DYSTOCIA OF THE POSTERIOR SHOULDER

Dystocia of the posterior shoulder may also result from inadequate rotation of the fetal shoulders as they enter the pelvic inlet. In this case, the posterior shoulder (nearest the mother’s spine) impacts against the promontory of the sacrum. This is the less common form of dystocia.

POSTERIOR SHOULDER DYSTOCIA

FETAL INJURIES RELATED TO DYSTOCIA

Brachial Plexus Injury

The most common injury associated with shoulder dystocia is injury to the brachial plexus. The brachial plexus is a group of nerves located in the lower neck, formed by 5 nerve roots emerging from the spinal cord as it runs through the cervical spine.

BRACHIAL PLEXUS ANATOMY

This plexus is responsible for all sensory and muscular function of the corresponding upper limb. Thus, there is a plexus on both the left and right side of the neck. During dystocia (both anterior and posterior), the forward propulsion of the fetus as a result of uterine contractions and maternal bearing down, and the impaction of the shoulder, causes the neck, and the nerve plexus within, to stretch. Repetitive and intensive stretching may damage the nerves of the plexus. The nerves may tear (avulsion), but neurological impairment can also arise as a result of simple stretching of the nerves. In these cases, there is undoubtedly microscopic internal tearing of nerve bundles.

 The mechanism of plexus injury resulting from operator traction on the the fetal head and neck in attempts to deliver an infant whose shoulder is impacted is controversial. Many researchers are skeptical of this mechanism, and point to the fact that 40% of all brachial plexus injuries occur in deliveries without known dystocia, where no undue force is exerted on the fetal head.

 Most commonly, the nerves of the C5 and C6 nerve roots are injured, resulting in the clinical findings of  Erb-Duchenne palsy: the infant loses the power to abduct the arm from the shoulder, rotate the arm externally, and supinate the forearm. The characteristic position consists of adduction and internal rotation of the arm with pronation of the forearm. Power to extend the forearm is retained, but the biceps reflex is absent; the. The outer aspect of the arm may have some sensory impairment. Power in the forearm and hand grasp are preserved unless the lower part of the plexus is also injured; the presence of hand grasp is a favorable prognostic sign.

Rarer brachial plexus injuries include Klumpke’s Palsy (injury of the C7, C8 and T1 nerve roots. This results in a clinical picture of. a paralyzed hand, and eyelid weakness and constricted pupil (ptosis and miosis - Horner syndrome) if the sympathetic fibers of the 1st thoracic root are also injured. Damage to the entire brachial plexus has occasionally been seen, as well as paralysis of the respiratory diaphragm,, and facial nerve injury.

One third of brachial plexus palsies are associated with a fetal bone fracture on the affected side, most commonly the clavicle (94%). Rarely, a fracture of the fetal radius (forearm) may result from either the dystocia or the various maneuvers employed to relieve dystocia.

MATERNAL INJURIES RELATED TO SHOULDER DYSTOCIA

Severe bleeding after delivery (post-partum hemorrhage) and extension of episiotomy incisions resulting in tears of the rectum (fourth-degree laceration) are the most common maternal injuries related to fetal shoulder dystocia.  Other reported complications include cervical and vaginal lacerations, loss of bladder function, uterine rupture, and separation of the maternal pubic symphysis & lateral femoral cutaneous nerve injury related to overzealous use of the McRoberts maneuver (described below).

FACTORS INCREASING THE RISK OF SHOULDER DYSTOCIA

 Studies have established a significant relationship between increasing birth weight and the risk of shoulder dystocia. In births not complicated by maternal diabetes, only 5% of infants weighing between 4000 and 4250 grams at birth typically experience dystocia, while for infants weighing 4750-5200 grams, almost 21% are so affected.   Despite these findings, attempts to predict dystocia based on previous history of over weight births (macrosomia),  pre-existing or pregnancy induced non-insulin dependent diabetes, excessive maternal weight gain, and delivery after expected dates, have proven  to be unreliable at best. Likewise, fetal birth weight estimates based on ultrasounds performed in the latter stages of pregnancy have not correlated well with the incidence of shoulder dystocia. The most recent recommendation of the American College of Obstetricians is that there is no contra-indication to attempted vaginal delivery for any infant estimated to be up to 5000g (where maternal insulin-dependent diabetes is not present).

Mothers with insulin-dependent diabetes whose babies are of high birth weight show a significantly higher risk for encountering fetal shoulder dystocia during labor. It is thought that this relates to changes in fetal body proportions, including larger chest and trunk circumferences, larger bisacromial diameters and chest to head ratios, all of which may contribute to inadequate shoulder rotation at the pelvic inlet.

Mothers who have had a prior history of fetal shoulder dystocia during labor should also be considered in a special risk category for recurrence with subsequent pregnancies.