Optimal Stimulation Rates in Sacral Neuromodulation Therapy

Patients with refractory symptoms of urge-incontinence, urgency-frequency and
urinary retention without obstruction who do not respond to conservative
treatment like medication and/or physical therapy, can get symptom relief by
sacral nerve stimulation (SNS). It is postulated that the mechanism of
neuromodulation is based upon restoring the correct balance between excitatory
and inhibitory impulses from and to the pelvic organs at a sacral and
supra-sacral level via pulses through the afferent small myelinated Aδ and
unmyelinated C nerve fibers.
Stimulation is achieved using an electrode, which is implanted unilaterally at
the site of the S3 nerve root and connected to a stimulator implanted at the
buttock or abdomen. The implantable pulse generator (IPG) has a variety of
settings. The pulse rate (PR) can be set between 2.1 and 130 Hz and the pulse
width (PW) between 60 and 450 ms. The amplitude can be changed from 0.05 to
10,55 V. Patients who are implanted with a neuromodulation system receive an
external programmer, which is used to change the amplitude of the stimulation
current.
It is customary in most clinics where SNS is applied to set the PR of the IPG
at 10 Hz and the PW at 210 ms. These parameters have been advised in the
beginning of SNS therapy development. The choice of these parameters is based
on animal experiments. Studies in animal models suggests that PR of more than
50 Hz might be detrimental to the stimulated nerve because of the occurrence of
early axonal degeneration.

To determine whether rate change is feasible and has a positive influence on
the effect of SNS on the symptoms measured with voiding diaries.
To investigate whether there are differences between optimal stimulation
settings between patients with SNS for urge symptoms or for retention symptoms.

study with single subject design

Change the pulse rate of the implanted neurostimulator in patients with
implanted neuromodulation system. Frequencies will be changed to 5.2Hz, 10Hz,
25Hz and 40Hz, each frequency will be used during one week.

The burden for these patients is minimal. Although it will cost the patient
some time to fill out the questionnaire and diaries and travel to and from
Maastricht. Furthermore, there is a possibility that the patient will
experience some discomfort because it is possible that certain frequencies will
have a decreased effectiveness.
The frequencies that has been chosen in this study are well within the safety
limits that are determined by animal experiments. Therefore we do not expect
any adverse effects from the stimulation on the patient*s sacral nerves.
However, there are clinical reports of a small amount of patients who find
stimulation with a frequency below 10Hz uncomfortable. If patients have
discomfort of the stimulation parameter settings during the study they are
advised to contact their urologist and to turn their IPG off.