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Anatomy clinical correlates: Female pelvis and perineum

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Obstetrics and gynecology

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The pelvis lies between the abdomen and the lower limbs, forming the lower part of the trunk. It supports and contains organs of the gastrointestinal system, the urinary system, and the reproductive system.

Furthermore, the structure and contents of the pelvis differs between biological male and biological female individuals.

These anatomic differences are important to understand as they have large clinical implications in biological females for things such as fertility and childbirth.

So, why don’t we make like a baby and dive into the clinical correlates of the female pelvis and perineum head first!

Speaking of babies, as cute and adorable as they are, pregnancy and childbirth can lead to a number of complications such as the risk of perineal or pelvic floor injury.

The pelvic floor holds the pelvic organs in a stable position, and during childbirth the pelvic floor makes every effort to support the fetal head.

During delivery the fetal head stretches the pelvic floor, frequently resulting in injury to the perineum, levator ani, and ligaments of the pelvic viscera.

Specifically, injury to the pubococcygeus and the puborectalis muscles of the levator ani often occur. These muscles surround and support the urethra, vagina, and anal canal.

So injury to these muscles can lead to decreased support for the vagina, bladder, uterus, or rectum.

Another important structure that can be damaged is the perineal body, which is the fibromuscular center point of the perineum, that serves as an attachment point for structures such as the muscles of the levator ani, fibers from the external urethral sphincter and urethrovaginal sphincter, and the external anal sphincter.

Damage to the pelvic floor or perineal body can lead to urethral hypermobility and stress urinary incontinence, which is when drops of urine come out when intra abdominal pressure increases.

Unfortunately, this can occur with coughing, lifting, laughing, and constipation. Also, if the injury to the pelvic floor is severe enough, a pelvic organ may lose its support and herniate into the vagina, called pelvic organ prolapse.

Examples of this would be prolapse of the bladder into the anterior vaginal wall, the rectum into the posterior vaginal wall, or the uterus through the vaginal orifice.

Childbirth can also cause damage to the pudendal nerve, which can be stretched and damaged along its course near the ischial spine, especially during prolonged labour, when there’s pressure on the pelvic floor for a longer period of time.

The pudendal nerve provides motor and sensory innervation to the perineal region, so damage to it can cause weakness in the perineal muscles.

This can lead to inability to control bowel movements, leading to fecal incontinence. Individuals may also experience urinary incontinence, as well as perineal pain, and sexual dysfunction.

Now, trying to prevent injury to the pelvic floor or management of stress urinary incontinence can be done through Kegel exercises, which aim to strengthen the pelvic floor muscles by contracting and relaxing the levator ani muscles a few times each day, which is similar to the contraction when trying to stop peeing midstream.

Regularly performing these exercises lowers the risk of them getting injured during childbirth, which lowers the risk of having a stress incontinence.

On the other hand, sometimes an episiotomy can be performed during childbirth. This is an intentional incision of the female perineum and the posterior vaginal wall.

This incision enlarges the opening for the baby to pass through, so it reduces the risk of severe perineal lacerations and helps accelerate delivery during the second stage of labor.

Episiotomy is typically done in cases of fetal distress or a large baby that is stuck and can’t get out. There are two main ways to perform an episiotomy.

First is median episiotomy, which runs from the posterior vaginal wall to the perineal body, and tries to spare the external anal sphincter and rectal muscles.

This type of episiotomy is done because it’s thought that the fibrous tissue that develops during healing is similar to that of the already tendinous perineal body.

However, even though care should be taken not to injure the perineal body, median episiotomy can cause damage to the perineal body.

Next is mediolateral episiotomy, which runs at 45 degrees from the middle of the vaginal opening to the buttocks.

This type has much lower risk of injuring the anal muscles, but it can potentially cause more pain, more bleeding, and a more difficult repair.

Now, childbirth can be painful, but for those soon to be mothers - don’t worry, because there are different anesthetic techniques which can be used for pain control. These techniques include general and regional anesthesia.

General anesthesia makes the mother become unconscious and is often used during emergency procedures, as it completely eliminates the pain and discomfort of delivery. Here, both the mother and the baby are continuously monitored, and childbirth happens passively.

Next is regional anesthesia, which is used for mothers who wish to participate in the delivery, but don’t want to experience the pain.

There are three types of regional anesthesia; spinal anesthesia, epidural block, and pudendal nerve block.

With spinal anesthesia, the anesthetic agent is introduced with a needle into the subarachnoid space at the L3-L4 vertebral level and anesthesia usually occurs within 1 minute.

The spinal anesthesia produces a temporary anesthesia below the waist, so individuals can’t feel pain in the perineum, pelvic floor and birth canal, or the sensation of uterine contractions, and the motor and sensory functions of the entire lower limbs.

