Sydney Pelvic Floor Health

Anal Sphincter Muscle

Obstetric Anal Sphincter Injury (OASI):

Third or Fourth Degree Perineal Tear

“When compared to the hands, the sphincter ani is far superior. If you place into your cupped hands a mixture of fluid, solid and gas and then, through an opening at the bottom try to let only the gas escape you will fail. Yet the sphincter ani can do it. The sphincter apparently can differentiate between solid, fluid, and gas. It apparently can tell whether the owner is alone or with someone, whether standing up or sitting down, whether its owner has his pants on or off. No other muscle in the body is such a protector of the dignity. A muscle like that is worth protecting.”

 Walter C. Bornemeier, a former president of the American Medical Association


Obstetric anal sphincter injury (OASI) is thought to be the most important risk factor for female anal incontinence (inability to control bowel movements, causing stool to leak unexpectedly from back passage). OASI is a severe maternal complication during a vaginal delivery and occurs even in otherwise normal, easy deliveries, but it's commonest after Forceps. In addition to anal incontinence, OASI may cause pain, discomfort and problems on intercourse. Reported incidences of OASI vary from 1 to 20% in different countries and between delivery units, but these tears are often overlooked. Of women who have sustained such an injury, 60-80% have no symptoms at 12 months, of whom most report incontinence of flatus only. Most women with faecal incontinence will not volunteer this information unless asked because of embarrassment and fear of stigma.

What is a perineal tear?

Many women (90%) experience tears to some extent during childbirth as the baby stretches the vagina, especially with the first baby. Most tears occur in the perineum, the area between the vaginal opening and the anus (back passage). First-degree tears are small and skin-deep and can heal without sutures. Tears that are deeper and affect the muscles of the perineum are known as second-degree tears. These usually require stitches. An episiotomy is a cut made by a doctor or midwife through the vaginal wall and perineum to make more space to deliver the baby, and it's always repaired after the birth. Figure 1 shows how the muscles of the perineum are stretched during crowning of the baby's head, which is when most of the damage occurs. Figure 2 shows the appearance of different degrees of perineal tears on the surface.


Figure 1: Crowning of the baby's head and its effect on the perineal muscles.

What is obstetric anal sphincter injury (OASI)?

If a tear is deeper it can affect the anal sphincter, and this is called a '3rd degree' tear. Third degree tears extend downwards from the vagina and perineum to the anal sphincter, the muscle that controls the anus. If the tear extends further into the lining of the anus or rectum it is known as a fourth-degree tear (see Figure 2).


Figure 2: Perineal tears as seen after childbirth.

How common are third or fourth degree tears?

Overall, a third- or fourth-degree tear occurs in 5-20% of women having a vaginal birth. It is more common with a first vaginal birth. Only about 1/4 or 1/3 of them are detected in Labour Ward. We're not sure why this is, but the better the training of a doctor or midwife, the better they are at picking up such tears.

What increases risk of a third or fourth degree tear?

These types of tears usually occur unexpectedly during birth and most of the time it is not possible to predict when it will happen. However, it is more likely if:

it's a first vaginal birth

One of baby’s shoulders becomes stuck behind the pubic bone delaying the birth of the baby’s body, which is known as shoulder dystocia

The second stage of labour is longer than expected

Large baby (over 8 pounds 13 ounces or 4kg)

Forceps or Vacuum (which is less risky)

Previous history of a third or fourth degree tear

previous Caesarean.

Could anything have been done to prevent this type of tear?

Usually, a third or fourth degree tear cannot be prevented because it cannot be predicted. Your midwife or obstetrician may protect the perineum as your baby’s head is delivering and this may help prevent a tear. Whether an episiotomy will prevent an OASI from occurring during a normal vaginal birth is unclear. An episiotomy will only be performed if necessary, and with your consent. If you have an assisted birth (ventouse or forceps), you are more likely to have an episiotomy as it may reduce the chance of a OASI occurring.

What will happen if I have an OASI?

When a third or fourth degree tear is suspected or confirmed, this will usually be repaired in the operating theatre. You will need regional (epidural or spinal) anaesthetic but occasionally a general anaesthetic may be necessary. The obstetrician will then stitch the tear including the damaged anal sphincter.

After the operation you will be:

offered pain-relieving drugs such as paracetamol, ibuprofen or diclofenac to relieve any pain

advised to take a course of antibiotics to reduce the risk of infection because the stitches are very close to the anus

advised to take laxatives to make it easier and more comfortable to open your bowels.

catheterised to drain the urine until you are able to walk to the toilet.

referred to the Physiotherapist or Continence Nurse Advisor for perineum/pelvic floor muscle strengthening exercises to prevent possible problems with bowel control.

referred to a Perineal Clinic (if such services exist in your area) to check on healing and the quality of the repair.

