This nested case-controlled study of 42 women matched for age and ethnicity was designed to assess the effect of repeat OASI in a subsequent pregnancy on anorectal function. We found that 12 weeks after delivery, although there was no significant difference in anorectal symptoms, women who had sustained two OASIs had significantly reduced anal manometry pressures and more severe residual anal sphincter injury diagnosed on endoanal ultrasound compared to the controls. This study, to our knowledge, is the first study evaluating the effects of a second OASI on symptoms, anorectal function and sphincter integrity using a matched control group.
It is interesting to note that both groups of patients did not report significant symptoms including urgency or anal incontinence following the subsequent delivery as demonstrated by the low St Mark’s scores. This is probably due to the relatively short-term follow-up period of 12 weeks. Although the average subsequent delivery interval in both the control and study group was only 3 years, it is possible that on longer term follow-up the symptoms in both groups may become evident as demonstrated by studies after a single [15, 16] and after recurrent OASI . Conversely, Bøgeskov et al. found no significant difference in the prevalence and severity of anal incontinence among women who sustained one OASI versus two OASIs at a follow-up duration of 10 years following OASI .
Although it is widely accepted that faecal incontinence is a complex, multifactorial condition, continence is partially dependent on normal sphincter anatomy and function . The IAS contributes to approximately 85% of the total anal canal pressure at rest and so it is mainly responsible for anal continence at rest, the MRP [18, 19]. This is reinforced by the EAS, which supports the pressure within the anal canal and so protects continence during rises in intra-rectal or intra-abdominal pressure . In addition, the EAS is responsible for voluntary anal squeeze, MSP . Our study demonstrated that both MRP and MSP were significantly reduced in women with a history of recurrent OASI compared to a single OASI. This supports the findings of Jango et al. that women with recurrent OASI are more at risk of anal incontinence  and compromised anal sphincter function may manifest with symptoms over time.
The Sultan classification is used to grade OASIs and describes the extent of anal sphincter involvement [9, 21]. The extent of sphincter thickness involved is synonymous with the “depth of defect” subset score within the modified Starck scoring system . In the present study, there was a significant difference in the modified Starck score found between women with a repeat OASI in a subsequent pregnancy compared to a single OASI. This is an important finding because in the literature it has been shown that OASIs of a higher degree are associated with worse anorectal symptoms in the long term . However, there is conflicting evidence describing the effect of the degree of anal sphincter involvement diagnosed on EAUS on anorectal symptoms. In keeping with the present study findings, the severity of anal sphincter injury defined by EAUS has been described to correlate with incontinence symptoms [23, 24]. However, it is important to note that a recent large retrospective study showed that severity of anal incontinence was not associated with the extent and location of anal sphincter injury diagnosed on EAUS .
Strengths and limitations
Strengths of this study include the assessment of anal incontinence symptoms and anal sphincter defects using validated tools, including the SMIS  and modified Starck score . In addition, concerning the prospective collection of the data, to date it is the largest study of its design of women who have sustained two OASIs.
The study groups were controlled for ethnicity and age. Also, all women had subsequently achieved a vaginal delivery following index OASI. The aim of doing this was to minimise possible known and unknown confounders. Age was chosen because it has been shown to be an independent risk factor for anal incontinence in large cross-sectional studies [26, 27]. Maternal age at both first and subsequent delivery is also a risk factor for repeat OASI . Ethnicity was chosen because the risk of OASI at index delivery and recurrent OASI is higher among Asian ethnic groups . However, mode of delivery of the subsequent vaginal birth was not chosen because first delivery is considered the greatest risk to anal sphincter injury, as primiparous women are at an increased risk of OASI in comparison to multiparous . In this study there was no significant difference in mode of delivery of the subsequent pregnancy. In addition, there was no difference in rate of mediolateral episiotomy between the study and control group, which removed mediolateral episiotomy as a potential confounder. This strengthens this study as mediolateral episiotomy has been shown to be protective against repeat OASI .
Although median infant birth weight was significantly higher in the study group (3612.5 g [3397.5–3967.5]), it has been described in the literature that a birthweight > 4 kg is associated with an up to three-fold increase in the risk of recurrent OASI [5, 29]. However, it has been reported that a birth weight of > 3.5 kg increases the risk of recurrent OASI by 1.5-fold . The median birthweights of both groups may be expected, since our Perineal Clinic population median (IQR) birthweight from 2009 to 2019 was 3460 (3154–3760) g.
The literature shows that women with a higher grade tear are more likely to have an anal sphincter defect diagnosed on endoanal ultrasound, lower anal manometry pressures and anorectal symptoms [22, 30]. Another strength of this study is that the initial OASI tear grade was also removed as a potential confounder as there was no significant difference in OASI grade at index delivery between the two groups.
Limitations to this study include the short-term follow-up after a subsequent pregnancy. There is a need for long-term follow-up of these patients to establish the effect of two OASIs on anorectal symptoms and function, as one would expect worsening of symptoms at long term. In addition, anal manometry and endoanal ultrasound results from the index delivery where the first OASI occurred were not available for analysis. A difference in these findings may have contributed to the significant differences in the results of this study.
This nested case-controlled study has shown that, at 12 weeks, although women who sustained two OASIs had no difference in anorectal symptoms, there was worsening in anal manometry and extent of the anal sphincter defect. In the absence of randomised studies, this is the largest nested case-controlled study providing subjective and objective information following two OASIs that would be clinically useful in counselling women. This study provides further evidence to support the current recommendation that clinicians should counsel women with a history of OASI of the potential risk of new or worsening anal incontinence following a subsequent delivery. However, it is important to note that although anal sphincter anatomy is important in maintaining continence, anal incontinence is a multifactorial condition.