range of implanted neural devices exist, including: spinal
cord implants which electrically stimulate the spinal column in
order to suppress chronic pain; cochlear implants which
electrically stimulate the auditory nerve to produce a hearing
sensation; deep brain stimulators which electrically stimulate
selected regions of the brain to treat conditions such as
Parkinson's disease or epilepsy; and neural bypass devices which
electrically stimulate either afferent sensory nerve fibres to
reproduce impaired sensory function or efferent motor nerve fibres
to reproduce impaired motor activity, or both.
[0004] Such devices require implantation of an electrode array
proximal to the neural pathway of interest, in order to enable
electrical stimuli to be delivered from the array to the nerve in
order to evoke compound action potentials, or neural responses. For
example, the typical procedure for implantation of a spinal cord
stimulator having a paddle electrode involves placing the patient
under general anaesthesia and performing a laminectomy or removal
of part of the dorsal process to access the epidural space. However
the success of spinal cord stimulation for pain relief, and of
neural device implantation in general, is strongly linked to the
accuracy of the placement of the implanted stimulating electrodes
during surgery. Physiologic midline placement of paddle leads is
important to avoid uncomfortable side-effects during stimulation as
a result of the activation of dorsal root fibers. One approach to
accurately position the electrode array is to temporarily wake the
patient from the general anaesthesia and to ask the patient to
report the location of paraesthesia produced by stimuli delivered
by the array. Temporarily waking a patient from a general can be
difficult, and even once the patient is awake the reports provided
by a drowsy patient are often unreliable. Because the patient is
not fully alert when temporarily awoken from general anaesthesia,
and is otherwise asleep during the remainder of the implantation
procedure, they can only provide limited feedback regarding the
location of the paraesthesia, or regarding any complications
arising from lead placement. Although complications are rare they
can be very serious.
[0005] Another option is to not wake the patient during surgery,
and to use anatomical targeting to guide the positioning of the
electrode array, by reference to anatomical markers that can be
imaged via fluoroscopy, instead of relying on unreliable patient
feedback. However, fluoroscopic imaging resolution is relatively
imprecise, compared to the accuracy requirements of lead placement.
Moreover, complications of implanting a surgical lead while a
patient is asleep can include damage to the spinal cord due to
direct pressure of the lead as it is placed into the epidural
space, or post-operative damage due to the development of a
hematoma over the lead, which can then create pressure on the lead
and damage the dorsal column axons.
[0006] Another situation requiring accurate electrode lead
placement is the case of paddle leads, which comprise a two
dimensional array of electrodes which when implanted into the
epidural space extend both along (caudorostrally relative to) and
across (mediolaterally relative to) the dorsal columns. Paddle
leads for example can be used to treat patients with bilateral pain
complaints, with the goal to provide paraesthesia to both sides of
the body. To accomplish this it is preferable to place the paddle
lead over the physiologic midline of the dorsal columns. However
the physiologic midline, being the centre line of the spinal cord
which demarcates between the fibres innervating the left side and
the right side of the body, may or may not be well aligned with the
anatomical midline as defined by anatomical markers that can be
imaged via fluoroscopy. Consequently, implanting a patient under a
general anaesthetic by reference to anatomical markers can result
in the paddle electrode array not providing equal stimulation and
paraesthesia to both sides of the body.
[0007] One technique for defining the physiologic midline is to use
somatosensory potentials, observed from electrodes placed on the
scalp. In this technique the stimulation of peripheral nerve
fibres, such as stimulation of the posterior tibial nerve by needle
electrode, evokes a response in the somatosensory cortex. By
simultaneously stimulating dorsal column fibres using the spinal
cord lead, a collision can be created between the peripherally
evoked response and the spinally evoked response. This collision
results in an observed depression of the somatosensory responses.
Both tibial nerves are stimulated, so that a symmetric depression
from left and right somatosensory cortex responses will indicate
that the stimulated electrode is above the midline.
[0008] Somatosensory response to stimulation of peripheral nerves
has also been used to identify the rostral caudal location of the
electrode with respect to peripheral locations. However, this has
been less successful as when considering a sensory homunculus the
representation of the legs for example on the sensory cortex is
small, and buried within the longitudinal fissure of the brain.
Since many chronic pain patients have lower extremity pain this
method has not proved to be useful. Another method has been to
record motor evoked potentials from the muscles in the periphery in
response to stimulation at the spinal cord. Although more
successful at activating muscle fibres, dorsal column motor
stimulation requires very high currents and as such does not
closely correspond to the area of sensory activation.
