Normal Female Anal Sphincter

Normal Female Anal Sphincter Difficulties in Interpretation Explained

R.C.Bollard,F.R.C.S.,*A.Gardiner,B.Sc.,?S.Lindow,F.R.C.O.G.,F.C.O.G.(S.A.),?K.Phillips,M.R.C.OG.,§G.S.Duthie,F.R.C.S.,M.D.?

From the*Royal Lancaster Infirmary,Lancaster,United Kingdom,?Department of Gastrointestinal Physiology,§Department of Obstetrics and Gynaecology,and?Academic Surgical Unit,University of Hull, Castle Hill Hospital,Cottingham,United Kingdom,and?Academic Department of Obstetrics and Gynaecology,The Maternity Hospital,Hull,United Kingdom

PURPOSE:Our aims were to quantify the nature,character-istics,and frequency of variations in female anal sphincter anatomy.METHODS:Nulliparous patients from the antena-tal clinic and healthy volunteers of both genders were studied.Sphincter length was determined by the position of the puborectalis sling.Defects in the external anal sphincter were defined at each level and recorded in degrees.Cylin-dric longitudinal images of the endoanal scans were created by a three-dimensional-representation software package. Manometry was performed by a pull-through technique. RESULTS:Fifty-seven nulliparous patients and18healthy volunteers were included in the study.The mean age was 39years for males and28.35years for females.There was no significant difference in overall sphincter length or in the internal anal sphincter length as a percentage of overall sphincter length between genders.All nine males had a complete ring of external anal sphincter along the full sphincter length.In the external anal sphincter below the level of the puborectalis sling,a natural gap occurred in43 nulliparous(75percent)and all9female volunteers.The greater the size of the defect,the greater its extent(mean 1.33cm for?90°and1.16cm for?90°;chi-squared P?0.008,eight degrees of freedom).Manometry provided con-firmatory evidence of the gaps seen.Anal manometry was analyzed by Mann-Whitney U test for continuous nonpara-metric data and t-test for comparison between genders. CONCLUSION:The female sphincter has a variable natural defect occurring along its anterior length.This makes inter-pretation of the isolated endoanal ultrasound difficult and explains previous overreporting of obstetric sphincter de-fects.[Key words:Female anal sphincter;Endoanal ultra-sound;Normal anatomy]

Bollard RC,Gardiner A,Lindow S,Phillips K,Duthie GS. Normal female anal sphincter:difficulties in interpretation explained.Dis Colon Rectum2002;45:171–175.

E ndoluminal ultrasound has been used to assess

anal sphincter morphology and integrity since the early1990s.1,2Studies correlating anal en-dosonography with in vitro and in vivo anatomy have also been important to help our understanding of the sphincter muscle complex.3,4Nevertheless,there are conflicting interpretations of anal sphincter anatomy. The outermost echogenic band is interpreted by most authors as the endosonographic representation of the external anal sphincter(EAS)3,5;however,the inclu-sion and significance of the longitudinal muscle layer in this outer echogenic band is a matter of some debate.1,3,6,7

Differences in the anal sphincter occur between genders.3,8In particular,there appears to be a gap anteriorly in the EAS in females.The nature,charac-teristics,and frequency of this variation in the EAS in females have not been quantified previously.It is important that we do so to understand the anatomy of the sphincter complex so that accurate interpretation of postnatal endoanal ultrasound can be made.

METHODS

Nulliparous patients were recruited from the ante-natal clinic in the first trimester of pregnancy.En-dosonography of the anal sphincter was undertaken with the Brujer and Kruger Medical axial10-MHz360°endoscopic probe(diameter,1.7cm;Copenhagen, Denmark).Examination required no patient prepara-tion.Patients were examined in the left lateral posi-tion.The probe was inserted into the anal canal,and a360°image was obtained.By performing standard 1-cm-level endoanal images from the anal verge,it was noted that there were natural defects in the EAS of these nulliparous patients.Definition was further obtained by concentric0.5-cm-level images,0.5to3 cm from the anal verge.Sphincter length was deter-mined by position of the puborectalis(PR)sling.De-fects in the EAS were defined at each level and re-corded in degrees.Hard copies were taken of the scan for independent review.The serial0.5-cm slices were processed in Adobe?Photoshop?(Adobe Sys-

Presented at the meeting of the Association of Surgeons of Great Britain and Ireland,York,United Kingdom,October16,1998,and York Medical Society Founders,York,United Kingdom,November 13,1998.

