Response to letter: “Nonlinear” burst stimulation by Richard North, MD

To the Editor: We thank Dr. North for his concern about introducing phrases into the neuromodulation lexicon that are not technically correct or meaningful. We agree that this practice can be misleading and may lead to the misinterpretation of the pre-clinical and clinical literature. We would like to take this opportunity to explain the rationale for using the term “nonlinear burst stimulation” as opposed to the trademarked name BurstDRTM. There are two primary issues at hand. First, the editor requested us to remove the trademarked name, except for the first description of the waveform. The generic term “burst” is inadequate to describe the unique and proprietary design of the waveform. Moreover, other manufacturers are using this generic term (burst) and it is creating confusion within the field. Therefore, we have chosen to use the nontrademarked term “nonlinear burst” to describe the unique characteristics of BurstDR. We have chosen nonlinear burst, as it describes the nonlinear relationship between access voltage and current (1). BurstDR delivers packets of stimulation (5 pulses at 500 Hz) at a frequency of 40 Hz with passive repolarization during the inter-burst interval. Nonlinear access voltage is needed to sustain constant current injection at the electrode–tissue interface during monophasic cathodal pulses (current-controlled, long pulse width) and as charge accumulates between pulses (within the burst packet). This accumulated charge then passively dissipates between bursts. North correctly states that passive discharge has a nonlinear decay, and that this is also present in lowfrequency tonic stimulation (thus not making it a distinguishing feature). However, this is not the only nonlinear feature of the waveform. The most distinguishing nonlinear feature of BurstDR is a function of both the passive discharge and the high frequency of the pulses (500 Hz) during the burst packet. Tissue capacitance at the electrode tissue interface has a nonlinear effect on the access voltage needed to maintain current delivery. This effect is further amplified with BurstDR due to the residual charge accumulated (but not fully dissipated) from the previous pulse (during the inter-burst packet) increases the access voltage in a nonlinear manner both during the pulse delivery, and across the multiple pulses within each burst packet. Thus, each component of the waveform has a nonlinear feature. Nonlinear access voltage during each pulse, amplification of this effect due to residual charge accumulation (intra-burst period), and nonlinear decay during the quiescent period between bursts (interburst) all support the use of the term “nonlinear burst. ” Due to the nonspecific use of the word “burst,” it is paramount that we differentiate it from BurstDR. Over 10 years of preclinical, imaging, and clinical data have been published on BurstDR. The summation of this literature, which includes five level 1 RCTs (2), supports the fact that BurstDR has a unique mechanism of action compared with other stimulations being referred to as “burst. ” These robust and replicated clinical results may be due to the unique postsynaptic and neuroplastic effects that are present with nonlinear burst, but not present during other stimulations referred to burst or active-discharge burst (3). Indeed, after years of marketing “burst,” which is more accurately described as “active discharge burst” or clustered tonic, with no RCT data to support its clinical efficacy, a crossover RCT using active discharge burst, tonic, and 1 kHz stimulation was published showing no significant differences between the three paradigms, and very low (21%) preference for active-discharge burst (4). This is in stark contrast to the robust body of literature on nonlinear burst (BurstDR). (5) In closing, we completely agree with Dr. North that use of the term BurstDR is ideal as it is well established, unique, and unambiguous. However, both the scientific community and this journal provide explicit instructions to minimize or remove trademarked or “commercial” terms. This has resulted in a need to use a term that most thoroughly and meaningfully distinguishes this proprietary form of neuromodulation from other forms that have attempted to use the robust clinical and preclinical recognition of BurstDR to confuse clinicians, scientists, and patients. If this was pharmaceutical research, the drug name would be used in place of the commercial name. We should strive to find a similar model in neuromodulation so that misuse of generic terms like “burst” cannot be used to create confusion within the community.