Preganglionic and Postganglionic Brachial Plexus Birth Injury Effects on Shoulder Muscle Growth

Purpose Brachial plexus birth injury can differ in presentation, depending on whether the nerve ruptures distal to, or avulses proximal to, the dorsal root ganglion. More substantial contracture and bone deformity at the shoulder is typical in postganglionic injuries. However, changes to the underlying muscle structure that drive these differences in presentation are unclear. Methods Seventeen Sprague-Dawley rats received preganglionic or postganglionic neurectomy on a single limb on postnatal days 3 and 4. Muscles crossing the shoulder were retrieved once the rats were sacrificed at 8 weeks after birth. External rotation range of motion, muscle mass, muscle length, muscle sarcomere length, and calculated optimal muscle length were measured bilaterally. Results Average shoulder range of motion in the postganglionic group was 61.8% and 56.2% more restricted at 4 and 8 weeks, respectively, compared with that in the preganglionic group, but affected muscles after preganglionic injury were altered more severely (compared with the unaffected limb) than after postganglionic injury. Optimal muscle length in preganglionic injury was shorter in the affected limb (compared with the unaffected limb: -18.2% +/- 9.2%) and to a greater extent than in postganglionic injury (-5.1% +/- 6.2%). Muscle mass in preganglionic injury was lower in the affected limb (relative to the unaffected limb: -57.2% +/- 24.1%) and to a greater extent than in postganglionic injury (-28.1% +/- 17.7%). Conclusions The findings suggest that the presence of contracture does not derive from restricted longitudinal muscle growth alone, but also depends on the extent of muscle mass loss occurring simultaneously after the injury. Copyright (C) 2021 by the American Society for Surgery of the Hand. All rights reserved.

Automated summary

The study found that shoulder range of motion was more restricted in the postganglionic group at 4 and 8 weeks compared to the preganglionic group. However, muscles affected after preganglionic injury exhibited more severe alterations. Optimal muscle length and muscle mass were also significantly lower in the affected limb after preganglionic injury than after postganglionic injury. The presence of contracture after brachial plexus birth injury may not only be due to restricted longitudinal muscle growth, but also dependent on simultaneous muscle mass loss.