STIMULATOR FOR TREATING POST-STROKE SHOULDER SUBLUXATION
Z.-P. Fang*,
*NeuroControl Corporation and **
ABSTRACT
A miniature stimulator was developed to drive a set of percutaneous wire electrodes implanted into the shoulder muscles for treating shoulder subluxation and pain in stroke survivors. The 8-channel stimulator was built around a low-power microcontroller and packaged into a pager-sized box. A single AA-size battery can power the device for about 50 hours of operation. Stimulation pulse trains with duty cycle of 10-sec on and 8-sec off were applied 6 hours per day for 6 weeks. Among the 7 chronic subjects enrolled in the study, shoulder subluxation was reduced from a median of 7mm to 3mm (p=0.05) as evidenced by radiograph; pain intensity decreased from 5 to 0 (p=0.03) as reported on the Brief Pain Inventory. The preliminary clinical tests showed that the device was well accepted by the users and the intervention may be effective for the indicated use.
Keywords:
stroke, hemiplegia, shoulder, subluxation, pain, electrical stimulation
INTRODUCTION
Shoulder pain is a common complication of hemiplegia occurring in 70-84% of stroke survivors with a paralyzed upper extremity [1]. Inferior subluxation of the glenohumeral joint is the major cause for the pain, accounting for 17-75% of the cases [2]. Conventional intervention on subluxation by using slings and arm supports is now considered controversial or even contraindicated because of the potential complications from immobilization, such as unwanted synergies and disabling contractures. Electrical stimulation of the shoulder muscles has been shown effective in managing post-stroke subluxation [3,4]. However, existing surface stimulation systems are not widely accepted because of stimulation-induced pain, poor muscle selectivity, and difficulty in daily application of electrodes. Based on our many years of experience in using percutaneous stimulation electrodes in patients with spinal cord injury, we expected that percutaneous stimulation would cause less pain, obtain better muscle selectivity, and be easier for daily use in the stroke survivors [5]. For ambulating stroke survivors to accept this treatment, however, a small, light stimulator must be developed, as the exiting bulky, wheelchair-mounted stimulator can not be used by this patient population.
DEVICE
DESIGN
The percutaneous intramuscular stimulation system consists of a pager-sized stimulator, a cable assembly, an interconnection block, one or more wire stimulation electrodes, and a disk reference electrode, as depicted in Figure 1. The wire electrode used for percutaneous stimulation is wound from a multi-strand, stainless steel wire insulated with Teflon. The tip of helix is deinsulated to form a stimulation surface. The electrode is loaded into a 19-gage hypodermic needle for insertion into the muscle belly, as shown in Figure 2.
Figure1
Stimulation system composition Figure2
Percutaneous stimulation electrode
The stimulator circuitry is designed around a low-power, high-performance microcontroller. The current driver, along with the amplitude controller, converts the digital commands to the charge-balanced, current-regulated biphasic stimuli. The impedance detector monitors the impedance of the cable-connector-electrode circuit and interrupts stimulation whenever the integrity of the electrodes or connection in compromised. The cumulative use hours and the number of sessions are logged by the stimulator for compliance checking. The circuit board of the stimulator is assembled using high-density, surface-mount technology to reduce size, allowing the device to be packaged in a plastic enclosure originally design for a pager. The specifications for the prototype device are presented in Table 1.
