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0022-5347/97/1574«1504$03.00/0
Vol. 157, 1504-1508, April 1997
Printed in U.S.A.
T h e J o u r n a l o f U ro lo g y
Copyright © 1997 by A m e r i c a n U r o l o g i c a l A s s o c i a t i o n , I n c
SELECTIVE DETRUSOR ACTIVATION BY ELECTRICAL SACRAL NERVE
ROOT STIMULATION IN SPINAL CORD INJURY
N. J. M. RIJKHOFF ,* H. WIJKSTRA, P. E. V. VAN KERREBROECK
and
F. M. J. DEBRUYNE
From, the Department of Urology, University Hospital Nijmegen, The Netherlands and the Center for Sensory-Motor Interaction, Aalborg
University, Denmark
ABSTRACT
Electrical sacral nerve root stimulation can be used in spinal cord injury patients to induce
urinary bladder contraction. However, existing stimulation methods activate simultaneously
both the detrusor muscle and the urethral sphincter. Urine evacuation is therefore only possible
using poststimulus voiding. Micturition would improve if the detrusor muscle could selectively be
activated. The purpose of this study was to demonstrate selective detrusor activation in patients
by ventral sacral root stimulation. The stimulation method involves selective activation of the
small diameter myelinated nerve fibers and consists of a combination of cathodal excitation and
selective anodal blocking using a tripolar electrode. To investigate anodal blocking, the intraurethral pressure response to stimulation was measured in acute experiments performed on 12
patients. The influence of both pulse amplitude and pulse duration on the pressure response was
analyzed. In 8 out of 12 patients anodal blocking of somatic motor fibers was possible. This study
also indicates the feasibility of selective detrusor activation by sacral root stimulation.
Key W ords: electrical stimulation, bladder, sacral nerve roots, anodal block
sor. The somatic fibers have a larger caliber than the para­
sympathetic fibers8 and as large diameter fibers need a
smaller stimulus for their activation than smaller ones,9»10
activation of the smaller fibers is always accompanied
by activation of the larger fibers.
Brindley was the first to describe that, despite the simul­
taneous contraction of both the detrusor muscle and the
urethral sphincter muscle, urine evacuation is possible by
taking advantage of differences in the biomechanical charac­
teristics of smooth and striated muscle tissue.11 The relax­
ation time of the striated external urethral sphincter muscle
is shorter than the relaxation time of the smooth detrusor
muscle so when stimulating with interrupted pulse trains,
voiding is achieved between the pulse trains due to the sus­
tained high intravesical pressure. This poststimulus voiding
principle is used in a commercially available system for blad­
der control (Finetech Medical Ltd., Welwyn Garden City,
UK) which has already been implanted in over 700 patients12
with good clinical results.13 However, the stimulus technique
has some drawbacks. Poststimulus voiding is an artificial
micturition pattern with voiding in spurts at supranormal
intravesical pressures. In addition movement of the lower
limbs occurs during stimulation as the ventral sacral roots
also contain fibers innervating leg musculature.
The micturition pattern would improve if the detrusor is
selectively activated. Therefore several attempts have been
made to prevent the sphincter from contracting. These in­
clude i.a. interruption of the somatic fibers,14»16 blocking
the transmission of motor signals through the pudendal
nerve,16-17 and fatiguing the sphincter muscle.18
A better method for selective detrusor activation without
extra surgery or additional electrodes would be selective ac­
tivation of the small diameter parasympathetic fibers in the
ventral sacral roots. This is possible when using a selective
anodal block.10»19>20 Selective small fiber activation is ob­
tained using a tripolar cuff electrode consisting of a cathode
Accepted for publication November 19, 1996.
flanked by two anodes (Fig. 1). Near the cathode all fibers
* Requests for reprints: Center for Sensory-Motor Interaction, Aal­
(small
and
large)
are
activated
while
near
the
distal
anode,
borg University, Fredrik Bajers Vej 7D, Building D-3, 9220 Aalborg,
the propagation of the action potentials (AP’s) in the large
Denmark.
