Authors:
David T. Yu, MD
Andrew S. Friedman, MD
Evan L. Rosenfeld, MD, JD

Pain

Affiliations:
From the Physical Medicine and
Rehabilitation (DTY, ASF), Virginia
Mason Medical Center, Seattle,
Washington; and Medical and
Regulatory Affairs (ELR), Bioness,
Inc., Valencia, California.

Correspondence:
All correspondence and requests for
reprints should be addressed to:
David T. Yu, MD, X-7 PMR, 1100
Ninth Avenue, Seattle, WA 98111.

CASE REPORT

Electrical Stimulation for Treating
Chronic Poststroke Shoulder Pain
Using a Fully Implanted
Microstimulator with Internal Battery

Disclosures:
The authors have disclosed that this
study is a “work for hire” funded by
Bioness, Inc. Two of the authors have
disclosed a financial conflict of
interest related to the conducting of
this study and the publication of this
article. David T. Yu is a consultant to
Bioness, Inc. and receives fees for
these services. Andrew S. Friedman is
the principal investigator for an
ongoing clinical trial evaluating the
intervention described in this report.
The clinical trial is funded by
Bioness, Inc. Evan L. Rosenfeld is a
full-time employee (Chief Medical
Officer and Vice President for Medical
and Regulatory Affairs.) at Bioness,
Inc. and receives salary, benefits, and
stock option grants on an annual
basis. This report has not been
previously presented in any form.
0894-9115/10/8905-0423/0
American Journal of Physical
Medicine & Rehabilitation
Copyright © 2010 by Lippincott
Williams & Wilkins
DOI: 10.1097/PHM.0b013e3181d8d06f

ABSTRACT
Yu DT, Friedman AS, Rosenfeld EL: Electrical stimulation for treating chronic
poststroke shoulder pain using a fully implanted microstimulator with internal
battery. Am J Phys Med Rehabil 2010;89:423– 428.
This case report describes the first stroke survivor with chronic poststroke
shoulder pain treated with electrical stimulation delivered via a fully implanted
microstimulator containing a rechargeable internal battery. In light of existing
efficacy data for similar types of treatment, the investigational system described
in this report was developed to address the limitations of previously evaluated
electrical stimulation devices. A 58-yr-old male stroke survivor with chronic
hemiparesis and chronic shoulder pain received up to 6 hrs of stimulation daily
over 12 wks. The microstimulator was implanted percutaneously near the axillary
nerve at the quadrilateral space, under local anesthesia during an outpatient
procedure. The implantation procedure was well tolerated. There were no adverse events related to the implantation procedure or treatment (implanted
peripheral nerve stimulation). Outcomes were obtained before treatment, after
12 wks of treatment, and at 3-mo follow-up. Question no. 12 of the Brief Pain
Inventory was used as the primary outcome measure to evaluate response to
treatment. Shoulder pain decreased from 8/10 before treatment to 4/10 after
treatment and decreased further to 3/10 at 3-mo follow-up. Passive range of
motion and motor function also improved after treatment. Sensation, shoulder
subluxation, activities, and quality-of-life did not change. The feasibility, safety, and
efficacy of implanted peripheral nerve stimulation to treat poststroke shoulder
pain should be evaluated further in clinical trials already underway.
Key Words: Electrical Stimulation, Peripheral Nerve Stimulation, Shoulder Pain, Stroke

S

houlder pain is a common complication after stroke that inhibits functional
recovery and reduces quality-of-life. Many types of shoulder pathology can occur
after stroke. Although prevention measures and prompt diagnosis followed by
early, appropriate management result in the most favorable outcomes,1 20%–
30% of stroke survivors with moderate to severe impairment develop chronic
poststroke shoulder pain (PSP) that is refractory to available treatments,2,3
underlining the need for the development of more effective treatments.
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Electrical Stimulation for Shoulder Pain

