Peripheral Nerve Stimulation for Back Pain

Abstract

Peripheral nerve stimulation (PNS) generally refers to stimulation of a named nerve via direct placement of a lead next to the nerve either via a percutaneous or open approach; in peripheral nerve field stimulation (PNFS), leads are subcutaneously placed to stimulate the region of affected nerves, cutaneous afferents, or the dermatomal distribution of the nerves which converge back to the spinal cord. Recently, there has been a renewed interest in using the PNS approach for many otherwise refractory pain conditions; however, PNFS appears to be more effective for the management of low back pain and therefore more attractive. Here we discuss procedural details of PNFS trial and implant, and provide scientific and clinical rationale for placing PNFS electrodes at a certain depth under the skin. We also summarize results of published studies on use of PNFS in the management of low back pain and list the criteria that are used for proper patient selection. Our experience and the published studies provide evidence that PNFS is a safe and well-tolerated pain control option for intractable pain conditions, including chronic low back pain. Notably, achieving efficacious pain relief relies on correct patient selection and the optimal placement of the leads, ensuring, in particular, a lead depth of 10-12 mm from the surface to maximize the target sensation (mediated by fast-adapting Aβ fibers) of PNFS, which is believed to be most effective for the pain relief.

Full text

Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
Abstract
Peripheral nerve stimulation (PNS) generally refers to stimulation of a named nerve via direct place-
ment of a lead next to the nerve either via a percutaneous or open approach; in peripheral nerve
field stimulation (PNFS), leads are subcutaneously placed to stimulate the region of affected nerves,
cutaneous afferents, or the dermatomal distribution of the nerves which converge back to the spinal
cord. Recently, there has been a renewed interest in using the PNS approach for many otherwise
refractory pain conditions; however, PNFS appears to be more effective for the management of low
back pain and therefore more attractive. Here we discuss procedural details of PNFS trial and implant,
and provide scientific and clinical rationale for placing PNFS electrodes at a certain depth under the
skin. We also summarize results of published studies on use of PNFS in the management of low back
pain and list the criteria that are used for proper patient selection. Our experience and the published
studies provide evidence that PNFS is a safe and well-tolerated pain control option for intractable
pain conditions, including chronic low back pain. Notably, achieving efficacious pain relief relies on
correct patient selection and the optimal placement of the leads, ensuring, in particular, a lead depth
of 10–12 mm from the surface to maximize the target sensation (mediated by fast-adapting Aβ fibers)
of PNFS, which is believed to be most effective for the pain relief. © 2016 S. Karger AG, Basel
Electrical neuromodulation generally involves the selective application of a program-
mable pulse waveform through a series of electrodes, within a lead, to stimulate affer-
ent nerve fibers and subsequently reduce the perception of pain.
Historically, spinal cord stimulation (SCS) has primarily been used for widespread
leg, buttock, and (to some extent) back pain, particularly following failed back surgery
[1–6] . Traditionally, however, SCS has not adequately covered and relieved axial back
pain. This has led to individuals experimenting with the placement of subcutaneous
leads within areas of significant pain. Peripheral nerve stimulation (PNS) generally
refers to stimulation of a named nerve via direct placement of a lead next to the nerve
Peripheral Nerve Stimulation for Back Pain
Paul Verrills a · Marc Russo b, c
a  Metro Pain Group, Melbourne, Vic. , and
b  Hunter Pain Clinic, and
c  Australian Pain Research Center,
Newcastle,N.S.W. , Australia
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128 Verrills · Russo
either via a percutaneous or open approach. In peripheral nerve field stimulation
(PNFS), leads are subcutaneously placed to stimulate the region of affected nerves,
cutaneous afferents, or the dermatomal distribution of the nerves which converge
back to the spinal cord
[7] . Sometimes, the two terms are used interchangeably in the
literature or not defined at all, making it difficult to ascertain the neurostimulation
technique used without close reading of the methods section ( table1 ).
Stimulation Principles
Much work remains to be done on determining how much of a pain area can reason-
ably be attempted to be covered by paresthesia from a subcutaneous cylindrical lead.
It is generally considered to cover a small area the size of a credit card or palm of the
hand, although it seems that more extensive coverage can be obtained by cluneal
nerve stimulation [Dr. Peter Courteney, pers. commun.]. Therefore, it is important to
think about how many leads will be required and in what position or array the leads
will be placed. We refer the reader to the excellent atlas produced by Dr. Giancarlo
Barolat
[12] detailing real case planning of peripheral leads. For most pain conditions
it would be typical to use 2–4 leads given current implantable pulse generator (IPGs)
offer 2–4 channels of stimulation.
