Shoulder Dystocia - There is no universal definition of shoulder dystocia (SD), hence the variable incidence noted in literature. 'True shoulder dystocia' was coined by Gross et al, as deliveries requiring, in addition to downward traction and episiotomy, manoeuvres to deliver the shoulders.
Incidence ranges from 0.25-1%
Fetal complications
Permanent neurological injury
Umbilical artery pH falls at a rate of 0.04/min following delivery of the head. Therefore acute total asphyxia of less than 10 mins is unlikely to lead to permanent damage. Fetal reserve may be less in diabetics.
Brachial plexus injuries
Follow 7-16% of cases of SD, infants of diabetic mothers being particularly prone.
Erb's palsy is avulsion of C5,6 nerve roots leading to paralysis of deltoid, supraspinatus, infraspinatus, biceps & brachioradialis. Biceps & Moro reflexes are reduced or abolished. 1/3 become permanent.
C4 injury is rare, and leads to hemidiaphragm paralysis.
C8,T1 injury - Klumpke's paralysis - weakness of intrinsic muscles of the hand leading to absence of palmar grasp reflex. Moro is usually present. T1 sympathetic outflow if damaged leads to an ipsilateral Horner's syndrome and eventual diminished pigmentation of the iris. 60% of lower root traumas become permanent.
Skeletal injury
The clavicle is the most frequently traumatised bone during birth and may be fractured in about 15% of cases of SD. Thorough paediatric neurological examination is mandatory, as a concommitent brachial pleus injury may be present (20%). Both clavicular and humeral fractures have an excellent prognosis.
Maternal morbidity
A prolonged 2nd stage complicated by shoulder dystocia often leads to significant blood loss (70%>1000ml). Vaginal and cervical lacerations may occur, and uterine rupture may follow erroneous fundal pressure.
Mechanism of shoulder dystocia
The problem lies at the pelvic inlet. The posterior shoulder enters the pelvis and the anterior shoulder remains hooked behind the symphysis pubis and fails to rotate into a larger pelvic diameter.
In a more serious form, both shoulders are packed into the pelvic brim in an anteroposterior or transverse/oblique diameter.
Manuevers to overcome shoulder dystocia
Recognition is usually straightforward, turtle-necking may or may not be seen. Delivery may have been preciptiate, the shoulders having undergone little 'moulding' (similar to that of the fetal head in normal labor).
Manoeuvres should be directed at the underlying problem - failure of shoulder rotation at the pelvic inlet. Pulling on the fetal head and fundal pressure are to be avoided, combination of the two are known to be associated with fetal neurological and orthopaedic complications.
1. Put the patient into the lithotomy position and flex the hips further to bring the maternal thighs to the abdomen (McRobert's manoeuvre)
2. Perform a generous episiotomy, if not already done - to allow the accoucher access to the pelvis.
3. Suprapubic pressure is applied, the head and neck of the fetus grasped and taken posteriorly (avoiding lateral flexion of the neck and traction on the brachial plexus)
Pressure should be directed slightly towards the side of the fetal chest. Maternal expulsive effort should be discouraged until the shoulders are disimpacted.
4. If this does not work, summon an anaesthetist (in the most difficult cases a GA may be required)
5. The hand of the accoucher enters the sacral hollow and the posterior humerus is identified. This is followed until the elbow is reached and this is flexed. The wrist is grasped and traction sweeps the arm over the fetal chest, allowing its delivery. Continued traction on the arm (clockwise, if the fetal back is to the mother's left) will convert the posterior shoulder to an anterior position. The previously anterior shoulder will enter the pelvis to lie in the sacral hollow. Delivery may now be achieved by spontaneous delivery of the new anterior shoulder under the symphysis, by delivery of the new posterior arm as before, or by continuing the circle of traction.
6. If the posterior shoulder cannot be delivered, the shoulders must be manually rotated. Wood's screw manoeuvre involves application of pressure to the anterior surface of the posterior shoulder. A turn of 180 degrees will disimpact the anterior shoulder from under the symphysis. Midline suprapubic pressure is applied in the direction of rotation. O'Leary suggested a reverse screw manoeuvre with the added advantage of shoulder adduction and reduction of the bisacromial diameter.
7. If all else has failed, and the fetus is still alive, then cephalic replacement with caesarean section and symphysiotomy are the final two options.
Cephalic replacement with tocolysis has a reportedly high success rate with good fetal outcome, although it is rare. Depression of the posterior vaginal wall is needed with constant and firm pressue on the head.
Symphysitomy carries the danger of long-term maternal morbidity. The bladder must be catheterised, the legs and hip supported by assistants and a solid bladed scalpel divides the symphysis from above whilst the urethra is displaced laterally. The angle between the legs should not exceed 80 degrees, as this places excessive strain on the sacroiliac joints and can tear the anterior vaginal wall and urethra. Complications include vesicovaginal fistula, osteitis pubis, retropubic abscess, stress incontinence and long-term walking disability/pain.
Antenatal predisposing factors
Although half of all shoulder dystocias occur during delivery of infants weighing less than 4kg, the incidence is poportional to birth weight.
23% of SD cases do not exhibit any classically associated features.
Infants of diabetic mothers not only are larger, they have significantly greater shoulder/head and chest/head ratios than infants of normal pregnancies of comparble weight.
Maternal weight greater than 80kg increases the risk of SD by a factor of 8 (via larger babies, increased incidence of diabetes and increased incidence of prolonged pregnancy)
Prolonged pregnancy increases the risk of SD by a factor of two.
Intrapartum associated features
Mid-pelvic instrumental delivery for prolonged second stage
Arrest disorder in the first stage (2 hour cessation of cervical dilatation in the active phase)
Primary dysfunctional labour
Secondary arrest
Prevention of shoulder dystoia
Preconceptial maternal weight reduction
Strict diabetic control will decrease the incidence of fetuses>4kg and reduce the incidence of shoulder dystocia.
Interventions unlikely to significantly reduce incidence of shoulder dystocia
Induction of labour for postmaturity - most cases of SD do not occur in prolonged pregnancy and fetal weights are not markedly reduced when such a policy is adhered to.
Caesarean section for suspected macrosomia:
- clinical screening for large babies suitable for USS fetal weight estimations is poorly sensitive (20% for BW>4500g)
- USS prediction of macrosomia is not accurate and weights of large infants are typically underestimated.
- Specificity of USS EFW is low and unnecessary section rate will be significant.
- macrosomia alone has a poor predictive value for SD.
- Most cases of SD occur in infants of average weight.
- 23% of cases of SD do not exhibit classical associated features.
- Most cases of SD can be overcome without trauma to mother or baby and abdominal delivery of a large infant is not without hazard (2.5% incidence of birth trauma in infants>4500g)
Adapted from 'shoulder dystocia' by L. Roberts, Progress in Obstetrics & Gynaecology
Volume 11 Edited by John Studd (ISBN 0261 0140)
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