© 2004 International Neuromodulation Society, 1094-7159/04/$15.00/0 Neuromodulation, Volume 7, Number 3, 2004 157–167

Blackwell Publishing, Ltd.

Real-Time Paresthesia Steering Using Continuous Electric Field Adjustment. Part I: Intraoperative

Performance

John Oakley, MD*;

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Clayton Varga, MD

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Elliot Krames, MD

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Kerry Bradley, MS

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Yellowstone Neurosurgical Associates, Billings, Montana;

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Pasadena Rehabilitation Institute, Pasadena, California;

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Pacific Pain Treatment Center, San Francisco, California; and

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Advanced Bionics Corporation, Valencia, California

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BSTRACT

We present data collected from a multicenter studyusing a new neurostimulation system. This new systemuses a current-shifting programming technique forspinal cord stimulation. The system maintains a continu-ous, suprathreshold stimulation field while adjustingthe distribution of anodic and cathodic current amongcontacts along a multi-contact array. The changingdistribution of current shifts the electric field along thespinal cord, resulting in real-time, dynamic paresthesiasteering. This process of adjusting the stimulation fieldhas been termed continuous electric field adjustment(CEFA). This technique has been used to assess pares-thesia coverage for patients undergoing implantationof stimulation contact arrays for chronic pain. This

multicenter study evaluated the performance of theCEFA technique. The results of the study showed thatparesthesia coverage could be shifted in real-time todifferent regions on the patient’s body in a comfort-able fashion, with the patient always feeling pares-thesia during the adjustment process. The resultsof the study also show that the process was time-efficient: intraoperatively, the median time to assess71 combinations on a single 8-contact lead across18 patients was 4.1 (3.6–5.0) minutes.

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chronic pain

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constant current

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contactcombination adjustment

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paresthesia

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spinal cordstimulation

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steering

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Financial support was provided by Advanced Bionics Corporation.Address correspondence and reprint requests to: John Oakley, MD,Yellowstone Neurosurgical Associates, 2900 12th Avenue North, Billings,MT 59101. Email: JoShir@aol.com.

INTRODUCTION

Spinal cord stimulation (SCS) has been employedfor more than 30 years for the treatment of

chronic pain syndromes (1,2). Technical advance-ments in SCS systems have improved the long-term efficacy of the therapy. These advancementsinclude an increase in the number of stimulatingcontacts and more flexibility in the programmabil-ity of the devices (1,3).

With these advances, however, comes the chal-lenge of identifying and maintaining an optimalset of stimulation parameters (contact combina-tion, amplitude, PW, rate, etc.) for each patient. InSCS, overlap of the pain with paresthesia has been

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identified as a critical factor for a successful out-come (1). Paresthesia location is determined bywhich spinal cord fibers are activated by the elec-trical field stimulation. To a first approximation,optimal paresthesia location involves identifyingthe stimulation contact combination that recruitsthe desired fibers.

SCS systems limited to “discrete” combinationsof anodes and cathodes without independentamplitude control of each contact can causepatient discomfort when switching to a new dis-crete combination. The programming of suchsystems requires the reduction of the stimula-tion amplitude to a subthreshold value beforechanging to the new discrete combination. Theneed to “ramp-up” the amplitude for each combi-nation reduces the number of contact combina-tions that can be investigated in a reasonableperiod of time. A recent report on methods ofSCS programming acknowledges that the task isrelatively slow, reporting a mean combinationassessment time of approximately two minutesper combination for manual discrete combinationprogramming (4).

Even motivated clinicians using well-definedoptimization algorithms or automated discretecombination programmers can find the search forthe best paresthesia to be a numerically over-whelming task. Either an exhaustive search isundertaken which may take hours (4), or a limited(and possibly suboptimal) subset of discrete com-binations are tried within an allotted, likely brief,period of clinical time. In contrast, a “continuous”combination SCS system with independent ampli-tude control on each contact might smoothlyswitch between combinations (while maintainingparesthesia sensation) and avoid the time consum-ing process of reducing amplitude between eachcombination.