423

In 1986, Baker and Parker published the first
article evaluating the use of electrical stimulation
(ES) for treating poststroke shoulder dysfunction
and pain. These authors used an external stimulator to deliver electrical current to paretic shoulder
muscles through electrodes placed on the skin
surface (transcutaneous ES [TES]). Several studies
evaluating TES for treating PSP have been published since that time. The potential clinical benefits of TES applied to the shoulder region in hemiparetic stroke survivors include reduction of
shoulder pain, improvement in range of motion,
reduction of subluxation, and recovery of motor
function.4 –7 However, the clinical practicality of
TES for treating PSP is questionable for several
reasons. First, TES in the shoulder region causes
pain through stimulation of cutaneous nociceptors, which limits user tolerance and compliance in
a significant number of cases.6 Although methods
to enhance tolerance of TES for treating PSP have
been developed, stimulation-induced pain prohibits clinical use in a significant number of cases.8
Second, stimulation of deeper muscles may be limited by unwanted stimulation of more superficial
muscles that are positioned between the electrode
and the target muscle. Third, many patients have
difficulty applying TES in the home environment
because of technical difficulty with electrode placement and proper adjustment of stimulation parameters. This is a particularly important consideration for treating PSP because studies suggest that
hours of daily treatment over weeks may be required to achieve clinical benefit, necessitating
treatments beyond those that can be practically
provided in a clinical setting. For these reasons,
even the first investigators of TES recognized the
need for implanted systems. In 1986, Baker and
Parker9 noted the limitations of TES, stating “until
implanted electrode systems become available,
however, long-term use of surface ES can be managed by only a few patients with hemiparesis and
their families.”
The limitations of TES led to the development
of a partially implanted ES systems for treating
PSP. Chae and Yu evaluated a system using flexible
wire electrodes placed near motor points of key
shoulder muscles. The wire electrodes were insulated along their length but deinsulated both at the
distal stimulating end located within the muscle
and at the proximal end connected to the external
stimulator. Thus, the wire electrodes traversed
from the outside to the inside of the body. A small
external stimulator was developed that could be
carried in a pocket or hooked onto a belt. A gel-type
surface electrode served as a common anode. This
partially implanted system had several advantages
over TES systems. First, cutaneous nociceptors
were bypassed, resulting in significantly less stim-

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Yu et al.

ulation-induced pain.10 Second, the entire system
could be worn on the body, giving users the freedom to ambulate and perform daily activities during stimulation. Third, the system could be applied
in the home setting because the electrodes remained in place for the duration (weeks) of treatment, thereby obviating the need for daily electrode placement and adjustment of stimulation
parameters. Adverse events, including accidental
electrode displacement, infection, and granuloma
formation occurred because of the electrodes’ percutaneous interface and external leads. In addition,
most users required assistance to don and doff the
system. A randomized controlled clinical trial demonstrated the efficacy of ES delivered through this
partially implanted system,11,12 but ultimately, the
system was not commercialized because of the
above-described limitations. Another system, reported in a case study, used an implanted microstimulator that required an external power source.
Power was delivered from an external generator to
an external coil worn over the affected shoulder
and transmitted to the implanted microstimulator
through a radiofrequency link. Although the results of the case report were promising with regard
to treatment of PSP, the authors did not discuss
clinical feasibility with respect to home-based
treatment.13
In this case report, we describe the first stroke
survivor with chronic PSP treated with ES to the
axillary nerve delivered through a fully implanted
microstimulator containing a rechargeable internal battery (Bioness Battery-Powered Microstimulator, Valencia, CA). The system described in this
report was developed to address the limitations of
previously developed ES systems in light of existing
efficacy data for implanted ES for treating PSP.

Preintervention Clinical Course
The patient is a right-handed 58-yr-old man with
a history of type 2 diabetes mellitus, dyslipidemia, and
coronary artery disease who sustained a left middle
cerebral artery infarct with hemorrhagic conversion
resulting in right hemiparesis 59 mos before implanted peripheral nerve stimulation (iPNS). PSP was
present for 42 mos. Previous treatments included use
of a swath-type sling, physical therapy directed toward improving shoulder range of motion, nonsteroidal anti-inflammatory drugs, and opioid analgesics.
Treatments before iPNS were administered at an unrelated outside institution. Thus, medical records detailing the patient’s pre-iPNS clinical course were not
available for review.
A baseline physical examination was performed
by a board-certified physiatrist-investigator 2 wks before initiating treatment. Significant findings included right hemiparesis, atrophy of the hemiparetic
upper limb, minimally increased muscle tone, hemi-

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sensory loss, reduced active and passive range of
motion of the shoulder in all planes, pain limiting
passive abduction and external rotation, and one fingerbreadth of shoulder subluxation. Manual muscle
testing revealed 2/5 shoulder abduction, 2/5 shoulder
flexion, 3/5 elbow flexion, 2/5 elbow extension and 0/5
wrist flexion, finger flexion, and finger abduction.
Muscle tone was ⬎1 at the shoulder and 1 at the
elbow as measured by the Modified Ashworth Scale.
No swelling was observed, and the shoulder region
was not tender to palpation. On the basis of this
examination, diagnoses of shoulder subluxation and
capsulitis were made.

