Revealing Neuroplasticity Following Sacral Neuromodulation Treatment with fNIRS

Objectives: Functional MRI studies have observed normalization and neuroplasticity of the micturition neural networks after successful treatment for urinary urgency incontinence (UI) including physical therapy and medications. Sacral neuromodulation (SNM) is a UI treatment which uniquely, directly effects afferent bladder nerve signaling. The prefrontal cortex (PFC) receives bladder afferent signals toward the decision to void. Functional near infrared spectroscopy (fNIRS) is a technique which can be used to quantify PFC oxyhemoglobin (O2Hb) concentration as a marker of neuroexcitation. fNIRS is non-invasive, portable, and a more cost-effective alternative to fMRI. The objective of this study was to quantify prefrontal cortical O2Hb, as a marker for neuroexcitation, response to natural filling in female participants at least six months after sacral neuromodulation treatment.

Methods: Female participants with medication-refractory UI and SNM implanted for at least 6 months underwent a natural filling protocol using a Sensation Meter to record bladder sensation (0-100%). Throughout filling the SNM unit was at the baseline programming while 24-channel fNIRS device continuously measured O2Hb in the PFC channels divided into brain regions: right (9), middle (6), and left (9). A “elevated sensation” period was defined from “first desire” to 100% bladder sensation. fNIRS signals were processed to remove motion artifacts and physiologic noise. Data were interpolated to a reference time to account for variability in fill time. Sociodemographic variables, validated surveys for urgency and bladder diaries were collected. fNIRS patterns were compared to previously published O2Hb signals in normal controls and undertreated UI patients, without SNM, collected via comparable protocol.

Results: A total of 11 female participants completed the study with mean age 58.4±15 years, mean BMI 33.2±9.5 kg/m2 and a median of 17 [6.8,46] months from SNM implantation. Mean O2Hb change during the elevated sensation period was positive demonstrating increasing neuroexcitation in all cortical regions (left 0.39±0.95, middle 0.31±0.57, right 0.49±0.83), Figure 1. The SNM cohort was compared to previously reported normal controls (n=9) with mean age 39.5±16.5 years and BMI 22.4±2.0 kg/m2 and an UI cohort (defined by ICIq-OAB) (n=11) with mean age 60.7±15.6 years and BMI 32.9±16.5 kg/m2. The SNM neuroexcitation pattern was most similar to normal participants, though not to the same magnitude, while undertreated UI neuroexcitation patterns were stable or decreasing, Figure 1. Mean difference in O2Hb during elevated sensation period did not correlate with ICIQ-OAB score, cues score, or months from implantation.

Conclusions: This study is one of the first to demonstrate neuroplasticity in the prefrontal cortex with SNM treatment using fNIRS. These data support the hypothesis that the prefrontal cortex plays an inhibitory role in voiding function and that the inhibitory control in women with UI can be restored with SNM. Additional research with prospectively collected data is need to establish timeline required for neuroplastic changes.