[0009] The dorsoventral position of the electrode array is also of
importance, as a large nerve-to-electrode distance can increase the
stimulus power required to evoke neural responses and thus decrease
battery life. A large electrode-to-nerve distance can also decrease
the strength of observed neural signals reaching sense electrodes,
in devices configured to measure the neural responses. On the other
hand, bringing the electrode array too close to the nerve can apply
pressure or direct trauma to the nerve and cause temporary or even
permanent nerve damage. However, the dorsoventral position is also
difficult to accurately determine during surgery. Occasionally a
surgeon may take a lateral view image with fluoroscope, however
these images are not of sufficient resolution to sufficiently
accurately judge the proximity of the array to the cord.
[0010] Any discussion of documents, acts, materials, devices,
articles or the like which has been included in the present
specification is solely for the purpose of providing a context for
the present invention. It is not to be taken as an admission that
any or all of these matters form part of the prior art base or were
common general knowledge in the field relevant to the present
invention as it existed before the priority date of each claim of
this application.
[0011] Throughout this specification the word "comprise", or
variations such as "comprises" or "comprising", will be understood
to imply the inclusion of a stated element, integer or step, or
group of elements, integers or steps, but not the exclusion of any
other element, integer or step, or group of elements, integers or
steps.
[0012] In this specification, a statement that an element may be
"at least one of" a list of options is to be understood that the
element may be any one of the listed options, or may be any
combination of two or more of the listed options.
SUMMARY OF THE INVENTION
[0013] According to a first aspect the present invention provides a
method of surgically positioning an electrode array at a desired
implantation location relative to a nerve, the method
comprising:
[0014] implanting a temporary probe electrode adjacent to the nerve
and at a location which is caudorostrally separate to the desired
implantation location of the electrode array;
[0015] temporarily fixing the implanted position of the probe
electrode relative to the nerve;
[0016] during implantation of the electrode array, applying
electrical stimuli from one of the temporarily fixed probe
electrode and the electrode array, to evoke compound action
potentials on the nerve;
[0017] sensing, from at least one electrode of the other of the
temporarily fixed probe electrode and the electrode array, the
compound action potentials evoked by the stimuli; and
[0018] determining from the sensed compound action potentials a
position of the electrode array relative to the nerve.
[0019] According to a second aspect the present invention provides
a system for positioning an electrode array at a desired
implantation location relative to a nerve, the system
comprising:
[0020] a temporary probe electrode configured to be implanted
adjacent to the nerve at a location which is caudorostrally
separate to the desired implantation location of the electrode
array, and configured to be temporarily fixed relative to the nerve
while implanted;
[0021] an electrode array configured to be implanted adjacent to
the nerve at the desired implantation location, and comprising at
least one electrode configured to evoke or sense compound action
potentials; and
[0022] a controller configured to: [0023] cause electrical stimuli
to be applied from one of the temporarily fixed probe electrode and
the electrode array to evoke compound action potentials on the
nerve during implantation of the electrode array; [0024] sense from
at least one electrode of the other of the temporarily fixed probe
electrode and the electrode array the compound action potentials
evoked by the stimuli; and determine from the sensed compound
action potentials a position of the electrode array relative to the
nerve.
[0025] A non-transitory computer readable medium for surgically
positioning an electrode array at a desired implantation location
relative to a nerve, comprising instructions which, when executed
by one or more processors, causes performance of the following:
[0026] computer program code means for, during implantation of the
electrode array, applying electrical stimuli from one of the
electrode array and a probe electrode which is temporarily fixed
adjacent to the nerve at a location which is caudorostrally
separate to the desired implantation location of the electrode
array, to evoke compound action potentials on the nerve;
[0027] computer program code means for sensing, from at least one
electrode of the other of the electrode array and the probe
electrode, the compound action potentials evoked by the stimuli;
and
[0028] computer program code means for determining from the sensed
compound action potentials a position of the electrode array
relative to the nerve.
[0029] In some embodiments of the invention, the probe electrode is
surgically introduced via the same incision as the electrode array.
In some such embodiments the probe electrode may be fed from the
incision in a first caudorostral direction which is opposite to a
second caudorostral direction in which the electrode array is
introduced. In further such embodiments, in which the nerve is the
dorsal column, the probe electrode may be temporarily fixed so as
to be positioned in the same or a nearby vertebral segment as the
electrode array. Temporarily fixing the probe electrode near the
electrode array, such as in the same vertebral segment or in an
adjacent vertebral segment, or nearby within a small number of
vertebral segments, is desirable because while the fibres of the
dorsal column run approximately parallel over the distances of a
few vertebral segments, any twist or rotation of or within the
spinal cord could produce a misalignment of the
electrophysiological midline relative to the anatomical midline and
this risk rises beyond a few vertebral segments, and this might
alter or make unclear the spatial representation of the
physiological midline of the nerve which is provided by the ECAPs
when first evoked. Temporarily fixing the probe electrode near the
electrode array is also advantageous when it permits a single
surgical incision to be used, such as a single laminectomy, to
implant both the probe electrode and the electrode array.