Outcome Measures
All outcome measures were obtained within 2
wks before starting iPNS, after completing the
12-wk treatment phase and 3 mos after completing
treatment. Question no. 12 of the Brief Pain Inventory served as the primary outcome measure. The
Brief Pain Inventory has shown reliability across
cultures and languages.14 The developers of the
Brief Pain Inventory have suggested that question
no. 12, the “pain worst” rating, may be selected as
the primary response variable. The question asks
subjects to rate their worst pain in the past week on
an 11-point numeric rating scale where “0” indicates no pain and “10” indicates the worst pain
imaginable.
Brief Pain Inventory question no. 23 assesses
the reactive component of pain by measuring the
degree to which pain interferes with daily activities
using numeric rating scales from “0” (no interference) to “10” (complete interference). The summary score is a composite of seven questions that
relate to the domains of general activity, mood,
walking ability, normal work, relationships, sleep,
and life enjoyment. Pain medication usage was
measured by average daily analgesic use normalized to ibuprofen for the week before each evaluation. All subjects were asked to restrict analgesic
medications to a single type during study participation. Motor impairment was measured using the
upper limb portion of the Fugl-Meyer Assessment,
a measure of motor impairment that has been
shown to be valid and reliable in hemiparetic subjects.15,16 Light touch sensation was measured over
the lateral shoulder in the sensory distribution of
the axillary nerve. Shoulder subluxation was assessed by the number of fingerbreadths that could
be inserted between the acromion and the superior
portion of the humeral head compared with the
normal side in seated position. Passive, pain-free,
shoulder abduction was measured using a handheld goniometer. Quality-of-life was measured using the Euroqol 5D, a standardized instrument for
use as a measure of health outcome over a wide
range of conditions and treatments.
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Intervention
The intervention protocol for 16 subjects was
approved by the U.S. Food and Drug Administration under an investigational device exemption and
by the local institutional review board. Informed
consent was obtained. The patient underwent a
baseline medical evaluation within 2 wks of the
microstimulator implantation procedure. Treatment was initiated 2 wks after the implantation
procedure to allow for tissue healing and stabilization of the microstimulator. The treatment was
iPNS administered independently by the patient at
home for 6 hr/d, daily over 12 wks. Pretreatment
outcome measures were obtained within a day of
treatment initiation, posttreatment outcomes were
obtained within 1 wk after completing the 12-wk
treatment period, and follow-up outcomes were
obtained 3 mos after completing treatment.
The microstimulator (Fig. 1) was implanted
near the axillary nerve within the quadrilateral
space under local anesthesia during an outpatient
procedure that required approximately 1 hr. The
axillary nerve was chosen based on studies suggesting that the middle and posterior deltoid muscles
provide significant reduction of glenohumeral subluxation in hemiparetic subjects, the possible neuromodulatory effect on this mixed nerve and the
desire to implant a single microstimulator.17 In a
clean procedure room, the patient was placed in a
semiprone position on the procedure table with the
coronal plane approximately 45° to the plane of the
table. Pillows were placed under the patient’s chest
and abdomen for support. Anatomical landmarks
and fluoroscopy were used to locate the quadrilateral space and the surgical neck of the humerus. A mark was made on the skin surface
overlying the quadrilateral space. The shoulder
region was prepped and draped in sterile fashion.
A 5-mm skin incision to subcutaneous fat was
made 3 cm inferior to the skin mark along the
belly of the posterior deltoid muscle. A sterile
tool kit (Fig. 2) designed specifically for implantation of the microstimulator was used. The
stimulation probe, connected to an external
pulse generator (Dakmed Peripheral Nerve Stimulator Model 750, Buffalo, NY), was directed into
the tissues toward the quadrilateral space. Stimulated muscle contraction of the three heads of
the posterior deltoid and teres minor muscles at

FIGURE 1 Microstimulator with absorbable suture
attached.

Electrical Stimulation for Shoulder Pain

425

FIGURE 2 Implantation tools. A, Sheath. B, Plastic
dilator. C, Ejector tool. D, Stimulation
probe. E, EPG anodal lead. F, EPG
cathodal lead.

FIGURE 3 Recharging apparatus.

RESULTS
the lowest possible stimulation intensity (current) was sought to confirm optimal localization
of the probe’s stimulating tip. Initially, stimulation was delivered at 30 Hz, 5 mA, and 200 ␮secs.
For treatment, the pulse width was unchanged, a
50% duty cycle was used, current was minimized
to provide visible muscle activation of the deltoid, and frequency was minimized to provide
fused muscle activation with minimal discomfort. The tip of the stimulation probe is rounded
rather than sharp, minimizing the risk of puncturing vascular or neural structures. When the best
possible probe position was achieved, the external
pulse generator was disconnected and the introducer
(dilator ⫹ sheath) was then slid over the probe. The
probe and dilator were removed while the sheath was
held in place. The microstimulator was then inserted
into the sheath, cathodal end first, and pushed by the
ejector to the tip of the sheath with the cathode
protruding slightly beyond the sheath as indicated by
a mark on the ejector. The microstimulator was then
activated by the external controller to confirm its
optimal position relative to the target nerve by
visualizing stimulated muscle contraction. The microstimulator was then implanted by withdrawing
the sheath upward to the hilt of the ejector. Manual
pressure was then applied over the microstimulator as the ejector and sheath were withdrawn. Test
stimulation was delivered again to ensure adequate
microstimulator placement. An absorbable suture
was tied to the proximal eyelet of the microstimulator before the implantation that permitted withdrawal of the microstimulator during and up to 10
days after the procedure. After implantation, the
microstimulator was controlled using a remote
control. The battery was recharged by the user at
home approximately once per week (Fig. 3).

