Though slowed motor conduction of the pudendal nerve and neuropathic changes in the pelvic floor muscles have been implicated in the pathophysiology of stress urinary incontinence (SUI) in women[1,2], impairments in sensory function are not commonly investigated. Previous reports on sensation in this population have had mixed results and have been limited due to a high risk of bias and a narrow focus (single nerve branch). Moreover, reported sensory testing protocols have relied on the method of limits, which can be influenced by participant reaction time, while the use of a stepwise method may provide a more accurate measure of sensory thresholds. Additionally, no previous study has reported on differences in light touch sensation with monofilaments in the regions innervated by the pudendal nerve between those with SUI and continent controls. The objective of this study was to use monofilaments and a stepwise method to quantitatively assess light touch sensation thresholds in skin regions innervated by branches of the pudendal nerve, the genitofemoral nerve and a remote site innervated by the same nerve roots among women with and without SUI. We hypothesized that, when compared with continent controls, women with SUI would have higher light tough sensation thresholds in regions innervated by the pudendal nerve, but not at regions innervated by other nerves.

This was an observational case-control study which received prior approval from the local institutional research ethics board. Women >18 years old were recruited into two groups (SUI and control) and provided informed consent before participating. Women were eligible to participate if they did not report signs or symptoms of pelvic pain or pelvic organ prolapse at the time of recruitment. Those in the SUI group reported urinary incontinence with predominant SUI symptoms (ratio of self-reported SUI to urgency urinary incontinence episodes per week was > 1) while controls reported no urine leakage over the previous year. Custom single-use monofilaments were made from surgical sutures following a previously established protocol[3] and ranged in application force from 0.0015g to 11.749g. Sensory thresholds were assessed at three locations normally innervated by the pudendal nerve: the skin overlying the posterior aspect of the urethral meatus, the middle of the perineal body, and the right side of the anal verge (overlying the external anal sphincter (EAS)), a site innervated by the genitofemoral nerve located at right anterior third of the labia majora, and a control site posterior to the right medial malleolus. Sensation was tested with participants in the lithotomy position. The monofilaments were applied using a 2-down-1-up staircase method, and each site was assessed three times. Sensory thresholds were defined as the average of the three trials. 
A mixed model analysis of covariance was planned with age and parity as covariates; however, the data violated the assumptions of this test; therefore, multiple linear regression models were used to assess the relationship between continence status and light touch sensation thresholds while accounting for age and parity. Outliers were defined as those > three standard deviations from the mean. While outliers may be considered signs of potential sensory impairment, they may also influence the regression models; therefore, two models were generated at each test site: one with and one without outliers included to allow for a robust interpretation.

Thirty women participated, but one dataset was removed as the participant reported having used an over-the-counter pain medication prior to testing (n = 29). Women with SUI (n = 16, 49 ± 11 years; p = .03) were significantly older than the controls (n = 13, 39 ± 13 years); the groups did not significantly differ in terms of parity (control parous n = 6, SUI parous n = 12; p = .11). Sensation thresholds determined at each site are presented in Table 1.
One outlier (high threshold/poor sensation) was identified at each genital site (Table 2). Each case in which an outlier was identified demonstrated no outliers at the other test sites: Case 1 at the labia majora (z = 3.82), Case 2 at the urethral meatus (z = 3.75), Case 3 at the perineal body (z = 3.56), and Case 4 at the EAS (z = 5.07). All outlier cases were parous and three of the four reported experiencing a perineal tear and/or an episiotomy during vaginal delivery. 
With outliers included, both the mid-perineal and EAS sites demonstrated no effect of continence status on sensory thresholds (R2 = .01, p = .96; R2 =.10, p = .46). However, with the outliers removed, continence status tended to be associated with higher sensory thresholds at the mid-perineal site (R2 = .24, p = .08) and significantly so at the EAS site (R2 = .29, p = .04). In both models, continence status was the only significant coefficient (p = .02; p = .04) indicating that SUI was associated with high light touch sensory thresholds independent of age and parity. Continence status was also significantly associated with sensation at the labia majora site (R2 = .16, p = .03). While continence status remained a significant model coefficient (p=0.04) at this site, the overall relationship was no longer significant once age and parity (R2 = .25, p = .07) were included in the model.
At the urethral meatus (R2 = .08, p = .58) and the right ankle (R2 = .03, p = .86), continence status was not associated with light touch sensory thresholds.

These findings indicate that the sensory branches of the pudendal nerve may be impaired in at least some cases of SUI, most notably those innervating the skin at the mid-perineum and over the EAS. These findings, along with an examination of the outliers, suggest that sensory impairments may be focal, where individual distal branches of the pudendal nerve may be implicated but not the pudendal nerve as a whole. Perineal tears or episiotomies may also influence focal sensation at these sites. The higher sensation thresholds at the labia majora among those with SUI compared to controls were unexpected and appears to be mediated by age and parity. High thresholds at the labia majora may reflect injury to the genitofemoral nerve or its branches during pregnancy and/or delivery; such injuries have been reported in case studies. The lack of difference in sensation thresholds at the urethral site between those with SUI and controls is not consistent with the findings at the other sites. This may be due to the focal nature of the observed sensory impairments, or because the stimulus type was not consistent with typical stimuli perceived by sensory receptors around the urethra.

These findings support the idea that changes in sensory nerve function may be associated with SUI, yet the observed differences appear to be associated with focal and superficial disruption of sensory branches at the mid-perineum and EAS rather than the pudendal nerve as a whole. While associations do not support causation, focal changes in sensation should be investigated further among women with SUI to determine whether they may influence symptom severity or treatment outcomes.

Snooks SJ, Badenoch DF, Tiptaft RC, Swash M. Perineal Nerve Damage in Genuine Stress Urinary Incontinence; An Electrophysiological Study. Br J Urol. 1985;57(4):422-426. doi:10.1111/j.1464-410X.1985.tb06302.x
Smith ARB, Hosker GL, Warrell DW. The role of pudendal nerve damage in the aetiology of genuine stress incontinence in women. BJOG An Int J Obstet Gynaecol. 1989;96(1):29-32. doi:10.1111/j.1471-0528.1989.tb01572.x
Pukall CF, Binik YM, Khalifé S, Amsel R, Abbott F V. Vestibular tactile and pain thresholds in women with vulvar vestibulitis syndrome. Pain. 2002;96(1):163-175. doi:10.1016/S0304-3959(01)00442-0

Continence 7S1 (2023) 100887
DOI: 10.1016/j.cont.2023.100887