Supported by grant C92.1249 from the Dutch Kidney Foundation.
fibers is blocked by a selective anodal block. The anodal
Normal function of the lower urinary tract is disturbed in
patients with spinal cord injury (SCI). In patients with a
suprasacral lesion detrusor hyperreflexia and detrusorsphincter dyssynergia develops.1 Hence a number of compli­
cations such as poor urine evacuation, urinary tract infec­
tions, incontinence, vesicoureteral reflux and hydronephrosis
are regularly found in SCI-patients. As restoration of the
neural control, e.g. by spinal cord regeneration, is not yet
possible, the goal of treatment is to prevent the urological
complications while preserving continence. This can be
achieved by creating a bladder with large volume, low pres­
sure urine storage and periodic evacuation of urine at low
intravesical pressure. Urine evacuation is usually obtained
by intermittent catherisation while a hypo- or areflexic blad­
der is created either by administration of drugs or by detru­
sor deafferentation. However, many patients are not able to
catheterise themselves or are not willing to depend on catheterisation.2 Furthermore, indwelling catheters are a cause
of urinary tract infections. It is therefore desirable to induce
voiding by eliciting a detrusor contraction.
To induce a detrusor contraction, artificial electrical stim­
ulation through implanted electrodes can be used. Four dif­
ferent sites are available where application of electrical stim­
uli results in a detrusor contraction: the bladder wall,3 the
pelvic nerves,4 the sacral nerve roots5’6 and the spinal cord.7
The sacral nerve roots appear to be the most attractive stim­
ulation site because the space within the spinal column fa­
cilitates mechanically stable electrode positioning and the
long intraspinal course of the nerve roots allows application
of insulated tripolar electrodes. However, eliciting a detrusor
contraction by ventral sacral nerve root stimulation results
in co-activation of the urethral closure mechanism leading to
little or no voiding. This is due to the composition of the
ventral sacral roots, which contain, among others, somatic
fibers innervating the external urethral sphincter and
preganglionic parasympathetic fibers innervating the detru-
1504
SELECTIVE DETRUSOR ACTIVATION IN SPINAL CORD INJURY
n
h
JT
12
Bladder
Anode
A.
Sphincter
Mr
X Mr
Cathode
K
K
Anode
3 Small fiber
3 Large fiber
Selective block
Excitation
F ig . 1. Principle of selective small fiber activation using a tripolar
electrode. Both nerve fiber groups (small and large) are excited near
cathode. Produced action potentials propagate away in both direc­
tions, although only propagation in distal direction has been drawn.
Near anodes propagation of action potentials may be blocked, de­
pending on amplitude of anodal current and pulse duration. As large
diameter fibers need less current for their blocking than smaller
ones, a selective block is possible.
current causes hyperpolarization of the fiber membrane.
When sufficiently hyperpolarized AP’s cannot pass the hy­
perpolarized zone and are arrested. The selective blockade
is possible since the large diameter fibers need less current
for their blocking then smaller fibers.10-20 As the AP’s in
the smaller fibers can pass unhindered, the net result is
selective small fiber activation. Since the motoneurons inner­
vating the lower limb musculature would also be blocked,
this stimulation method would also reduce the stimulation
induced lower limb movement.
Simulations with a computer model10 showed the theoret­
ical possibilities of this stimulation technique for activation
of the detrusor without activation of the urethral sphincter.
Based on these theoretical results, tripolar cuff electrodes
have been developed and successfully tested in acute exper­
iments on dogs.21,22 So far, the method of anodal blocking has
never been demonstrated in humans. Only Brindley et al.
described the use in patients but their paper does not contain
any results apart from the conclusion “we have not yet suc­
ceeded in making the method well enough in patients for
every day use.”23
In this study we investigated the feasibility of a selective
anodal block in patients to obtain selective detrusor activa­
tion and determined the effect of pulse amplitude and pulse
duration on the anodal block.
MATERIALS AND METHODS
Patient preparation. In acute experiments, measurements
were performed on 12 spinal cord injury patients, who un­
derwent implantation of a Finetech-Brindley sacral anterior
root stimulator. During the operation, access to the intra­
dural sacral nerve roots was gained after a laminectomy.
