Note: Descriptions are shown in the official language in which they were submitted.
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COMBINED STIMULATION OF VENTRAL AND DORSAL SACRAL
ROOTS FOR CONTROL OF BLADDER FUNCTION
CROSS REFERENCE TO RELATED APPLICATIONS
This is a utility patent application taking priority from provisional patent
application 60/100,524, filed September 16, 1998, which is incorporated
herein by reference.
to
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates generally to selective nerve stimulation for bladder
control, and, in particular, to a technique by which micturition can be
achieved in
spinal cord injured patients without dorsal root section.
2. Description of the Prior Art
Retention of urine, leading to complications such as urinary tract infection
and
urinary calculi, remains a major factor leading to morbidity in spinal cord
injured
patients. In high cord injury, with upper motor neuron damage, the lower nerve
pathways to the bladder are intact. The aim of micturition control in these
individuals is to enable them to contract the bladder musculature without
activating
structures in the urethra that may impede urine flow. The procedure should
leave
an acceptable post-void residual volume within the bladder and should also be
able
to prevent overflow incontinence.
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The difference in the size of the nerve fibers to the bladder or bowel and the
urethral or anal sphincter allows the development of techniques to selectively
activate the nerves to the bladder and bowel without the activation of the
sphincters.
Previously; electrical stimulation has been applied to control the bladder and
bowel: The previous attempts have focused on three techniques: direct
stimulation
of the detrusor muscle, activation of the detrusor by stimulation of the conus
medullaris, and activation of the detrusor by sacral root or nerve stimulation
with
extensive dorsal rhizotomy. All three of these methods suffer from the same
problem. They all cause contraction of the bladder to expel urine concurrently
with
contraction of the external urethral sphincter blocking urine flow. The
rhizotomy
technique also results in the loss of erection for the male. It would be
advantageous
if contraction of the sphincter could be selectively blocked.
Techniques available for blocking nerve impulses are discussed, for example,
in "A Technique for Collision Block of Peripheral Nerve: Single Stimulation
Analysis",
van den Honert and Mortimer, IEEE Transactions on Biomedical Engineering,
Volume
BME-28, No. 5, May 1981, pages 373-378, and "Generation of Unidirectionally
Propagated Action Potentials in a Peripheral Nerve by Brief Stimuli", van den
Honert
and Mortimer, Science, Volume 206, December 1979, pages 1311-1312. With the
van den Honert and Mortimer techniques, a nerve impulse or action potential is
generated which travels toward the brain. When the artificially generated
nerve
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impulse meets a motor impulse travelling from the brain, the motor impulse is
collision blocked. That is, the artificially generated action potential
cancels the
motor action potential. If one were to apply the van den Honert and Mortimer
techniques, it could be used to cause concurrent relaxation of both the
bladder
contracting muscles and the urethral sphincter.
Sacral nerve stimulation for electrical control of bladder function has been
attempted for many years; however, virtually all attempts have been plagued by
problems associated with co-activation ~of contractile structures in the
urethra that
i0 impede urine flow. One such attempt is described in U.S. Patent No.
4,607,639,
which issued to Tanagho, et al. This patent describes a technique in which the
sacral nerves are separated to isolate the ventral and dorsal roots thereof,
and the
inferior somatic nerve Sl is sectioned to isolate the external sphincter on
one side.
The dorsal root of S2 is then sectioned unilaterally to isolate the sensory
function
25 thereof. An electrode is positioned on the S3 sacral nerve to stimulate the
detrusor
muscles of the bladder. However, it is suggested that sphincter response may
be
reflexly produced using this technique, and mentions the necessity for the
rhizotomy
of the dorsal roots.
20 U.S. Patent No. 5, 199,430 teaches a system for selectively arresting
propagation of action potentials in large diameter fibers without arresting
propagation in small diameter nerve fibers using a quasitrapezoidal waveform.
This
waveform, which is disclosed in U.S. Patent No. 4,608,985, and its stimulation
scheme made it possible to install electrodes on the sacral roots that could
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differentially activate the small fibers to the detrusor and rectum without
activating
the large fibers to the sphincter, thus allowing bladder and bowel activation
without
the sphincter tone being raised. However, this procedure often involved dorsal
rhizotomy to minimize any reflexogenic response.
The present invention contemplates a new technique for bladder function
control in which a dorsal rhizotomy is unnecessary.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a system and
method for inducing micturition in spinal cord injured patients.
