Brachial plexus birth injury and cerebral palsy lead to a common contracture phenotype characterized by reduced functional muscle length and strength

Muscle physiology in spasticity and muscle stiffness

This paper examines the physiological changes in spastic muscles contributing to spasticity and muscle stiffness, focusing on the underlying mechanisms and their clinical implications. Spasticity, which is prevalent in neurological conditions such as multiple sclerosis, cerebral palsy, spinal cord injury, stroke, and traumatic brain injury, is characterized by disordered sensorimotor control and often results in increased muscle stiffness and resistance to movement. Recent developments in the understanding of spasticity suggest the importance of architectural changes in muscles that may contribute to increased passive resistance, potentiate reflex mechanisms, and progression to fibrosis, with hyaluronan (HA), a glycosaminoglycan, playing a pivotal in modulating the properties of the muscle extracellular matrix (ECM). The hyaluronan hypothesis of muscle stiffness postulates that the accumulation and biophysical alteration of HA in the ECM of muscle increases its viscosity, resulting in increased passive mechanical resistance. This is turn mayincrease muscle sensitivity to stretch, potentiating spasticity, and lead to cellular differentiation of myofibroblasts to fibroblasts ultimately leading to fibrosis and contracture. A deeper understanding of HA's role in ECM dynamics offers promising avenues for novel treatments aimed at mitigating stiffness and preventing long-term disability in patients with spasticity.

The use of mathematical modelling in hand surgery

Consistent explanations of hand and wrist anatomy, how it relates to function and leads to dysfunction, remain elusive. Recent research using new capabilities in computer-aided and three-dimensional (3D) modelling with mathematical tools have advanced our understanding in areas where conventional understanding does not align with clinical observation. Translating the complex biomechanical and clinical realities into mathematical models of the hand and upper limb, however, poses significant challenges; yet when successful, these models can provide answers to questions difficult to address in two-dimensional analyses. Mathematical tools can also measure forces across joints, mark fracture planes in comminuted fractures, enabling the virtual reduction of fractures, and analyse the properties of denervated muscles. High mathematical quality data, when validated carefully, facilitate the creation of models that can be used to provide new general information in hand surgery as well as the potential to personalize patient care.

Effect of Muscle Strength on Functionality after Shoulder Tendon Transfer in Brachial Plexus Birth Injury: Is There a Relationship between Them?

BACKGROUND/OBJECTIVES

Secondary problems in BPBI occur due to decreased muscle strength in the upper extremities. Comprehensive assessment methods are necessary to understand structural problems and to plan appropriate interventions in children with BPBI. We investigated the relationship between distal muscle strength, range of motion (ROM), and functionality by comparing distal muscle strength on the affected and unaffected sides in patients with BPBI who underwent shoulder tendon transfer.

METHODS

A total of 25 children with BPBI, 13 (52%) girls and 12 (48%) boys, aged 4-7 years (mean age: 5.98 ± 1.27 years), who had undergone shoulder tendon transfer surgery at least one year prior to the study were included. The muscle strength of the elbow, forearm, and wrist were assessed using the MicroFET®2 Digital Hand Dynamometer. The ROM of the elbow, forearm, and wrist were measured using the universal goniometer. The Pediatric Evaluation of Disability Inventory (PEDI) was used for functionality assessment.

RESULTS

The strength of the elbow flexor-extensor, forearm pronator-supinator, and wrist extensor muscles on the affected side was greater than on the unaffected side in all children (p < 0.001). No correlation was found between muscle strength, ROM, and functionality in the affected extremity (p > 0.005).

CONCLUSIONS

Although children with BPBI have good shoulder function after shoulder tendon transfer, structural problems in the distal joints may affect their functionality during daily life. Distal joint strengthening and ROM exercises, as well as bimanual functional activities, should be included in the rehabilitation programs of children with BPBI after shoulder tendon transfer.

Prevalence and predictors of elbow flexion contractures during early childhood following brachial plexus birth injury

BACKGROUND

Elbow flexion contracture development in school-age children with a brachial plexus birth injury (BPBI) is common. Reports indicate onset between 2 and 4 years; however, little is known about early childhood prevalence, development, and trajectory of these contractures.

PURPOSE

To determine the prevalence and predictors of BPBI elbow flexion contractures during early childhood.

STUDY DESIGN

A retrospective cross-sectional study.

METHODS

Demographic, diagnostic, treatment, and elbow contracture data were collected for children with a BPBI <4 years between 2015 and 2019 from a prospectively collected database. Spinal root motor contributions and injury were determined using Active Movement Scale (AMS) scores at 6 weeks of age and used to predict contracture development.

RESULTS

Of the 171 children that met inclusion criteria, 87% (n = 149) had upper plexus injuries. The mean age at the time of evaluation for an elbow contracture was 21.4 ± 12.7 months. The prevalence of elbow flexion contractures was 22% (n = 38), with mean onset at 13.4 ± 11.0 months. Mean contracture degree was -10.8 ± -6.9 degrees with 76% (n = 29) <-10 degrees. AMS shoulder abduction, flexion, and external rotation; elbow flexion; forearm supination; and wrist extension scores at a mean 2.3 ± 1.4 months were significantly lower in children who developed elbow flexion contractures (p < 0.001). Logistic regression found that low AMS elbow flexion with high elbow extension scores were a significant (p < 0.003) predictor of elbow contracture development.

CONCLUSIONS

The prevalence of elbow flexion contractures in early childhood is greater than previously understood. These findings indicate that C5-C6 injury affecting elbow flexion with relative preservation of elbow extension is a predictor of contracture development. Further research is needed to investigate the nature and sequelae of C5-C6 injury and its effects on elbow flexion contracture development.