[0030] In some embodiments of the invention, the desired
positioning of the electrode array is relative mediolaterally to a
physiologic midline of the nerve. For example, the desired
mediolateral positioning of the electrode array may be centrally
over the midline of the nerve. In such embodiments the probe
electrode is preferably configured to simultaneously stimulate an
even distribution of fibres mediolaterally across the nerve. This
may be achieved by the probe electrode comprising a wide electrode
element, or a plurality of electrode elements, which extend(s)
across substantially an entire mediolateral extent of the nerve,
and/or by applying probe stimuli which are sufficiently large, such
as being a multiple of 1.5, two or more of the threshold stimulus
level, so as to evoke responses in most or all fibres of the nerve.
In such embodiments the probe electrode thus launches a compound
action potential along the fibres of the nerve which is
substantially electrically centred on the nerve, even though the
probe electrode itself will not necessarily be precisely centrally
positioned. Identification of the physiologic midline of the nerve,
and positioning of the electrode array relative to the identified
midline, may then be achieved by providing two laterally spaced
apart sense electrodes on the electrode array, and monitoring a
relative strength of the compound action potential sensed by each
of the sense electrodes. If one sense electrode senses a stronger
compound action potential, that electrode is likely closer to the
physiologic midline and the electrode array can be mediolaterally
moved by the surgeon accordingly. If the sense electrodes sense
equally strong CAPs, they are likely equidistant mediolaterally
from, i.e. centrally positioned over, the physiologic midline of
the nerve.
[0031] In additional or alternative embodiments of the invention a
radial spacing of the electrode array from the nerve, such as a
dorsoventral position of a dorsal column stimulator, may be
determined. In such embodiments, the probe electrode preferably
comprises first and second stimulus electrodes each at distinct
radii away from the nerve. For example where the probe electrode
comprises a sheet substrate, first and second electrodes may be
formed on opposing outer surfaces of the sheet and may thereby be
positioned at radii from the nerve which differ by the thickness of
the sheet. The first and second probe electrodes may then be used
to deliver stimuli of equal intensity, at different times. A sense
electrode of the electrode array being implanted is then used to
sense a first intensity of the CAP evoked by the first probe
electrode, and a second intensity of the CAP evoked by the second
probe electrode. A difference between the first intensity and the
second intensity may then be used to estimate a radial spacing of
the electrode array from the nerve. Notably, even though a height
of the probe electrode above the nerve may not be known, such
embodiments permit a relative height of the electrode array to be
monitored by comparing the first and second intensity measurements
over time as the electrode array is moved during implantation.
[0032] The probe electrode may comprise multiple elements which are
caudorostrally spaced apart along the nerve, for example to
facilitate embodiments in which the probe electrode senses ECAPs
evoked by the electrode array, and/or to enable an optimally
caudorostrally positioned probe electrode element to be selected in
order to maximise recruitment and or measurement sensitivity.
[0033] Because the ECAPs produced by the probe electrode are being
used as a point of reference during ongoing positioning of the
electrode array, the probe electrode needs to be in a fixed
location throughout the procedure. The probe electrode may be fixed
by being temporarily anchored upon a vertebra, within the epidural
space. Alternatively the probe electrode may be fixed to an
external structure such as a surgical stabilising arm and have
suitable longitudinal rigidity to maintain a substantially constant
implanted position relative to the nerve for the duration of the
procedure, or may be fixed by any other suitable temporary fixing
means.
[0034] In some embodiments of the invention the probe electrode is
a peripheral nerve stimulator delivering stimuli to evoke CAPs on
peripheral nerve(s) at a location of interest such as a desired
site of paraesthesia. In some such embodiments, the electrode array
which is being implanted may comprise both stimulus electrodes and
sense electrodes, whereby an array location at which the sense
electrodes sense a maximal collision of CAPs evoked by the stimulus
electrodes with the CAPs evoked by the peripheral nerve stimulator
is taken to be an optimal caudorostral position of the stimulus
electrodes relative to the location of interest. Collision of CAPs,
being the reduced recruitment achieved by a given stimulus due to
some or all of the adjacent population of fibres being in their
refractory period because of the peripherally evoked CAP, may be
determined by a depression in the overall amplitude of sensed CAPs.
Preferably the timing of the delivery of the dorsal column pulse is
adjusted to uniquely detect collision.
[0035] The present invention thus recognises that sensing compound
action potentials by use of electrodes of an electrode array, can
be used to monitor the placement of the electrode array during
surgery. The present invention thus provides a method to better
assess the position of the electrode array, in the dorsoventral,
caudorostral and/or mediolateral direction, quickly and simply
while the patient is under general anaesthesia, without requiring
scalp electrodes for somatosensory cortex monitoring, for
example.
