Article Title: Intraoperative direct cortical stimulation motor evoked potentials: Stimulus parameter recommendations based on rheobase and chronaxie
The choice of stimulation parameters for direct cortical stimulation is based primarily on observational data. This data has suited us well over time but there were no specific true basic neurophysiologic studies that could better help define the optimal stimulation parameters.
An important property of a nerve and more specifically neural tissue is the strength duration (S-D) curve. This curve relates how much energy is needed to activate a nerve based on the stimulation intensity and the pulse width of a square shaped pulse, which is what we in the neurophysiology community use to stimulate the nerve or neural tissue.
When evaluating the strength duration curve there are two key properties of this curve, the Rheobase (Rb) and the Chronaxie (Cx). The Rb is the smallest stimulation amplitude that can cause a nerve (axon) to generate an action potential (AP). It is defined for a infinitely long pulse width (DC current pulse), all pulse width’s smaller than this value will require a greater stimulation amplitude to generate an action potential.
The Cx is the pulse width value on the strength duration curve that crosses the stimulation amplitude value that is two times the Rb current. The Cx is a excitability time constant. The Cx point on the curve also defines the minimal energy point for AP generation.
In the paper by Abalkhail et. al. they investigate the Rb and Cx to help define the optimal interstimulus interval (ISI) and the pulse width (D) for direct cortical stimulation. Common parameters used in the operating room for DCS are an ISI of either 2 or 4 mSec and a pulse width of 500 uSec.
Key points from this paper
- Standard S-D curves are based on single pulse stimulation. This study evaluated the S-D using a pulse train. They were not able to determine if the S-D curve was based on the complete pulse train or a set of individual pulses. Additionally, the S-D curve for this study is a composite of the axon in the CST and the alpha-motor neuron (AM). First, a AP needs to be generated in the CST and second that AP, or set of APs needs to cause the AM to fire. If neither of those occurs there will be no MEP. Yet, the data is still valuable since we are activating this network and the network still has a minimal energy point even if individual elements of that network are not being activated at their minimal energy points.
- Evaluation of basic neurophysiological parameters of neural tissue can come from standard tools that we are already using in the operating room. By just varying pulse width and amplitude of the stimulus it is possible to optimize the stimulation parameters to each patient. But given the data in this paper the values are relatively constant. In Abalkhail the values of Cx varied between 160 uSec and 200 uSec which is more than half of the common 500 uSec pulse that is most commonly used for DCS.
- The strength duration curve does not take ISI into consideration thus the authors evaluated these values for multiple ISI values. This is important because they used the Rb to define the lowest stimulation current needed to generate an action potential. This value was for an ISI of 4 mSec which is a common value used in the OR presently.
- It is critical to note that using these changes in parameters the actual values that indicate safety distances may vary and this was not part of their study.
- Abalkhail TM, et. al. Clinical Neurophysiology. 2017;128:2300-2308
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