Abstract
The ischial tuberosities
(IT) are the most vulnerable sites for debilitating pressure ulcers (PU) in SCI
patients. Interestingly, functional electrical stimulation (
1 Introduction
Pressure ulcers in the gluteal
region constitute one of the most serious complications in the wheelchair based
spinal cord injured population [1]. It affects up to 56% of this group of patients [2]. Prolonged localized pressure loaded on wasted gluteal muscles coupled with insensitivity to ischemia
below the level of the lesion have long been considered to be the most
important factor [3][4]. At present, specialised cushions to reduce interface
pressures combined with pressure relief by the patient performing ‘push-ups’ or
‘leaning forward’ is the best option for preventing PU in spinally injured
patients. Performing pressure relief requires sufficient upper limb strength,
which may not present in a high level lesion, and continued motivation. Surface
functional electrical stimulation (FES) of gluteus maximus
has been reported to change the shape of loaded buttocks and reduce interface
pressure in gluteal region [5][6], however the muscles have to be stimulated in a
cumbersome manner by repeatedly sticking large electrode pads on the skin. The
long-term practicality and patient compliance with this technique is a problem.
However, an implanted
The aim of this study was to 1) investigate
the acute effects of sacral FMS on seating interface pressure changes in
healthy volunteers and demonstrate its utility as an assessment tool and 2)
show that similar effects are possible with
2 Methods
2.1 Subjects
Local ethical approval and informed consent was obtained; 5 able-bodied male subjects and 1 female SCI patient (T7/T8 complete) with a SARS implant were recruited for this study.
2.2 Functional Magnetic Stimulation (FMS)
FMS was delivered using a Medtronic Dantec MagPro with a large circular coil. Stimulation frequencies in the range of 15-25pps at 50-70% intensity for 2 seconds were utilised.
The optimal coil position for sacral nerve
root stimulation was determined by mapping the gluteal
muscle response [7]. The coil was placed over various points of a grid
pattern ranging from the line of the iliac crest to 10 cm below and 8 cm either
side of midline.
2.3 Stimulation via SARSI implant
Electrical stimulation was applied bilaterally via implanted Finetech-Brindley Sacral Anterior Root Stimulator Implant (SARSI). The S2 nerve root was stimulated using a frequency of 20pps and amplitude ‘1’, the pulse width was varied from 8-800μs
2.4 Ischial pressure measurement
Each subject sat in a wheelchair with
standard cushion (high resilience foam, density 45kg/m3). Ischial pressures were measured using interface pressure
map (Xsensor
2.5 Phantom Study
To identify possible interference of magnetic stimulation on interface pressure map acquisition hardware, we carried out a phantom study, previous studies have reported artefacts due to magnetic stimulation. We applied magnetic stimulation in gel buttocks to measure the effects of the magnetic pulses on the interface pressure mapping system. Magnetic stimulating coil was applied against the ‘sacrum area’ of gel buttocks and energized at single pulse and frequencies of 5-20Hz and intensity of 30% to 90% of maximum expected stimulator output during mapping interface pressure at 10% intervals. We compared the results of interface pressure measurements made with the stimulator off and on in gel buttocks and detected no effect of magnetic stimulation on the results of pressure values.
3 Results
3.1 Results from able-bodied subjects
All able-bodied subjects tolerated FMS well. Figure 1 shows typical interface pressure distribution before and during stimulation in one subject; Figure 2 shows bilateral IT peak pressure traces before, during and after stimulation, where the coil was placed at midline of S2 level. The difference between resting and stimulated pressures at the ischial was statistically significant in both sides of all subjects.
For the group (n=5), the average peak pressure drop was 20.3±7.5mmHg; IT peak pressures decreased during FMS as compared to baseline (127.5.0 ± 17mmHg vs. 107.7 ± 18mmHg, p<0.0001) (Figure3). The optimal response was achieved with the coil located at S2 level (about 6cm below iliac crest). For bilateral stimulation the optimal coil position was midline, and for unilateral stimulation the optimal position was at 2-4cm lateral to midline. Increased intensity of FMS was associated with a greater reduction in peak interface pressure
3.2 Results from SARSI patient
Bilateral S2 stimulation caused a palpable contraction of the gluteus maximus and associated hip and leg movement. Peak pressures under ischial tuberosities were significantly decreased (109.5 ± 5.6 mmHg at resting vs 64.07 ± 4.6 mmHg during stimulation, p<0.0001) (Figure4). Maximum reduction of IT peak pressure was obtained at a stimulation pulse width of 256 μs, frequency of 20 pps .
4 Discussion and Conclusions
Our preliminary results indicated that reduction of ischial pressures during sitting could be achieved by sacral nerve root stimulation, either by using FMS or implanted sacral nerve stimulator. In the able-bodied subjects, maximal effects were obtained approximately over S2 by using magnetic stimulation; in 1 SCI patient with SARSI implant, maximum reduction of IT peak pressure was obtained when stimulating S2 with pulse width of 256 μs, frequency of 20 pps.
Sacral Functional Magnetic Stimulation or through an implanted stimulator effectively induced gluteal muscle contraction and pelvic tilt, which significantly reduced the seating pressures. The magnitude of peak pressure reductions in this study were similar to those obtained by Ferguson et al [8] with electrical stimulation of quadriceps but greater than those produced by Levine [7] with stimulation of gluteal muscles. In our study, IT peak pressure produced a 42% reduction in 1 SCI patient and nearly 20% reduction in 5 able-bodied subjects. This difference between healthy males and a single female SCI patient may simply reflect the variation of individual’s body weight, differences in pelvic anatomy or small sample sizes. More likely it is the potential for greater benefit in patients already suffering from muscle wasting of the buttocks.
While the formation of a
pressure ulcer is undoubtedly a multifactorial event,
the application of a prolonged pressure loading to exceed the tissue tolerance
is the main factor that allows ulcers to develop. Therefore, a sensible
approach of preventing ischial pressure ulcers is to
redistribute seating interface pressure and reduce pressures under ischial tuberosity by regular
stimulation of S2 sacral nerve roots to achieve gluteal
muscle contraction and increase muscle bulk.
There is a further important
issue concerning tissue perfusion now being studied in our Centre. Although the
pressure changes recorded in this study were statistically significant, the
application in a clinical setting is a more complex issue. Ischial
blood perfusion during the stimulation is presently under investigation by
using Tissue Reflectance Spectrometer and Laser Doppler Flowmetry.
Gluteal muscle thickness after chronic stimulation is
also measured by ultrasound image. If
the pressure reductions can improve local vascular circulation, an implanted S2
sacral nerve roots stimulator (SARSI currently used for bladder emptying) may
form the basis of an alternative method of ischial
pressure ulcer prevention. We expect to
present these results when ready to a future meeting of the Society.
References
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Acknowledgements
This work is funded by the