Introduction

Shoulder dystocia

Shoulder dystocia is an obstetric emergency.

 

It occurs when the baby’s shoulders do not spontaneously deliver after the head. The most common reason is the anterior shoulder being trapped behind or above the pubic symphysis. Alternatively, the posterior shoulder can become trapped in the hollow of the sacrum or above the sacral promontory.

 

Mechanism

In the full-term fetus, the bisacromial diameter (the distance between the outermost points of the fetal shoulders) is greater than the biparietal diameter (the diameter of the fetal head between the two parietal eminences). Flexibility of the shoulders allows passage through the pelvis. As the head passes through the pelvic outlet, the shoulders enter the pelvic brim in the oblique diameter. The posterior shoulder leads via the sacral bay or sacro-sciatic notch; the anterior shoulder is accommodated by the obturator foramen. If the fetal shoulder dimensions are too large, or the maternal pelvis is too narrow, the shoulders can become obstructed, impeding delivery. This leads to shoulder dystocia.

 

Once the head is delivered, the uterus contracts down leading to a decrease or cessation in blood flow to the intervillus space. The fetal chest is compressed which makes it impossible for an adequate respiratory effort, even with an unobstructed mouth and nose. The supply of oxygen decreases, and the pH in the umbilical artery steadily drops. There is an estimated 4-6 minute window for safe delivery of the body following the head without hypoxic brain damage.

 

Risk Factors

These can be divided into antepartum and intrapartum.

 

Antepartum:

  • Macrosomia: clinically or on ultrasound scan (abdominal circumference > 95th centile). Incidence of shoulder dystocia is proportional to birth weight. Having previously had a large baby (> 4.5kg) is also a risk factor.
  • Diabetes mellitus: babies of diabetic women have an increased shoulder:head-circumference ratio due to the insulin-sensitive nature of tissues contributing to shoulder girth; this is in contrast with brain growth which is not affected by hyper-glycaemia and hyperinsulinism.
  • Maternal obesity: women with a BMI > 30kg/m2 have an increased incidence of gestational diabetes and prolonged pregnancy. There is also a link with fetal macrosomia.
  • Induction of labour.
  • Previous shoulder dystocia.

 

Intrapartum:

  • Prolonged or arrested 1st stage of labour.
  • Prolonged or arrested descent in the 2nd stage of labour.
  • Assisted mid pelvic delivery.

 

Presentation

Suspect shoulder dystocia if:

  1. There is difficulty with delivery of the face and chin.
  2. The head remains tightly applied to the vulva or even retracting (‘turtle neck’ sign).
  3. There is failure of restitution of the fetal head.
  4. There is failure of the shoulders to descend.

 

If gentle traction downwards and backwards fails to deliver the anterior shoulder, further traction should be abandoned, as the most common reason for brachial plexus injury is excessive movement of the head and neck.

 

Management

Once shoulder dystocia has been recognised:

 

Call for help: further midwifery assistance, an obstetrician, a paediatric resuscitation team and an anaesthetist.

 

Discourage pushing: this may lead to further impaction of the shoulders. Manoeuvre the woman to bring the buttocks to the edge of the bed.

 

McRoberts’ manoeuvre (see diagram below): flexion and abduction of maternal hips, positioning her thighs on her abdomen. This straightens the lumbosacral angle, rotates the maternal pelvis superiorly and increases uterine pressure and amplitude of contractions. It is a highly successful intervention (90% success rate in some reports) and has a low complication rate.

 

Apply suprapubic pressure: in conjunction with McRoberts’ manoeuvre. This reduces the bisacromial diameter and rotates the anterior shoulder into the oblique pelvic diameter. The shoulder can then slip underneath the pubic symphysis with routine traction. Apply it in a downward and lateral direction to push the anterior shoulder towards the fetal chest, for around 30 seconds.

 

Other manoeuvres: internal manipulation, removal of the posterior arm, and all-fours manoeuvre.

 

These steps can be remembered by the mnemonic 'HELPERR':

McRoberts' manoeuvre HELPERR mnemonic

Complications

These can be divided into fetal and maternal:

 

Fetal:

  • Brachial plexus injury: affects 5-15% at least temporarily. This is normally of the Erb-Duchenne type (nerve roots C5 and C6).
  • Fractured clavicle: affects approximately 15%. This heals well with no long-term sequellae if diagnosed and treated correctly.
  • Cerebral palsy: from hypoxia.
  • Death.

 

Maternal:

  • Genital tract lesions: associated with generous episiotomy and additional manoeuvres.
  • Post-partum haemorrhage: multi-factorial, including uterine atony, prolonged labour, large infant, and increased blood loss from lacerations and episiotomy.
  • Uterine rupture.
  • Bladder or urethral damage.
  • Psychological trauma.

 

References

Chamberlain G, Hamilton-Fairley D. Lecture notes on obstetrics and gynaecology. 8th ed. Wiley-Blackwell.

 

Bonnar J. Recent advances in obstetrics and gynaecology: v 22. RSM books.

 

eMedicine: Shoulder dystocia.

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