What is OASIS?

Obstetric anal sphincter injuries, otherwise known as OASIS, may occur during a vaginal delivery.  OASIS occurs when the degree of perineal trauma is categorized as a third or fourth degree tear.  

A third degree tear involves injury to the perineum and the anal sphincter complex.  Third degree tears can be further subcategorized:

  • 3a Less than 50% of the external anal sphincter (EAS) thickness torn 
  • 3b More than 50% of EAS thickness torn 
  • 3c Both EAS and internal anal sphincter (IAS) torn 

Fourth degree perineal tears are considered the most severe and involve the entire anal sphincter complex (EAS and IAS) and anal epithelium.

The World Health Organization’s International Classification of Diseases reports an OASIS incidence of 4-6.6% of all vaginal births.  It is reported that there is a higher incidence occurring with assisted deliveries (6%) compared to vaginal births in which no instrumentation was used to assist the birth (5.7%).  Instrumentation would include the use of forceps and/or vacuum.  

Functionally, the anal sphincter complex is very important for fecal continence.  The EAS and IAS maintain a tonic, baseline resting pressure within the anal canal to ensure feces or gas does not involuntarily exit the anus.  When the brain decides the timing is right to allow for a bowel movement or release of gas, these structures relax to facilitate defecation and passing gas.

The potential consequences of injury to the anal sphincter complex include the risk of postpartum pelvic floor dysfunction and symptoms such as anal incontinence, rectovaginal fistula (definition below), urinary retention, difficulty evacuating bowels, and perineal pain.  These associated symptoms can have a significant distressing impact on those who have just given birth by impairing their quality of life.  Long term symptoms can develop such as pain with intercourse and pelvic organ prolapse.

What are the Risk Factors for OASIS?

The factors that have been identified as increasing the risk for OASIS include asian ethnicity, first time births, a birthweight of greater than 8 pounds, shoulder dystocia, baby positioned in occipito-posterior position, prolonged second stage of labour, and use of vacuum or forceps.  There is more risk associated with forceps deliveries compared to vacuum-assisted vaginal deliveries.  There is very limited evidence to provide any information on the risk of sustaining recurring OASIS for multiparous individuals.   



The Society of Obstetricians and Gynecologists of Canada (SOGC) clinical practice guidelines include various recommendations to reduce the incidence of OASIS during vaginal delivery.  One such recommendation suggests obstetrical care providers should slow down fetal head crowning by not coaching the labouring individual to push but to allow the expulsive efforts of the uterus to dominate at this moment in the birthing process.  A 2011 Cochrane Review recommended a warm compress on the perineum as well as the technique of perineal massage to be performed during labour to further help decrease the risk of OASIS.  Upright positions are often encouraged to support gravity assistance when delivering a baby.  However, some research suggests that standing with buttocks unsupported may slightly increase the risk of OASIS versus birthing upright with sit bones supported.  There is significant debate in the literature over whether or not an episiotomy may help protect from OASIS versus lead to an increased risk of OASIS.  The general rule is an obstetrical care provider must follow a policy of only performing an episiotomy if indicated versus as a routine procedure.  A consensus that has been made is that when an episiotomy is used, a midline/vertical incision increases the risk of OASIS versus instituting a diagonal or mediolateral approach.  If instrumental delivery is indicated, vacuum extraction carries less risk to injuring the anal sphincter than forceps.


Pelvic Physiotherapy

The SOGC guidelines recommend pelvic floor physical therapy as one of the first lines of treatment to address any symptoms associated with OASIS.  Pelvic floor physical therapists are able to assess and guide a comprehensive exercise program with the goals of regaining strength within the anal sphincter complex.  Therapists are able to identify and treat any areas of connective tissue restriction or scarring in the perineal area that may be contributing to pelvic muscle weakness and pain.


****A rectovaginal fistula occurs when the perineal tear has created an opening within the muscular wall that connects the vagina to the rectum.  Symptoms associated with this is the passive leakage of liquid feces or gas from the vaginal opening.  There is an associated increased risk of development of a vaginal infection.   





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