Table 1.

Original studies on sacral neuromodulation. PCOS—Polycystic Ovary Syndrome, CRDs—complex repetitive discharges, DBs—decelerating bursts, EUS—external urethral sphincter, TRUS—transrectal ultrasonography, SNM—sacral neuromodulation, FS—Fowler’s syndrome, PAG—periaqueductal grey, DRS—dorsal root stimulation, INUR—idiopathic nonobstructive urinary retention, TNS—tibial nerve stimulation, PNS—pudendal nerve stimulation.

Author	Type of Study	Aim	Primary Outcome
Fowler et al., 1988	Cross-sectional	To reveal the cause of unexplained urinary retention in young women	Abnormal electromyographic activity of the external urethral sphincter in women with urinary retention associated with PCOS
Fowler et al., 1985	Observational	To elucidate the electromyotonic activity of the external urethral sphincter in women with urinary retention	Complex repetitive discharges and decelerating bursts, ephaptic spread of excitation between muscle fibers
Tawadros et al., 2015	Cross-sectional	To investigate the presence of CRDs and DBs in the EUS during the menstrual cycle in females with no urinary symptoms	CRDs and DBs in the EUS are present in 53% of asymptomatic women, more commonly in the luteal phase of the cycle
Ramm et al., 2012	Cross-sectional	To compare the proportion of women with CRDs among females with and without urinary disorders	CRDs detected in 30% of asymptomatic women
Noble et al., 1995	Cross-sectional	TRUS assessments of the EUS volume in women with obstructed voiding and abnormal electromyography (EMG) activity and in asymptomatic women	The volume of the EUS in the control group was significantly lower than in the obstructed group
Andrich et al., 2005	Cross-sectional	Morphological description of the urethral rhabdosphincter in women with and without urinary retention	No difference in rhabdosphincter fiber diameter was found in women with urinary retention and control
DasGupta, Fowler 2004	Cross-sectional	SNM in restoring voiding function in women with urinary retention	SNM has no direct relaxant effect on the sphincter
Kavia et al., 2010	Etiology (case series)	To examine brain responses to bladder filling in women with FS treated with SNM	Overactive urethra inhibits afferent signals blocking bladder afferent activity at the sacral level, deactivates the periaqueductal grey (PAG) and higher center, causing a loss of bladder sensation. SNM blocks inhibition by urethral afferents at the sacral level
DasGupta et al., 2005	Etiology (cross-sectional)	To investigate how SNM acts on brain centers involved in bladder function in women with FS and control	Enhanced limbic cortical activity with no significant activity in the brainstem was detected while bladder fulness was present in women with FS. SNM restores the midbrain activity and decreases cortical activity in this group
Karmarkar et al., 2015	Case–control study	To assess the prevalence of gynecological pathologies in women with FS	Statistically significant relationship between FS and endometriosis (p = 0.003), and FS and PCOS (p = 0.003), was found
Panicker et al.,2012	Observation from Prospective Clinical Study	To identify the impact of opiates on urinary retention in women	Exogenous opiates may compound any functional abnormalities predisposing women to urinary retention
Li et al., 2018	Animal study	To test the hypothesis that DRS blocks pudendal afferent inhibition of the micturition reflex	DRS blocks pudendal afferent inhibition and restores bladder capacity to control level
Swinn et al., 2000	Case–control study	To evaluate the efficacy of SNM in the treatment of FS	Success rate 68% Reoperation rate 21%
Mehmood et al., 2017	Retrospective study	To determine the safety and efficacy of SNM in the treatment of INUR	Improvement rate 83.3% Reoperation rate 41.6%
Li et al., 2020	Animal study	To determine the effects of TNS on reflex bladder activity	Repeated application of TNS produced long-lasting bladder underactivity
Mohapatra et al., 2021	Animal study	To determine the effects of PNS on reflex bladder activity	Repeated application of PNS resulted in long-lasting bladder underactivity