Once you have opened your bowels and your stitches have been checked to see that they are healing properly, you should be able to go home. None of the treatments offered will prevent you from breastfeeding

What can I expect afterwards?

It is normal to feel pain or soreness around the tear or cut for two to three weeks after giving birth, especially when walking or sitting. Passing urine may also cause stinging. You can continue to take your painkillers when you go home. The stitches usually dissolve within a couple of weeks and full healing can take up to 6-8 weeks. As healing takes place, the stitches can irritate or you may notice some stitch material fall out but this is normal. Some women feel that they pass wind more easily or need to rush to the toilet to open their bowels. Most women make a good recovery, particularly if the tear is recognised and repaired at the time: 6–8 in 10 women will have no symptoms a year after birth.

What can help me recover?

Keep the area clean. Have a bath or a shower at least once a day and change your sanitary pads regularly. This will reduce the risk of infection.

An ice pack may be helpful if your perineum is bruised and/or swollen.

You should drink at least 2–3 litres of water every day and eat a high fiber diet (fruit, vegetables and cereals). This will ensure that your bowels open regularly and will prevent you from becoming constipated.

Avoid heavy lifting, pushing or pulling while allowing the perineum to heal. Also, avoid straining while emptying your bowel.

Strengthening the muscles around the vagina and anus by doing pelvic floor exercises can help healing. It is important to do pelvic floor exercises as soon as you can after birth. You should be offered physiotherapy advice about pelvic floor exercises to do after surgery.

Looking after a newborn baby and recovering from an operation for a perineal tear can be hard. Support from family and friends can help.

When should I seek medical advice after I go home?

You should contact a doctor, preferably a gynaecologist, if:

the stitches become more painful or smelly – this may be a sign of an infection

you cannot control your bowels or flatus (passing wind).

Talk to your obstetrician/ gynaecologist if you have any other worries or concerns. You can be referred back to the hospital before your follow-up appointment if you wish, and if you're lucky there will be a dedicated service ('perineal clinic').

When can I have sex?

Normally, many women feel sore in the weeks after having a vaginal birth. If you have had a tear, sex can be uncomfortable for longer. You should wait to have sex until the bleeding has stopped and the tear has healed which may take up to 6-8 weeks. After that you can have sex when you feel ready to do so. A small number of women may continue to have painful sex. Talk to your gynaecologist if this is the case so that you can get the help and support you need.

Your follow-up appointment

You may be offered a follow-up appointment at the hospital 6–12 weeks after you have had your baby to check that your stitches have healed properly. If facilities are available, follow-up of women with OASIS should be in a dedicated perineal clinic with access to perineal ultrasonography and anal manometry as this can aid decision making regarding future delivery. You will be asked questions about whether you have any problems controlling your bowels. You will also have the opportunity to discuss the birth and any concerns that you may have.

Can I have a vaginal birth in the future?

Most women go on to have a straightforward birth after a third- or fourth-degree tear. However, there is an increased risk of this happening again in a future pregnancy. Between 5 and 7 in 100 women who have had a third- or fourth-degree tear will have a similar tear in a future pregnancy. If you have recovered well and do not have any symptoms, you can consider a vaginal delivery. If you continue to experience symptoms from the third or fourth degree tear, you may wish to consider a planned caesarean section.

You will be able to discuss your options for future births at your follow-up appointment or early in your next pregnancy. Your individual circumstances and preferences will be taken into account.

What is Anal Sphincter Imaging?

This involves taking ultrasound pictures of the muscles around your back passage. It is a quick and simple test. It is painless and does not require any sedation or anaesthetic. You do not need to take any laxatives before the test. It is often carried out at the same time as anorectal physiological tests.

The endoanal ultrasound (EAU) (Figure 3), using high resolution probes with a field of vision of 360 degrees, is an internal examination. EAU is not universally available and distorts the anatomy. It does not allow dynamic evaluation of the anal sphincter and mucosa on sphincter contraction, which seems to enhance the definition of muscular defects. The invasiveness of the test makes routine imaging after childbirth near- impossible and has hampered uptake into clinical practice.


Figure 3: Endo-Anal Ultrasound


Exoanal or transperineal ultrasound imaging (Figure 4) does not have these disadvantages and is increasingly used to evaluate the anal sphincters. It is not internal- the transducer is placed on the outside as seen in Figure 4. A recent study comparing three-dimensional (3D) transperineal ultrasound (TPU) and two-dimensional endoanal ultrasound (EAU) for the detection of anal sphincter defects showed good agreement between the two techniques. TPU also allows assessment of the pelvic floor muscles i.e. levator ani and permits quantification of pelvic organ descent.


Figure 4: Transperineal (pelvic floor) Ultrasound: transducer placement, schematic drawing and ultrasound image.

Dr Nisnamini Subramaniam