Computational Model of SNS (Sacral Nerve Stimulation) Induced Electrical Current Flow Using Tractography Imaging

Patients who have had successful temporary SNS and awaiting permanent SNS will be approached. An assessment will be made to determine whether the patient meets the inclusion/exclusion criteria. Patients will be given a consent form to sign.

The investigators will recruit from the outpatient clinic a total of ten patients for this study.

A physical examination will be performed as per standard practice. Patients will be asked to fill in questionnaires probing the severity of faecal incontinence symptoms. Included questionnaires are ICIQ-B4, Wexner's and St Mark's.

Incontinence score (see appendix). It will also evaluate any existing pain symptoms.

The patient will undergo an MRI scan (3 Tesla) of the pelvis using sequences including anatomical sequences and diffusion tensor imaging technique for construction of sacral nerve tractography prior to permanent SNS. This will be performed at Mount Vernon Hospital over an hour: first 30 minutes for anatomical sequences and the second 30 minutes for DTI sequencing.

A surgical procedure (implant of SNS) will be performed as per standard practice.

Briefly, an antibiotic will be given at induction of anesthesia prior to skin incision. All types of equipment for SNS implantation will be soaked in antibiotic solution (gentamicin 80mg in 500ml of normal saline or equivalent). Under general anesthesia without neuromuscular junction blockade and the patient in prone position, a permanent quadripolar lead (model 3889) will be inserted aiming to observe contraction of the anus and/or toe flexion, which are signs to confirm that the lead is nearby third sacral nerve root. The lead position will be confirmed by an x-ray and the lead will be connected to an implantable neurostimulator which will be placed under the skin but deep enough in the upper outer area of the buttock region. The procedure will be performed according to the manufacturer's manual as per our clinical standard.

After the implantation, patients will be given one generic starting program to start the therapy.

Three to 4 weeks after the procedure the patient will undergo a limited CT scan of the pelvis to visualise the position of the SNS lead. The scan will focus only on the sacrum, implanted lead, and rectum and will not be extended beyond this area. Imaging from this CT will be superimposed to the MRI imaging the patient had pre-operatively, and a computational simulation will be performed (see details below).

After the scan, a clinician will test stimulation settings systematically to find an optimal program. Briefly, as the lead as 4 electrodes, each electrode will be tested, followed by a combination of electrodes. The patient's subjective responses (type and site of sensation, if there is any pain/discomfort site and nature, a degree of pain using a Lickert scale,) will be recorded. The investigators will try and obtain motor response if possible, at the level when patients' anus contracts slightly if this is achievable without causing significant distress and pain to the patients. This is also a routine process and the clinician will note if there is any specific setting that causes adverse pain and identify settings that are comfortable and give stimulation sensation around the anus.

The computational model will be done in a computer laboratory and the record will be compared and evaluated for any clinical relevance. The same set of questionnaires used during the baseline evaluation will be repeated at follow-ups, as per routine care.

Inclusion criteria

• Patients who are psychologically stable and suitable for intervention and able to provide informed consent
• Patients with symptoms of faecal incontinence for solid or liquid stool
• Patients who have had successful outcome from temporary SNS
• Patients who are willing and competent to fill in questionnaires and undergo an extra CT and MRI scan during the study

Exclusion criteria

• Patients who are claustrophobic to undertake MRI scan
• Patients who had previous devices implanted that may be magnetically, electrically, mechanically activated or affected by MRI scan
• Pregnant patients
• Patients who are breastfeeding
• Patients who are psychologically unstable and unsuitable for intervention and unable to provide informed consent
• Patients who did not successfully pass the trial of SNS temporary SNS)