The increased spatial resolution of stimuluscurrent fields from a continuous combination SCSsystem may provide an opportunity to furtheroptimize paresthesia coverage. Contact arrayswith increased numbers of more closely-spacedcontacts, however, are required to fully realize thegreater spatial adjustment available with the stimu-lation field. In typical SCS electrodes, the inter-contact edge-to-edge spacing varies from 4–12 mm(5). If the neural target resides underneath theregion between contacts on an SCS array, the

amplitude must be increased significantly torecruit the desired nerves. This typically results inextraneous or unwanted stimulation and, as aside effect, may limit the use and success of thetherapy. SCS contact arrays with reduced intercon-tact spacing may avoid this side effect and allowa continuous combination SCS implant to fullyoptimize the paresthesia for a given contact arrayplacement.

An SCS system with continuous combinationprogramming and a closely spaced contact arrayhas been designed. The purpose of this paper isto describe the system and its intraoperative appli-cation during the implantation of SCS contactarrays and present data collected from multipleclinical sites.

GLOSSARY

For the purposes of this paper, we define the fol-lowing (as adapted from Ref. 6):

Lead: the collection of conductors, contacts, andconnectors, encased within insulating material(typically polyurethane and/or silicone rubber)that provides a current delivery path between thepulse generator circuitry and the neural targets.

Contact: the small piece of metal at the end of alead that interfaces with the body and deliversstimulation current to the neural targets; typi-cally made of platinum-iridium.

Array: the collection of contacts at the distal endof the lead.

Electrode: the term “electrode” has historicallybeen used to refer to both leads and contactsand thus will not be employed in this paper.

Channel: an individual timing generator, pro-grammable with unique pulse amplitude, pulsewidth, and pulse type (asymmetric or sym-metric biphasic).

Discrete: a method of programming in which stimu-lation parameters are established while stimu-lation is not perceived, then stimulation energyis adjusted until the effect of the stimulation isperceived.

Continuous: a method of programming in whichstimulation parameters are adjusted while theeffect of stimulation is perceived.

Combination: a distinct programming of anodicand cathodic current distribution among active

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contacts, that is, if three contacts E1, E2, andE3 are programmed as a “guarded cathode” thenthese three contacts may be programmed intonine distinct tripolar combinations, where 100%cathodic current is equal to 100% anodic cur-rent (Table 1).

MATERIALS AND METHODS

Stimulation System

The prototype stimulation system used in the studyemployed a pulsatile, controlled-current stimulator.The stimulator assigned a single programmablecurrent source to each stimulating contact. Thestimulator could sink/source up to 12.7 mA fromeach contact and drive a total of 16 contactssimultaneously. The pulse width was program-mable from 20–1000

µ

s. The stimulation rate wasadjustable from 2–1200 Hz. The stimulation wave-forms employed both passive and active recharge,depending upon programmed stimulation para-meters. Charge balance was maintained at all timesboth algorithmically in the system programmingsoftware as well as by the use of coupling capacitorson each contact. The stimulator had four program-mable channels that could operate simultaneouslywith arbitration to avoid pulse overlap.

The use of individual controlled-current sourcesfor each contact enabled a unique programmingtechnology. The programming software used a“continuous” method of programming: stimulation

combinations were adjusted in small steps suchthat the electric field could be shifted graduallyalong the contacts, making the process of chang-ing spatial neural recruitment smooth. For thisintraoperative study, the specific method ofprogramming was termed continuous electricfield adjustment (CEFA) (described in detailbelow).

For this study, the stimulation electronicsresided in a custom aluminum chassis with con-nector outputs that mated to Medtronic (MDT;Minneapolis, MN), Advanced NeuromodulationSystems (ANSI; Plano, TX), and Advanced Bionics(AB; Valencia, CA) percutaneous connector sys-tems for delivery of stimulation to the patient (seeFig. 1). These outputs were gated by a fail-safeswitch, which controlled output relays. For thisintraoperative study, only AB connectors wereused. The stimulation system was completelybattery-powered. A laptop/notepad computer (DellInspiron 7500; Dell Computer Corp., Austin, TX/LT-C 600; Fujitsu, San Jose, CA) with a touchscreen was used to program and control thestimulation circuitry through wireless infrared andRF communication links. The computer ran thestimulator programming software, which wascontrolled both by keyboard input and input fromthe touch-screen on a graphical user-interface (seeFig. 1).

Prior to use, the system was bench-tested (peran extensive protocol) to guarantee safe operationand accuracy within 5% of the programmed values

Table 1. Programmable “Guarded Cathode” Com-binations when using Continuous Contact CombinationProgramming of Anodic Current Distribution

Possible “guarded cathode” combinations

E1 % anodiccurrent

E2 % cathodiccurrent

E3 % anodiccurrent

10 100 9020 100 8030 100 7040 100 6050 100 5060 100 4070 100 3080 100 2090 100 10

Figure 1. Prototype Stimulation System used for Intraoper-ative Testing.