[0036] It is to be appreciated that embodiments of the present
invention may be implemented in respect of any suitable
neurostimulator such as spinal cord stimulators, cardiac
pacemakers/defibrillators, functional electrical stimulators (FES),
pain stimulators, etc.
[0037] The stimuli may be delivered by the probe electrode, and
evoked ECAPs may be sensed by the electrode array. Alternatively,
the stimuli may be delivered by the electrode array, and evoked
ECAPs may be sensed by the probe electrode, and it is to be
understood that in all embodiments described herein the positioning
roles of the probe electrode and the electrode array may be
reversed, within the scope of the present invention. Moreover, over
time the source of stimuli may alternate between the probe
electrode and the electrode array, which may assist with position
resolution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] An example of the invention will now be described with
reference to the accompanying drawings, in which:
[0039] FIG. 1 schematically illustrates an implanted spinal cord
stimulator;
[0040] FIG. 2 is a block diagram of the implanted
neurostimulator;
[0041] FIG. 3 is a schematic illustrating interaction of the
implanted stimulator with a nerve;
[0042] FIG. 4 is a view of the spinal cord through a conventional
laminectomy;
[0043] FIG. 5a illustrates a probe electrode, and an electrode
array being positioned, FIG. 5b illustrates the probe electrode
evoking a neural response, FIG. 5c illustrates measurement of the
evoked response to locate the physiologic midline of the nerve, in
accordance with a first embodiment of the invention, and FIGS.
5d-5f illustrate experimental verification of the principles of
FIGS. 5a-5c;
[0044] FIG. 6a illustrates electrode array positioning and channel
allocations in accordance with another embodiment of the invention,
and FIG. 6b shows ECAP signals recorded from the arrangement of
FIG. 6a;
[0045] FIGS. 7a and 7b show recordings obtained from electrodes
13-16 during the implantation procedure;
[0046] FIGS. 8a and 8b show recordings obtained from electrodes
13-16 during closing,
[0047] FIG. 9a illustrates electrode array positioning and channel
allocations in accordance with another embodiment of the invention,
and FIGS. 9b and 9c illustrate ECAP signals obtained, during the
procedure, at 3.39 mA of stimulation;
[0048] FIG. 10a is a plot of ECAP signal strength obtained on
Channel 16, during the procedure, as the stimulus current was
increased from zero to 2.2 mA, and FIG. 10b is a plot of ECAP
signal strength obtained on Channel 16, during closing, as the
stimulus current was increased from zero to 2.2 mA;
[0049] FIG. 11 is a post operative CT image illustrating the
lateral location of the lead causing the late responses;
[0050] FIG. 12A illustrates variation of the amplitude of the
observed ECAP response with the distance of the axon from the
recording electrode, FIG. 12b illustrates an embodiment for
assessing dorsoventral electrode position, and FIG. 12c illustrates
observed ECAPs at differing electrode heights;
[0051] FIG. 13 illustrates a probe electrode arrangement for
assessing electrode array height; and
[0052] FIGS. 14a and 14b illustrate another embodiment of the
invention in which ECAPs evoked directly on the spinal cord are
combined with peripheral nerve stimulation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0053] 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 abdomen and an
electrode assembly 150 implanted within the epidural space and
connected to the module 110 by a suitable lead.
[0054] 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, 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 and the electronics module 110.
[0055] 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 120 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. 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.
[0056] FIG. 3 is a schematic illustrating interaction of the
implanted stimulator 100 with a nerve 180, in this case the spinal
cord. Electrode selection module 126 selects a stimulation
electrode 2 of electrode array 150 to deliver a current pulse to
surrounding tissue including nerve 180, and also selects a return
electrode 4 of the array 150 for current recovery to maintain a
zero net charge transfer.
[0057] 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 spinal cord stimulator for chronic
pain is to create paraesthesia at a desired location.
[0058] 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 2 and 4, 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.
[0059] FIG. 4 is a view of the spinal cord through a conventional
laminectomy. Some embodiments provide for insertion of a probe
electrode through such a surgical incision and in the caudal
direction within the epidural space, and the simultaneous
implantation of an electrode array through the same incision and
then in the rostral direction within the epidural space.
[0060] Referring to FIG. 4, the lamina is a posterior arch of the
vertebral bone lying between the spinous process (which juts out in
the middle) and the more lateral pedicles and the transverse
processes of each vertebra. The pair of laminae, along with the
spinous process, make up the posterior wall of the bony spinal
canal. A conventional laminectomy involves excision of the
posterior spinal ligament and some or all of the spinous process.
