In Reply: Longevity of Deep Brain Stimulation in Essential Tremor: A Comparison of Omnidirectional and Directional Leads

Letter to the Editor: Dear Editor, we thank the authors for their thoughtful critique of our study, “Longevity of Deep Brain Stimulation in Essential Tremor: A Comparison of Omnidirectional and Directional Leads.”1 Their insights highlight important considerations regarding study design, methodological limitations, and challenges within the broader context of deep brain stimulation (DBS) research.2 Below, we address these points while providing additional context and data. As noted, our study's retrospective nature carries inherent selection bias risks. To mitigate this, we controlled for baseline demographics, including age, tremor severity, and energy utilization, finding no significant differences between groups (eg, baseline tremor scores: omnidirectional 3.19 ± 0.17 vs directional 3.1 ± 0.2; P = .68). However, as the authors correctly point out, unmeasured confounders—such as surgeon preferences or institutional protocols—could still influence the observed outcomes. These variables merit further exploration in future prospective studies. The issue of sample size and limitations on subgroup analyses is a valid concern; however, we believe our cohort of 62 leads (24 directional and 38 omnidirectional) represents one of the largest cohorts comparing omnidirectional and directional leads for essential tremor (ET) to date. Although we agree multicenter collaborations are needed to generalize findings and assess lead performance across diverse patient populations, previous studies on long-term DBS outcomes in ET have provided valuable insights, despite similarly modest single-center sample sizes.3,4 Achieving larger cohorts is inherently challenging due to the limited number of ET patients undergoing DBS, the relatively recent adoption of directional DBS technology, and the need for longitudinal data collected under consistent, standardized protocols to minimize confounding variables. This issue is not unique to our study but reflects a broader challenge faced by DBS research. An important limitation of our study, as the authors have pointed out, is the disparity in follow-up duration between the directional and omnidirectional lead groups, as it is indeed a limitation that we identified in our study. As noted in our manuscript, the shorter mean follow-up period for directional leads (3.71 years) compared with omnidirectional leads (7.54 years) reflects the more recent adoption of directional DBS technology. To account for this disparity, we excluded data from omnidirectional leads beyond 6 years to ensure temporal comparability. This adjustment allowed us to standardize the analysis and focus on the shared follow-up period for both groups. Although longer follow-up may naturally favor omnidirectional leads in detecting amplitude tolerance or battery depletion, our findings of reduced amplitude drift with directional leads within the first 6 years provide meaningful insights into their early stability and energy efficiency. These results align with findings in Parkinson disease, where directional leads have demonstrated improved targeting and reduced current spread, contributing to lower energy requirements and improved clinical outcomes5 with reduced need for higher amplitude settings over time.6 We fully acknowledged these limitations in the discussion section of our paper and emphasized the need for extended follow-up in future studies to better understand the long-term differences between these technologies. Incorporating objective outcome measures, such as accelerometer-based tremor assessments,7 is an important consideration that could help mitigate variability introduced by subjective scoring systems. Although our study relied on validated clinical outcomes such as tremor scores and energy utilization, these measures remain standard in DBS research and provide meaningful insights into lead performance. In addition, as the authors note, technological advances introduce a potential confounder. In the manuscript, we discuss that newer implantable pulse generator (IPG) models have shorter battery life compared with earlier devices and how changing technology is a possible limitation with potential influence on findings such as the shorter time to first IPG replacement for directional leads (P = .062). Regarding the absence of a control group for alternative therapies such as pharmacotherapy or focused ultrasound, our study was specifically designed to compare outcomes between directional and omnidirectional DBS leads, and the inclusion of other treatment modalities was beyond its scope. We agree that future studies exploring comparative effectiveness across multiple therapies and incorporating advancements in DBS programming and device materials would provide broader clinical insights. Despite the noted limitations, our findings provide important insights. Directional leads demonstrated significantly less year-to-year amplitude change, particularly in the first year, suggesting improved adaptability and energy efficiency. This aligns with prior reports of directional leads, widening the therapeutic window and reducing stimulation-related side effects.8,9 However, the lack of significant differences in tremor scores or IPG longevity underscores the need for longer follow-up and more refined metrics to capture subtle clinical advantages. We thank the authors for their critical appraisal of our work. Their commentary highlights the need for prospective, multicenter studies with longer follow-up, larger cohorts, and objective outcome measures to clarify the long-term benefits of directional DBS technology.