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

Abstract

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.

Clinical relevance: This study expands our understanding of differences in muscle architecture and the role of muscle structure in contracture formation for preganglionic and postganglionic brachial plexus birth injury.

Keywords: Brachial plexus birth injury; contractures; muscle mass; range of motion; sarcomere length.

FIGURE 1:

Schematic figure of preganglionic and postganglionic injury models.

FIGURE 2:

Passive external rotation of the affected limb relative to the unaffected limb at 4- and 8-week points. Postganglionic injury resulted in significant restriction of passive external rotation in the affected limb (relative to unaffected), and the restriction was significantly more pronounced (more negative) than after preganglionic injury. *P < .05 for affected versus unaffected limb. ^†P < .05 for postganglionic versus preganglionic group.

FIGURE 3:

Optimal muscle length for muscles in the affected limb relative to the unaffected limb. Postganglionic injury resulted in significant restriction of muscle length in the affected limb (relative to unaffected), and the restriction was significantly more pronounced (more negative) in more muscles after preganglionic injury. *P < .05 for affected versus unaffected limb. ^†P < .05 for postganglionic versus preganglionic group.

FIGURE 4:

Muscle mass for muscles in the affected limb relative to the unaffected limb. Postganglionic injury resulted in significantly lower muscle mass in the affected limb (relative to the unaffected limb), and lower mass was significantly more pronounced (more negative) in more muscles after preganglionic injury. *P < .05 for affected versus unaffected limb. ^†P < .05 for postganglionic versus preganglionic group.

FIGURE 5:

Passive muscle force capacity of a representative muscle with altered architecture. Isolated shortening cases (dark and light gray solid lines), with optimal muscle length reduced by 5% and 10% relative to the nominal muscle (black solid line), increase passive force. Isolated mass loss (dark and light gray short dashed lines), with maximum isometric force reduced by 30% and 60% relative to nominal, reduce passive force. Combined changes to length and mass (dark and light gray long dashed lines) result in variable passive force magnitude and slope.