This means that the mother is conscious, but they depend on electronic monitoring of uterine contractions to coordinate pushing.

However, bear in mind that with extended labor or inadequate anesthesia, it’s almost impossible to readminister the spinal anesthesia.

Furthermore, the anesthetic agent ascends proximally and circulates into the cerebral subarachnoid space in the cranial cavity when the individual lies flat, so the most common side effect of this type of anesthesia is a severe headache.

Patients may also experience a decrease in blood pressure during administration of the anesthesia, as well as pain around the injection site.

Next is the epidural block, which uses a catheter to inject an anesthetic agent into the epidural space within the vertebral canal, and does not pierce the dura or subarachnoid mater to enter the subarachnoid space like the spinal anesthetic.

Similar to the spinal anesthetic, this method affects all the nerves below the level of administration, especially trying to target the S2-S4 nerve roots.

This causes anesthesia of the superior vagina and the uterine cervix, as well as the structures supplied by the pudendal nerve which include the entire birth canal, the pelvic floor, and the majority of the perineum.

Epidural anesthesia can be titrated based on delivery, but is typically not as potent as spinal anesthesia, so the individual can often still feel and move their legs.

Now, the body of the uterus lies above the pelvic pain line, so visceral pain from the uterine body will travel with sympathetic fibers to the spinal cord at the level of T10 to L1, where epidural anaestheia is usually given at the L2 to L4 level.

But since the epidural space is a continuous space along the length of the vertebral canal, the anaesthetic agent can travel superiorly and often blocks those sympathetic pathways at the T10 to L1 level, which is great for blocking all of the relevant pain pathways in labour!

Furthermore an epidural block when done correctly doesn’t cause a headache as the spinal and epidural spaces are not continuous.

However, headache may be a potential side effect if the needle is inserted too far and penetrates the dura.

Epidural anaesthetic must be administered in advance of delivery as it takes time to become effective.

Finally we have the pudendal nerve block, which is used when a woman has progressed too far into labour and is unable to receive an epidural.

Here a needle is used to inject the anesthetic agent into the tissues that surround the pudendal nerve. This is done by palpating the ischial spines intravaginally, and injecting the anesthetic intravaginally, through the sacrospinous ligament, medial to the ischial spine.

This anesthetizes the area supplied by the pudendal nerve, which includes some of the area supplied by dermatomes S2-S4 and the lower quarter of the vagina.

This helps in the second stage of labor, or when the baby is descending through the birth canal. Now, the upper part of the vagina, the uterine cervix, and the uterine body are not anesthetized, so the mother still feels uterine contractions during delivery.

Complications include incidental injection or injury to the pudendal artery and vein which run in close proximity to the nerve, causing a hematoma.

Okay, now let’s take a deep breath and have a quick quiz! Can you name each of these types of regional anesthesia?

That’s right, the first area is where spinal anesthesia and epidural blocks can be administered. Both spinal and epidural anesthesia will block everything inferiorly, where spinal anesthesia works quicker and is typically more potent.

Yup, next is the pudendal nerve block. Note how the anesthetic agent is pushed into the tissues surrounding the pudendal nerve.

Alright, now, some bleeding is normal after delivery. However, when there’s more bleeding than expected, this is called postpartum hemorrhage, or PPH for short.

Postpartum hemorrhage is defined as losing more than 500 milliliters of blood in a vaginal delivery, or more than 1000 milliliters in a cesarean section.

There are two types of postpartum hemorrhage; primary PPH, which happens in the first 24 hours of delivery, and secondary PPH, which happens after the first day up to six weeks after delivery.

The main causes of postpartum hemorrhage can be remembered using the 4 T’s. Lack of uterine Tone, Trauma such as lacerations or uterine damage, Thrombin which refers to issues with coagulation, and Tissue which refers to retained products of conception.

Okay, postpartum hemorrhage is mainly caused by uterine atony which is loss or uterine tone, one of our four T’s, and happens when the muscles of the uterus fail to contract after delivery.

Sources
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  2. "Miller's Anesthesia" Elsevier Health Sciences (2009)
  3. "Περίς, πηρίς and περίναιος" Emerita (2016)
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  5. "The prevention and treatment of postpartum haemorrhage: what do we know, and where do we go to next?" BJOG: An International Journal of Obstetrics & Gynaecology (2014)
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  7. "Diagnosing ectopic pregnancy and current concepts in the management of pregnancy of unknown location" Human Reproduction Update (2013)
  8. "The erogenous zones: their nerve supply and its significance" Proc Staff Meet Mayo Clinic (1959)
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