Hypothesis / aims of study
Background
The rate of obstetric anal sphincter injury (OASI) in the United Kingdom has risen over the last two decades and is currently estimated at 2.9%.(1) Some have suggested this is due to greater awareness and training amongst doctors and midwives, whilst others cite advancing maternal age in the primiparous, obesity, type 2 diabetes mellitus and overdiagnosis estimated at 7% as contributory factors.(2)
Importantly, despite primary repair, the rate of anal canal defects identified over 12 months post-delivery remains high at 50-85% with 36-39% reporting anal incontinence (3).
Aim
This study aims to assess the rate of defects between symptomatic and asymptomatic individuals following a 3a tear. Secondary aims include assessing the rate and degree of defects found upon endoanal ultrasound scan (EA USS), and rate of symptoms in patients with defects in 3a compared to higher degree tears.
Study design, materials and methods
The notes of 305 patients with a history of OASI referred to an outpatient clinic at a UK teaching hospital between January 2020- December 2022 were retrospectively analysed. Patients were seen from 3 months postnatally and/or in the third trimester of a subsequent pregnancy. Exclusion criteria included: clinic non attendees, did not consent to EA USS , and degree of tear unknown.
OASI diagnosed clinically was grouped as per Sultan classification and compared with EA USS findings from 3 months postnatally or in the third trimester of a subsequent pregnancy. Anal canal defects (either partial or full thickness) were noted by a single assessor if hypoechoic areas in the external (EAS) or internal anal sphincter (IAS) were seen. Patients reporting faecal or flatal incontinence for over 3 months were deemed symptomatic.
Results
Asymptomatic patients made up 76% of those with a 3a tear but the likelihood of defects compared to symptomatic patients was not statistically significant (x2= 2.56, p=0.10). Overall, 62% of patients had a defect on EA USS post primary repair; 46% of 3a, 74% of 3b, 63% of 3c and 92% of 4th degree tears. Out of the total number, patients with a 3a tear were noted to have the least risk of defects on scan; RR=0.633 [CI 0.478-0.838].
There were no significant differences in the degree [level, thickness, or involvement of more than 1 portion of the canal] of defect between 3a and higher degree tears (p = 0.72), although majority occurred in the upper canal (69%).
The rate of faecal incontinence symptoms was 35% overall and patients with 3a tears with a defect were just as likely to have symptoms 3 months postnatally as higher degree tears (x2=1.18, p=0.278).
Interpretation of results
The overall defect rate (62%) and percentage of patients with faecal incontinence symptoms for our cohort (35%) is comparable to existing literature rates.
As 3a tears had a significantly lower defect rate compared to higher degree tears, there may be an element of over-diagnosis occurring. However, as there was an insignificant correlation between asymptomatic individuals having an intact anal canal, patients should continue to be offered EA USS post OASI to guide future management.
Interestingly, as majority of defects occurred in the upper canal, this has identified training needs as incomplete inclusion of all torn fibres in the repair or inadequate healing secondary to infection may have contributed to these findings.
Our study was limited by the sample size and therefore large cohort studies are recommended to avoid type 2 error.