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of pulse amplitude, pulse width, and pulse rate forthe entire range of programmable parameters.

Stimulation Contact Array

While the stimulation system could be pro-grammed to work with any commercially availablestimulation leads, the CEFA process was designedto work with an in-line contact array with agreater contact density than market-released per-cutaneous arrays. This array (custom prototypedesign; Advanced Bionics Corp., Valencia, CA) hadeight contacts with a contact length of 3 mm andan intercontact edge-to-edge spacing of 1 mm (seeFig. 2). We believed that such spacing would allowfor a smoother combination adjustment along thearray, due to the likelihood that neural thresholdsusing closely-spaced contacts will be more similarthan for contacts that are more widely spacedapart (7). Additionally, the electric field generatedby the stimulation current will have a more con-tinuous spatial shifting along closely-spaced con-tacts than along widely-spaced contacts.

Patient Population

A total of eighteen patients from three centerswere enrolled. Study patients were selected fromstandard SCS candidates at each clinical centerinvolved in this study. Informed consent wasobtained from all patients prior to testing. Theconsent form and the protocol were approved bythe IRB of record for each center.

Intraoperative Study Description

For intraoperative testing, the patient was testedduring the implantation procedure of a previouslyprescribed commercially available SCS system. Thepatients were placed prone on the operating tableand were prepped and draped per each center’sstandard SCS implantation practice. Patients wereintravenously sedated and locally anesthetizedduring surgical procedures. The epidural spacewas accessed using a 14 Ga needle, using a “hang-ing drop” or “loss-of-resistance” technique, as perthe implanting physician’s normal technique.

Once the epidural space was accessed, prior toimplantation of the prescribed and intended finalcontact array(s), the AB 8-contact leads were intro-duced through the percutaneous needles into theepidural space of each patient enrolled in thestudy. Once the implanting physician was satisfiedwith the visual location of the array (via fluoro-scopy), these leads were then connected to theprototype stimulation system for stimulation test-ing, that is, a CEFA trial.

Prior to the CEFA trial, an impedance measure-ment was made for all connected contacts. Theseimpedance measurements were used to verify theintegrity of all connections from the stimulationsystem to the contacts. All impedances for allpatients indicated that all contacts were deliveringstimulation current into the body. The impedancemeasurements, for all patients, took less than 15seconds.

A CEFA trial was performed as follows: an initialcontact combination was established at subthresh-old amplitude with the cathode at the most distalcontact on the array. The amplitude was thenslowly increased until the patient reported feelingparesthesia at a medium intensity. The contactcombination was then changed in small steps toshift the cathodic field down the contact array.While this shifting was taking place, the patientwas continually asked about the intensity of thestimulation as well as the location of the pares-thesia. If the intensity became too strong or tooweak, contact combination adjustment was tem-porarily ceased and amplitude adjustments weremade to assure a comfortable intensity; the com-bination adjustment was then continued.

During the CEFA trial, the patient reported thelocation of the paresthesia, which often shifted to

Figure 2. Medtronic Pisces Quad 3487A (above) andAdvanced Bionics 8-Contact Stimulation Lead (below).

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various regions of their body. If the patient hap-pened to report that paresthesia covered theirpainful area satisfactorily, the clinician control-ling the stimulation reported the location of thecathode(s) to the implanting physician. The physi-cian noted the location of these cathodic contactsrelative to the vertebral anatomy and saved thefluoroscopic image for later reference whenimplanting the prescribed and intended market-released contact array.

The CEFA trial was completed when thecathodic field reached the most proximal contacton the array. Stimulation was then turned off and,at the physician’s discretion, the lead was movedand a CEFA trial was again performed. In 10 cases,two AB 8-contact leads were placed epidurally andtested. The contact arrays were typically placed in“perfect parallel” (no stagger of contacts betweenarrays) and the interlead transverse separationvaried from 1–5 mm across patients. At least oneCEFA trial was performed for each implantedAB 8-contact array. No “cross-lead” stimulationcombinations were programmed in these dual-leadcases.