It is a further object of the present invention to provide a method of
controlling bladder draining by stimulating nerve cuff electrodes implanted on
the
sacral motor roots and also the sacral sensory roots.
It is a still further object of the present invention to provide a method of
2o efficiently voiding the bladder of a patient without performing a dorsal
rhizotomy.
These and other objects are accomplished in the present invention by a
method and system for selectively controlling activation of a patient's
bladder by
applying trains of quasitrapezoidal pulses with appropriate current amplitudes
on
both the ventral and dorsal sacral roots to subdue urethra) reflexes and
enhance
voiding.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates the placement of electrodes for controlling
the
bladder in the present invention;
FIG. 2 is a graphic representation showing average voided volume from
different combinations of stimulus;
FIG. 3 is a graphic representation showing average voided volume when
combined dorsal and ventral root stimulation is used; and
FIG. 4 illustrates a stimulation scheme which may be used for combined
stimulation of the dorsal and ventral roots in the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is shown an illustrative embodiment of the
present invention within the environment of the human body. Kidney 10 is
connected to the bladder 12 via the ureter 14, which carries away urine from
kidney
iQ to bladder 12. Urine is expelled from the body through bladder neck 16 and
urethra 18 and out from urethral sphincter 20. Bladder 12 and sphincter 20
function
is controlled by action potentials traveling from spinal cord 19 primarily,
but not
limited to, on a pair of sacral roots 21 which consists of a segment of
ventral sacral
roots 22 and a segment of dorsal sacral roots 23. Dorsal roots 23 are
primarily
s
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sensory (afferent) to transmit sensation to spinal cord 19, while ventral
roots 22
primarily transmit motor pulses (efferent) from spinal cord 19 to bladder i2
and
sphincter 20. Although illustrated as being separated, the dorsal and ventral
roots
for each nerve are, in fact, normally joined together and their fibers mixed
to
progress as a single trunk.
Ventral roots 22 include nerve bundles 22a which include larger diameter
nerve fibers and nerve bundles 22b which include smaller diameter fibers.
Larger
fibers 22a connect between spinal cord 19 and sphincter 20, while smaller
fibers 22b
connect between spinal cord i9 and bladder 12. Action potentials flowing along
larger fibers 22a cause sphincter 20 to contract, blocking the outlet from
urethra 18.
When the bladder is to be emptied, the flow of action potentials through Fbers
22a
is stopped, allowing sphincter 20 to relax.
Smaller fibers 22b usually carry no action potentials until the person desires
to evacuate the bladder; action potentials are then sent along fibers 22b
concurrently with the stopping of action potentials along fibers 22a, causing
sphincter 20 to relax and allowing bladder neck 16 to open concurrently with
bladder
12 muscles contracting, thus expelling urine.
Spinal cord injuries and various other medical conditions can cause a loss of
control of the bladder function. To reinstitute this control, a cuff electrode
30 can be
mounted surrounding sacral ventral root 22. Cuff electrode 30, which is
preferably a
self curling spiral electrode that is biased to curl around the selected root
and is
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described in U.S. Patent No. 4,602,624, is configured to accommodate nerves of
varying diameters and can electrically excite action potentials on smaller
fibers 22b
while blocking naturally occurring and electrically activated action
potentials from
travelling downstream on larger fibers 22a. An example of this procedure is
described in detail in U.S. Patent No. 5, 199,430, which issued in April 6,
1993, and
is hereby incorporated by reference in its entirety.
The present invention also includes an additional cuff electrode 32 which is
implanted on a dorsal sacral root 23. Application of trains of
quasitrapezoidal
pulses, which are described and taught in U.S. Patent No. 4,608,985, which
patent is
incorporated herein by reference, that have appropriate current amplitudes and
are
applied concurrently to electrodes 30 and 32 to stimulate the dorsal and
ventral
roots can result in the voiding of bladder contents without increasing
sphincter
pressures. A controller 34 is electrically coupled to electrodes 30 and 32 to
provide
the necessary signals for this desired bladder control.
Several experiments were conducted to confirm that combined dorsal and
ventral root stimulation provides an effective low pressure bladder evacuation
without requiring dorsal rhizotomy.