Address reprint requests to Mr.Duthie:Honorary Reader in Sur-gery,Academic Surgical Unit,University of Hull,Castle Hill Hospi-tal,Castle Road,Cottingham,North Humberside HU165JQ,United Kingdom.

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tems Incorporated,San Jose,CA)then stacked to-gether by use of Macromedia FreeHand 10(Macro-media,Inc.,San Francisco,CA)to create a cylindric three-dimensional representation of the anal sphinc-ter.Having defined natural gaps in nulliparous pa-tients,we then went on to study healthy volunteers with no history of surgery trauma or anorectal dis-ease.

Anal manometry was performed with a solid-state transducer (Gaeltec,Isle of Skye,UK)mounted in a flexible catheter by use of a station pull-through tech-nique.Resting anal pressures and squeeze pressure were recorded at 1-cm intervals from the anal verge.

Statistical Methods

Anal manometry was analyzed by Mann-Whitney U test for continuous nonparametric data and Student’s t -test for comparison between genders.Categoric data were assessed by Pearson chi-squared analysis.

RESULTS

A total of 57nulliparous patients were studied.There was no history of surgery,anorectal disease,or trauma to the anal canal in any of the patients.Mean age was 27.8(standard deviation,4.21)years.Mean

sphincter length as determined by the level of the PR sling was 2.7(standard deviation,0.48)cm.

Thirteen patients had a full 3-cm-long cylindric sphincter without any gap in the sphincter.Figure 1illustrates the six serial 0.5-cm slices and the same slices stacked by use of the freehand software pack-age to form a three-dimensional representation in the form of a cylinder.

Below the PR,a natural gap was seen in 44of 57of nulliparous patients (Table 1).The longer the anal canal,the larger the potential defect was.The extent of defect varied,but the median extent of defect was 2(interquartile range,1–2.5)cm.The larger the angle of defect,the greater was the overall extent of defect,and this reached statistical significance (Pearson chi-squared,degrees of freedom ?10,P ?0.000).

Eighteen healthy volunteers evenly split between genders were then studied.The mean age was 39years for males and 28.35years for females.There was no significant difference in sphincter length (median 3.23cm for males and 3.0cm for females),internal anal sphincter (IAS)length (median 2.75cm in both genders),or the IAS length as a percentage of overall sphincter length (median,91.5percent in males and 100percent in females)between genders.All nine males had a complete cylindric ring of EAS along the full sphincter length.In the EAS below the level of the PR,a natural gap occurred along the anterior longi-tudinal sphincter length in all females studied (Table 1).

Figure 2A demonstrates the serial 0.5-cm slices in a nulliparous woman in whom there was an easily de-termined PR sling at 3and 2.5cm.Below the PR entering the sphincter,there is a definite gap anteri-orly,which is also present,although not as wide,at 1.5cm.At 0.5cm,there is a complete ring of EAS,but no IAS is seen.Figure 2B illustrates the construction of the same slices to form the three-dimensional image;the longitudinal appearance of the anterior

hypo-

Figure 1.Three-dimensional reconstruction of a full cy-lindric anal sphincter.

Table 1.

Extent of Natural Gaps

Sphincter Length (cm)No Gap,Nulliparous

Gap ?90°

Gap 90–180°

Nulliparous

Normal Nulliparous

Normal 3.0131321652.5031312.0040401.501100Total

13

21

4

23

6

172BOLLARD ET AL Dis Colon Rectum,February 2002

Vol.45,No.2NORMAL FEMALE ANAL SPHINCTER DIFFICULTIES173

Figure2.A.Serial6?0.5-cm slices of nulliparous patients with gap.B.The stacked three-dimensional image with anterior gap highlighted.

echogenic defect can be seen clearly and is high-lighted.

