Intrapartum pelvic floor magnetomyography correlates with Kegel strength

Objectives: The levator ani muscles (LAMs) are integral to pelvic floor support and LAM injury has been associated with pelvic floor disorders, but ability to evaluate their neuromuscular integrity is limited. Magnetomyography (MMG) is a novel, non-invasive tool to passively measure the electromagnetic field generated by depolarization activity of muscles and offers a unique method to evaluate LAMs. Our objective was to correlate active and passive intrapartum LAM MMG signals with clinical pelvic floor muscle strength.

Methods: We collected MMG data on a convenience sample of 3rd trimester gravidas using SARA (Superconducting Quantum Interference (SQUID) Array for Reproductive Assessment) system during rest and voluntary contraction of LAMs (Kegels). Women performed increasing Kegel intensity: small (SmK), medium (MK), strong (StK) for 10 seconds (s) with intervening 10s rest. We collected 10s of isolated abdominal and thigh contraction. All subjects had clinical confirmation of Kegel ability (Brink score) and wore surface electromyography (sEMG) sensors: abdomen (A), thigh (T), perineum (P) to evaluate for accessory muscle activation. Brink pressure score (B1) was utilized for direct comparison with MMG. MMG data were collected with sampling rate of 1.2kHz and signals band-pass filtered between 20-200 Hz using 4th order Butterworth filter and notch filtered for ambient noise removal (60Hz). SARA sensors were selected for analysis given positional proximity to LAMs. Ambient noise and cardiac artifact were removed. To isolate MMG Kegels, we applied independent component analysis method. 10s MMG epochs were extracted with baseline correction using previous rest period (3s). Power spectrum density (PSD) and total PSD was calculated using Welch’s method. Correlation calculated using Pearson’s correlation coefficient.

Results: We recorded 3rd trimester MMG data for 14 women with mean age 27.0±5.0 years, mean gestational age 30.3±1.5 weeks. 79% were White, 21% Black, all non-Hispanic. 13 women were nulliparous and 1 had previous unlabored Cesarean delivery. Median B1 pressure score was 2 (IQR 2,3). Figure 1 depicts single subject MMG data and averaged PSD distributions across MMG sensors with total power maps by Kegel intensity. Power maps show sum of power across frequencies of MMG activities by sensor and are color-coded and overlaid onto a 151 channel sensor map for spatial distribution. Maps demonstrate increasing power with increasing Kegel intensity. Figure 2 depicts box and whisker plot of B1 score versus MMG amplitude and PSD. B1 measures demonstrated strong correlation ( >0.6) with MMG amplitude and PSD during MK and rest.

Conclusions: MMG offers a novel, passive method to measure LAM strength and function. 3rd trimester Kegel strength is strongly correlated with LAM MMG activity during active contraction and resting state. Our future work will collect MMG data intrapartum and postpartum in an expanded cohort to explore MMG’s unique ability to characterize birth trauma to the pelvic floor.