Individual nerve roots were identified by their size and by the
response of several muscle groups to electrical stimulation
using a hook electrode. The intradural ventral sacral nerve
roots (S2-S4) were placed in a standard book electrode24
while the dorsal sacral nerve roots (S2-S4/S5) were cut to
abolish the detrusor hyperreflexia. The book electrode can be
described as contacts mounted in grooves cut into a block of
insulating silicone rubber. The grooves have a rectangular
cross section of 2 X 3 mm. Each pair of roots (left and right)
was placed in a separate groove so that one root pair could be
stimulated independently from the others. After electrode
implantation the operation proceeded with closure of the
dura and tunneling of the electrode leads to a subcutaneous
pocket in the flank. Following closure of the skin, the patient
was turned over and the leads were prepared to be connected
to the implantable receiver/stimulator. At this time, the leads
were connected via an aseptic cable to a self made computer
controlled stimulator and for 15-20 minutes we tested if
anodal blocking can be realized. After this period of experi-
1505
mental stimulation the leads were disconnected from the
stimulator and the normal operation procedure resumed
with implantation of the receiver/stimulator.
Stimulation and recording. The self-made stimulator con­
sisting of two synchronized current sources with a common
cathode was used to drive the tripolar book electrodes. The
ratio between the two anodal currents (Fig. 1), which was
adjustable, was set to 1 because the electrode configuration is
symmetrical. Monophasic rectangular pulses without active
charge balancing and without the use of a series capacitor
were used. Pressure responses were elicited using pulse
trains of 3-5 s length, containing identical pulses at a
pulse rate of 25 pulses/s. As the S3 and S4 ventral roots
contain most of the motor fibers innervating the lower uri­
nary tract, experimental stimulation was limited to these two
root pairs. As the roots pairs (left and right) were placed in
the same electrode groove the bilateral roots were always
simultaneously stimulated. The toes and feet were visually
observed during the experiment to see whether the nerve
block reduced the stimulus induced lower limb movements.
A two channel transurethral pressure catheter (7F,
Gealtec, UK) was used to measure intravesical and intraurethral pressure. The urethral pressure sensor was positioned
at the level of the external sphincter so that in response to
suprathreshold stimulation, a maximum pressure response
was measured. Pressures were sampled at 8 Hz, displayed on
a monitor and stored in a portable datalogger. After the
experiment the data were transferred to a computer for off­
line analysis. Prior to the stimulation the bladder was filled
with approximately 200 ml. saline using a 16-ch transure­
thral filling catheter.
RESULTS
As expected, in all 12 patients toe and foot movements
were recorded and the intravesical and intraurethral pres­
sure increased upon stimulation of at least one sacral root
pair, using a small duration stimulus pulse (200 / a s ) with a
sufficient high amplitude (5 mA), This shows that the effer­
ent neuromuscular system was intact and demonstrates the
undesired movements of the lower limbs during stimulation.
In 8 patients the stimulation induced movements of toe and
feet significantly decreased or were complete absent upon
stimulation with long duration pulses (700 ¡is), depending on
the pulse amplitude, due to the occurrence of an anodal block.
This demonstrates that anodal blocking is capable of reduc­
ing the undesired lower limb movements.
Fig. 2 shows a typical intravesical and intraurethral pres­
sure response to stimulation of an S3 root pair for different
pulse amplitudes using a 700 /xs pulse duration. The intra­
vesical and intraurethral pressure responses differ as the
muscle characteristics of detrusor and sphincter differ. The
striated sphincter muscle has a relative fast response to
stimulation resulting in a rectangular shaped pressure re­
sponse as a function of time (Fig. 2, bottom trace). The
smooth detrusor muscle has a relative slow response result­
ing in a slower rise and fall of the pressure response (Fig. 2,
top trace). This slow detrusor response to stimulation causes
that at a relative high stimulation current (e.g. 6 mA) the
maximum intravesical pressure is not reached during the
stimulation time and the elicited intravesical pressure de­
pends largely on the stimulation time (see discussion).
The stimulus threshold to elicit a urethral sphincter con­
traction was between 0.1 and 0.2 mA. A maximum intraure­
thral pressure response is already elicited 0.3 mA and a
farther increase of the stimulus to 0.5 and 1.0 mA did not
result in a larger intraurethral pressure response since all
motoneurons innervating the sphincter are activated and the
maximum obtainable pressure response has been reached.
