The Upper Extremity Flexion Synergy Is Minimally Expressed in Young Individuals With Unilateral Cerebral Palsy Following an Early Brain Injury

Abstract

Hemiparetic stroke in adulthood often results in the grouped movement pattern of the upper extremity flexion synergy thought to arise from an increased reliance on cortico-reticulospinal pathways due to a loss of lateral corticospinal projections. It is well established that the flexion synergy induces reaching constraints in individuals with adult-onset hemiplegia. The expression of the flexion synergy in individuals with brain injuries onset earlier in the lifespan is currently unknown. An early unilateral brain injury occurring prior to six months post full-term may preserve corticospinal projections which can be used for independent joint control and thus minimizing the expression of the flexion synergy. This study uses kinematics of a ballistic reaching task to evaluate the expression of the flexion synergy in individuals with pediatric hemiplegia (PH) ages six to seventeen years. Fifteen individuals with brain injuries before birth (n = 8) and around full-term (n = 7) and nine age-matched controls with no known neurological impairment completed a set of reaches in an admittance controlled robotic device. Descending drive, and the possible expression of the upper extremity flexion synergy, was modulated by increasing shoulder abduction loading. Individuals with early-onset PH achieved lower peak velocities when reaching with the paretic arm compared to controls; however, no differences in reaching distance were found between groups. Relative maintenance in reaching seen in individuals with early brain injuries highlights minimal expression of the flexion synergy. We interpret this conservation of independent control of the paretic shoulder and elbow as the use of more direct corticospinal projections instead of indirect cortico-reticulospinal pathways used in individuals with adult-onset hemiplegia.

Between Limb Muscle Co-activation Patterns in the Paretic Arm During Non-paretic Arm Tasks in Hemiparetic Cerebral Palsy

“…In contrast to maintained ipsilateral CST projections for upper limb control, the corticoreticulospinal tract is implicated in the abnormal flexion synergies of the upper extremity in adults after stroke (Schwerin et al, 2008;McPherson et al, 2018). Pronounced flexion synergies driven by shoulder abduction during reaching are seen in adult-onset hemiplegia, whereas our previous work in pediatric-onset hemiplegia largely showed an absence of this synergy pattern in those with early lesions while completing a ballistic reaching task (Hill and Dewald, 2020). This provides evidence against the activation of brainstem mediated pathways during volitional movements and would support the hypothesis that mirrored activation could be driven by ipsilateral CST connections.…”

“…Individuals with HCP were identified through the Cerebral Palsy Research Registry ( Hurley et al, 2011 ), local clinics, and parent support groups. In brief, participants were; (1) at least 6 years of age at time of testing; and (2) had a diagnosis of CP with unilateral motor impairment of the upper extremity; full criteria have been reported previously ( Hill and Dewald, 2020 ). A cohort of age-matched controls without known neurological impairment (typical development; TD) was recruited for comparison.…”

“…Active differential surface electrodes with 1-cm inter-electrode distance (Delsys, 16-channel Bagnoli EMG System, Boston, MA, United States; 1,000×gain, 20 and 450 Hz bandpass filter) were placed by a licensed physical therapist on the following six upper extremity muscles on each arm: biceps brachii (BIC; elbow flexor), lateral head of the triceps brachii (TRI; elbow extensor), intermediate deltoid (IDL; shoulder abductor), brachioradialis (BRD; elbow flexor), extrinsic wrist/finger flexors (WFL; flexor carpi radialis and flexor digitorum profundus), and extrinsic wrist/finger extensors (WEX; extensor carpi radialis, extensor digitorum communis). After electrode placement, the forearm of the arm randomized as the first Active_Isometric Arm was casted with a participant specific cast from just distal to the epicondyles of the humerus to the fingertips ( Hill and Dewald, 2020 ). This arm was positioned at 85° shoulder abduction, 40° shoulder horizontal flexion, and 90° elbow extension as previously described ( Hill and Dewald, 2020 ).…”

“…After electrode placement, the forearm of the arm randomized as the first Active_Isometric Arm was casted with a participant specific cast from just distal to the epicondyles of the humerus to the fingertips ( Hill and Dewald, 2020 ). This arm was positioned at 85° shoulder abduction, 40° shoulder horizontal flexion, and 90° elbow extension as previously described ( Hill and Dewald, 2020 ). Participants were cued to produce and hold a 5 s maximum effort in five joint torque directions: shoulder abduction, elbow flexion, elbow extension, wrist flexion, and wrist extension.…”

“…Additionally, our methods allow us to determine whether there are differences in amount of mirroring within the limb. While mirror movements, or associated reactions ( Brunnstrom, 1970 ), observed in adults following stroke are thought to be mediated by pathways synapsing in the brainstem ( Ejaz et al, 2018 ), the absence of abnormal flexion synergies detected in individuals with HCP indicates that use of a brainstem pathway is unlikely to significantly influence arm control in this population ( Hill and Dewald, 2020 ). We hypothesized that individuals with HCP would show mirroring in the paretic arm primarily when activating the NP arm and not the reverse (P_Mirror) explained by maintained ipsilateral projections branching from the CST descending from the non-lesioned cortical hemisphere ( Staudt et al, 2002 ; Alawieh et al, 2017 ).…”