neuromodulation has been established as an effective treatment for patients with overactive bladder ( oab ) where first - line therapies , such as the muscarinic antagonists , do not provide sufficient efficacy.1 currently , the two most common approaches to neuromodulation are sacral spinal nerve ( sn ) stimulation using the interstim device and percutaneous tibial nerve ( tn ) stimulation using the urgent pc. sacral neuromodulation is included in the oab treatment guidelines of american urological association ( aua ) , european association of urology , and international continence society.2,3 percutaneous tn stimulation is listed as optional therapy in the aua treatment guidelines.2 using a rat model in which reflex bladder rhythmic contraction ( brc ) is induced by filling with saline , we have characterized the relationship between inhibition of reflex bladder contraction and the frequency and intensity of sn stimulation.4 we have also compared the effects of stimulation of different bladder nerves , including spinal , tibial , and genital.5 dorsal genital nerve stimulation has not been approved clinically for oab treatment and is not included in this study .
we now report on a further characterization of the response of the isovolumetric rat bladder to tn and sn stimulation , including a comparison of bilateral and unilateral stimulation , evaluation of desensitization to continuous and repeated stimulation , and the effects of simultaneous stimulation of spinal and tns .
results in the rat brc model appear to correlate with data from other animal models and with clinical observations .
the characterization of the bladder effects of tn stimulation , and comparison of these effects to those of sn stimulation , as well as the interaction of stimulation at these two sites , could identify mechanistic differences and provide information useful in the use of nerve stimulation for the treatment of oab .
female sprague  dawley rats ( 200300  g ) were anesthetized with urethane ( i.p . ,
1.2  g / kg , 200  mg / ml in saline , sigma 
anesthetized rats were maintained at 37c with a heating pad and were euthanized by co2 asphyxia upon completion of experiments .
the experimental protocols were approved by the institutional animal care and use committee of medtronic and non - clinical research board of medtronic ( minneapolis , mn ) . to deliver unipolar tn stimulation , a bared portion of a teflon - coated , 40-guage , stainless steel wire ( cooner wire co. , chatsworth , ca ) was placed bilaterally under each tn , which was exposed on the medial side of both hindlimbs above the ankle ( 72 rats , fig .
electrodes were also placed under both left tn and left l6 sn ipsilaterally ( 21 rats ) , and right tn and left sn contralaterally ( 26 rats ) .
the wire electrode(s ) were positioned , secured with silicone adhesive , and connected to a grass s88 stimulator ( grass medical instruments , warwick , ri ) , through stimulus isolation unit(s ) ( siu - bi , grass medical instruments ) .
experimental model for tibial ( tn ) and spinal nerve ( sn ) stimulation ( s , stim )
unilateral stimulation was delivered on the left ( l ) side of the tn or sn and bilateral stimulation was applied simultaneously on both left and right ( r ) side of the nerve .
stimulation ( 10  hz , pulse width 0.1  msec ) intensities varied from 0.8 times motor threshold intensity ( tmot ) up to six times tmot .
d , e : histogram of motor threshold ( tmot ) distribution to tn and sn stimulation . in each rat , the threshold current ( tmot ) for biphasic pulses ( pulse width  =  0.1  msec , 10  hz for 26  sec ) stimulation was defined as the lowest current required to evoke the first , barely observable , muscle contraction ( hind - toe twitches and/or pelvic floor muscle contraction , 4 , 5 ) . for bilateral stimulation ,
the tmot was measured on each side separately , to allow differentiation of muscle responses to left or right nerve stimulation . a cannula ( pe50 )
was inserted into the bladder via the urethra , and secured with a suture tie for intravesical pressure recording and saline infusion .
the urethral cannula was connected via a t - type connector to a pressure transducer of the data acquisition system ( ad instruments mlt0380d , colorado springs , co ) and the signal of intravesical pressure was put through a dc amplifier ( ad instruments , ml119 ) .