After all stimulation testing, the AB 8-contactleads were withdrawn from the epidural spaceand the patient was then implanted with the pre-scribed and intended lead(s). After intraoperativetesting, the patient was followed up via standardpractice of their attending physician.

During CEFA trials, the same contact combinationadjustment sequence was used for all patients.There were 71 unique contact combinations thatwere programmed in this sequence. For compari-son to commercially available stimulation systems,of the 71 unique contact combinations in theCEFA sequence, there were 15 discrete combina-tions that utilized either 100% cathodic or 100%anodic current on a single contact (“

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100%, 0,0,

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100%, 0,0,0,0”), or a 50/50% split of cathodic/anodic current to two contacts on the AB 8-contact array (“

+

50%,

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50%, 0,

−

50%,

−

50%, 0,0,0”).

Data Analysis

The primary variables measured in the intraoperativestudy were: CEFA time per lead position, stimula-tion parameters, and stimulation threshold “maps”.

Quantitative variables were transferred intoMicrosoft Excel (Microsoft, Redmond, WA) for

statistical description and analysis. Statistics in thetext, tables, and charts are reported as “Median(25% quartile

−

75% quartile)” as appropriate to thedistribution of the data.

CEFA Time

Elapsed CEFA time was recorded automatically bythe stimulation system in a log file. The time toCEFA test a single array (one trial) was defined as:

CEFA time is reported in minutes. The CEFA timewas further divided into two variables: Ramp-UpTime and Combination Adjustment Time, where:

Ramp-Up Time is defined as the time required toramp up the stimulation amplitude on the startingcontact combination to reach comfortable pares-thesia intensity. Combination Adjustment time wasdefined as the time to traverse through all 71CEFA combinations (see Fig. 4 for graphical defini-tion of these variables).

Stimulation Parameters

The stimulation parameters were also derivedfrom the log file recorded during the CEFA trial.The parameters measured were minimum (MinmA) and maximum (Max mA) stimulation currentamplitude, stimulation pulse width, and pulse rateused during a trial. In all cases, a fixed pulse widthand rate were used during a CEFA trial. Only stimu-lation parameters which generated paresthesia(during Combination Adjustment time) wereincluded in the statistics.

Stimulation Threshold Maps

Stimulation Threshold Maps are charts that showthe variation in the current amplitude of each con-tact as the contact combination is changed duringthe CEFA trial. These values were also recordedautomatically by the stimulation system in the logfile for each CEFA trial. Since the goal was to main-tain paresthesia during the CEFA process, theStimulation Threshold Maps show the variability of

CEFATime

Time of Completionof Contact

Time of First Programming of the Starting Combination

in the CEFA Combination Sequence

= −

CEFATime

Ramp-UpTime

CombinationAdjustment Time

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the medium-intensity paresthesia threshold spa-tially along the spinal cord.

RESULTS

Population

Of the eighteen patients enrolled, eleven patientshad unilateral pain conditions (see Table 2 for adescription of the patients enrolled in the study).All enrolled patients completed at least one CEFAtrial. There were no acute or chronic neurologicsequelae due to the stimulation testing. There wasa single adverse event in one patient, whichoccurred two weeks after the study that was notattributable to the CEFA testing. The patient suf-fered no injury due to the adverse event. Nopatients reported jolting or abrupt stimulationduring any CEFA trial such that stimulationneeded to be immediately ceased.

CEFA Time

In many patients, several CEFA trials were per-formed, many of these were “partial” trials, wherethe combination adjustments focused on only afew stimulating contacts. Partial trials were com-mon towards the end of the testing in each

patient. After the first CEFA trial, the lead wasoften longitudinally repositioned based uponpatient feedback of paresthesia location. Sincethese lead movements were usually less than thespan of an entire AB array, only a partial CEFA trialwas often necessary to determine if lead reposi-tion had improved paresthesia targeting.

In order to standardize the analyzed measure-ments, only trials in which

>

85% of the combina-tions were programmed were included in thesummary statistics. In all 18 patients, a total of 35CEFA trials were analyzed. The median number oftrials analyzed per patient was 2.0 (1–2.75). Therewere three analyzed trials in which less than 71combinations were used. For these three trials,the mean combination transition rate was calcu-lated and used to estimate the time to test the full71 combinations.

The median time for one CEFA trial (to shift thestimulation field down a single 8-contact array)in a single position for each patient is shown inFigure 3 below. The time statistics for all trials forall patients is shown in Table 3.