PNS for low back pain obviates the difficulty of generating paresthesia from the
dorsal columns into the low back and allows direct activation of peripheral nerve
fibers.
It has been postulated that the efficacy of PNFS is reliant on several key factors,
including eliciting the appropriate sensation within the region of pain. In this con-
text, understanding the effects of electrode placement, and specifically lead depth, on
Table 1. Applications of peripheral nerve field stimulation in treatment of low back pain
Author/year Patients, n Outcome
Paicius et al. [8], 2007 6 Case series – patients reported reduction in pain and
analgesic use and increase in quality of life measures
Verrills et al. [9], 2009 13 An overall 50% reduction in pain was observed at an
average follow-up of 7 months
Sator-Katzenschlager
et al. [10], 2010
68 Case series – pain reductions were reported by 92%
of patients, as well as significant reduction in their
analgesic use
Verrills et al. [11], 2011 44 Consecutive case series – patients reported average
reduction in pain of 3.3±2.3 NPRS; reduced analgesic use
was reported in 72% of patients with improvement in
functional scores and employment capacity
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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PNS for Back Pain 129
nerve types is crucial to achieving optimal outcomes for pain relief. Verrills et al. [13]
investigated the stimulation of cutaneous nerves at different electrode lead depths,
using the ‘pin-drop’ technique. Following measurements, patients underwent a se-
ries of 16 electrode programming combinations to compare lead depths with pares-
thesia felt at both maximum discomfort and at 70% of the usage range intensity. Pa-
tients were divided into four groups according to lead depth, ranging from superficial
to deep leads (<10, 10–12, 12–20, >20 mm). Paresthesia descriptions were classified
into 5 groups based on cutaneous fiber type: Aβ fast adapt (Aβ_FA), Aβ slow adapt
(Aβ_SA), Aδ, C nociceptive fiber stimulation (C), and sensation block (C/Blk) and
other ( table2 ).
For any depth, the most commonly experienced sensation was that typical of Aβ_
FA fiber stimulation (tingling, tickling, vibration, etc.). Maximal Aβ_FA sensation
was observed at depth range of 10–12 mm. Therefore, a depth of 10–12 mm from the
surface maximizes the target sensation (Aβ_FA) of PNFS, which is believed to be ef-
fective for pain relief ( fig.1, 2 ).
Patient Selection Criteria for Peripheral Nerve Field Stimulation
Appropriate patient selection has also been suggested to be a key element in optimiz-
ing PNFS outcomes. Most of the patients selected for trials have either suffered from
failed back surgery syndrome or had failed other minimally invasive and conservative
treatments. The important inclusion criteria are described below:
• A clearly defined, discrete focal area of pain with a neuropathic (or combined
somatic/neuropathic) pain component characterized by burning and increased
sensitivity
• Failure to respond to other conservative treatments including medications,
psychological therapies, physical therapies, surgery, and pain management programs
• Psychological clearance (including a psychologist ruling out major drug addictions
or significant psychiatric disorders that might impact on successful treatment)
• Informed consent
Table 2. Sensation classification
Cutaneous fiber Sensation
Aβ_FA Tingling, tickling, vibrating, massaging, tapping
Aβ_SA Hammering, pressure, cramping
Aδ Pinch, stabbing, pin-pricking
CBurning (hot), cool, Itching
C/Blk Dull, numb
Underlined descriptors denote the most common reported sensations.
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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130 Verrills · Russo
Trialing Procedure
All procedures are performed within a sterile operating room. Based on clinical as-
sessment, the patient’s area of pain is clearly outlined. Patients are minimally sedated
and routinely given intravenous antibiotics (typically, 1 g cefazolin). Under live
100 70% of usage range
Observations n = 128
Depth = 4–10 mm
70% of usage range
Observations n = 61
Depth = 10–12 mm
70% of usage range
Observations n = 79
Depth = 12–20 mm
50
5
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ADŽ_FA
ADŽ_SA
Depths of 10–12 mm can maximize
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Fig. 1. Distribution of categorized sensations at 70% usage range.
100 Maximum discomfort
Observations n = 128
Depth = 4–10 mm
Maximum discomfort
Observations n = 61
Depth = 10–12 mm
Maximum discomfort
Observations n = 79
Depth = 12–20 mm
50
5
Aİ
ADŽ_FA
ADŽ_SA
C
10
12
20
Depth (mm)
50
0
100
Probability to stimulate
sensory nerves
ADŽ_FA
ADŽ_SA
Aİ
C
C/Blk
Other
ADŽ_FA
ADŽ_SA
Aİ
C
C/Blk
Other
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ADŽ_SA
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Other
50
0
100
Observations (%)
50
0
Fig. 2. Distribution of categorized sensations at maximum discomfort.
