Week 3 - Anatomy Tutorial - Anatomy of Anaesthesia for labour

Question 1

What are the fontanelles of the foetal skull? What are the sutures of the foetal skull coming from the fontanelle?

Answer 1

• Anterior fontanelle - frontal suture, coronal suture, saggital suture
• Posterior fontanelle - lamboid suture, saggital suture
• Sphenoidal fontanelle (anterolateral fontanelle)
• Mastoid fontanelle (posterolateral fontanelle)
• Squamous suture joins these two fontanelles

Question 2

After the anterior fontanelle closes, what is it known as? When does the anterior fontanelle close?

Answer 2

Anterior fontanelle is known asbregma after its closure It closes around 18 months

Question 3

What bony features form the pelvic inlet and outlet?

Answer 3

Pelvic inlet - sacral promontory, ala of sacrum, linea terminalis (arcuate + pectineal line + pubic crest), pubic symphysis Pelvic outlet - pubic symphysis, inferior pubic ramus to ischial tuberosities,, sacrotuberous ligament, coccyx

Question 4

What is the vertex of the foetal head formed by?

Answer 4

Vertex- diamond shape between the anterior and psoterior fontanelles and the parietal eminences The vertex point is the highest point on the foetal head and if midway between the two fontanelles

Question 5

WHta is the ideal position for presentation of the foetal head in the birth canal?

Answer 5

Ideally the baby’s head presents in the birth canal in the occipito-anterior position. This means that the vertex is facing the anterior aspect of the mother, and the baby’s face is looking down toward the floor.

Question 6

If the babies head is attempting to exit the pelvis in a transverse position - the occipitofrontal diameter of the foetal head will be too wide to pass through the transverse diameter of the pelvic outlet What can be done to get around this?

Answer 6

manual rotation, or using a vacuum extraction, or Kielland’s rotational forceps Image shows vaccum assisted delivery

Question 7

On vaginal examination the posterior fontanelle, sagittal suture, vertex and anterior fontanelle are palpable.

• * What is the presentation and position of the foetal head?

On vaginal examination the anterior fontanelle and orbital margins are palpable.

• * What is the presentation and position of the foetal head?

Answer 7

On vaginal examination the posterior fontanelle, sagittal suture, vertex (and sometimes) anterior fontanelle are palpable.

• Vertex presentation of the foetus - head in occiptioanterior position

On vaginal examination the anterior fontanelle and orbital margins are palpable.

• This is face presentation of the foetus (face aa brow aka mentum) - the neck is very extended

Question 8

Delivery of the fetal skull through the maternal pelvis The normal process involves the fetal head entering the pelvic inlet (widest diameter transverse) in an occipitotransverse position. It then rotates to an occipitoanterior position as it reaches the pelvic outlet Why is this? What happens to the foetal head as it descends beyond the ischial spines?

Answer 8

The foetal head is in the occipitoanterior position as it reaches the pelvic outlet because the widest diameter of the pelvic outlet is the anteroposterior diameter (presentation is as the foetal head exits the pelvic outlet) As the foetal head extends beyond the ischial spines, the head should be in extension to aid with a normal vaginal delivery

Question 9

The sacro-iliac joints are synovial with a fibrous capsule supported by very strong anterior, posterior and intra-articular ligaments. Movement is extremely limited. The ligaments relax a little during pregnancy, allowing a wider pelvis for delivery, but possibly causing back pain (also caused by arthritis of the joints). The body weight tends to tilt the upper sacrum down and forward, so that the lower sacrum would swivel up and backwards WHat ligaments prevent the lower sacrum from this?

