Failure of SCS MR-Conditional Modes Due to High Impedance: A Review of Literature and Case Series

A retrospective review of elevated lead impedances in impedance-dependent magnetic resonance-conditional spinal cord stimulation devices

OBJECTIVES

Advances in Spinal cord stimulation (SCS) device technology in recent years have led to the development of SCS systems that are magnetic resonance imaging (MRI)-conditional, most of which are dependent on normal lead impedances. The objective of this study was to retrospectively analyze the rate of elevated lead impedance in these devices to determine the rate of failure of MR-conditional modes.

MATERIALS AND METHODS

This was a single-center, retrospective, chart-based review conducted during a five-year period. Patients were included if they had been implanted with an impedance-dependent MR-conditional SCS and had a documented impedance check at least 6 months after implantation. A Kaplan-Meier survival analysis was performed to map the survival of MR-conditionality over time.

RESULTS

There were 363 cases included between 2015 and 2020, which corresponded to a total of 602 SCS leads. Nevro was the most common manufacturer (67.8%), followed by Boston Scientific (22.3%) and Abbott (9.9%). The average overall follow-up time was 2.25 years. Overall, 67 (18.5%) of patients had lead impedances over 10,000 Ω at follow-up with a total of 186 electrode contacts (3.9%). Leads most commonly had either one (40%), two (22%) or three (12%) electrode contacts out of range. Risk of failure of lead impedances increased by 35.4% with each successive year to a peak of 43% of all leads by year 5. Mean overall survival time of normal lead impedances was 4.77 years (CI 4.40-5.13). There was no statistically significant difference in mean overall survival time between Abbott (M = 4.0 years, SD = 1.25), Boston Scientific (M = 4.64 years, SD = 1.75) and Nevro (M = 4.80 years, SD = 3.28), χ2 (2, N = 358) = 1.511, p = 0.47; however, Abbott leads had a greater total number of failed impedance contacts (50/568, 8.8%), in comparison to Nevro (124/3064, 4.0%), χ2 (1, N = 3630) = 23.76, p < 0.00001, at a similar follow-up time.

CONCLUSION

This retrospective study identified elevated impedances in 18.5% of MR-conditional SCS devices at an average of 2.25 years follow-up resulting in loss of MR-conditionality and a mean overall lead survival time of 4.77 years for normal lead impedance.

Neuromodulation Techniques in Chronic Refractory Coccydynia: A Narrative Review

Refractory coccydynia is a condition characterized by severe coccygeal pain and poses a challenging management dilemma for clinicians. Advancements in neuromodulation (NM) technology have provided benefits to people experiencing chronic pain that is resistant to standard treatments. This review aims to summarize the spectrum of current NM techniques employed in the treatment of refractory coccydynia along with their effectiveness. A review of studies in the scientific literature from 2012 to 2023 was conducted, revealing a limited number of case reports. Although the available evidence at this time suggests significant pain relief with the utilization of NM techniques, the limited scope and nature of the studies reviewed emphasize the need for large-scale, rigorous, high-level research in this domain in order to establish a comprehensive understanding of the role of NM and its effectiveness in the management of intractable coccydynia.

Bioheat Model of Spinal Column Heating During High-Density Spinal Cord Stimulation

INTRODUCTION

High-density (HD) spinal cord stimulation (SCS) delivers higher charge per time by increasing frequency and/or pulse duration, thus increasing stimulation energy. Previously, through phantom studies and computational modeling, we demonstrated that stimulation energy drives spinal tissue heating during kHz SCS. In this study, we predicted temperature increases in the spinal cord by HD SCS, the first step in considering the potential impact of heating on clinical outcomes.

MATERIALS AND METHODS

We adapted a high-resolution computer-aided design-derived spinal cord model, both with and without a lead encapsulation layer, and applied bioheat transfer finite element method multiphysics to predict temperature increases during SCS. We simulated HD SCS using a commercial SCS lead (eight contacts) with clinically relevant intensities (voltage-controlled: 0.5-7 Vrms) and electrode configuration (proximal bipolar, distal bipolar, guarded tripolar [+-+], and guarded quadripolar [+--+]). Results were compared with the conventional and 10-kHz SCS (current-controlled).

RESULTS

HD SCS waveform energy (reflecting charge per second) governs joule heating in the spinal tissues, increasing temperature supralinearly with stimulation root mean square. Electrode configuration and tissue properties (an encapsulation layer) influence peak tissue temperature increase-but in a manner distinct for voltage-controlled (HD SCS) compared with current-controlled (conventional/10-kHz SCS) stimulation. Therefore, depending on conditions, HD SCS could produce heating greater than that of 10-kHz SCS. For example, with an encapsulation layer, using guarded tripolar configuration (500-Hz, 250-μs pulse width, 5-Vpeak HD SCS), the peak temperature increases were 0.36 °C at the spinal cord and 1.78 °C in the epidural space.

CONCLUSIONS

As a direct consequence of the higher charge, HD SCS increases tissue heating; voltage-controlled stimulation introduces special dependencies on electrode configuration and lead encapsulation (reflected in impedance). If validated with an in vivo measurement as a possible mechanism of action of SCS, bioheat models of HD SCS serve as tools for programming optimization.

Patient Selection for Spinal Cord Stimulation in Treatment of Pain: Sequential Decision-Making Model - A Narrative Review

Despite the well-known efficacy of spinal cord stimulation (SCS) in chronic pain management, patient selection in clinical practice remains challenging. The aim of this review is to provide an overview of the factors that can influence the process of patient selection for SCS treatment. A sequential decision-making model is presented within a tier system that operates in clinical practice. The first level incorporates the underlying disease as a primary indication for SCS, country-related reimbursement rules, and SCS screening-trial criteria in combination with underlying psychological factors as initial selection criteria in evaluating patient eligibility for SCS. The second tier is aligned with the individualized approach within precision pain medicine, whereby individual goals and expectations and the potential need for preoperative optimizations are emphasized. Additionally, this tier relies on results from prediction models to provide an estimate of the efficacy of SCS in the long term. In the third tier, selection bias, MRI compatibility, and ethical beliefs are included, together with recent technological innovations, superiority of specific stimulation paradigms, and new feedback systems that could indirectly influence the decision-making of the physician. Both patients and physicians should be aware of the different aspects that influence patient selection in relation to SCS for pain management to make an independent decision on whether or not to initiate a treatment trajectory with SCS.