the other end of the t connector was attached to a syringe pump . to induce brc ,
saline was infused into the bladder via the syringe pump at a rate of 50  l / min to induce a micturition reflex ( here defined as bladder contraction of a magnitude > 10  mmhg ) .
the infusion rate was then lowered to 10  l / min and continued until three to five consecutive contractions were established . at this time
each trial of recording lasted for up to 50  min including 15  min control , 515  min nerve stimulation , and 20  min post - stimulation .
two trials of the testing were performed with a random stimulation parameter in some rats .
the bladder was emptied after finishing the first trial and brc was re - established by saline infusion .
electrical stimulation at a fixed frequency of 10  hz , which has been shown to be optimal for inhibition of bladder contractions by both low and high intensity stimulation,4,5 was tested unilaterally or bilaterally for 15  min ( fig . 1a - a , b and b - a , b ) .
the repeated 5  min stimulations were also assessed from variable length of 520  min between stimulations to allow recovery of the bladder inhibitory responses to tn stimulation from desensitization ( fig .
finally , the combination of unilateral stimulation of sn and tn was examined using a 5-min stimulation protocol .
the stimulation period of 5  min was chosen based on the results of a sustained 15-min tn stimulation which showed that bladder contractions were shut down only during the first 5  min of stimulation ( see results section ) . with unilateral sn stimulation on the left side ( 0.82 times tmot ) ,
the stimulation of either the ipsilateral and contralateral tn ( 23 times tmot ) was tested for a total of 10 possible combinations ( fig .
the stimulation intensity ranges were chosen based on the intensity dependent bladder inhibitory response curve to stimulations on either sn or tn alone . either lower current intensities below which no response occurs or can be measured , or higher intensities above those producing a maximum response , were not tested .
sn or tn stimulation did not reduce the amplitude of bladder contractions,4,5 therefore only effects on frequency / interval of brc were studied .
data were calculated in 5  min bins , each having three control periods , two periods during stimulation , and four periods after stimulation .
all data were normalized to the mean response during the 5  min immediately prior to stimulation .
time course for the brc response to stimulation was analyzed using repeated measures anova ( prism 5 , graphpadsoftware , inc .
bonferroni post - test was used to determine the statistical significance between individual time points .
sn or tn stimulation did not reduce the amplitude of bladder contractions,4,5 therefore only effects on frequency / interval of brc were studied .
data were calculated in 5  min bins , each having three control periods , two periods during stimulation , and four periods after stimulation .
all data were normalized to the mean response during the 5  min immediately prior to stimulation .
time course for the brc response to stimulation was analyzed using repeated measures anova ( prism 5 , graphpadsoftware , inc .
bonferroni post - test was used to determine the statistical significance between individual time points .
the threshold current ( tmot ) at which first visible motor contraction occurred to tn stimulation was 0.17    0.01  ma ( n  =  253 ; range : 0.010.6  ma ; 95% confidence interval [ ci ] : 0.150.19  ma , fig .
tmot for sn stimulation was 0.17    0.01  ma ( n  =  121 ; range : 0.010.7  ma ; 95% ci : 0.140.20  ma , fig .
the tmot is distributed approximately normally , irrespective of tn and sn or left side and right side of the nerve roots .
figure 2 shows typical results of bilateral , electrical stimulation of the tn or sn on brc ( 3 times tmot , 10  hz , 15  min ) .
there was no functional delay between application of the stimulus and effect on the bladder , that is , the next contraction expected to occur was abolished when the current was applied
. however , the contractions re - appeared before tn stimulation was terminated ( desensitization ) .
in contrast , inhibition of brc using the same parameters ( 3 times tmot at 10  hz ) of sn stimulation was sustained for 2  min post - stimulation .
typical experimental records showing the bladder rhythmic contraction ( mmhg ) to sustained 15-min bilateral tibial nerve stimulation ( a ) and spinal nerve ( b ) at three times motor threshold intensity ( 10  hz , pulse width 0.1  msec ) .
if stimulation on the tn was applied continuously for 15  min , the frequency of spontaneous bladder contractions returned to pre - stimulation values before the end of the stimulation period ( fig .
significant inhibition to tn stimulation was produced by the first 5  min stimulation at intensities equal or greater than three times tmot ( vs. without stimulation , n  =  29 , p  <  0.05 , unpaired student 's t - test ) , and by the second or third 5  min stimulation at intensities five and six times tmot ( vs. same time control without stimulation , n  =  29 , p  <  0.05 , unpaired student 's t - test , fig .