Increasing the stimulus above 1.0 mA results in a decrease in
the intraurethral pressure as the current at the distal anode
1506
SELECTIVE DETRUSOR ACTIVATION IN SPINAL CORD INJURY
o<N
E
ü
,
CL)
l- r
3
æ
e
PM
6
CD
u.
e
>5
o
1
8
6
Cathodal current [mA]
Fig. 2. Intraurethral (Puret) and intravesical (Pblad) pressure
responses to stimulation of S3 sacral root pair for pulse amplitudes
between 0.1 and 6.0 mA. Each response was obtained by applying
stimulus train for approximately 5 s (pulse duration^ 700 /xs; pulse
rate: 25 pulses/s). At 6.0 mA selective detrusor activation is obtained
as external urethral sphincter does not respond to stimulation (com­
plete block). Stimulation with 200 fxs wide pulses results in normal
intraurethral pressure response, demonstrating that pressure re­
sponse is not caused by muscle fatigue or nerve damage.
exceeds the threshold for blocking of the largest somatic
motoneurons. When increasing the current, more and more
fibers are blocked until at 6.0 mA all somatic motoneurons
are blocked and the sphincter is prevented from contraction.
Although the square shaped sphincter pressure response
has disappeared at 6 mA, still an increase in intraurethral
pressure was visible. As this pressure response has the same
shape as the intravesical pressure response it is assumed
that the measured intraurethral pressure response is trans­
mitted from the intravesical pressure. This reflection of the
intravesical pressure in the intraurethral pressure results in
two peaks in the response to 4.0 mA. The first peak stems
from the sphincter contraction while the second one is due to
the detrusor contraction.
To prove that the reduction of the intraurethral pressure
response is caused by anodal blocking and is not due to other
causes such as muscle fatigue or nerve damage, stimulation
at 6.0 mA has also been preformed with a pulse duration of
200 /¿s. Since 200 jus is usually too short to obtain blocking
(see below), the intraurethral pressure response should
roughly be the same as the maximum response obtained with
0.5 and 1 mA. Fig. 2 shows that just by switching the pulse
duration between 200 and 700 jlls the block could be switched
on and off, demonstrating that the urethral pressure re­
sponse reduction was undoubtedly caused by the anodal
block.
The stimulus threshold to elicit a detrusor contraction was
about 1.5 mA. Beyond the 1.5 mA the intravesical pressure
response increases as more and more parasympathetic motor
fibers are activated. At 6.0 mA, 700 jus selective activation of
the detrusor muscle was obtained since the detrusor was
activated while there was no square shaped increase in in­
traurethral pressure.
In 8 patients the intraurethral pressure response could be
reduced by anodal blocking. However, in only 2 patients was
the recording sufficiently stable (see Discussion) to measure
the relation between pulse parameters and intraurethral
pressure. Fig. 3 shows the maximum intraurethral pressure
to stimulation of an S3 root pair as a function of the cathodal
current. Both curves have the same shape. Once the excita­
tion threshold has been exceeded, there is a steep increase in
the intraurethral pressure response until all motor fibers
have been activated and maximum pressure plateau has
been reached. Beyond the 1-1.5 mA the response gradually
decreases as the current at the distal anode exceeds the
F ig. 3. Intraurethral pressure response (deviation from baseline
pressure) to bilateral stimulation of S3 root pair as function o:
cathodal current. Pulse duration: 700 ju,s; pulse rate: 25 pulses/s
data from two patients.
blocking threshold for fibres innervating the sphincter,
Above 6.0 mA the maximum pressure reduction was reached
and no further reduction could be obtained.
The occurrence of anodal blocking depends besides on the
anodal current also on the pulse duration. The anodal cur­
rent causes hyper polarization of the nerve fiber membranes
and when sufficiently hyperpolarized, AP’s cannot pass the
hyperpolarized zone and are arrested. But, as it takes time
for an AP to propagate from the excitation point (near the
cathode) to the hyperpolarized zone, the hyperpolarizing
stimulus must at least stay until the AP has reached the
hyperpolarized zone. Thus anodal blocking only occurs using
a sufficient large pulse duration. To analyze the effect of the
pulse duration on the anodal block, the pulse duration was
varied while the cathodal current was kept constant at 7.0
mA. The maximum intraurethral pressure response as a
function of the pulse duration is shown in Fig. 4. Below 200
jus no blocking effects could be observed. When increasing the
pulse duration, the pressure response gradually decreased as
more and more motoneurons innervating the urethral
sphincter are blocked. Above 600 ¡is the maximum pressure
reduction was obtained.
Although stable recordings could be obtained in only two
patients, the same trends in the relation between pulse pa­
rameters and intraurethral pressure were noted in the other
6 patients in whom anodal blocking could be achieved.