The Relative Efficacy of Available Proteasome Inhibitors in Preventing Muscle Contractures Following Neonatal Brachial Plexus Injury

BACKGROUND

Contractures following neonatal brachial plexus injury (NBPI) are associated with growth deficits in denervated muscles. This impairment is mediated by an increase in muscle protein degradation, as contractures can be prevented in an NBPI mouse model with bortezomib (BTZ), a proteasome inhibitor (PI). However, BTZ treatment causes substantial toxicity (0% to 80% mortality). The current study tested the hypothesis that newer-generation PIs can prevent contractures with less severe toxicity than BTZ.

METHODS

Unilateral brachial plexus injuries were surgically created in postnatal (5-day-old) mice. Following NBPI, mice were treated with either saline solution or various doses of 1 of 3 different PIs: ixazomib (IXZ), carfilzomib (CFZ), or marizomib (MRZ). Four weeks post-NBPI, mice were assessed for bilateral passive range of motion at the shoulder and elbow joints, with blinding to the treatment group, through an established digital photography technique to determine contracture severity. Drug toxicity was assessed with survival curves.

RESULTS

All PIs prevented contractures at both the elbow and shoulder (p < 0.05 versus saline solution controls), with the exception of IXZ, which did not prevent shoulder contractures. However, their efficacies and toxicity profiles differed. At lower doses, CFZ was limited by toxicity (30% to 40% mortality), whereas MRZ was limited by efficacy. At higher doses, CFZ was limited by loss of efficacy, MRZ was limited by toxicity (50% to 60% mortality), and IXZ was limited by toxicity (80% to 100% mortality) and loss of efficacy. Comparisons of the data on these drugs as well as data on BTZ generated in prior studies revealed BTZ to be optimal for preventing contractures, although it, too, was limited by toxicity.

CONCLUSIONS

All of the tested second-generation PIs were able to reduce NBPI-induced contractures, offering further proof of concept for a regulatory role of the proteasome in contracture formation. However, the narrow dose ranges of efficacy for all PIs highlight the necessity of precise proteasome regulation for preventing contractures. Finally, the substantial toxicity stemming from proteasome inhibition underscores the importance of identifying muscle-targeted strategies to suppress protein degradation and prevent contractures safely.

CLINICAL RELEVANCE

Although PIs offer unique opportunities to establish critical mechanistic insights into contracture pathophysiology, their clinical use is contraindicated in patients with NPBI at this time.

ISB clinical biomechanics award winner 2023: Medial gastrocnemius muscle and Achilles tendon interplay during gait in cerebral palsy

BACKGROUND

The interplay between the medial gastrocnemius muscle and the Achilles tendon is crucial for efficient walking. In cerebral palsy, muscle and tendon remodelling alters the role of contractile and elastic components. The aim was to investigate the length changes of medial gastrocnemius belly and fascicles, and Achilles tendon to understand their interplay to gait propulsion in individuals with cerebral palsy.

METHODS

Twelve young individuals with cerebral palsy and 12 typically developed peers were assessed during multiple gait cycles using 3D gait analysis combined with a portable ultrasound device. By mapping ultrasound image locations into the shank reference frame, the medial gastrocnemius belly, fascicle, and Achilles tendon lengths were estimated throughout the gait cycle. Participants with cerebral palsy were classified into equinus and non-equinus groups based on their sagittal ankle kinematics.

FINDINGS

In typically developed participants, the Achilles tendon undertook most of the muscle-tendon unit lengthening during stance, whereas in individuals with cerebral palsy, this lengthening was shared between the medial gastrocnemius belly and Achilles tendon, which was more evident in the equinus group. The lengthening behaviour of the medial gastrocnemius fascicles resembled that of the Achilles tendon in cerebral palsy.

INTERPRETATION

The findings revealed similar length changes of the medial gastrocnemius fascicles and Achilles tendon, highlighting the enhanced role of the muscle in absorbing energy during stance in cerebral palsy. These results, together with the current knowledge of increased intramuscular stiffness, suggest the exploitation of intramuscular passive forces for such energy absorption.

The role of sympathetic innervation in neonatal muscle growth and neuromuscular contractures

Neonatal brachial plexus injury (NBPI), a leading cause of pediatric upper limb paralysis, results in disabling and incurable muscle contractures that are driven by impaired longitudinal growth of denervated muscles. A rare form of NBPI, which maintains both afferent and sympathetic muscle innervation despite motor denervation, protects against contractures. We have previously ruled out a role for NRG/ErbB signaling, the predominant pathway governing antegrade afferent neuromuscular transmission, in modulating the formation of contractures. Our current study therefore investigated the contributions of sympathetic innervation of skeletal muscle in modulating NBPI-induced contractures. Through chemical sympathectomy and pharmacologic modification with a β2 -adrenergic agonist, we discovered that sympathetic innervation alone is neither required nor sufficient to modulate contracture formation in neonatal mice. Despite this, sympathetic innervation plays an intriguing sex-specific role in mediating neonatal muscle growth, as the cross-sectional area (CSA) and volume of normally innervated male muscles were diminished by ablation of sympathetic neurons and increased by β-adrenergic stimulation. Intriguingly, the robust alterations in CSA occurred with minimal changes to normal longitudinal muscle growth as determined by sarcomere length. Instead, β-adrenergic stimulation exacerbated sarcomere overstretch in denervated male muscles, indicating potentially discrete regulation of muscle width and length. Future investigations into the mechanistic underpinnings of these distinct aspects of muscle growth are thus essential for improving clinical outcomes in patients affected by muscle disorders in which both length and width are affected.