Poster E1 - Effects of brachial plexus birth injury in muscle spindle quantity of biceps, supraspinatus, and subscapularis muscles

Thursday, October 24, 2024

2:45 PM - 3:45 PM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Joshua D. Farrelly

Undergraduate Researcher
North Carolina State University
Jacksonville, North Carolina, United States

Co-Author(s)

Kyla Bosh (she/her/hers)

Graduate Research Assistant
Joint Department of Biomedical Engineering at North Carolina State University and University of North Carolina at Chapel Hill
Clayton, North Carolina, United States

Primary Investigator(s)

Katherine Saul, PhD (she/her/hers)

Professor
North Carolina State University, United States

Last Author(s)

Jacqueline Cole, PhD (she/her/hers)

Associate Professor
North Carolina State University and University of North Carolina at Chapel Hill, United States

Introduction: Brachial plexus birth injury (BPBI) is a neuromuscular injury that can occur during childbirth due to excessive stretching of the infant’s neck, damaging the brachial plexus nerve bundle. BPBI is a common perinatal nerve injury, occurring in 1 out of every 1,000 live births. Although 60-90% of patients diagnosed with BPBI make a complete or almost complete spontaneous neural recovery, 30-40% suffer permanent damage to their upper limb. Individuals who do not recover often experience muscle paralysis, reduced shoulder range of motion, musculoskeletal growth deformities in the glenohumeral shoulder joint, and loss of sensation in the hand and arm that persist for the rest of their lives. Deficits are based on the severity and location of the injury, whether the nerve ruptures distal from the spinal column to the dorsal root ganglion (postganglionic injury) or avulses from the spinal column proximal to the dorsal root ganglion (preganglionic injury).

To improve BPBI treatment, we require a better understanding of the separate contributions to muscle composition changes from altered neuromusculoskeletal signaling and abnormal mechanical loading, as well as how these compositional changes contribute to varying shoulder deformities at different developmental stages. Changes in muscle spindle quantity from different injury locations could be a result of or a contributing factor to muscle imbalance and bone deformation. Developing a better understanding of how compositional changes in the muscle progress following BPBI could contribute to the development and application of more effective therapies.

Materials and

Methods: Sprague Dawley rats were divided into 4 surgical groups (postganglionic neurectomy, preganglionic neurectomy, forelimb disarticulation, or sham surgery), received surgery on one forelimb at postnatal days 3 to 6, and were sacrificed at one of 5 endpoints (2, 3, 4, 8, or 16 weeks post-injury). For postganglionic neurectomy (n=8/endpoint), a C5-C6 nerve root excision was made distal to the dorsal root ganglion via transverse infraclavicular incision and splitting of the pectoralis major muscle. For preganglionic neurectomy (n=8/endpoint), C5-C6 nerve roots were excised proximal to the dorsal root ganglion via supraclavicular incision. For forelimb disarticulation (n=8/endpoint), the right forelimb was amputated at the elbow to imitate limb disuse without nerve injury. The sham group (n=6/endpoint) received an incision through the pectoralis major muscle, but the brachial plexus was not transected. Intact contralateral limbs served as added controls.

Injured and uninjured biceps brachii, supraspinatus, and subscapularis muscles were dissected the day of sacrifice for all surgical groups (2, 3, 4, 8, and 16 weeks post-surgery) and were embedded in OCT, snap-frozen, and stored at -80℃. Muscles were longitudinally cryosectioned at -24℃ at 10 micrometers thick. Three tissue slices per muscle were taken from the belly and stained using a H&E staining kit for the identification and counting of muscle spindles. Muscle sections were imaged at 20x magnification, and images were analyzed for spindle number. Spindle counts were averaged across sections for each muscle.

Results, Conclusions, and Discussions: Analyses are ongoing to understand the progression of altered muscle composition following BPBI. Preliminary results at 2, 3, 4, and 16 weeks indicate that muscle spindle quantity was greater in uninjured biceps muscles following postganglionic neurectomy than forelimb disarticulation at 3 weeks (p < 0.05), tended to decrease in uninjured supraspinatus muscles from 2 to 16 weeks (p < 0.1) following forelimb disarticulation, and tended to be greater in injured than uninjured subscapularis muscles at 3 weeks following forelimb disarticulation (p < 0.1). Greater muscle spindle quantity in uninjured limbs following postganglionic neurectomy could indicate more compensatory use of these limbs relative to forelimb disarticulation. This is further supported by the favored usage of uninjured limbs observed during gait analyses and notable increase in muscle mass of uninjured limbs following preganglionic and postganglionic injury versus forelimb disarticulation. Decreases in muscle spindle quantity over time could indicate that more muscle spindles are present in early musculoskeletal development when growth is more rapid. While few significant differences were observed, this could be attributed to the current small sample size. Ongoing work will complete analyses for 2-, 8-, and 16-week time points to better establish muscle spindle progression at varying musculoskeletal developmental stages. This study is the first to characterize progressive changes to muscle spindle quantity after BPBI injury, which may inform optimal timing and strategies for treatment.

Acknowledgements (Optional):