8.1 Introduction

Deep brain stimulation (DBS) is a well-established therapy for a variety of medication refractory movement disorders. Successful DBS outcomes depend on appropriate patient selection, post-operative programming, management of non-DBS related issues, and recognition of potential complications of the therapy. Accurate electrode placement is a pivotal step to produce positive outcomes.

Accurate electrode placement depends on appropriate targeting combined with MER. The exclusive use of image-guided indirect stereotactic targeting may affect outcome because of poor image acquisition technique, brain shift and pneumocephalus. Judicious use of MER can precisely map the motor territory and identify target boundaries that in turn can be verified by macrostimulation through the DBS lead. The final position of the DBS lead can be selected by identification of intraoperative thresholds that optimize clinical benefit and limit unwanted side effects.

MER identifies neuronal action potentials and facilitates testing of kinesthetic cells, cells with movement-evoked neuronal responses. It can characterize the topography and territory within a specific nucleus or cerebral region and determine its boundaries. It is dependent on clinical expertise. There are three major elements to the recording technique:

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  Target verification– pass a single microelectrode to verify that physiology matches the expected region based on imaging. This technique is the one most prone to error.

  
  
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  Ben Gun approach– named after Professor Alim-Louis Benabid, this approach depends on passing multiple microelectrodes simultaneously and has the advantage of fixing brain tissue in place to avoid shift during a procedure. The technique depends on using the best of the five simultaneous penetrations. Several groups have modified this technique and use two or three MER passes instead of five.

  
  
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  MER mappingThe most accurate and detailed of the approaches uses single MER passes and determines the next pass based on the previous pass. This allows the physician to map a structure in three dimensions and to choose the best site for the electrode. Although more accurate, this technique can be more time consuming and more vulnerable to brain shift. The benefits of additional MER passes, however should be weighed against the risk of injury, usually hemorrhage.

This chapter will review basic intraoperative MER and macrostimulation techniques for the treatment of movement disorders. It will focus on the most common conditions and targets, discuss anatomy, boundaries and tips for ideal target placement. ▶ Table 8.1 summarizes important principles for MER mapping and anatomic localization.