| Use of ASA-Lab recording system in assessment of internal and scalp EEG in psychiatric patients undergoing deep brain stimulation treatment |
Background Information In the Academic Medical Center in Amsterdam, Deep Brain Stimulation (DBS) has been used since 2006 to treat patients suffering from psychiatric diseases including Obsessive Compulsive Disorder (OCD) and depression. Currently, drug-addiction patients are also being implanted. DBS is a humanitarian treatment for refractory patients, as an alternative to surgical brain lesions. As such, DBS is currently being tested in treatment of psychiatric diseases. The DBS system works as a 'brain pacemaker' stimulating regions related to the patient's disease in order to change the way they work.This experimental treatment involves implantation of two electrodes, each one with four contact points, in the nucleus accumbens of one brain hemisphere. Electrical current is provided to this structure through a battery implanted in the chest. The nucleus accumbens is a collection of neurons in the brain thought to play an important role in reward, pleasure, laughter, addiction, aggression and fear. Neurosurgeons place the two electrodes in their precisely correct locations guided by surgical navigation systems. After electrodes surgery, and before battery implant, two leads exit the head on each side. Every cable has four electrical contacts each one connected to a monopolar lead near or inside the nucleus accumbens. Before battery implant, these externalized leads can be used to record electrophysiological signals from the nucleus accumbens while the patients perform psychological test related to their disease. |
Recording setup In order to record EEG from both scalp electrodes and implanted leads in the nucleus accumbens NIN / AMC approached ANT for an EEG system customized for this purpose. After several discussions with the customer a project was started to enable simultaneous recording of EEG and intracranial EEG (so-called 'iEEG') and to carry out ERP studies within their selected patient group. It was agreed to supply the customer with a partially customized 64 channel ASA-Lab system.Two main kinds of customization to the system were implemented in cooperation with the customer. |
Customized software When operating the ASA-Lab recording system, the impedance check mode will send a small current through all attached electrodes. The resulting voltages at the different electrodes sites will be recorded and using Ohm's law the impedances at all site will be calculated and displayed. However, for the 8 implanted leads, this is not an option: no currents should be sent directly into the brain and therefore no impedance check can be carried out. In order to allow simultaneous recording of EEG and iEEG but to prevent impedance check a customized data acquisition toolbar was designed and realized. In this customized data acquisition toolbar the opening screen forced the user to choose between a recording that did or did not include iEEG channels (for recording that only contain scalp electrodes the impedance check of course needs to work). After proceeding into a recording containing iEEG channels, the normally appearing impedance check screen was taken out of the workflow, as well as the button in the EEG toolbar that can normally be checked to run impedance check. As a result, impedance check was disabled in this situation. When proceeding with a recording not containing iEEG, impedance check was enabled again. To avoid human errors before actually starting any impedance checks, message boxes warn the user in the opening screen about a potentially wrong choice of options. This procedural workflow was designed and implemented together with our customer several times and was followed by training. |
Customized hardware Since the Waveguard cap system works with sintered Ag / AgCl electrodes while the implanted leads are made of Pt / Ir, initial recordings demonstrated that due to incompatible electrode materials, a combined EEG / iEEG recording without hardware modifications would not be possible. Therefore, a passive 0.16 Hz high-pass hardware filter was designed and made with proper connections to implanted electrodes and our amplifier. The hardware filters make sure that the iEEG channels do not have a DC offset voltage anymore. |
Patient recording sessions After installation of the system at the customer's site, two combined EEG / iEEG pilot recordings have been performed. To assist the customer in using the customized ASA-Lab recording system for combined EEG / iEEG recording, ANT was present during the first pilot recordings. In between two surgeries (the first for placing the leads the second for implanting the stimulator) the implanted electrodes were available for iEEG recording. During a 90-minute recording session the patient was confronted with a stimulation paradigm in which several pictures appeared on the screen that all had to be rated in terms of level of arousal or valence. For all appearing images it was also determined whether they related to the type of disorder the patient suffered from. The study is currently being continued by the user. We will keep you posted on the outcome once the publication is prepared. |  |
| written by Jaap van der Spek |
|