Overactive Bladder (OAB) is defined as a condition with frequent feelings of needing to urinate during the day, during the night, or both. Percutaneous tibial nerve stimulation (PTNS) was initially introduced by Stoller in 1999, and later on, in 2000, received FDA approval for office based OAB treatment, and in 2006, received NICE approval. This process is performed by a needle electrode inserted about 5 cm above the medial malleolus in the leg. The current of electricity is continuous with a duration of 200 µs and a frequency of 20 Hz. The patients usually go through one 30-min session of PTNS per week for 12 weeks. 
Transcutaneous tibial nerve stimulation (TTNS) is a rather newer method, and is acclaimed to be more viable and easier to use, due to the fact that it only uses surface electrodes and therefor, can be self-administrated by patients, making adherence to treatment more likely . However, using this method usually requires a bigger number of stimulation sessions per week, often reaching the dosage of one 30-min session per day. There have been other reviews previously that have studied PTNS or TTNS efficacies individually, or studies that have focused on OAB or PB separately. In this systematic review, we tend to give a wholesome overview including all four of these concepts in one study, in order to determine and compare the efficacy of PTNS and TTNS on both PB and OAB, and update the results of previous reviews.

We searched studies available on PubMed, Embase, and Cochrane, from the oldest possible articles to the ones published up to Mar 31, 2021. Searching strategy for each database was planned according to the 4 main elements of the PICO formation of the study topic, including both the pre-determined database-specific searching terminology (MeSH and Emtree) and manually added synonym terms. The PICO formation comprising the inclusion criteria are as follows:
-Population: patients suffering from OAB, or PB;
-Intervention: PTNS, and/or TTNS;
-Control: placebo (sham) treatment, or other actual treatment methods used for treating OAB and PB;
-Outcome: efficacy of treatment based on Daily voiding frequency, Mean voiding volume, post void residual volume (PRV), Max cystometric capacity (MCC), Daily leakage episodes (Incontinence), Urgency episodes, Nighttime voiding frequency, and Urge incontinence. Exclusion criteria for the studies were being in languages other than English, study types other than quasi-experimental, and randomized-controlled trials, or unavailability of the full text version for the article.Meta-analysis of the data was done by using the MantelHaenszel method with Comprehensive Meta-Analysis (CMA) software (ver. 2.2; Biostat, Englewood, NJ, USA JBI quality checklist was used to assess the internal and external validity of the selected studies.

We assessed the overall outcomes as well as subgroup analysis based on intervention, control and neurogenic bladder subgroups; and Daily voiding frequency, mean voiding volume, post void residual volume (PRV), Max cystometric capacity (MCC), Daily leakage episodes (Incontinence), urgency episodes, Nighttime voiding frequency and urge incontinence were analyzed. 
Daily voiding frequency
Of the 17 studies, 12 used PTNS and 5 used TTNS. Our meta-analysis found both of nerve stimulations a significant intervention (overall SMD: -2.40, 95% CI: -3.06 to -1.73, p-value<0.01). In neurogenic bladder sub-group, we meta-analyzed 6 studies and the intervention were also significant in this sub-group (p-value<0.01) 
Mean voiding volume
Of 10 studies, 7 of them used PTNS and 3 used TTNS as intervention, our meta-analysis found both of methods significant (overall SMD: 32.38, 95% CI: 24.18 to 40.58, p-value<0.01), the intervention was also effective in neurogenic bladder sub-group (p-value=0.02) 
Daily leakage episodes (Incontinence)
Of 9 studies, 4 of them used TTNS and 5 used PTNS. The result of meta-analysis came up with overall SMD: -1.09, 95% CI: -1.48 to -0.71 and p-value<0.01 which indicates that the interventions were significant. As the same in NB sub-group that showed p-value<0.01. 
Nighttime voiding frequency
Of 14 studies, 11 used PTNS and 3 used TTNS. Our meta-analysis found them both efficacious interventions (overall SMD: -1.04, 95% CI: -1.39 to -0.69, p-value<0.01). Also in neurogenic bladder subgroup, the therapies seemed to be useful (p-value<0.01)

In context of existing evidences, we found that TNS might also be useful in treating bowel incontinence as well as urinary incontinence, and this intervention could play an important role in neurological patients with both lower urinary tract and bowel dysfunction. There is a need for a well-designed multi-central for approving the findings.

Overall, nerve stimulations appear to be effective interventions in treating the symptoms of OAB and also neurogenic bladder subgroup.