Removal of these structures with an open technique requires
disconnecting the many muscles of the back attached to them. After
the laminectomy is performed the electrode is then positioned in
place with forceps or other tool by sliding the electrode in the
rostral direction into the epidural space. Conventionally, direct
visual or radiographic examination is used to determine the
position of the electrode.
[0061] The arrangement shown in FIG. 5a separates the probe
electrode from the recording electrode. The probe electrode can be
arranged on a surgical tool, which can be positioned over the
dorsal column and, importantly, kept stationary while the recording
electrode is moved. The probe electrode may be placed caudally or
rostrally of the electrode array. The probe electrode(s) are
designed to stimulate a large area of the cord and are temporarily
placed at the time of surgery.
[0062] FIGS. 5a-5c illustrate such an arrangement. In FIG. 5a the
electrode array 150 is inserted rostrally, while probe electrode
500 is inserted caudally. The probe electrode 500 is preferably
attached to a handle on surgical tool to allow for simple
placement. The insertion tool(s) used allow both the electrode
array and the probe electrode to be placed with a relatively steep
angle of surgical approach through a shared incision. Such an
approach can be achieved by performing a standard surgical
laminectomy or using a surgical tubular retractor system, such as
the MetRX or the Swivel retractor, modified if required to provide
appropriate guides and anchors to facilitate the placement of both
the probe electrode and the insertion tool for the SCS
electrode.
[0063] In other embodiments, percutaneous implantation of a paddle
lead may be performed, as follows. A standard 14 gauge tuohy needle
is used to access the epidural space. A guide wire is then inserted
through the needle to allow access to the epidural space. The
standard needle is then removed; a custom needle is then passed
over the guide wire with the tip just entering the epidural space.
The tip has a sleeve to prevent coring of the tissue. The guide
wire and sleeve are removed allowing the custom paddle lead to pass
into the epidural space. As the folded lead enters the epidural
space it is separated to allow it to unfurl and lie flat over the
dorsal columns. A stylet is used to help position the lead.
[0064] As shown in the cross sectional view of FIG. 5b, probe
electrode 500 comprises an electrode element 502 which extends
widely in the mediolateral direction relative to the spinal cord
180. Further, a stimulus intensity delivered by the probe electrode
element 502 is set to be significantly above a stimulus threshold.
The stimulus threshold for the recording of ECAPs on the electrode
array 150 can be identified in accordance with any suitable
technique. Delivery of a sufficiently large stimulus from element
502 will create a region of recruitment 504 which is sufficiently
large to recruit action potentials within most if not all of the
ascending fibres of the dorsal column 182. As can be seen the wide
extent of element 502 means that, even though the probe electrode
500 and the associated region of recruitment 504 will not
necessarily be centrally positioned about the physiologic midline
184 of the spinal cord 180, most if not all of the ascending fibres
of the dorsal column 182 will nevertheless be recruited. It is to
be appreciated that any other configurations of the probe electrode
which achieve a corresponding effect are within the scope of the
present invention.
[0065] Because probe electrode 500 has been inserted caudally of
electrode array 150 in the manner shown in FIG. 5a, the orthodromic
rostral propagation of the compound action potential evoked by a
single stimulus delivered by probe electrode 500 will take such an
action potential past electrode array 150. Alternative embodiments
may position the probe electrode 500 rostrally of the electrode
array 150, and exploit antidromic caudal propagation of the
compound action potential along the dorsal column 182 from the
probe electrode 500 to the electrode array 150.
[0066] Once again, due to the difficulties of accurate
implantation, electrode array 150 will not necessarily be centrally
positioned over the physiologic midline 186 of the spinal cord 180.
It is further noted that that midline 186 at the location of the
array 150 may or may not align precisely with the midline 184 at
the location of probe 500.
[0067] The compound action potential evoked by the probe electrode
500 propagates rostrally within the dorsal column 182 and passes
electrode array 150, as shown in the cross sectional view of FIG.
5c, where it is simultaneously sensed by sense electrodes 156 and
158. Because most if not all of the ascending fibres of the dorsal
column 182 have been recruited by probe electrode 500, the electric
field of the compound action potential can be considered to be
centrally located on the physiologic midline 186. Consequently, a
first field strength of the compound action potential sensed by
sense electrode 156 depends on the distance of the sense electrode
156 from the midline 186, and a second field strength of the
compound action potential sensed by sense electrode 158 depends on
the distance of the sense electrode 158 from the midline 186. The
first field strength and second field strength may then be compared
to determine which sense electrode is closer to the midline 186,
and an indication may be given to a surgeon as to which direction
mediolaterally the array 150 should be moved in order to improve
the position of the array during surgery.