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No treatment-related adverse events occurred
during the implantation procedure, treatment period,
or follow-up period. The patient reported pain in the
contralateral shoulder starting 6 wks after initiation
of treatment that resolved spontaneously and was not
deemed to be related to the implantation procedure
or treatment. Table 1 summarizes outcome measures
obtained before treatment, after the 12-wk treatment
period, and at 3 mos after completing treatment.

TABLE 1 Pretreatment, posttreatment, and
3-mo follow-up outcome measures
Follow-up
Pretreatment Posttreatment (3 mos)
Paina
Analgesicsb
(mg)
Motorc
Sensationd
Range of
motione
Subluxationf
Activitiesg
Quality-ofLifeh

8
514

4
400

3
400

17
0
95

36
0
130

35
0
140

1.0
29
11

1.0
29
10

1.0
30
10

a
Brief Pain Inventory no. 12: worst pain in the past week
on 0 –11 numeric rating scale.
b
Average daily ibuprofen for the week before evaluation.
Patient was restricted to the use of a single type of analgesic
during the study period.
c
Fugl-Meyer Assessment: upper limb portion only (maximum, 66).
d
Light touch sensation over the lateral shoulder (0 ⫽
none, 1 ⫽ abnormal, and 2 ⫽ normal).
e
Passive shoulder abduction (0 –150°).
f
Fingerbreadths of glenohumeral subluxation.
g
Brief Pain Inventory no. 23: pain interference with 7 daily
activities (0 ⫽ no interference; 70 ⫽ maximum interference).
h
EuroQol 5D.

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Shoulder pain decreased from 8/10 before treatment
to 4/10 after treatment and decreased further to 3/10
at 3-mo follow-up. Passive range of motion and motor
function also improved after treatment. Sensation,
shoulder subluxation, activities, and quality-of-life did
not change.

DISCUSSION
The mechanism by which ES confers reduction of PSP remains unknown. Suspected mechanisms include sensory neuromodulation and improved joint stability. The mechanisms that
underlie sensory neuromodulation of pain resulting from ES have not been rigorously demonstrated through experimentation, but a theoretical
basis has been postulated by Melzack and Wall.18
Joint stability may result from reduction of subluxation during stimulation (i.e., as an orthotic), improvement in passive properties of muscles that
stabilize the shoulder through stimulated conditioning, and recovery of volitional motor function.
More than one mechanism may be involved. In this
case, greater reduction of PSP 3 mos after completing treatment is particularly difficult to explain. Three months exceeds the expected neuromodulatory effect. Although motor impairment
decreased during the course of treatment and follow-up, subluxation did not change. This is a similar dilemma encountered in other studies evaluating implanted ES in which pain reduction
persisted for more than a year beyond treatment
yet shoulder subluxation did not change.11,12 Other
possible mechanisms such as improved range of
motion remain speculative. Further investigation
to elucidate the mechanisms of ES for treating PSP
are needed.
The high degree of motor recovery observed in
this case is difficult to explain. The patient began
iPNS almost 5 yrs after stroke onset, making spontaneous recovery unlikely. The patient did not receive any additional therapies that could account
for recovery of motor function during treatment or
follow-up. Although motor recovery resulting from
ES to the upper limb after stroke has been well
documented,19,20 recovery has generally been limited to those muscles directly receiving stimulation. In this case, the only muscles directly receiving stimulation were those innervated by the
axillary nerve; however, motor recovery was observed not only in those muscles receiving direct
ES but also in nonstimulated muscles of the distal
upper limb.
This report describes the first stroke survivor
with chronic hemiparesis and chronic PSP treated
with ES delivered through a fully implanted microstimulator containing a rechargeable internal
battery. The treatment described in this report also
differs from previously studied transcutaneous and
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partially implanted systems in that a single microstimulator was used to direct stimulation solely to
the axillary nerve, a mixed peripheral nerve, in
contrast to other systems that target one or more
motor points of key shoulder muscles. The results
suggest that treatment of PSP using the novel
system described in this report may be clinically
feasible, including both the outpatient implantation procedure and the home-based treatment regimen. Further clinical study, already ongoing, is
needed to more fully evaluate the feasibility, safety,
and efficacy of this treatment.

ACKNOWLEDGMENTS
We thank Dr. Ross Davis for his assistance in
developing the microstimulator implantation
procedure.
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