3b ) . the first 5-min bilateral tn stimulation at 10  hz , three times motor threshold decreased the frequency of contractions to 69.49    8% of control ( n  =  6 ) .
effects of tibial nerve ( a , b ) and spinal nerve stimulation ( c , d : 10  hz , pulse width 0.1  msec ) on the frequency of the bladder rhythmic contraction .
a , c : time course response of frequency of the bladder rhythmic contraction to bilateral tibial nerve ( a ) and spinal nerve ( c ) stimulation at motor threshold ( tmot ) and threefold of tmot ( 3 times tmot ) .
shaded areas are responses during electrical stimulation . + , p  <  0.05 , versus control without stimulation ; repeated measures anova , bonferroni post - test .
b , d : intensity dependent effects of unilateral ( uni ) and bilateral ( bi ) tibial nerve ( b ) and spinal nerve ( d ) stimulations on frequency of bladder contractions during electrical stimulation .
x - axis denoted increasing current intensity relative to multiples of motor threshold ( tmot ) stimulation .
the mean contraction frequency during stimulation is expressed as a percentage of the control response prior to stimulation ( % control ) . * , p  <  0.05 , versus values without stimulation , unpaired student 's t - test ; # , p  <  0.05 , first 5-min stimulation versus second and third 5-min stimulation , * , p  <  0.05 , unilateral versus bilateral , unpaired student 's t - test . the number of animals is indicated in each symbol .
the decrease in bladder contraction frequency produced by the first 5-min stimulation at three and four times tmot was significantly greater than that produced by the second or third 5  min stimulation ( p  <  0.05 , paired student 's t - test ) . these data again show decreased response to sustained tn stimulation .
the inhibitory response was equal , regardless of whether unilateral or bilateral stimulation was applied ( fig .
3b ) . figure 3c and d shows the effect of sn stimulation at 10  hz on brc at different stimulation intensities .
the inhibition of the contraction frequency to sn stimulation was greater than that of tn stimulation .
the lowest intensity of sn stimulation that produced a statistically significant bladder inhibition ( p  <  0.05 , vs. without stimulation , n  =  29 , unpaired student 's t - test ) was 0.8 times tmot for bilateral stimulation and two times tmot for unilateral stimulation , to 67.46    15% ( n  =  9 ) , and 32.52    16% ( n  =  6 ) of controls , respectively . the first 5-min bilateral sn stimulation at three times motor threshold decreased the frequency of contractions to 9.42    9% of control ( n  =  7 ) . the inhibitory effect to bilateral stimulation at tmot is significantly greater than that produced by unilateral sn stimulation ( p  <  0.05 , unpaired student 's t - test , fig .
a comparison of the bladder inhibitory response to tn and sn stimulation shows that with either of continuous ( 15  min ) or repeated 5  min stimulation , there is a clear difference between the responses to stimulation at the two sites . with only a 5-min interval between two 5  min stimulations , the second sn stimulation ( tmot intensity , bilateral ) produced a response ( 38.33    22% control ) equal to the first ( 46.56    27% control , n  =  4 , p  >  0.05 , paired student 's t - test ) .
in contrast , the second tibial stimulation ( three times tmot intensity ) produced no inhibition ( 105.98    12% control , n  =  16 , combined data from 9 bilateral with 7 unilateral stimulations ) while the first stimulation reduced bladder contraction to 65.44    11% control ( p  <  0.05 , paired student 's t - test , fig .
