Improved Selectivity in Eliciting Evoked Electromyography Responses With High-Resolution Spinal Cord Stimulation

Abstract

Background and objectives: As spinal cord stimulation (SCS) offers a therapy for increasing numbers of patients with chronic pain and spinal cord injury, it becomes increasingly important to better understand its somatotopy. In this prospective study, we investigate whether high-resolution SCS (HR-SCS) offers improved selectivity assessed through elicitation of evoked electromyography (EMG) responses as compared with commercial paddle leads.

Methods: Vertical tripole configurations were used to elicit EMG responses in both types of paddles placed for standard-of-care indications between T6 and T10. In HR-SCS, evoked EMG responses in lower extremity/abdominal muscle groups were monitored at 6 to 8 mediolateral sites. All commercial paddle columns were tested. Percentage change in the maximum root mean square value was calculated at a group level. Heat maps were generated to identify responders for each muscle group. Responders were considered patients who had a >50% change in root mean square over baseline.

Results: We demonstrated significantly greater motor responses across medial and lateral contacts and greater responder rates consistently at the T6 and T9 levels with HR-SCS as compared with commercial paddles in 18 patients. Distal muscle groups (gastrocnemius and tibialis anterior) and proximal muscle groups (biceps femoris and quadriceps) were selectively activated at both levels.

Conclusion: We demonstrate that HR-SCS has greater selectivity in eliciting evoked EMG responses in an intraoperative setting. HR-SCS offers recruitment of muscle groups at lateral contacts concurrently with medial contacts. We provide data that HR-SCS may provide higher spatial resolution, which has the potential to allow for personalization of care and treatment of pain syndromes/symptoms which to date have not been effectively treated.

Trial registration: ClinicalTrials.gov NCT05459324.

Graphical abstract

FIGURE 1.

Images taken of the commercial and HR-SCS have been overlaid on an intraoperative fluoroscopic image. The dashed white line indicates the anatomic midline. We see that the orientation of both paddles differs in relation to midline at T8 and T9. The white arrow shows that column 2 (from left) in the commercial paddle overlaps with column 4 (from left) on the HR-SCS paddle. This normalization and alignment of contacts were essential for data comparison. HR-SCS, high-resolution spinal cord stimulation.

FIGURE 2.

Average maximum RMS change (%)for baseline at 6 mA or lower. The data were then aligned for the physiological midline (0—Contact), where left-sided contacts have negative values and right-sided contacts have positive values. The mean values from both left- and right-sided muscles were used in contact 0 calculations. (Top Left) Data from 3 patients were recorded at T6. (Middle Left) Data from 4 patients were recorded at the T7 spinal level. (Bottom Left) Data from 2 patients were recorded at the T8 spinal level. (Top Right) Data from 6 patients were recorded at the T9 spinal level. (Middle Right) Data from 11 patients were recorded at the T10 spinal level. HR-SCS, high-resolution spinal cord stimulation; RMS, root mean square.

FIGURE 3.

Comparison of HR-SCS and commercial paddles for all muscle groups at T6 to T10 spine levels. The contacts corresponding to the anatomic midline column were labeled Contact 0. Contacts 0 and |1| were considered medial. |1| includes both −1 and 1, and a similar absolute value expression was used for all contacts. Contacts |2| and |3| were assumed to stimulate lateral structures. Recordings were taken from 18 patients. The number of recordings for each contact at each paddle varied from 0 to 11. Maximum RMS group averages of evoked EMG responses for medial and lateral contacts with stimulation at amplitudes of (top) 0.5–6 mA and (bottom) 0.5–10 mA were calculated. Significance was determined using a linear mixed-effects model. From T6 to T9, HR-SCS paddle RMS responses were higher than commercial paddle responses. For T6 and T9, significant differences changed for lateral contacts from 6 to 10 mA stimulation. Commercial paddles had higher muscle responses (AH and MG) than HR-SCS paddles for amplitudes of 0.5–6 mA and AH, MG, and QUAD for amplitudes of 0.5–10 mA at the T10 level (indicated by dotted squares). ADD, adductor magnus; AH, abductor hallucis; BF, biceps femoris; GLUT, gluteus; HR-SCS, high-resolution spinal cord stimulation; MG, medial gastrocnemius; QUAD, quadriceps; RMS, root mean square; TA, tibialis anterior.

FIGURE 4.

Motor recruitment heatmaps for adductor hallucis (AH) at amplitudes of (left side) 0.5–6 mA and (right side) 0.5–10 mA at T6 to T10. Responders were determined based on values >50% RMS change for baseline, and nonresponders were classified as values <50% RMS change for baseline. The responder group was then separated into low (51-100 %RMS change), mid (101-500 %RMS change), and high (500-max %RMS change) responders. These values were decided based on the change in percentage threshold. AH, adductor hallucis; HR-SCS, high-resolution spinal cord stimulation; RMS, root mean square.