[0068] The above described actions can then be incorporated into an
implantation process, as follows: [0069] 1. Surgical approach and
placement of the probe electrode 500; [0070] 2. Insertion of the
tip of the electrode array 150 and connection of the array 150 to
the recording system; [0071] 3. Stimulation amplitude adjustment of
the probe electrode 500 by increasing the amplitude, until the
threshold for ECAP generation is reached, as measured by the
electrodes on the inserted tip of array 150. The amplitude is
further increased to be 1.5.times. or 2.times. the threshold
current; [0072] 4. The electrode array 150 is then further inserted
in the epidural space by manipulation with forceps or other
appropriate surgical tool; [0073] 5. The amplitude of the ECAPS is
continuously monitored and displayed. The implanting surgeon
manipulates the electrode to achieve a balance of ECAP amplitudes
from electrodes on opposing lateral sides of the electrode array
150. [0074] 6. When the left and right most lateral electrodes 156
and 158 are producing the same amplitude ECAP responses, the
electrode array 150 is aligned with the electrophysiological
midline.
[0075] FIGS. 5d and 5e illustrate experimental verification of the
principles of FIGS. 5a-5c. Data was obtained from a patient
implanted with a St Jude Penta.TM. lead 550 shown in FIG. 5d. FIG.
5e shows that the amplitude of the ECAPs recorded on the electrodes
in line with the stimulation were larger compared to those on
either side (more lateral), while FIG. 5f shows that the latency of
the N1 peaks remained the same. FIGS. 5d-5f further illustrate that
the electrically evoked compound action potential can be used to
locate the midline of the dorsal column with a single electrode
array that has a number of lateral contacts. Stimulating at the
centre of the electrode and then measuring the amplitudes at each
of the lateral contacts thus reveals the electrophysiological
midline. The midline is identified by comparing the amplitudes of
the responses at the various contacts and identifying the maximum
amplitude. This requires an electrode with a large number of
lateral spaced contacts and a stimulating electrode that produces a
predominantly midline response.
[0076] Notably, the method of FIG. 5 does not require patient
feedback so that the patient can remain under general anaesthetic
throughout. Moreover, this method avoids the need for more complex
recording of somatosensory cortex potentials. Further, because the
patient is under general anaesthetic the possible recruitment of
motor and/or pain fibres by the large stimulus delivered by probe
electrode 500 will not cause patient discomfort.
[0077] FIGS. 6 to 11 illustrate the detection of lateral lead
position by reference to the production of late responses, or motor
activity, in accordance with another embodiment of the present
invention. A patient had been previously approved for the
implantation of a spinal cord stimulator to treat their pain. The
patient was anaesthetised and prepared for paddle lead
implantation. Once in place the lead was connected to a stimulating
and recording system and ECAPs were monitored during surgery. The
S4 Lamitrode electrode array 602 was inserted rostrally and was
connected to channels 1 to 4 of the stimulating and recording
system, while the S8 Lamitrode electrode array 604 was inserted
caudally and connected to channels 9 to 16 of the stimulating and
recording system, in the manner shown in FIG. 6a.
[0078] ECAPs were recorded on the S8 Lamitrode both during the
procedure and while closing, with stimulation on either the S4 or
S8 Lamitrode. FIG. 6b shows ECAP signals recorded from the caudal
end (i.e. from channels 9-11) of the S8 Lamitrode 602, while
stimulating at the rostral end (i.e. channels 14-16, in tripolar
configuration).
[0079] FIG. 7a shows recordings obtained from electrodes 13-16
during the procedure, while FIG. 7b is an enlarged view of the
recordings of FIG. 7a during the time period 0-5 ms. Notably, no
late responses can be seen in the time period 5-25 ms in FIG. 7a.
In FIG. 7b, small ECAP signals can be seen propagating from CH16 to
CH13.
[0080] FIG. 8a shows recordings on electrodes 13-16 during closing,
while FIG. 8b is an enlarged view of the recordings of FIG. 8a
during the time period 0-5 ms. Strong late responses are visible in
FIG. 8a in the time period 5-25 ms, which corresponded with
observed patient twitching. In FIG. 8b, no ECAP signals can be seen
propagating from Ch16-13.
[0081] Due to the strong twitching observed in the patient during
closing, the current was not increased beyond 2.2 mA, while it was
previously increased beyond that level during the procedure without
issue. During the procedure late responses were observed at 3.39
mA, although these were significantly smaller (<50%) than those
observed during closing at 2.2 mA (less than 60% of that
current).
[0082] FIG. 9a shows the electrode configurations used to obtain
the data of FIGS. 9b, 9c, 10a and 10b. In particular, stimuli were
delivered by channels 1-3 on electrode array 602, while recordings
were taken from channels 16-13 on electrode array 604.
[0083] FIG. 9b illustrates ECAP signals propagating down the S8
lead 604, during the procedure, at 3.39 mA of stimulation. FIG. 9c
is an enlarged view of the recordings of FIG. 9b, during the period
0-5 ms.