O
1
<u
C/3
w
PU
J3
<U
§
h
Ih
>S
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Pulse duration [ms]
Fig. 4. Intraurethral pressure response (deviation from baseline
pressure) to bilateral stimulation of S3 root pair as function of pulse
duration. Cathodal current: 7 mA; pulse rate: 25 pulses/s; data from
two patients.
SELECTIVE DETRUSOR ACTIVATION IN SPINAL CORD INJURY
DISCUSSION
Both anodal blocking of the large diameter fibers in the
sacral nerve roots19,21,22 and selective detrusor activation
by sacral root stimulation25 have been demonstrated in ani­
mal experiments but have never been demonstrated in man.
The results of this study demonstrate the feasibility of anodally blocking of the somatic nerve fibres in ventral sacral
roots in patients. When used in combination with cathodal
excitation, it results in selective activation of the detrusor
muscle, depending on the pulse parameters.
In 4 patients anodal blocking could not be demonstrated.
This might be due to the more than average mechanical
manipulation of the nerve roots during identification and
electrode positioning. In comparable animal experiments we
often noticed that, after excessive manipulation of the
nerves, anodal blocking does not occur immediately after
placement of the electrodes. However, after leaving the elec­
trode and nerve untouched, the blocking effects reappears
after a few hours. In this study there was about one hour
between electrode placing and stimulation, which could ob­
viously not be extended if blocking did not occur.
To evaluate the effect of anodal blocking on the activation
of the external urethral sphincter, the intraurethral pressure
at the level of the urethral sphincter was measured. The
pressure responses should be stable and reproducible
throughout the experiment but it proved to be difficult to
ensure complete immobility of the pressure sensor. The cath­
eter tends to move during stimulation and as a result dislo­
cating the intraurethral pressure sensor. This happened in 6
patients and the relation between pulse parameters and in­
traurethral pressure response could not be measured as the
setup did not allow for repositioning of the sensors. Therefore
in only two patients a stable recording could be obtained.
However, the occurrence of anodal blocking could also be
observed visually as an absence of foot or leg movement
during stimulation.
Monophasic rectangular current pulses without active charge
balancing and without the use of a series capacitor were used.
However, in chronic implants monophasic pulses may induce
neural damage and electrode contact corrosion due to a net
transfer of electrical charge at each contact. This can be pre­
vented by adding a secondary reversed pulse. However, during
the reversed pulse the anodes become cathodic which causes
excitation if the excitation threshold is exceeded. The reversed
current can be kept below excitation threshold when the com­
plete time space between two pulses is used. To avoid the
excitation by the reversed pulse, monophasic pulses were used
exclusively in this study.
In experiments on anodal blocking of peripheral nerves,
excitation of large fibers occurred near the anode at the end
of a long rectangular blocking pulse.20»26 A gradual decay at
the end of the pulse was necessary to suppress this so-called
anodal break excitation. However, break excitation did not
occur in our experiments. In animal experiments, we also
often observed that anodal blocking was possible without
anodal break excitation, using a current just above the block­
ing threshold.22 A further increase of the stimulus usually led
to break excitation which could be suppressed by a gradual
current decrease at the end of the pulse. It is therefore likely
that break excitation will also appear in patients when using
rectangular pulses at higher amplitudes.
Results of anodal blocking of large diameter nerve fibers
when stimulating the sacral roots of baboons using chronic
implanted book electrodes have been reported by Brindley
and Craggs.19 They found initial blocking of fibers innervat­
ing tail musculature a tt4.5 mA while at 9.0 mA a complete
block was obtained. These currents are roughly twice as large
compared to our results. The discrepancy may be caused by a
larger cross-section of their electrode or by the connective
tissue between the contacts and the nerve roots in the chronic
1507
implant. In acute experiments, stimulating the sacral nerve
roots in dogs using a tripolar cuff electrode (inner diameter:
1.5 mm.), Rijkhoff et al. measured initial blocking at 0.3-0.7
mA while a maximum block was obtained at 0,7-1.1 mA.22
These relative low currents are caused by the smaller crosssection of the cuff-electrodes compared to the book electrodes.