The extent of defect varied,but the median extent of defect was1.5(interquartile range,0.75–1.5)cm. The greater the size of the defect,the greater was its extent(mean1.33cm for?90°and1.16cm for?90°, chi-squared,degrees of freedom?8,P?0.008). Manometry confirmed differences between genders (Table2).Differences in resting and squeeze pressures between genders became more significant the more proximal the recording in the anal canal.Mean squeeze pressure and resting pressures in the proximal and mid anal canal were significantly lower in those with larger defects than in those with no defect(Table3).When the extent of defect reached1.5cm,the differences in ma-nometry also became significant(analysis of variance). Thus,we have manometric evidence of the gaps we are defining using endoanal ultrasound.IAS widths were measured at12,3,6,and9o’clock in the mid anal canal. The only significant difference between genders was that at12o’clock,the mean was1.64mm in males and 2.8mm in females(P?0.03by t-test).There were obvious differences between these gaps and the sphinc-ter ruptures that have been reported previously(Table4, Fig.3).

DISCUSSION

The EAS has been defined in several ways.1,3,5,7,9Use of the PR sling as the landmark below which the true sphincter is identified makes interpretation of this sling as a defect less likely.We have shown that despite this, in the true sphincter(i.e.,below the PR),the female EAS varies,and uniformly hypoechogenic areas with smooth,regular edges can be seen.A greater IAS width anteriorly in females may reflect a compensatory mech-anism.Manometric differences between genders are greater in the proximal and mid sphincter areas,i.e.,the same sites as natural gaps,which provides confirmatory evidence of gaps in the EAS.

Two studies have examined the anal sphincter in healthy volunteers.8,9Differences between genders in the morphology of the anterior part of the EAS were illustrated on MRI.8Brunese et al.9studied16subjects and defined three levels for endoanal sonography, i.e.,deep,intermediate,and superficial planes.Al-though they found greater IAS widths in individuals older than44years,there was no reported difference in EAS.

Our findings suggest that great care needs to be taken in the interpretation of the isolated postnatal

Table2.

Manometry Pressures Depending on Level of Anal Canal

Level of Canal

(cm)

Mean Resting Pressure(cm H

2

O)Mean Squeeze Pressure(cm H

2

O) Male Female t-Test Male Female t-Test

13554.50.253981170.52 298.8864.50.0461931160.093 376.2523.330.004160450.001

Table3.

Comparison of Manometry with Large Natural Gaps Compared with No Gap

Level of Canal

(cm)

Mean Resting Pressure(cm H

2

O)Mean Squeeze Pressure(cm H

2

O)

No Gap

Large Gap

(?90°)

Mann-Whitney

P Value

No Gap

Large Gap

(?90°)

Mann-Whitney

P Value

135450.263981130.557 295560.0441621210.059 376.25220.012160480.017

Table4.

Comparison of Natural Sphincter Gaps and Ruptures

Natural Gap Rupture

Anterior uniform hypoechogenicity Anterior mixed hyperechogenicity

Edges smooth and uniform Edges ragged and irregular

Symmetric Asymmetric

174BOLLARD ET AL Dis Colon Rectum,February2002

endoanal scan.Natural gaps can be discriminated from a sphincter rupture by the above criteria.EAS rupture,in contrast,has irregular edges;the anterior sphincter has mixed echogenicity,which is in keep-ing with scarring and is often associated with IAS defects.In addition,natural gaps appear to occur in the upper part of the sphincter,and therefore any obvious irregularity of the EAS in the distal anal canal should be treated with suspicion.These interpretative difficulties may explain the high false-positive transanal ultrasounds reported in the literature.10

Without the above definitions of normal anatomy,we would have reported a false-positive rate of sphincter injury of 10percent of nulliparous scans when assessing the lower 1.5cm,and this rate in-creased when the sphincter was examined above this level.Finally,studies to date have used a 7-MHz transducer.It is widely accepted that the higher-fre-quency 10-MHz transducer gives better resolution of both the IAS and EAS sonographic image.2This may

explain why we have been able to more accurately define these natural defects and traumatic disruptions.

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Figure 3.The difference between a natural gap and a ruptured sphincter.

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