Answer 9

The sacrotuberous and sacrospinous ligaments prevent the lower sacrum from swivelling up and backwards during the weight in pregnancy - keep the sacrum in place

Question 10

The ischiopubic ramus passes from the ischial tuberosity to the inferior aspect of the pubic body. The greater sciatic foramen is above the ischial spine; the lesser sciatic foramen is between the ischial spine and the ischial tuberosity, therefore between the sacrotuberous and sacrospinous ligaments What muscle passes through the greater sciatic formane to connect where? The sciatic nerve passes inferior to this muscle

Answer 10

The pirifiomris msucle passes through the greater sciatic foramen to attach to the greater trochanter of the femur - sciatic nerve runs inferiorly to supply posterior thigh and leg

Question 11

Damage to the anal sphincters may be avoided when a vaginal tear is likely to extend, by carrying out an episiotomy. This is a deliberate incision of the vagina and pelvic floor performed during delivery, which requires careful suturing after delivery. What is the most commonly carried out episiotomy?

Answer 11

Medio-lateral episiotomy is carried out most commonly (aka posterolateral)

Question 12

The urethra is immediately anterior to the vagina while the anal canal with its all-important sphincters is immediately posterior. Where does the mediolateral episiotomy start and extend to?

Answer 12

The episiotomy starts at the posterior foruchette (where the labia minora meet) and extends into the fat filled ischio-anal fossa - This avoids the incision extending into the rectum. A rectal examination is usually performed after delivery and suturing of the episiotomy to ensure no damage has occurred to the anal canal and sphincters.

Question 13

What is the major structure incised during a median episiotomy and what potential issues are faced if further tearing occurs?

Answer 13

The major structure incised is the perineal body (a major support structure for the pelvic floor) and if further tearing occurs - it could extend to the anal sphincter leading to faecal incontinence

Question 14

Describe the course of the pudendal nerve to exiting and re-entering the pelvic cavity

Answer 14

The internal pudendal vessels and pudendal nerve (S2, 3, 4) emerge from the pelvis, below piriformis, through the greater sciatic foramen - it then curves over the sacrospinous ligament and ischial spine to reenter the pelvic cavity via the greater sciatic foramen

Question 15

Once the pudendal nerve reenters the pelvic cavity it travels within the pudendal canal to reach the perineum What is the pudendal canal also known as? Wat muscle does the pudendal canal lie within?

Answer 15

The pudendal canal is also known as Alcock’s canal It runs within the obturator internus muscle

Question 16

To carry out an anaesthetic block on the pudendal nerve the ischial spine may be palpated vaginally. The pudendal nerve is immediately inferior to the tip of the spine and an anaesthetic needle may be passed through the vaginal wall, or through the overlying skin, aimed just below the ischial spine and at the pudendal nerve. How are the ischial spines palpated?

Answer 16

The ischial spines are felt in the 4 and 8 oclcok in a vaginal examination

Question 17

Describe the motor and sensory innervation provided by the pudendal nerve

Answer 17

The pudendal nerve is the main motor innervation to the perineum, as well as providing sensory innervation to most of the skin of the perineum and the external genitalia. It provides innervation to the perineal muscles, the external anal sphincter and external urethral sphincter.

Question 18

Does a pudendal nerve block affect the uterine contractions - motor or sensory?

Answer 18

The pudendal nerve block does not block pain from the uterus, cervix or superior vagina as t only supplies sensory innervation to the perineum It also has no control over the uterine contractions and therefore they are not affected either

Question 19

Why may the facial nerve be injured during foreceps delivery of the foetus? What could happen if the arm of the feotus is deluvered first? If the head is delivered, but the baby is “stuck” by a shoulder (shoulder dystocia), then pulling on the head, particularly at an angle, may force the shoulder and neck apart. What may this cause?

Answer 19

Facial nerve may be injured as the foetus does not have a formed mastoid process - therefore facial nerve may be innjured during forceps delivery If the foetus arm is delivered first and pulled - this may cause forceful abduction of the arm leading to brachial plexis injury in the C8/T1 regions - Klumke’s palsy If shoulder dystocia - can cause Erb’s palsy - C5/C6

Question 20

Where would sensory loss be in Klumpke’s palsy? Where would sensory loss be in Erb’s palsy?

