"Neuromonitoring during lesser pelvis surgery is still uncharted territory"

Interview with Prof Klaus-Peter Hoffmann, Head of Medical Engineering & Neuroprosthetics, Fraunhofer Institute for Biomedical Engineering IBMT in St. Ingbert, Germany

The bowel is surrounded by a dense plexus of nervous tissue that presents problems for surgeons. On the one hand, it is difficult to distinguish from the surrounding tissue; while on the other hand, sometimes portions of it need to be removed when parts of the colon are removed. Yet injuring these nerves can result in permanent damage, since they control bladder function, anal sphincter muscles and sexual function.


Photo: Klaus-Peter Hoffmann

Prof. Klaus-Peter Hoffmann; ©Fraunhofer IBMT

Intraoperative neuromonitoring could prevent this by warning surgeons before they damage nerves. In the interview with MEDICA.de, Prof Klaus-Peter Hoffmann explains how this works in the lesser pelvis and what problems the sacrum causes in this.

Prof Hoffmann, what challenges do surgeons encounter during lesser pelvis surgery?

Klaus-Peter Hoffmann: One of our project partners, Prof Werner Kneist from the Mainz University Medical Center Department of General, Visceral and Transplantation Surgery always states that the nerve tissue in this area "lines the pelvic floor like wallpaper on the inside and is simultaneously directly located near pelvic organs, such as the rectum for example." It is essential for physicians performing these types of surgeries to detect potential damages to the autonomic nervous system early. Of course, surgeons want to protect the nerves in general during surgery; they require a quality assurance tool and want to be warned ahead of time before damages occur.

Intraoperative neuromonitoring can issue such a warning. What is this actually?

Hoffmann: Neuromonitoring is the continuous monitoring of nerve function. In herniated disc surgeries for example, the sensory and motor tracts of the spinal cord, that is to say the afferent nerve fibers after peripheral and the efferent nerve fibers after central stimulation, are being monitored in terms of their excitability. We want to transfer this idea to surgery in the lesser pelvis. We stimulate the sacral nerve roots and their extensions. Our vision is an assistance system that warns the surgeon with a signal before he is about to damage nervous tissue.

What is so special about sacral nerve roots is that neuromodulation can excite an autonomic plexus. The complex part about this is that surgeons can damage portions of the plexus, while maintaining function. However, they might only minimally damage something and the function is disturbed. This is difficult to recognize, which is also why neuromonitoring during lesser pelvis is still uncharted territory.
Photo:Surgery, an electrode is embeddes in tissue

Continous intraoperative stimulation of N. pelvici splanchnici during mesorectal dissection at a patient with rectal cancer. A tripolar electrode that has been developed during the IKONA project is used; ©University Hospital Mainz/ Prof Werner Kneist

Our first project titled IKONA (Continuous Intraoperative Neuromonitoring as a Microtechnological Navigation Instrument) was meant to demonstrate that intraoperative neuromonitoring is feasible at this juncture. The continued development of a minimally invasive procedure with the autoPIN project (Assistance System for the Stimulation of Autonomous Pelvic Nerves with the Aim of Intraoperative Neuromonitoring in Laparoscopy) is the next, fascinating step in this development.

How should we envision the use of this system during surgery?

Hoffmann: At the beginning of the surgery, the electrodes are positioned for stimulation. A catheter is being inserted into the bladder and the bladder is filled with liquid. A pin electrode is placed in the M. sphincter ani internus, the internal anal sphincter. Later it is meant to be replaced by an electrode that is developed especially for this application. The signals can be subsequently measured: when the sacral nerve roots are stimulated, the internal bladder pressure is increased and the sphincter is increasingly activated.

After preparation, the surgery is performed in the usual way. The surgeon is able to see a signal analysis on the monitor. The system also analyzes the signal, so the surgeon does not need to look at the monitor the entire time and concentrate on his work instead. The goal is to get an acoustic and optic warning should the surgeon mechanically and thermally affect the nervous tissue and causing a risk of injury. This can alter the values measured in the bladder and the sphincter.
Photo: The inside of the body via endoscope

Laparoscopic removal of the rectum because of a carcinoma. Smallest nerves can be seen magnified at the pelvic wall. Neurostimulation to monitor pelvic nerves is especially suited to perform gentle, minimal-invasive surgery; ©University Medicine Mainz/ Prof Werner Kneist

You have developed specific electrodes for stimulation. Can you describe them in more detail?

Hoffmann: At the beginning phase of the project, stimulation during open abdominal surgery took place with electrodes the surgeon had to hold. This is why continuous monitoring was not possible yet. In the IKONA project that we coordinate, we developed electrodes that can be positioned on the pelvic autonomic nerves for the duration of the surgery. They enable continuous simulation. We envision the development of an array of multiple silicone electrodes to stimulate through the intact skin during laparoscopic surgeries.

Here we would externally stimulate from the spine, which is a big challenge. The current does not just need to penetrate layers of skin and tissue, but also the sacrum. It has several foramina, meaning holes. We want to modulate the electric field to where it penetrates the foramina. We have already tested various field geometries and electrode arrangements on models and in animal testing in preparation for this.
Graphic: human sacrum

The human sacrum has several foramina. The electric field used in neuromonitoring has to be guided through them; ©panthermedia.net/krishna creations

Why do you use silicone electrodes?

Hoffmann: When the patient lies down, the electrodes need to perfectly adapt to the shape of the body. If the contact area is too small, the current density is too high which can in turn cause injuries. Unlike metal electrodes, silicone electrodes fully rest against the body, even without conductive gel that could dry out during the course of extended surgery. The silicone we use is filled with conductive particles and has the same electrochemical properties as a silver/silver chloride reference electrode. Within the array, the respective optimal electrodes are subsequently being determined and activated, with which we are able to guide the field through the foramina.

What does the schedule for this development look like?

Hoffmann: Over the next two years, we want to demonstrate in animal studies that our concept works. This is also about the assessment of usability in the operating room. Ultimately, we want to show that the concept can be transferred onto humans.
We collaborate with several partners, in particular Prof Kneist of the Mainz University Medical Center Department of General, Visceral and Transplantation Surgery, as well as Dr Thilo Krüger of the inomed Medizintechnik GmbH, which coordinates the overall project. This company has many years of experience in neuromonitoring. autoPIN is a close collaboration and we mutually work out many device-related details. Being a manufacturer, inomed ultimately also wants to implement the results in a device.
Foto: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.