effects of bladder inhibitory response to repeated 5-min stimulations ( 10  hz , pulse width 0.1  msec ) .
a , b : typical experimental records showing the bladder rhythmic contraction ( mmhg ) to two 5  min tibial nerve ( tn ) stimulations at three times motor threshold intensity ( 10  hz , pulse width 0.1  msec ) .
the contraction traces in ( a ) illustrate that with an interval of 10  min or less between tn stimulations , the response to a second stimulation was completely blunted ; with 15  min between stimulations , the first and second stimulations were equieffective ( b ) .
c : inhibition of the bladder rhythmic contraction to first and second tn stimulation at three times motor threshold intensity and spinal nerve ( sn ) at motor threshold intensity with increasing interval between stimulations ( x - axis ) .
the number of animals is indicated in each symbol . * , p  <  0.05 , first 5-min stimulation versus second 5-min stimulation , paired student 's t - test .
the effect of combined stimulation of spinal and tns is shown in figure 5 . with unilateral sn stimulation ,
the stimulation of either the ipsilateral or contralateral tn was tested . with 10 possible combinations tested , there was no significant additive effect of stimulation at the two sites . only in the case of low intensity spinal ( tmot ) and high intensity tibial ( 3 times tmot )
stimulation was there any additive effect , but the difference between spinal  +  tibial stimulation and tibial stimulation alone was not statistically significant .
intensity - dependent effects of combination of spinal nerve ( sn ) and tibial nerve ( tn ) stimulation on the frequency of bladder contractions .
unilateral tibial nerve stimulation was tested at intensity of two times motor threshold ( tmot , a ) and three times tmot ( b ) .
the responses are represented as a percentage of pretreatment values ( % control ) , where the baseline response before stimulation is defined as 100% .
the rat brc model has been used for prediction of the effect of nerve stimulation on bladder function.4,5 since the urethra is ligated , actual voiding is not measured .
furthermore normal rats are used , so bladder muscle and its innervations are normal . nevertheless ,
considering the limitation of this animal model , the ability of nerve stimulation to inhibit the frequency of reflex bladder contractions is in good agreement with the ability to normalize micturition parameters in other animal models , such as cystometry in bladder irritated rats6 and rhythmic contraction and cystometry in anesthetized cats,7 as well as with clinical observations in oab patients.8 we have reported that both tn and sn stimulation was effective for inhibition of bladder reflex contractions.4,5 in those experiments , a single wire electrode was positioned under both sides of the nerve and current intensities were not controlled separately , and would depend on the functional impedance at each implantation site . in the present study , we specifically tested and controlled the delivery to both sides independently allowing a more complete characterization of stimulation thresholds for unilateral and bilateral stimulation of the sn and tn .
even though stimulation pulses were delivered differently in these three studies , it was consistently observed that the brc had a different sensitivity to stimulation of the tn and sn and that the response had a different time course .
sn stimulation produces bladder inhibition with a longer duration and greater efficacy than tn stimulation .
the rat sn is composed of nerve fibers emerging from the tn and along pelvic nerve , and other somatic nerve bundles ; the tn originates from the sns l4l6.9,10 it is possible that tn neuromodulation activates some , but not all of the fibers involved in sn neuromodulation .
tn stimulation triggers only toe twitches but sn stimulation evokes additional pelvic floor contraction and urethral sphincter contractions , which may produce further reflex detrusor inhibition , although effects on the urethra are unlikely since the urethra was expanded with a catheter in this preparation.5 in addition , sn has higher fiber density than tn,10 which may be responsible for the lower stimulation currents required for activation of a sufficient number of fibers for bladder control and may also contribute to the higher effectiveness of sn mediated bladder neuromodulation . high intensity stimulation of the tn or sn induces a strong skeletal muscle contraction .
previously we found that the marked inhibition of brc frequency by sn stimulation occurs in rats pretreated with pancuronium to block skeletal muscle contractions.4 therefore , such  unwanted  contraction does not seem to contribute to the bladder inhibition observed in response to sn or tn stimulation .
there were some differences in the relationship between stimulation intensity and bladder inhibition ; in particular , tn stimulation was effective over a narrow range ( 34 times tmot ) and the duration of the inhibition was less than that from sn stimulation . these results may be a consequence of the more rapid diminution of the response to tn vis -  - vis sn stimulation which we observed in the current study ( see fig .