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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PNS for Back Pain 131
C-arm fluoroscopy, octapolar leads are placed subcutaneously within the area of
maximal pain using a 14-gauge vascular access catheter (e.g. Angiocath) and a small
amount of lidocaine 1% at the insertion site. Between 1 and 4 leads may be used in
the trial. On-table stimulation is performed to determine that paresthesia is felt in the
area of pain and that it is comfortable. The leads are then sutured to the skin and
dressings applied. Patients are monitored in the recovery suite, while initial stimula-
tion parameters are programmed. The patient wears an external power source for
5–7 days attached to the leads (2 leads can be tested at a time), and careful monitor-
ing occurs over the following days to determine whether adequate pain relief ensues.
Verrills et al.
[11] , in a series of 100 patients, reported that 72% of all patients under-
going a PNFS trial achieved successful results of >50% pain reduction and progressed
to a permanent implant.
Pulse Generator Implant Procedure
As with the trial procedure, the patient’s skin is marked for area of pain, the position
of successful trial leads, and placement of the IPG. A small incision is made outside
the area of pain and the leads are inserted via a 14-gauge vascular access catheter (e.g.
Angiocath). On-table stimulation is performed to confirm pleasant paresthesia cover-
age of the area of maximal pain. The leads are sutured to the deep fascia and tunneled
to the site of the IPG where blunt dissection is used to create a tight pocket approxi-
mately half an inch deep. The most common site for IPG placement is the upper out-
er buttock (below the belt line but not impeding sitting positions). The leads are con-
nected to the IPG, impedance is verified and leads are secured. Refinement of the
stimulation parameters is performed over the following weeks.
Lead Depth
There appears to be an ideal depth for the placement of electrode leads to achieve ef-
fective stimulation of the affected nerves. Evidently, more superficial lead implanta-
tion in the dermal layers results in painful stimulation, while deeper tissue placement
results in muscle recruitment and uncomfortable sensations. Though PNFS is one of
the newest paradigms of neuromodulation – providing effective pain relief with low
risks and minimal insult during a trialing phase – the ideal depth of the lead implant
and its reproducibility are among several persisting issues that remain problematic
[14, 15] . To date, there have been only a handful of published studies that have ad-
dressed this issue.
Most investigators describing subcutaneous lead implants in occipital nerve stim-
ulation or PNFS do not report the lead depth; however, there is now a general consen-
sus on the optimal depth of the electrode lead placement in PNFS. In a recent study,
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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132 Verrills · Russo
Abejón et al. [16] utilized a radiofrequency probe (range of 0.5–1.0 V at 50 Hz) to
identify optimal depth of the electrode lead in patients with PNFS for low back pain.
This novel concept enabled greater reproducibility in achieving optimal stimulation
with subcutaneous leads and has potential advantages over other techniques such as
ultrasound guidance. However, this technique is invasive, requires additional equip-
ment, and does not measure the electrode lead depth reliably enough.
In another study, presented in 2010, McRoberts and Cairns
[17] measured the
depth of the electrode lead implant in patients with PNFS for lower back pain using
ultrasound guidance. Despite the authors reporting fairly accurate and comparable
values for low back PNFS electrode lead depths, ultrasound is known to yield poor
estimation of actual electrode depth because of tissue compression from the ultra-
sonic probe, as well as varied scatter from intervening tissues such as bone. These fac-
tors can potentially reduce this method’s accuracy and reproducibility.
Verrills et al. [13] reported on lead depth measurements taken for 17 patients pre-
viously implanted subcutaneously with a PNFS system in their low back. The ‘pin-
drop’ technique uses a device with freely moving multiple pins that sits on the skin
directly above the implantation site. Using fluoroscopic imaging at 30° cephalad and
caudad, the depth of each lead was estimated by measuring the depth between the
contacts and the reference pins, with an accuracy of ∼ 0.5 mm ( fig.3 ; table3 ) [13] .
Distribution of electrode lead depth ranged between 4 and 19 mm, with an average
depth of 10.5 mm.
Across all three studies
[13, 16, 17] , which utilized different depth measurement
techniques, the average electrode lead depth that delivered effective subcutaneous
paresthesia in low back pain PNFS was found to be remarkably consistent and mea-
sured at 9.2–10.5 mm ( table4 ; fig.4 ). Furthermore, the lead depth was found not to
correlate with percentage pain relief achieved or BMI. The results of these studies
corroborate the importance of having a unifying reference point for the depth of sub-
cutaneous lead implantation with the goal of delivering the most adequate nerve
stimulation.
