A total of 139 patients (110 women, median age 63 years [range 22–82 years]) diagnosed with FI refractory to medical treatment were enrolled in the study. The demographic characteristics and comorbidities of the study population are described in Table 1. Fifty-nine patients (42.5%) were diagnosed with FI between 1 and 5 years after symptom onset, the majority with 2 to 3 normal bowel movements per day (22.1%). Some associated symptoms included urinary incontinence (29.8%), sexual dysfunction (6.4%) and pelvic pain (3.5%).

Table 1 Demographic and examination data

The rectal examination findings were normal in 28 (21.2%) patients, and 7 patients (5.1%) had hypotonic sphincters. Perineal scars were present in 21 patients (15.3%), and 10 (7.3%) patients had celes that were not considered appropriate for surgery. Endoanal ultrasound at baseline showed that 49 patients (35.2%) had no external anal sphincter defect, while 48 patients (34,6%) had a defect of < 90°, and 42 (30.2%) had defects of 90–180°.

Clinical outcomes (Fig. 2)Fig. 2

Clinical outcomes. OR optimal responders, PR partial responders, IR insufficient responders

At the 3-month follow-up, 4 (2.9%) patients were optimal responders, 93 patients (69%) were partial responders, and 36 patients did not improve after treatment (insufficient response)). Therefore, 97 patients were allowed to progress to the second phase of treatment.

At 6 months, the four patients who had been optimal responders continued to respond positively. Of the initial 93 patients with partial response, most became optimal responders (n = 62), and 24 remained partial responders.

The third and final phase was conducted for all optimal responders (4 from the first phase and 62 from the second phase) and partial responders (n = 24). The 66 optimal responders maintained an optimal response and 23 partial responders were classified as optimal responders at the 12-month follow-up. Only 1 patient was classified as an insufficient responder in that phase.

Hence, at the end of the treatment, optimal response was observed in a total of 89/139 (64.0%) patients, and (93.3%) of the patients initially classified as partial responders succeeded to achieve optimal response.

Patients’ progression is described in Fig. 2. At the 36-month follow-up, all optimal responders (n = 89) were revaluated, and 64 (71.91%) patients still had an optimal response without supplementary treatment. Specifically, 3 of the 4 patients who with an optimal response after the first phase of treatment continued to show benefit at the 36-month follow-up. Of the 62 patients with optimal response after the second phase, 45 patients maintained continence and 16 patients of the 23 with an optimal response after the third phase continued to show benefits at the final evaluation. The differences between groups were not statistically significant.

No major complications or side effects were observed. Six patients were lost to follow-up due to pulmonary and oncological comorbidities and personal reasons; these patients were considered patients with an insufficient response.

Wexner score (Table 2)Table 2 Wexner score and bowel habit diary results

The median Wexner score decreased significantly from 11 (range 7–14) at baseline to 6 (range 2–9) at the 6-month follow-up (p < 0.001), and this magnitude of decrease was the largest observed. The scores continued to decrease to the end of the treatment, with a score of 6 (range 3.5–10) at 12 months. At the 36-month follow-up, after 2 years without additional treatment, the median Wexner score with previous optimal response was still 4 (range 1–8).

Severity of incontinence (bowel habit diary) (Table 2)

Most patients (57.7%) had mild incontinence at baseline. While 17 patients (12.4%) showed severe incontinence at baseline, this rate decreased to four (0.5%) at the end of the study. Patients gradually turned from moderate and severe incontinence to mild incontinence (79.3%) at the end of treatment, and kept improving at 36 months reassessment (79.5%).

RAFIS score (Table 3)Table 3 Rapid Assessment for Faecal Incontinence Sore (RAFIS), and faecal urgency results

Prior to treatment, the mean RAFIS score was 4.5 (± 2.7). The score gradually increased to 6.1 (± 3.6) at the end of treatment (p = 0.16). However, the RAFIS score decreased to 5.9 (± 1.1) at the latest evaluation.

Faecal urgency (Table 3)

A total of 94 (67.7%) patients could not delay defecation for more than 1 min at diagnosis, and 29 (21.3%) could delay defecation for 1–5 min. Eighth patients (5.5%) could delay defecation for 5–10 min, and 8 (5.5%) could delay defecation for more than 10 min.

The percentage of patients with severe faecal urgency (< 1 min) rapidly decreased from 94 patients (67.7%) to 61 (44%) at the first follow-up and even improved during the following phases of treatment. At the 6-month follow-up, twenty-four patients (25.1%) could delay defecation for more than 10 min, which was even higher at the 12 and 36-month follow-up (56.3% and 89.8%, respectively). These improvements were statistically significant in all phases (p < 0.001).

Anal manometry

At diagnosis, the mean maximum resting pressure (MRP) was 31.8 mmHg (SD ± 18.4), and the maximum squeezed pressure (MSP) was 55 mmHg (SD ± 26.9). At the 6-month follow-up, the MSP remained stable at 59.1 mmHg (SD ± 27.8; p > 0.05), and the MRP increased to 35.2 mmHg (SD ± 15.9; p = 0.04).

Quality of life (FIQLs) (Table 4)Table 4 Faecal Incontinence Quality of Life scale (FIQLs)

The data obtained from the QoL questionnaires showed that “embarrassment” was the domain that was rated worst by patients at the moment of diagnosis. At the 3-month follow-up, improvements were observed in behaviour and depression domains. At the 6-month follow-up, the lifestyle domain was particularly relevant, with a mean score of 36 (previously 27 and 25). At the 12-month follow-up, “embarrassment” was the domain with the largest improvement (5 points vs. 10 points), and the change was statistically significant (p = 0.03).

CART (Fig. 3)Fig. 3

CART diagram. CART classification and regression tree

The CART procedure allowed the investigators to generate a tree containing the baseline Wexner score, symptom duration and obstetric history as predictive factors for PTNS success.

The results showed that a baseline Wexner score ≤ 10 suggests a high probability (79%) of obtaining a long-term optimal response. For a baseline Wexner score > 10, if symptom duration was ≤ 1 year, it is highly likely (82%) that the patient will exhibit optimal response in the long term. In contrast, if the baseline Wexner score is > 10, associated with symptom duration > 1 year and a history of obstetric conditions, the chance of the patient not exhibiting an optimal response is > 60%.