The pulse duration needed to block the propagation of an
induced action potential using a tripolar electrode depends
on a number of variables including the propagation velocity
of the action potential and the distance between the cathode
and the anode. Rijkhoff and co-workers simulated the effect
of pulse duration on the blocking thresholds of a 12 fxm. fiber
using a cuff electrode with 6 mm. contact spacing and found
a minimum pulse duration of220 ¡is for blocking at minimum
current while below 155 /xs no blocking occurred.10 Experi­
mental data on minimum pulse duration for blocking in
animal experiments have been reported by Brindley and
Craggs19 and Rijkhoff et al.22 Brindley and Craggs reported
maximum blocking with pulse durations of 0.5 and 1 ms but
found no blocking at 0.2 ms when stimulating the sacral roots
in baboons.19 In canine experiments Rijkhoff et al. found
initial blocking at 150 /xs while maximum blocking was ob­
tained above 400 ¡is .22 Compared with these theoretical and
experimental data the minimum pulse duration for blocking
in patients (maximum blocking at a pulse duration beyond
600 /xs) is relative large. The difference is probably due to the
lower propagation velocity of the AP’s in humans as AP
propagation velocities in motor fibers are 40% lower man
than in cat and dog.27
The stimulation time of 3 to 5 s was sufficiently long to
elicit the maximum intraurethral pressure as the contraction
time of the striated muscle is less than 0.5 ms. The detrusor
muscle contracts more slowly and the stimulation time was
too short to elicit the maximum intravesical pressure at 6
mA. The stimulation time should have been at about 10 s for
eliciting a maximum intravesical pressure. Nevertheless the
stimulus time was kept relatively short for two reasons.
Firstly, the intravesical pressure can become very high
(>250 cm. H20) which can lead to vesicoureteral reflux. A
short stimulus duration reduces this risk. The second reason
has to do with the limited time available for the experiment.
It can take up to 50 s for the intravesical pressure to return
to the baseline pressure when high pressures have been
elicited and this would limit the total number of stimulations
during the experiment.
This study shows that the technique of anodal blocking can
be used in human sacral root stimulation for bladder control.
The combination of cathodal excitation and selective anodal
blocking leads to selective activation of the detrusor muscle.
In addition to blocking the fibers innervating the urethral
sphincter, the large motor fibers innervating muscles in the
lower limbs are also blocked. Hence movement of lower limbs
during stimulation is reduced. When in time this technique
can be used in implanted systems, bladder emptying by sa­
cral root stimulation will be more physiological and at lower
intravesical pressures because the outlet resistance is largely
reduced.
REFERENCES
1. Thomas, D. G. and Lucas, M. G.: The urinary tract following
spinal cord injury. In: Scientific Foundations of Urology. Ed­
ited by G. D. Chisholm and W. R. Fair. Oxford: Heinemann
Medical Books, 3rd edition, chapt. 35, pp. 286-299, 1990.
2 . Lapides, J., Diokno, A. C. and Silber, S. J.: Clean intermittent
self catheterisation in the treatment of urinary tract disease.
J. Urol., 107: 458, 1972.
3. Magasi, P. and Simon, Z.: Electrical stimulation of the bladder
and gravidity. Urol. Int., 41: 241, 1986.
4. Kaekenbeeck, B.: Electrostimulation de la vessie des para­
plégiques. Technique de Burghele-Ichim-Demetrescu. Acta
Urol. Belg., 47: 139, 1979.
1508
SELECTIVE DETRUSOR ACTIVATION IN SPINAL CORD INJURY
5. Brindley, G. S., Polkey, C. E., Rushton, D. N. and Cardozo, L.:
Sacral anterior root stimulators for bladder control in paraple­
gia: the first 50 cases. J. Neurol. Neurosurg. Psych., 49:1104,
1986.
6 . Tanagho, E. A., Schmidt, R. A. and Orvis, B. R.: Neural stimu­
lation for control of voiding dysfunction: a preliminary report
in 22 patients with serious neuropathic voiding disorders.
J. Urol., 142: 340, 1989.
7. Grimes, J. H. and Nashold, B. S.: Clinical application of elec­
tronic bladder stimulation in paraplegics. Br. J. Urol., 46: 653,
1974.
8 . Schalow, G. and Barth, H.: Group conduction velocities and
nerve fibre diameters of a and 7 -motoneurons from lower
sacral nerve roots of the dog and humans. Gen. Physiol.
Bioph., 11: 85, 1992,
9. Blair, E. and Erlanger, J.: A comparison of the characteristics of
axons through their individual electrical responses. Am. J.