Outcomes
Pain Indices
In a study of 44 low back pain patients, with a mean age of 60.6 years (33–88) and
follow-up of 8.1 ± 4.7 months, a statistically significant reduction in pain was observed
following lumbosacral PNFS with a reduction of 3.3 ± 2.3 pain scale points (p ≤ 0.001)
( fig.5 ) [11] . To assess whether results obtained shortly after PNFS implantation were
sustained over a longer period, patients were divided into two groups: those who were
followed up for less than 8 months after treatment (average: 4.6 ± 1.4 months; range:
1–7), and those who were followed up for 8 months or longer (average: 12.3 ± 3.8
months; range: 8–23; fig.6 ). Pre-PNFS pain scores were similar among the two groups,
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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PNS for Back Pain 133
ab
cde
X-ray image
Lead
X-ray
Skin surface
X-ray image
a:b = Pin length:depth
Pin Art Pin Art under X-ray
(reference shot)
Pin Art under X-ray
(30º shot)
a
bDepth = b × Pin length
a
Fig. 3. Principles of lead depth measurement. a Under X-ray fluoroscope, a metal pin and electrode
would be shown as a line and a square, respectively. The length of the pin image and the distance
from the pin tip to contact would be proportional to real distances. Therefore, if we measure the real
length of the pin (31 mm) and distance from the pin tip to contact of the X-ray image, we can esti-
mate the depth of contact. b Mathematical equation to compute the depth of contact. Under X-ray
image, length (a) of the pin image and distance (b) of the pin tip to contact should be measured. The
lead depth can be estimated by the equation (b × pin length/a). c Pin Art device with multiple pins
touching the surface of hand. Individual pin touches perpendicular to the surface of the skin. d X-ray
image with 90° to identify reference pins (red/dark arrows) and contacts. e X-ray image with 30° ro-
tation of fluoroscope. The identified pins (red/dark arrows) and contacts (yellow/light arrows) are
used to measure the contact depth.
Table 3. Patient characteristics: PNFS for low back pain
Variable Male Female All
Sample size, n 10 7 17
Age, years 65±18.7 46.9±13.4 57.4±18.6
Implant duration, months 12.6±11.8 17±12.2 14.4±11.8
Height, cm 176.2±9.7 167.5±9.6 172.5±10.4
Weight, kg 97.6±26.1 76±21.6 88.5±26.1
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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134 Verrills · Russo
negating any potential impact on the follow-up results. Both groups demonstrated the
same reduction in pain scores, confirming that there was no deterioration in thera-
peutic response over time
[11] .
In an Austrian PNFS series, Sator-Katzenschlager et al.
[10] reported on patients
with failed back surgery syndrome and low back pain (n= 68), with a mean age of
Table 4. Comparison of lead depth studies
Factor Verrills et al.
[13], 2013
McRoberts and
Cairns [17], 2010
Abejón et al. [16],
2011
Depth
measurement
method
‘Pin-drop’ technique
under c-arm fluoroscopy
Ultrasound Radiofrequency
probe to identify
appropriate depth
Results Average depth of
10.5 mm (4.0–19.3)
Average depth of 9.2 mm
(5.6–15.1)
Average depth of
10.5 mm (9.8–11.3)
0
510152025
17 points
33 leads (1 measurement missed)
30 35 40 45
5
Observations (n)
10
Low back pain (subcutaneous stimulation)
Fig. 4. Low back subcutaneous paresthesia was observed at an average lead depth of 9.2–10.5 mm.
10
Lumbosacral
Pre-PNFS
Post-PNFS
p = 0.000
7.0
3.7
Pain (NPRS)
8
6
4
2
0
0
NRS before STS NRS after STS
2
4
6
8
10
Fig. 5. PNFS significantly reduces Numeric Rating Scale (NRS) pain scores [10, 11] . Pain intensity
score before and after PNFS (NRS; 0= no pain, 10= unbearable pain) (modified from Verrills et al.
[11] , with permission from John Wiley and Sons).
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
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PNS for Back Pain 135
59years and average duration of pain around 13 years, whose pain scores reduced in
a similar proportion from 8.2 to 4.0. Patient age and gender were not predictors of
outcome following PNFS.
Medication Use
Over 30% of patients report a decrease in their analgesic intake at 3 months following
implant, with this number increasing to approximately 70% of patients at a follow-up
duration of 8 months or greater. Approximately 20% of the patients state that their
analgesic regimen is not altered following PNFS, with <5% reporting an increase
[10,
11] .
Satisfaction
Verrills et al.
[11] showed that while 30% of all patients were completely satisfied with
their outcome, a combined 86% of patients were satisfied, very satisfied, or complete-
ly satisfied with their results.