Stimulation Parameters

Table 4 details the statistics of the stimulationparameters that were recorded during the first

Table 2. Intraoperative Study Patient Demographicsa

Patient # GenderPrimary pathology/pain

locationVertebral location ofarrays during testing

# of leads implantedfor testing

1 M ULE T9-T11 12 F Post-mastectomy pain T3-T5 13 F Complex (unilateral trunk, ULE) T8-T9 24 M UUE C5-C7 15 M Low back/ULE T8-T10 26 F RSD/CRPS (UUE) C5-C7 27 M UUE C6-C7 18 F Low back/BLE T9-T11 19 M FBSS T9-T10 210 F RSD/CRPS (UUE) C5-C7 211 F FBSS T10-T11 112 F FBSS T9-T10 213 F RSD/CRPS (ULE) T9-T10 114 M FBSS T8-T9 215 F ULE T9-T10 216 M ULE T8-T9 217 F FBSS T9-T10 118 F Post-viral neuralgia (unilateral trunk) T5-T7 2

aULE, unilateral lower extremity pain; UUE, unilateral upper extremity pain; BLE, bilateral lower extremity pain.

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CEFA trial for each patient (for simplicity, only theresults of the first trial are shown; in thosepatients with multiple trials, subsequent trials hadsimilar results as the first).

Stimulation Threshold Maps

Stimulation Threshold Maps are shown for a singleCEFA trial in three individual patients (Figs 4–6).The bold line in each chart is the overall ampli-tude “Level”; a single control, proportional to thetotal cathodic current, used to adjust the overallamplitude of stimulation during the CEFA trial.The dashed lines are the pulse amplitude valuesfor each contact during the trial. Negative valuesof the pulse amplitude value for the contactsrepresent cathodic current flow, where positive

values represent anodic current flow. Along theordinate are the Level (unit-less) and the pulseamplitude (mA). Along the abscissa is “CEFAprogression.” CEFA progression is defined as the

Figure 3. Median CEFA Time per Patient.

Table 3. CEFA Time Statistics Across All Patients

Ramp-up time (min)

Combination adjustment time

(min) CEFA time (min)

0.61 (0.42–0.94) 3.28 (2.38–4.34) 4.09 (3.61–4.95)

Table 4. Stimulation Parameter Statistics for All Patientsduring CEFA Testinga

Patient Number Min mA Max mA PW, µs

1 2.6 3.9 2102 4.1 4.5 2103 2.4 7.3 2104 2.8 4.5 2105 2.2 4.8 2106 1.0 2 2107 0.9 3.8 2108 4.6 7.0 2109 0.7 3.8 36010 1.6 2.4 21011 4.8 15.2 100012 2.9 11.4 45013 1.8 4.6 15014 3.4 6.2 21015 3.6 6.0 21016 2.4 4.0 21017 4.1 5.6 21018 1.7 3.4 210

aPulse rate used was always continuous 50 Hz, no burst cycling was used.

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sequence of events that occur during a trial. Pro-gression is made when either a Level change or acontact combination change is made. Thus, CEFAprogression roughly corresponds to progressionof time during the trial.

When interpreting the Stimulation ThresholdMaps, the following simple features can be deter-mined. The initial increase of amplitude on thefirst contact combination is shown in the ramp-upof Level in the beginning of the sequence (“Ramp-Up” Time). Sections of the chart where the Levelis flat indicate that paresthesia intensity was rela-tively stable and only the contact combination wasbeing adjusted. Sections of the chart where theLevel increases or decreases indicate regions wherethe patient requested that stimulation intensitybe increased (increasing Level) or decreased(decreasing Level). It can be seen in these mapsthat the threshold variability along the array had adistinct inter- and intrapatient variability.

DISCUSSION

Electric field “steering” is a relatively new tech-nique in neuromodulation. It has been employedin changing the selectivity of recruitment of differ-ent fascicles within nerve bundles (7–10). In SCS,one system has been investigated quite thoroughlyand reported upon in the recent literature.The “transverse tripole stimulation” (TTS) systemachieved “paresthesia steering” via a paddle-typearray with transversely-oriented contacts (11–14).The TTS system was successful in shifting theparesthesia to various dermatomes of the bodyby selectively stimulating segments of the medialand lateral dorsal columns. This system, however,required a laminectomy for implantation of thepaddle and the paresthesia steerability was stillsomewhat sensitive to the placement of the paddlerelative to the midline and upon the distance fromthe contacts to the dorsal surface of the spinal cord(dCSF) (14). None of these variables could beassessed intraoperatively due the nature of theimplant surgery. Only during the office follow-upcould the true paresthesia steerability be determined.