3 ) . compared with stronger bladder inhibition to bilateral vis -  - vis unilateral sn stimulation , bilateral stimulation of the tn failed to produce more effective attenuation of bladder contractions .
bilateral symmetry of nerve responsiveness to electrical stimulation was demonstrated by equal motor threshold distribution between the left and right nerve roots . though there is a difference in the results of bilateral stimulation on the sn and tn
, the lack of a clear additive effect between spinal and tn stimulation would suggest that stimulation at either of these two sites inhibits the bladder via a similar effect on the micturition reflex arc .
however , it is still possible that tn and sn stimulation target different points on the reflex arc and that the impulses from the different stimulation sites may travel through different neuronal pathways before reaching the brainstem .
both tn stimulation and low intensity sn stimulations do not alter the brc amplitude , suggesting their common mechanism of action through afferent limb of the micturition reflex arc.5,11 higher intensities of sn stimulation may directly depress the contractility of detrusor smooth muscle through the efferent limb.4 both tibial and spinal neuromodulation have been found to be effective in relieving symptoms in patients with oab .
no side - by - side comparison of the two therapies in patients has been reported .
it is difficult to compare reported results from individual patients since spinal stimulation is applied continuously12 and tibial stimulation is used intermittently ( one 30-min stimulation once per week13 ) .
tibial stimulation is applied intermittently since it is done trans - cutaneously as an office procedure rather than by a permanently implanted device .
it is possible however , that stimulation at this site would not be effective if applied continuously , due to the potential for desensitization . a significant additive effect between spinal and tn stimulation was not observed ( fig .
however , there was a suggestion that the higher intensity tibial stimulation ( 3 times tmot ) could be additive with a low intensity spinal stimulation ( fig .
further experiments could determine whether this interaction is a consistent effect and if it could be enhanced by using different stimulation parameters .
based on this rat model , it would appear that neuromodulation of bladder activity by sn stimulation is preferable to tn stimulation .
the data generated in this study does not suggest that stimulation at both sites would offer a therapeutic advantage over spinal stimulation alone .
 aimsto determine time course of the bladder inhibitory response to unilateral or bilateral stimulation of the tibial nerve ( tn ) and spinal nerve ( sn ) as well as the interaction of stimulation at these two sites.methodsin anesthetized female rats , a wire electrode was placed under either one or both of the tn or l6 sn . 
 a cannula was placed into the bladder via the urethra . 
 saline infusion induced bladder rhythmic contraction ( brc).resultscompared to sn neuromodulation , tn neuromodulation is less efficacious . 
 the first 5-min stimulation at three times motor threshold on the sn and tn decreased the brc frequency to 9% and 69% of controls , respectively . 
 in contrast to sn stimulation , bilateral tn neuromodulation is not more effective than unilateral and sustained tn stimulation results in an apparent desensitization of the bladder response . 
 if a 15-min tn stimulation was applied , brcs were shutdown only during the first 5  min of stimulation . if a 5-min stimulation , using sufficient current to abolish brc , is repeated , at least 20  min between stimulations was required in order for the responses to the first and second stimulations to be equivalent . 
 finally , stimulation of the tn combined with sn never produced a significantly greater effect than tn or sn stimulation alone.conclusionsbased on the current experiments , it would appear that sn neuromodulation of bladder activity is preferable to tn stimulation and there is no evidence to suggest that stimulation at both sites would offer a therapeutic advantage over spinal stimulation alone . 
 neurourol . urodynam . 34:9297 , 2015 . 
  2013 the authors . 
 neurourology & urodynamics published by wiley periodicals , inc . 