Function
In Verrills et al.
[11] , patients younger than 60 years were asked to assess their capac-
ity for paid employment following PNFS. Overall, 43% of patients reported an in-
crease in their capacity for paid employment, with half of this group assessing their
improvement as extreme. No surveyed patients reported a decrease in their capacity
for paid employment following PNFS, and only 14% of the cohort stated that capac-
ity for paid employment was not applicable to their situation.
10
8
6
Average pain scale points
4
2
0
<8 months
3.19 3.18
7.37 7.39
Pre-PNFS
Post-PNFS
8 months
Fig. 6. Combined region pain scores following PNFS (n= 100): short-term pain relief is sustained.
Pain scores of patients followed soon after PNFS implantation and up to 7 months of follow-up were
compared with pain scores obtained from patients followed up for 8 months or longer. No difference
was noted between the two groups. Data expressed as means ± SEM. Data were analyzed using the
nonparametric Mann-Whitney U test, with p set at 0.05 (modified from Verrills et al.
[11] , with per-
mission from John Wiley and Sons).
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
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136 Verrills · Russo
Likewise, chronic low back pain patients receiving a PNFS implant in their lumbo-
sacral region were assessed for degree of disability using the Oswestry Disability Index
(ODI). These patients demonstrated a statistically significant reduction in their de-
gree of disability as measured by the ODI ( fig.7 ).
Complications
Complication rates for PNFS have been reported to occur in 9–24% of patients, with
lead migration and lead dislocation being the most frequent, affecting up to 13% of
patients. Lead fractures and equipment failure occur in 2–5% of patients, infections
in up to 6%, and lead erosions in approximately 5% of all PNFS patients
[10, 11] . These
results are in line with other large series of peripheral neurostimulation applications
[18] .
Conclusion
In patients with chronic low back pain, opioids have been shown to reduce pain by an
average of 10 points on a 100-point pain scale, while having very little impact on im-
proving the function or psychological condition
[19] . Other drugs, such as tramadol,
have been shown to provide moderate relief in 35% of patients; however, the observed
time frame for efficacy was only 4 weeks
[20] . In contrast to these observations, PNFS
has been shown to provide pain relief with a mean decrease of 3.3–3.8 NPRS points,
and a sustained effect observed in 8- to 23-month follow-up cohorts in the two key
publications
[10, 11] . In addition, up to 72% of patients are able to reduce their anal-
gesic use following PNFS.
Pre-PNFS Post-PNFS
*p  0.03
40
30
20
ODI (raw score)
10
0
Fig. 7. PNFS significantly reduces ODI-assessed
disability. Statistical differences were tested
using a t test comparison of paired-means, with
p set at 0.05 (modified from Verrills et al.
[11] ,
with permission from John Wiley and Sons).
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
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PNS for Back Pain 137
PNFS has a number of advantages over the traditional neuromodulation approach
of SCS, namely that it does not carry the same neurological risks such as epidural hem-
orrhage, paralysis, and meningitis. Given the low invasiveness of PNFS and its revers-
ibility, testability, and adjustability, it is also a preferable option when compared to
the more intrusive surgical alternatives available. The main issue, as with any implant-
able device, is the risk of infection. Sator-Katzenschlager et al.
[10] report an infection
rate of only 6%, occurring during the first 2 weeks following implantation. Other com-
plications such as lead migration and hardware failure are reported to have occurred
in up to 13 and 5% of the patients, respectively, in the two key publications, with no
patients reporting a serious or severe adverse event. This combined evidence suggests
that PNFS is a safe and well-tolerated pain control option for intractable pain condi-
tions
[10, 11] . Notably, achieving efficacious pain relief relies on correct patient selec-
tion and the optimal placement of the leads, ensuring, in particular, a lead depth of
10–12 mm from the surface to maximize the target sensation (Aβ_FA) of PNFS, which
is believed to be most effective for pain relief.
The future likely holds significant advances for both PNS and SCS, and hybrid
techniques that combine the two are also a possibility. Optimization of parameters
and implant strategies will determine how much of a role the PNFS approach will
have.
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Paul Verrills, MD
Metro Pain Group
Level 1, 544 Hawthorn Road
Caulfield South, VIC 3162 (Australia)
E-Mail pverrills@metropain.com.au
Slavin KV (ed): Stimulation of the Peripheral Nervous System. The Neuromodulation Frontier.
Prog Neurol Surg. Basel, Karger, 2016, vol 29, pp 127–138 ( DOI: 10.1159/000434666 )
Downloaded by:
University of Newcastle
134.148.5.68 - 8/25/2016 12:56:29 AM