To our knowledge, the present study representsthe first report of “real-time” paresthesia steering(that is, with suprathreshold stimulation continu-ously generating paresthesia sensation duringprogramming changes). The architecture of thestimulation system used in this study enabledthe real-time shifting of the electric field along thecontact array by continuous, small adjustments ofcontact combinations. The use of a controlled-current source for each contact allowed precise

Figure 4. Stimulation Threshold Map for Patient 3, Lead 1,Trial 1.

Figure 5. Stimulation Threshold Map for Patient 7, Lead 1,Trial 2.

Figure 6. Stimulation Threshold Map for Patient 12, Lead 2,Trial 1.

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distributions of anodic and cathodic currents tobe programmed while maintaining the stimulationcurrent at suprathreshold amplitude. In this acuteintraoperative study, all patients reported smoothshifting of the paresthesia to different dermatomeson the body as the contact combination was ad-justed. Paresthesia would grow weaker or strongerdepending upon the specific contact combinationand the underlying neural anatomy, but patientfeedback on stimulation intensity allowed for thestimulation amplitude to be adjusted continuouslyby the clinician.

The primary goal of intraoperative stimulationtesting in SCS is to ensure that the position of thecontacts will allow adequate paresthesia-painoverlap during post-operative programming. Thisentails obtaining good paresthesia coverage usingcontacts near the center of the array, to allow forslight migration in the early phase of implant (15).Historically, during the implantation of percutane-ous leads, two modes of stimulation testing havebeen typically performed. The first type is the dis-crete combination adjustment, where, once thelead is positioned, a specific contact combinationis programmed and the amplitude is increasedwhile the patient feels paresthesia. If the locationof the paresthesia is not satisfactory (does notcover the patient’s painful areas), the stimulationis turned off. The contact combination is thenchanged; the amplitude is then slowly increaseduntil paresthesia is felt, etc. If no concordant pares-thesia is felt with further testing using that leadlocation, the array is moved and the process isrepeated. This can be a time consuming processand implies that few combinations can be tried fora single lead position during implantation.

In response to this “clinical time problem,” asecond method used intraoperatively is “mechani-cal trolling.” In this technique, the lead is generallyplaced at a position cephalad to the assumedtarget spinal segment. A contact combination isprogrammed on the lead and the amplitude isthen increased until the patient feels paresthesia.The physician then gently pulls the lead from thespinal needle, thus dragging the lead caudallyalong the dura and shifting the electric field alongthe nerves while the patient reports paresthesialocation. While the physician is performing this“mechanical trolling,” the stimulation amplitude isadjusted by the clinician controlling the stimulator

to maintain a comfortable paresthesia intensity.This method can be a general improvement overdiscrete combination programming in terms oftime efficiency, but carries the risk of significantoverstimulation if the clinician control of theamplitude is not well synchronized to the actionsof the physician

and

the verbal feedback of thepatient. In the event of overstimulation, thepatient can become refractory to paresthesiasensitivity for several minutes after the incident.This can further increase the time for implant.

With the CEFA technique, the inefficiencies andrisks of the previous two methods are generallyavoided: the electric field is shifted smoothlyalong the array in small steps by the clinician,who is also controlling the stimulation amplitudein response to the patient’s verbal feedback. Thisallows the implanting physician to focus on thecomments of the patient who is reporting thechanging location of the paresthesia. In this man-ner, the “stimulatable space” in the vicinity of thecontact array is broadly determined and the physi-cian gains insight into the therapeutic possibilitiesof the array placement: 1) whether the painfularea is well covered by stimulation using contactsnear the center of the array; and 2) whether aspecific combination in the CEFA sequence mightbe explored further, for example, by increasingthe stimulation amplitude to the maximum com-fortable level to obtain more elusive paresthesiatargets, such as the low back. By allowing therapid exploration of many combinations, the CEFAmethod approaches a more exhaustive testing ofthe contact combinations on an array, which canresult in a better acute clinical outcome, as sug-gested by North et al. (4).