EXPERIMENT 1
Combined sensory and motor stimulus was applied to an animal under 2.5%
halothane inhalation anesthesia. An efferent motor stimulus was applied by a
spiral
nerve cuff electrode implanted on S2 motor roots. Either a quasitrapezoidal,
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balanced biphasic, 20 Hz pulse, with current amplitude set at a volume
determined
to selectively block sphincter activity, or a conventional rectangular supra-
threshold
stimulus was used. The efferent sensory stimulus was applied to the S2
dermatome
with surface electrodes using a 20 Hz balanced biphasic rectangular pulse at
10 to
15 ma. The surface stimulus was varied to be continuous or intermittent (1
second
on/1 second off). The bladder was flied before each trial run with 60 ml of
sterile
saline.
FIG. 2 shows the average volume of fluid voided during a 10 second pulse
to train for each combination of stimulus trains. The results show that a
combination
of selective motor root activation by a quasitrapezoidal pulse train and
intermittent
surface stimulation to the S2 determatome enhanced bladder emptying.
EXPERIMENT 2
Combined sensory and motor stimulus was applied to an animal under 2.5%
halothane inhalation anesthesia. The motor stimulus was applied by a spiral
nerve
cuff electrode implanted on the S2 motor roots. A quasitrapezoidal, balanced
biphasic, 20 Hz pulse, with current amplitude set at a value determined to
selectively
block sphincter activity, was used. The sensory stimulus was applied to S1,
S2, or
S3 dermatome with surface electrodes using a 20 Hz balanced biphasic
rectangular
pulse at 10 to 15 ma. The intermittent surface stimulation was varied to be
either 1
second on/1 second off or 0.5 seconds on/0.5 seconds off. The tests were
randomized for stimulus combinations. The test results show that bladder
emptying
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was enhanced only when the combined sensory stimulus was applied to the S2
dermatome. The i second on/1 second off intermittent pattern was more
effective
than the O.S seconds on/0.5 seconds off pattern.
EXPERIMENT 3
Combined stimulation of the dorsal and ventral sacral roots was applied to an
animal by implanting spiral nerve cuff electrodes on the sacral ventral motor
roots at
S2 and S3, while leaving the dorsal roots intact and implanting a spiral nerve
cuff
to electrode on one dorsal S2 root. The ventral sacral roots were stimulated
with
quasitrapezoidal pulse trains at 20 Hz with current amplitudes sufficient to
selectively
activate the bladder or with 20 Hz conventional rectangular pulses. At the
same
time, a 20 Hz intermittent stimulus, 1 second on/ 1 second off was applied to
the S2
dorsal root.
Table 1 shows the average results from three trials for each stimulus pattern.
For 10 seconds of motor stimulation, an average increase of 66% in flow rate
was
observed during combined motor and sensory stimulation, while FIG. 3 shows the
average volumes voided.
TABLE 1
Pulse Type Pv (Qmax) Pu (Qmax) Pv max Pu max
Qmax V(ml)
20 Hz R 2.3 6.0 50.0 40.0 117.5 160.0
20 Hz Q 2.4 7.0 78.8 57.5 111.3 99.4
Aff + 20 4.0 27.0 59.2 51.7 85.0 80.0
Hz Q
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Qmax = Average maximum flow rate in ml/sec
V(ml) = Average voided volume in ml
Pv (Qmax) = Maximum bladder pressure in cm of water at maximum flow
Pu (Qmax) = Average sphincter pressure in cm of water at maximum flow
Pv max = Maximum bladder pressure in cm of water
Pu max = Maximum sphincter pressure in cm of water
FIG. 4 illustrates the stimulus pattern for the combined dorsal and ventral
root stimulation of the present invention. An intermittent pulse train 100 is
applied
to dorsal root 23 via cuff electrode 32, while a continuous pulse train 102 is
simultaneously applied to ventral root 22 via cuff electrode 30. In this
manner, low
pressure bladder activation can be achieved by modulating the reflexes
associated
with the neural system for micturition control and eliminate the need for
dorsal
rhizotomies.
Pulse train 100 preferably consists of poises delivered at a frequency of 10
to
35 Hz, with each pulse having a nominal amplitude of less than 1 ma and a
pulse
2o duration of 10 to 100 Irsec. Pulse train 100 is generated intermittently,
with a
pattern of 0.25 to 1 second on/ 0.25 to 1 second off. Pulse train 102
preferably
consists of a continuous series of quasitrapezoidal pulses of 350 to 500 psec
duration and a nominal amplitude of 1 ma delivered at a frequency of 15 to 30
Hz.
While the invention has been shown and described in terms of several
preferred embodiments, it will be understood that this invention is not
limited to
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these particular embodiments and that many changes and modifications may be
made without departing from the true spirit and scope of the invention as
defined in
the appended claims.
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