[0084] FIG. 10a is a plot of signal strength obtained on Channel
16, during the procedure, as the stimulus current was increased to
2.2 mA. FIG. 10b is a plot of signal strength obtained on Channel
16, during closing, as the stimulus current was increased to 2.2
mA. FIG. 10b shows the appearance of the late response at
approximately 1.7 mA during closing, between 5 and 15 ms, which
continues to increase with increasing current above 1.7 mA. In
contrast, no late response is observed during the procedure, i.e.
in FIG. 10a.
[0085] To explain the results of FIGS. 9 and 10, the
electrophysiological position was correlated with an anatomical
post-operative CT image, shown in FIG. 11. The CT image confirmed
that the S4 lead was lateral on the left side, in particular being
15 mm left of midline at the top of C6, and being 9 mm left of
midline at the bottom of C7. The proximity to the dorsal roots
coincides with an early onset of the late response and lack of ECAP
signals. Thus, stimulating on the S8 Lamitrode 604 showed no
significant difference in ECAP amplitude for similar current
amplitudes, and no sign of a late response. Stimulating on the S4
lead showed a decrease in the amplitude of the ECAP and a
subsequent increase in the late response during closing. The
appearance of late responses coincided with an increase in muscle
activity--observed as twitching in the patient.
[0086] FIGS. 6 to 11 thus illustrate that monitoring the amplitude
and latency of the ECAP as well as late response during lead
insertion is a useful, accessible tool to aid lead placement. The
data shows that it is possible to determine if the lead is lateral,
near the dorsal roots and estimate its orientation with respect to
the physiological midline of the spinal cord. Examining the
presence of late responses can identify the mediolateral location
of the lead. Late responses are related to the activation of roots
and therefore if two leads are implanted and late responses are
only seen on one side this would indicate that that lead is closer
to the roots.
[0087] FIG. 12 illustrates another embodiment of the invention, in
which the dorsal-ventral depth of the electrode, or its relative
position from the surface of the spinal cord, is determined. In
this embodiment, the probe electrode comprises two sets of
stimulating contacts, each set being at a unique height above the
dorsal column.
[0088] FIG. 12A illustrates how the amplitude of the observed ECAP
response, as measured by the negative amplitude of the N1 peak,
varies with the distance of the axon from the recording electrode.
As can be seen from FIG. 12A, larger responses are observed if the
fibres are closer to the electrodes, and the amplitude of the
observed response varies with the distance r from the fibre by
approximately 1/r.sup.2. The present embodiment recognises that a
relative measure of the distance (x) of the electrode 1256 from the
spinal cord 1280 can be obtained in the following manner. Consider
a probe electrode 1260 with a least two electrode contacts 1262,
1264, which are separated by a vertical distance h above the spinal
cord, as shown in FIG. 12b. The probe electrode height separation h
can be precisely known. As discussed above in relation to FIG. 5,
the electrode contacts 1262 and 1264 can each be made to extend
mediolaterally from one side of the cord to other in such a manner
as to recruit the maximal amount of fibres of the dorsal column of
the cord 1280.
[0089] The probe electrodes 1262, 1264 are preferably mounted on a
surgical tool and inserted in the retrograde manner in the epidural
space opposite to the direction of the insertion of the SCS
electrode 1256, in the manner shown in FIG. 5a. The probe electrode
is stimulated in an alternate manner between the two electrode
positions from the upper position 1262 to the lower position 1264.
The frequency of the stimulation will allow the convenient
measurement of the ECAP responses by the SCS electrode 1256 from
both stimulating electrodes 1262, 1264.
[0090] The distance x between the SCS electrode 1256 and the spinal
cord 1280 can vary with insertion, or patient movement such as
breathing. The height r of the stimulating electrode 1264 is
unknown, but remains fixed with respect to the spinal cord 1280 due
to the temporary fixing of the probe electrodes throughout the
procedure. As illustrated in FIG. 12c, the relative distance from
the cord 1280 to the SCS electrode 1256 can be determined by
examining the difference in the observed ECAP amplitudes evoked by
delivering the same intensity stimuli from the respective
electrodes 1262, 1264.
[0091] Suitable adjustment of FIG. 12c may allow for the curve to
be stepped to account for the transition of the propagating
electric field from tissue, to the dielectric substrate material
bearing electrodes 1262, 1264. Moreover, while electrode 1262 is
sensing/stimulating, electrode 1264 should be electrically floating
to minimise shielding of the interaction between electrode 1262 and
the spinal cord 1280.
[0092] As shown in FIG. 12c, the amplitude difference a of the ECAP
as measured by the N1 peak from the two different height probe
electrodes is sensitive to the height of the electrode 1256 above
the spinal cord 1280. The closer the measurement electrode 1256 is
to the cord 1280, the larger the amplitude a of the differences,
noting a.sub.2>a.sub.1 in FIG. 12c.