Figure 3
Circuit block diagram of the percutaneous stimulator
Table 1 Technical specifications of percutaneous stimulator
|
Parameter |
Value |
Resolution |
Default |
|
Number of channels |
8, Monopolar |
|
|
|
Pulse waveform |
Charge-balanced biphasic |
|
|
|
Pulse frequency |
5 – 50 Hz, ± 10% |
1 Hz |
12 Hz |
|
Pulse duration |
5 – 200 msec, ± 2 msec |
1 msec |
5 msec |
|
Pulse amplitude |
1 – 20 mA, ±- 0.5 mA |
1 mA |
20 mA |
|
Maximum load resistance |
1.3 kW |
|
|
|
Session length |
0 – 10 hour |
1 min |
6 hour |
|
Cycle duration |
2 – 100 sec |
1 sec |
30 sec |
|
Stimulation delay duration |
0 – 100 sec |
1 sec |
0 sec |
|
Stimulation ramp-up duration |
0 – 100 sec |
1 sec |
5 sec |
|
Stimulation hold duration |
0 – 100 sec |
1 sec |
10 sec |
|
Stimulation ramp-down duration |
0 – 100 sec |
1 sec |
5 sec |
|
Stimulation pattern |
4 patient-selected modes |
|
|
|
Command/Programming input |
3 push buttons |
|
|
|
Display |
LCD, 12 characters |
|
|
|
Battery |
Single AA alkaline for 50 hrs |
|
|
|
Package size |
74 mm x 50 mm x 30 mm |
|
|
|
Total weight |
75g including battery |
|
|
CLINICAL
TEST
Seven stroke survivors, who were at least 6 months post stroke and manifested shoulder subluxation of at least one fingerbreadth, were enrolled in the pilot study. The wire electrodes, loaded inside an insertion needle, were introduced through the skin at the superior-medial aspect of the shoulder and tunneled subcutaneously to the motor point of the target muscles. The two muscle commonly used were the posterior deltoid and the supraspinatus. Stimulation treatment started one week after implantation. The intensity of the stimulation was adjusted to induce strong muscle contraction to elevate the humerus into the glenoid fossa, yet not too strong to cause arm abduction or shoulder shrugging. The instantaneous effect of stimulation on the reduction of subluxation, as evidenced by radiograph, is shown in Figure 4. A treatment protocol of 6-hour stimulation per day for 6 weeks was prescribed to the study subjects. Stimulation duty cycles consisting of a 4-sec ramp-up phase, a 10-sec on phase, a 4-sec ramp-down phase, and an 8-sec off phase were used.
Figure 4 Instantaneous effect of stimulation on shoulder subluxation
Inferior subluxation of the glenohumeral joint was assessed by comparing anteroposterior radiographs of the affected and unaffected shoulder; while shoulder pain was assessed by the Brief Pain Inventory. A representative set of radiographs depicting the effects of the treatment in reducing subluxation is shown as Figure 5. The outcome from all seven subjects are summarized in Table 2. Significant reduction in both subluxation and pain measurements was achieved in this small group of chronic patients. A user survey indicated that the stimulator was well accepted by the patients and the stimulation did not cause pain sensation. These promising results warrant further development of the stimulator and larger-scale studies in both chronic and acute patients.
Figure 5 Treatment effect of stimulation on chronic shoulder subluxation
Table 2 Results of stimulation on chronic shoulder subluxation and pain
|
Outcome |
|
Pre-treatment |
Post-treatment |
3-month follow-up |
|
||
|
|
m |
7 |
3 |
2 |
|
||
|
|
p |
0.05 |
0.35 |
|
|||
|
|
m |
5 |
0 |
2 |
|
||
|
|
p |
0.03 |
0.92 |
|
|||
REFERECES
1.
Van Ouwenaller,
et al. (1986). Painful shoulder in
hemiplegia. Arch Phys Med Rehabil, 67: 23-36
2.
Zorowitz
RD, et al. (1995). Shoulder pain and
subluxation after stroke: correlation or coincidence? Am J Occup Ther, 50: 194-201
3.
Faghri
PD, et al. (1994). The effects of
functional electrical stimulation on shoulder subluxation, arm function
recovery, and shoulder pain in hemiplegic stroke
patients. Arch Phys Med Rehabil, 75: 73-79
4.
Baker LL, Parker K (1986). Neuromuscular electrical stimulation of the
muscles surrounding the shoulder. Phys Ther, 66: 1030-1037
5.
Memberg
WD, et al. (1993). An analysis of the
reliability of percutaneous intramuscular elecrodes
in upper extremity FNS applications.
IEEE Trans Rehabil Rng,
1: 126-132
This
work was supported by Grant R43-HD34996 from the National Institutes of Health.