Answer 20

Klumpke’s palsy, loss of the function of all the small muscles of the hand. There would be clawing of all the fingers and sensory loss on the medial aspect of the upper limb. Erb’s or “Waiter’s Tip” palsy, where the arm is adducted and internally rotated at the shoulder and the elbow is extended and pronated. Sensory loss is to the lateral aspect of the upper limb.

Question 21

WHat can keeping the mother in the lithotomy position cause due to impingemen of a nerve?

Answer 21

Due to the knees being in stirrups - if the lateral aspect of the knee is impinged for too long, can lead to common peroneal nerve palsy leading to a foot drop

Question 22

What forms the vertebral arch? (this is what encloses the vertebral foramen)

Answer 22

The spinous process, laminae, pedicle and vertebral arch make the vertebral arch

Question 23

Describe the anatomical structures the needle passes through to insert a spinal anaesthetic At what level is the spinal anaesthetic usually carried out

Answer 23

Needle passes through: Skin at L3/4 spinal cord level then Subcutaneous tissue (fat) followed by Supraspinous ligaments - connects the tips of spinous processe and Interspinous ligaments- runs between the spinous processes. then Ligamentum flavum- runs vertically from lamina to lamina. Then Epidural space contains fat and blood vessels and finally the dura.and the arachnoid mater into the subarachnoid space

Question 24

When may a giving/popping sensation be felt during a spinal anaesthetic? What is the other name for the dural sac? What level does the spinal cord end at in adults and infants?

Answer 24

Popping sensation as needle pierces the ligamentum flavum and the dura mater Dural sac- thecal sac Spinal cord ends at L1/L2 in adults Spinal cord ends at L2/3 in children

Question 25

What is the main side effect of a spinal anaesthetic? What can happen if it reaches the cervical region in high enough concentration? When may it be carried out?

Answer 25

Main side effect is post - spinal headache If it reaches the cervical region in high enough concentration then can cause respiratory arrest May be carried out in emergency C-section

Question 26

How is the L3L4 space found?

Answer 26

A line is drawn between the most superior points on the anterior superior iliac spines and this is the intercristal line - meant to run through the L4 spinous process therefore above this is L3/4 intervetebral space

Question 27

The epidural space is the potential space between the dura mater and the overlying bones and ligaments of the vertebral (spinal) canal. The “space” has a small amount of fat within it, and the intervertebral venous plexus. How does the insertion of the needle differ between spinal and epidural aneathetic? Which is faster onset and longer lasting?

Answer 27

Epidural anaesthetic only passes through skin, subcutaenous fat, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space and no further Epidural has a slower onset but lasts longer - The anaesthetic spreads within the space to anaesthetise the emerging nerve roots. Such an epidural anaesthetic has quite a slow onset, but is long lasting. This can be used as pain relief during labour.

Question 28

Why is the position of the patient important when inserting a spinal needle?

Answer 28

It is important that the patient has their head lowered as to increase the intervertebral space and make it easier to identify the L3/4 space

Question 29

What are the potential side effects of a spinal or epidural anaesthetic?

Answer 29

Spinal anaesthetic - the spinal anaesthetic may cause post-spinal headache, and if it reaches the cervical cord in high enough concentration, may cause respiratory arrest. An epidural needle is inserted at L3/4 and the anaesthetic must block T11-L2 in the first stage of labour, but extend to include S2 to S4 in the second stage. Such a block may well cause temporary, neurological side effects in the bladder and in the lower limbs. The patient may need urinary catheterisation, and be unable to walk.

Question 30

What nerve routes travel back to T11-L2 and S2-4?

Answer 30

Superior aspect of pelvic organs use visceral afferentss that travel alongside sympathetic fibres to to T11-L2 Inferior aspect of pelvic organs use visceral afferents that travel alongside parasympathetic fibres to S2-4 Perineal organs use the pudendal nerve to travel back to S2-4