[0093] The design of the probe electrode 1260 needs to be
considered carefully. It is required to stimulate the same fibres
of the spinal cord 1280, from two (or more) different heights. The
stimulation location in the caudal rostral direction for the two
stimulating electrodes should ideally be at the same caudal-rostral
location or as close to each other as possible so as the ECAP
responses produced have the same distance to propagate to avoid the
problem of different propagation distances resulting in different
amplitudes of response. An electrode contact 1260 that achieves
this arrangement is depicted in FIG. 13, in both elevation and plan
view. It consists of interposed electrode contacts, whereby one set
1262 of contacts is present on the surface and the other set 1264
is separated by distance (h) at another plane in the electrode. The
digits are connected together and form a single large stimulating
electrode of a wide extent mediolaterally, and with two alternative
heights above the dorsal column. Such embodiments thus recognise
that not only is it important to be able to position the lead in
the dorsolateral and rostrocaudal direction to stimulate the
appropriate dermatome, it also important to know where the lead is
in the dorsal ventral direction. The distance from the spinal cord
to the electrode in the dorsal ventral direction affects both the
power consumption and the degree to which adjustments of the
stimulation current can control the location and strength of the
paraesthesia or level of pain relief. For closed loop control of
SCS, the closer the lead is to the spinal cord the smaller the
current that is required to stimulate the target. In turn this
corresponds to a larger amplitude of the actual compound action
potential generated by a similar size current. Sense electrodes
closer to the spinal cord will sense a stronger observed signal for
a given ECAP, as compared to sense electrodes further away,
improving signal to noise quality in ECAP measurements. Increasing
the amplitude of the ECAP is desirable to allow finer closed loop
control. Positioning electrodes closer to the dura also results in
lower currents required for stimulation and lower corresponding
artifacts of stimulation in ECAP measurement.
[0094] FIG. 14a illustrates a further embodiment of the invention,
in which ECAPs evoked directly on the spinal cord are combined with
peripheral nerve stimulation, whereby the rostrocaudal location of
the lead can be identified. In this embodiment, it is desired to
position a stimulus electrode 1452 of an electrode array 1450
physiologically adjacent to a selected nerve root 1470 with an
associated dermatome within which paraesthesia is required. A TENS
machine 1490 is used to stimulate the peripheral nerve(s)
associated with nerve root 1470, thereby evoking compound action
potentials which propagate rostrally to the brain via nerve root
1470. TENS machine is operated at a fixed location and at a fixed
intensity so as to produce a train of substantially constant action
potentials. Simultaneously, the chosen stimulus electrode 1452
directly stimulates the spinal cord 1480. Sense electrodes 1456 and
1458 sense the resultant neural activity produced from 1490 and
1452, as it continues to propagate rostrally. The present
embodiment recognises that the ECAPs evoked from stimulus electrode
1452 collide with, or interfere with, the compound action
potentials evoked at the periphery by TENS device 1490, and
further, that the maximal interference between the two types of
ECAPs occurs when the location of electrode 1452 is optimal
physiologically relative to nerve root 1470. Accordingly, the
method can be performed while adjusting the caudorostral position
of array 1450 to seek an array location at which maximal ECAP
interference occurs. In other embodiments the sense electrode(s)
may be positioned on a separate sense electrode array and for
example may be temporarily implanted only for the duration of the
implantation procedure.
[0095] FIG. 14b illustrates such ECAP interference or collision.
FIG. 14b shows the observed response 1402 from a single electrode
in response to tibial nerve stimulation alone, the response 1404
from tibial nerve stimulation simultaneously with spinal cord
stimulation, and the response 1406 observed when performing spinal
cord stimulation only, without peripheral stimulation. The delay
time to the dorsal column stimuli which produces the most
attenuation allows estimation of the total length of the fibre from
the point where the stimulus is presented.
[0096] The ability to monitor, and control optimisation of, the
mediolateral, caudorostral and/or dorsoventral location of the
electrode, relative to physiological characteristics of the dorsal
columns rather than anatomical markers, will thus enable a much
higher precision of implantation. The present invention may thus
provide feedback to a surgeon that allows the lead to be steered to
optimize the final implanted location of the spinal cord
stimulation lead. To do so requires surgical tools to assist in the
steering and placement of electrodes. Some embodiments may
therefore involve a lead comprising a longitudinal pocket or
similar parts designed to receive an insertion tool.
[0097] In all described embodiments the determined position
information can be presented to the surgeon by any suitable means,
such as by an acoustic tone with pitch indicating relative height
or position, or a visual indicia, or otherwise.
[0098] 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.
* * * * *