(E-176) Effects of Strain on Neonatal Brachial Plexus Functional Properties in Piglets

Introduction:: Neonatal Brachial Plexus Palsy (NBPP) is a condition caused by the over-stretching of the brachial plexus during difficult deliveries. The most common cause of injury is when the brachial plexus is stretched during the second stage of obstructed labor. The two main risk factors of NBPP are shoulder dystocia and above-average birth weight. The use of forceps during delivery, prolonged labor, history of NBPP in a previous birth and prematurity are all factors that can be potential risk factors1. The incidence of NBPP is 0.1 to 8.1 out of every 1000 births, so it is a very rare condition2. Of those affected, 75-95% of cases show advances towards complete recovery within the first 2 months1. Even with most cases showing almost a full recovery, there are still 20-30% of cases that don’t recover after the first 2 months1,3. It has been proven that the use of electrodiagnostic techniques produces better outcome predictions of brachial plexus injury, but future advances in electrodiagnosis and imaging equipment will continue to improve outcome prediction accuracy2. The effect that stretch and strain has on nerves, specifically how exceeding threshold can influence functional deficits, is not completely understood4. The goal of this study was to determine the percentage strain that will cause a significant decrease in nerve conduction velocity in neonatal animal models. My hypothesis is that as strain increases, the conduction velocity will decrease and eventually at a high enough strain (30%), there will be a conduction block.

Materials and Methods:: The piglets used in the study were 3-5 days old and were studied under hypoxic conditions. The piglets were anesthetized, intubated, hooked up to an IV and blood pressure monitor. The brachial plexus was exposed, and a pre-stretch recording was taken to get the baseline data. A bipolar stimulating electrode (SE) was placed proximally, and 2 bipolar recording electrodes (RE) were placed distally, and the nerve was stimulated. A picture was taken of the electrodes and a distance marker was placed on the nerve for scale and to assist with conduction velocity calculations. Once the pre-stretch recordings were taken, the electrodes were removed. The nerve was then clamped and stretched to a pre-determined strain as reported previously5,6. Once the stretching was complete, the electrodes were placed in the same position as mentioned above. After stretch and 1 hour after stretch recordings were taken and the process was repeated for the median and ulnar nerves on the other side of the body. After data collection, a MATLAB code was opened and executed. Several screens populated, following all the steps of opening the files where the experiment data was stored for each of the nerves, the experiment dates and the baseline, after and 1 hour after data for the different thresholds. When the images of the nerve responses for each of the stretch data populated, the first peaks of each image were selected and the xy data associated with the selected points were put into an excel spreadsheet for later calculations.

Results, Conclusions, and Discussions:: The effect of strain on the neonatal brachial plexus functional properties was investigated in this study. The strain data was split into two groups: less than 15% strain (< 15%) and greater than 15% strain ( >15%). In the < 15% strain group, a decrease in the conduction velocity can be observed as the strain increases in the after-stretch data. The 1-hour recovery also exhibits a decrease however, the slope of 1 hour after stretch trendline is not as steep as the after stretch trendline. At strains >15%, for each nerve sample tested, there is a decrease in conduction velocity immediately after stretch and a spontaneous recovery is observed 1 hour after stretch. Conduction velocity can be affected by a number of factors, including strain. Figure 1 shows the strain versus conduction velocity data, for strains less than 15%. The after stretch data shows a decrease in conduction velocity as strain increases. However, the 1 hour after stretch data shows a slight recovery in the conduction velocity as the strain increases. This trend of increase also happens with the strain versus conduction velocity data, with strains greater than 15%, which is shown in Figure 2. From the completed data analysis, we can conclude that there is a decrease in conduction velocity as the percentage strain on the nerve increases. We can also conclude that there is a trend of spontaneous recovery on the stretched nerve. In future work, both primary nerves – the nerves being stretched – and secondary nerves – the nerves that share the same anatomical cord – should be studied. Finally, spontaneous recovery beyond 1 hour, like 3 hours or 6 hours of recovery, should be studied. Data obtained from these studies can be used to develop training modules while utilizing novel educational tools. These modules can help achieve a better understanding of the NBPP injury mechanism7.

Acknowledgements (Optional): : This project was supported by the Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health R15HD093024; R01HD104910-01A1 and NSF CAREER grant Award #1752513.

References (Optional): : 1. Abid, A. Orthopaedics & Traumatology: Surgery & Research 102:125-132 (2016).
2. Mehlman, C.T. The Pediatric Upper Extremity 589-605 (2015).

3. Abzug, J.M. Orthopedics 33: 430-435 (2010).

4. Johnson, E.O. Injury 44(3), 293-298 (2013).

5. Singh, A. Journal of Neurotrauma 26: 627-640 (2009)

6. Bain, A.C. Journal of Neurotrauma 18: 499-511 (2001).

7. Hanna, A.S. J Anat 240: 172-181 (2022).