THE TIMING OF CONGENITAL BRACHIAL-PLEXUS INJURY - A STUDY OF ELECTROMYOGRAPHY FINDINGS IN THE NEWBORN PIGLET

OBJECTIVE: Permanent congenital brachial plexus palsy is a recognized serious complication associated with shoulder dystocia. The timing and etiology of this injury remains controversial. Previous authorities h ave used adult-derived, non-brachial plexus data to extrapolate the an ticipated timing for electromyographic denervation changes to date suc h injuries in the newborn. With use of a domestic swine model, this in vestigation tests the hypothesis that electromyographic evidence of br achial plexus denervation in the newborn is temporally different than that in the adult. STUDY DESIGN: Five healthy 2-day-old and two adult pigs underwent unilateral sharp transection of the brachial plexus. Da ily electromyographic studies were performed in brachial plexus innerv ated muscle groups on the involved and contralateral (control) front l imbs. Postmortem measurements of the transected nerve segments were ob tained in one piglet and one adult animal. Representative hard copy re cordings of individual electromyographic studies were collected. RESUL TS: Immediately after surgical transection of the brachial plexus, no electromyographic evidence of denervation was observed. Uniformly in t he newborn piglets, at 24 hours after transection, denervation in the form of fibrillation potentials, positive sharp waves, and complex rep etitive discharges was seen. Serial testing demonstrated proximal to d istal gradients of denervation over the next 24 to 48 hours. A delay i n electromyographic evidence of denervation was observed in the two ad ult pigs until days 5 and 8, respectively. Control limb studies remain ed normal throughout the study period. Nerve length measurements for i ndividual muscle groups were as follows for the adult and newborn pigs , respectively: deltoid 11.4 cm, 2.5 cm; cleidobrachialis 16.0 cm, 4.0 cm; triceps 15.5 cm, 4.5 cm; forelimb flexors 26.0 cm, 6.5 cm; and ex tensor carpi radiails 31.0 cm, 9.0 cm. CONCLUSION: Electromyographic e vidence of brachial plexus denervation after surgical transection diff ers between the newborn and the adult pig. Consistent with wallerian d egeneration, a correlation exists between length of the distal nerve s egment and timing for electromyographic signs of denervation. These fi ndings suggest it would be inappropriate to extrapolate the anticipate d timing for electromyographic changes in the newborn on the basis of previously established adult non-brachial plexus data.