Volker A. Coenen

Burkhard Mädler

The concept of high-frequency electric stimulation in the brain (deep brain stimulation [DBS]) to alleviate a variety of neurologic and psychiatric, otherwise therapy-refractory disorders has gained wide acceptance. DBS continuously delivers electrical pulses, variable in amplitude, pulse width, and frequency, through permanently implanted electrodes, which are connected to an internal pulse generator. The electrodes are placed by using stereotactic methods based on imaging protocols (CT, MR imaging) combined with real-time intraoperative x-ray and electrophysiology. Depending on the diagnosis and patient-specific symptoms, different anatomic targets have been proposed. Currently DBS is performed with the patient awake, to assess the effectiveness and the side effect spectrum of stimulation.

Although progress has been made in understanding the optimal anatomic structures as target areas for DBS, little effort has been put into modeling and predicting electromagnetic field properties of activated DBS electrodes and understanding their interactions with the adjacent tissue. Those targets have been empirically well-defined and proven to be beneficial in alleviating the neurologic and/or psychiatric symptoms, but the true mechanism of DBS is still poorly understood.

Recent advances in the development of key technologies like optogenetic neuromodulation (an experimental technique used in animals to differentially influence neuronal tissues on the basis of genetic

manipulations and light of different wavelengths) and DTI-based fiber tracking have shifted the focus of attention from a lesion-simulating high-frequency inhibition of brain nuclei to a mere activation of the afferent axonal fiber environment of the nuclei. While mentionable progress has been made in improving the optimal anatomic target areas for DBS, only a few groups have devoted their efforts to the specific aim of modeling and predicting the electromagnetic field of an activated DBS electrode and understanding its interaction with the electromagnetic properties of the surrounding tissue.

This is why we, a physicist (B.M.) and a functional neurosurgeon and neuroscientist (V.A.C.), became interested in working together to tackle the problem of a more suitable and practical way for the neurosurgeon to predict and visualize the effective electric fields generated by the DBS electrodes. Our attempt entailed using a simple model that relies only on preoperative data (anatomic and diffusion-tensor imaging) and impedance measurements directly performed during the DBS procedure, to estimate the region and extent of tissue that will be influenced under real brain stimulation settings. The core surgical procedure currently can take up to 4–8 hours and is physically as well as psychologically quite demanding for the patient, but this new technique might give us the potential to perform this procedure under full anesthesia in the near future.

Apparently, the simplified method of simulating the effective electric field surrounding an active DBS electrode is applauded by a lot of researchers and clinicians. Our presentations at conferences as well as our published book and peer-reviewed journal articles have received positive feedback from the neuroscience community.

With our current research strategies we hope to further advance these technologies and to be able to take the burden of awake surgery from the patient by accurately defining the target region of the electric field and hence the accurate and achievable electrode position. DBS with the patient under general anesthesia is desirable; it is achievable and at arm’s length now.

 

Read this article at AJNR.org . . .