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

Abstract

Introduction: Magnetic resonance imaging (MRI) conditional modes are a novel feature for certain Food and Drug Administration (FDA)approved spinal cord stimulation (SCS) devices. However, there is a paucity of literature around the limitation of MRI-conditional modes (''MRI safe''), specifically in clinical scenarios where urgent MRIs may be needed. One such limitation is load impedance, referring to the circuit's resistance to the current being generated by the system. High impedance can limit the MRIconditional mode capability, presenting potential harm to a patient undergoing an MRI or make an MRI unable to be completed. Methods: Three cases were identified, and informed consent was obtained. All information was obtained via retrospective chart review. Results: In this case series of three patients where MRI-conditional SCS systems were unable to be placed in ''MRI safe'' settings, preventing timely MRI study completion in the setting of high impedance, all three were required to undergo alternative imaging including CT scans, and two patients ultimately had the system explanted and one chose to be re-implanted after completion of scans. Conclusion: This case series highlights the need for further investigation of impedance in SCS systems and potential limitations for future MRI usage. The review of literature of impedance in SCS shows both device-and physiologic-related etiologies for changes in impedance that warrant consideration by the implanting physician.

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

“…Boston Scientific Abbott series described three cases where high lead impedances precluded timely MRI in impedance-dependent MRconditional SCS systems, with these patients instead undergoing alternative imaging with computerized tomography (CT). 16 Two of these cases had the SCS explanted in order to facilitate MRI, with one undergoing re-implantation after MRI. The issue is of particular concern where urgent MRI is required, such as in suspected cerebrovascular accident or acute disc herniation, or in cases where multiple MRIs may be required over a period of time for disease surveillance in cancer or multiple sclerosis.…”

“…Advice on impedance testing varies across manufacturers and the effects of the MR environment may vary depending on the device. 16 There are different safe impedance thresholds quoted for Nevro (> 10 kΩ) and Boston (> 4.5 kΩ) but no specific advice on impedances mentioned for other devices. Further safety studies on impedance thresholds in SCS would help physicians with contingency planning and deciding whether it is safe to proceed with MRI.…”

“…Other factors include epidural fibrosis, tissue impedance, vertebral level of electrodes, time since implant, patient age and previous spinal surgery. 16,20,21 Mean lead impedance has been shown to rise over time, possibly due to the development of fibrotic tissue around lead electrodes. 21 The vertebral level of electrodes can affect impedance due to CSF volume and crowding of the thecal sac at higher vertebral levels with lower thoracic regions associated with higher impedance compared to mid-cervical leads by approximately 200 Ω.…”

“…15 Aside from this study and a case series of three patients, elevated SCS lead impedances affecting MRI conditionality have been neglected within the literature and the rate of lead impedance failure over time or across different manufacturers is unknown. 16 The objective of this study was to retrospectively analyze the rate of lead impedance failure in impedance-dependent MR-conditional SCS devices over time and across different manufacturers to determine the rate of failure of MR-conditional modes of SCS.…”

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

“… 70 Counseling of patients regarding the use of MRI-conditional SCS systems should be considered prior to implantation. 69 , 71 , 72 Chronic pain is well known to interfere with daily activities and level of functioning; however, the choice of SCS device is unequivocally linked to day-to-day life. Ability to operate a vehicle is considered an essential component of an individual’s quality of life and productivity.…”