This study documented favourable results in terms of anorectal functional outcomes after TaTME. When interviewed after surgery, the mean LARS score was lower than values from existing literature [7] and also the vast majority of our patients reported minor faecal incontinence according to the Wexner score.
Aside from the well-known relevant impact of NAD [15], even age, operative time and time to stoma reversal significantly correlated with the incidence of bowel dysfunctions. However, and since it was not possible to define a clear trend for all the categories, we computed this information to create an algorithm that could be useful to classify categories of patients undergoing TaTME.
Also, with the comparative study, we explored the impact of the learning curve on the functional results: the cut-off values were based on a previous study from our group which identified as 27 the number of TaTME required to significantly decrease the anastomotic leak rate [21]. This threshold was considered appropriate since leakage is a well-known factor impairing post-operative bowel function [22] and thus it was adopted also in the present analysis. Interestingly, the TaTME learning curve had not an impact on post-operative bowel function, although this comparison was limited by the small numbers.
With respect of the secondary outcomes, post-operative urinary function significantly correlated to neoadjuvant therapy, independently from age. The same applies to sexual outcomes, even though, as expected, an increasing age was a relevant risk factor.
According to literature, 19 to 52% of patients who underwent sphincter preserving rectal surgery for cancer experience an altered defecation status known as LARS syndrome [7, 8, 14, 16, 23, 24]. The results of the present series disclosed that our patients could be ranked in the best-performing groups, with 13.4% of minor LARS and 25.8% of major LARS after TaTME, in support of the benefits the transanal approach can offer.
Undoubtedly, the transanal approach has been shown to provide a better visualization of five key zones where branches of the pelvic plexus are located [25]. It therefore allows to preserve the autonomic innervation of the internal anal sphincter, the main responsible for faecal passive continence [16]. However, the positive anorectal outcomes reported in our series could be explained by the strict selection of patients to be treated with restorative resections, in line with the low number of coloanal anastomosis, intersphincteric resections and pouchs ncluded in the cohort.
The height of the anastomosis and consequently the length of remnant rectum was shown to correlate with the risk of major LARS [11–14], since an important loss of rectal volume can lead to an increased frequency and urgency to defecate [26], as is the thickening of the rectal wall due to neoadjuvant radiation damage [26] due to nerve impairment [27].
Despite many concerns related to the use of the transanal platform, with the consequent anal stretching, prolonged dilatation and risk of sphincter damage [8, 16, 28], different systematic reviews [7, 8, 24] reported that laparoscopic TME and TaTME offer similar results in terms of functional outcomes.
A multicenter observational study [29] reported that the robotic approach could be superior in preserving post-operative anorectal function comparing TaTME, even though this result may be influenced by the consistent difference in the proportion of patients who underwent NAD among the groups used for comparison of data.
Consistently with this evidence, patients from our series who underwent neoadjuvant therapy report higher mean LARS and Wexner scores that those treated with upfront surgery, even though NAD doesn’t represent a distinct crossroads in our algorithm. This is probably due to its negative effect on postoperative anorectal outcomes that homogenously affects all the subsets of our study population.
Using a machine learning approach, we developed a statistical model to classify patients at risk of postoperative bowel impairment based on clinical and operative data.
This Random Forest, combined to the existing preoperative risk scores such as the POLARS [30], may represent a valid clinical tool to offer a proper preoperative counselling. It could be particularly useful in high-risk subsets of patients, and could also guide a tailored therapeutic program (i.e TAI-transanal water irrigation, biofeedback, electrostimulation, pelvic floor muscle training and Kegel exercises [31]) in cases of delayed stoma reversal surgery. Evidence from literature [32] reported that their prompt application (< 18 months from surgery) resulted in a greater improvement in fecal incontinence.
This study has, however, few limitations: firstly, it is a single-center experience and secondly patients were assessed exclusively once after primary surgery or stoma reversal, without a baseline evaluation. However, our group has a strong and consistent experience in TaTME as documented by several publications in the field [19, 21, 33, 34], and all the patients had similar a follow-up after surgery, so they can be regarded as homogeneous for long-term results. Also, the algorithm here proposed will require an external validation.
In conclusion, when performed in a high-volume center, TaTME surgery can provide good long-term results for the anorectal functions. Sub-groups of patients with high-risk clinical features are at risk to develop major LARS syndrome, however, an algorithm with specific risk categories was developed and could be useful in the decision-making process, especially with respect of the timing of stoma reversal.