The CEFA technique used stimulation energiesthat are typical for SCS (16). The stimulationparameters statistics in Table 4 show that theminimum recorded stimulation amplitude (MinmA) was within a reasonable range for regardlessof vertebral location (0.7–4.8 mA). The Max mAvalues across patients had a somewhat widerrange (2.0–15.2 mA). Since the CEFA techniquemaintains medium-intensity paresthesia, Min mAand Max mA values are useful only in defininggross boundaries of the stimulation thresholds(Perception and Maximum Comfortable) for anygiven lead placement. The Stimulation ThresholdMaps are more revealing than the Min mA and

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Max mA data in that they suggest the variability ofanatomy within a patient and from patient-to-patient. Figures 4–6 show that the medium-intensity paresthesia threshold, as quantified bythe “Level,” could be quite variable during theCEFA process. Additionally, Figures 4–6 show thatthe Level profile had a different shape for differentpatients.

Several factors may have contributed to thisobserved variability. In this study, when viewed onthe fluoroscope, most lead orientations were notsignificantly oblique to the radiologic midline,yet the variability of the medium-intensity pares-thesia thresholds may suggest that the neuro-anatomy was not necessarily in a parallel plane tothe lead plane. Both modeling results and clinicaldata have shown the threshold variability fordifferent contact combinations (17,18) on midline-positioned arrays, for different rostrocaudal posi-tions in the spinal column (19), for relativelyasymmetrical placements of contact arrays (17,20).More recently, Holsheimer and Manola have dis-cussed the effect of the dorsoventral position ofthe stimulating contact in the epidural space onparesthesia thresholds (21). All of these effectsmay play a role in the observed threshold variabil-ity, as may relative patient satisfaction with theparesthesia generated by specific contact combi-nations, that is, if the paresthesia satisfactorilycovers the painful area, the patient may tolerate ahigher intensity than for combinations that do notgenerate good coverage.

The Stimulation Threshold Maps also show thatthe initial Ramp-Up time on the starting combina-tion represents a significant portion (17%) of thetotal CEFA time. In fact, this is one of the majortime limitations of discrete combination program-ming systems. If it is assumed that the Ramp-Uptime represents the minimum amount of timerequired to assess a discrete combination, then agross comparison of the time efficiency of dis-crete vs. continuous combination programmingcan be made. As stated above, there are 15 dis-crete combinations present in the CEFA sequencethat are programmable on market-released dis-crete programming systems. The product of theRamp-Up time and these 15 discrete combinationsis 11.6 minutes. Even if the CEFA technique isused only to transition between these 15 discretecombinations, the CEFA method takes only about

4 minutes; this is a 2.5-fold time savings and sug-gests that the CEFA method is more efficient.

There were several limitations to this study.First, neither the “percent pain coverage” byparesthesia nor the paresthesia steerability couldbe objectively or quantitatively recorded due tothe patient’s inability during surgery to provideprecise drawings of paresthesia coverage. All thepatients were prone and sedated to varyingdegrees. Additionally, acquisition of such datawould have been too time consuming in the intra-operative environment. Second, this study wasacute. The long-term outcome of patients in whichthe CEFA technique was used to finalize lead place-ment was not determined. A prospective, con-trolled study would be required to assess whetherthe CEFA method provides greater benefit overthe long-term by improving lead placement. Third,most of the patients were naive regarding stimula-tion and paresthesia. While most patients reportedsignificant shifts of paresthesia over their bodyduring the CEFA process, reporting by suchpatients is known to be unsophisticated (22).

CONCLUSION

A new neurostimulation programming system,coupled with a closely-spaced 8-contact lead, hasbeen shown to be a rapid and comfortable tool forassessing lead placement during the implant ofSCS leads for chronic pain. The use of continuouselectric field adjustments, or CEFA, allowed 71combinations to be assessed in approximately fourminutes. This rapid stimulation adjustment, cou-pled with the verbal patient feedback during theCEFA technique, informed the implanting physi-cian about the therapeutic possibilities of the leadlocation.

ACKNOWLEDGEMENTS

The authors would like to thank Carla Woods,James Thacker, Dave Peterson, Sridhar Kothandara-man, and John King for their contribution to theCEFA concept and development; Holly Segel forher assistance in data acquisition; and JoanieSmith, Morayma Erazo, and Liz Buick for theircoordination efforts in performing the studies.Financial support was provided by Advanced BionicsCorporation.

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