Physiol., 106: 524, 1933.
10 . Rijkhoff, N. J, M., Holsheimer, J., Koldewijn, E. L., Struijk, J. J,,
van Kerrebroeck, P. E, V., Debruyne, F. M. J. and Wijkstra, H.:
Selective stimulation of sacral nerve roots for bladder control;
A study by computer modeling. IEEE Trans. Biomed. Eng., 41:
413,1994.
11 . Brindley, G. S.: Emptying the bladder by stimulating the sacral
ventral root. J. Physiol., 237: 15, 1973.
12 . Creasey, G. H.: Electrical stimulation of the sacral roots for
micturition after spinal cord injury. Urol. Clin. North Am., 20:
505, 1993.
13. Brindley, G. S.: The first 500 patients with sacral anterior root
stimulator implants: general description. Paraplegia, 32: 795,
1994.
14. Hohenfellner, M., Paick, J.-S., Trigo-Rocha, F., Schmidt, R. A.,
Kaula, N. F., Thtiroff, J. W. and Tanagho, E, A,: Site of deafferentation and electrode placement for bladder stimulation:
clinical implications. J. Urol., 147: 1665, 1992.
15. Schmidt, R. A., Bruschini, H. and Tanagho, E. A.: Sacral root
stimulation in controlled micturition (peripheral somatic neu­
rotomy and stimulated voiding). Invest. Urol., 17: 130, 1979.
16. Ishigooka, M., Hashimoto, T,} Sasagawa, I., Izumiya, K. and
17.
18.
19.
20.
21 .
22 .
23.
24.
25.
26.
27.
Nakada, T.: Modulation of the urethral pressure by highfrequency block stimulus in dogs. Eur. Urol., 25; 334, 1994.
Sweeney, J. D., Mortimer, J. T. and Bodner, D. R.: Acute animal
studies on electrically induced collision block of pudendal
nerve motor activity. Neurourol. Urodynam., 8 : 521, 1989.
Li, J.-S., Hassouna, M., Sawan, M., Duval, F. and Elhilali, M. M.:
Electrical stimulation induced sphincter fatigue during void­
ing. J. Urol., 148: 949, 1992.
Brindley, G. S. and Craggs, M. D.: A technique for anodally
blocking large nerve fibers through chronically implanted elec­
trodes. J. Neurol. Neurosurg. Psych., 43: 1083, 1980.
Fang, Z.-P. and Mortimer, J. T.: Selective activation of small
motor axons by quasitrapezoidal current pulses. IEEE Trans.
Biomed. Eng., 38: 168, 1991.
Koldewijn, E. L., Rijkhoff, N. J. M., van Kerrebroeck, P. E. V ,3
Debruyne, F. M. J. and Wijkstra, H.: Selective sacral root
stimulation for bladder control: acute experiments in an ani­
mal model. J. Urol., 151: 1674, 1994.
Rijkhoff, N. J. M.} Koldewijn, E. L., van Kerrebroeck, P. E. V.,
Debruyne, F. M. J. and Wijkstra, H.: Acute animal studies on
the use of an anodal block to reduce urethral resistance in
sacral root stimulation. IEEE Trans. Rehab. Eng., 2 : 92,1994.
Brindley, G. S., Polkey, C. E. and Rushton, D. N.: Sacral anterior
root stimulators for bladder control in paraplegia. Paraplegia,
20: 365, 1982.
Brindley, G. S.: An implant to empty the bladder or close the
urethra. J. Neurol. Neurosurg. Psych., 40: 358, 1977.
Rijkhoff, N. J. M., Koldewijn, E. L., van Kerrebroeck, P. E. V.,
Debruyne, F. M. J. and Wijkstra, H.: Selective activation of the
detrusor muscle by sacral root stimulation in a canine model.
Neurourol. Urodyn., 12 : 381, 1993.
van den Honert, C. and Mortimer, J. T.: A technique for collision
block of peripheral nerve: single stimulus analysis. IEEE
Trans. Biomed. Eng., 28: 373, 1981.
Schalow, G., Bersch, U., Gocking, K and Zach, G. A.: Detrusorsphincteric dyssynergia in paraplegics compared with the synergia in a braindead human by using the single-fibre action
potential recording method, J. Autonom. Nerv. Syst., 52: 151,
1995.