The Strength of the Corticospinal Tract Not the Reticulospinal Tract Determines Upper-Limb Impairment Level and Capacity for Skill-Acquisition in the Sub-Acute Post-Stroke Period

 

Absolutely nothing here is of any use at all. NOTHING ON INCREASING THE STRENGTH OF THE CORTICOSPINAL TRACT!  Useless. What the fuck was the point of this research? I'd fire you immediately for even suggesting this crapola. And until we get survivors in charge stroke survivors will be screwed.

The Strength of the Corticospinal Tract Not the Reticulospinal Tract Determines Upper-Limb Impairment Level and Capacity for Skill-Acquisition in the Sub-Acute Post-Stroke Period

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Ulrike Hammerbeck, PhD, Sarah F. Tyson, PhD, Prawin Samraj, MS, ...

First Published July 4, 2021 Research Article 

https://doi.org/10.1177/15459683211028243

Article information

Abstract

Background. 

Upper-limb impairment in patients with chronic stroke appears to be partly attributable to an upregulated reticulospinal tract (RST). Here, we assessed whether the impact of corticospinal (CST) and RST connectivity on motor impairment and skill-acquisition differs in sub-acute stroke, using transcranial magnetic stimulation (TMS)–based proxy measures.  

Methods. 

Thirty-eight stroke survivors were randomized to either reach training 3-6 weeks post-stroke (plus usual care) or usual care only. At 3, 6 and 12 weeks post-stroke, we measured ipsilesional and contralesional cortical connectivity (surrogates for CST and RST connectivity, respectively) to weak pre-activated triceps and deltoid muscles with single pulse TMS, accuracy of planar reaching movements, muscle strength (Motricity Index) and synergies (Fugl-Meyer upper-limb score).  

Results. Strength and presence of synergies were associated with ipsilesional (CST) connectivity to the paretic upper-limb at 3 and 12 weeks. Training led to planar reaching skill beyond that expected from spontaneous recovery and occurred for both weak and strong ipsilesional tract integrity. Reaching ability, presence of synergies, skill-acquisition and strength were not affected by either the presence or absence of contralesional (RST) connectivity.  

Conclusion. 

The degree of ipsilesional CST connectivity is the main determinant of proximal dexterity, upper-limb strength and synergy expression in sub-acute stroke. In contrast, there is no evidence for enhanced contralesional RST connectivity contributing to any of these components of impairment. In the sub-acute post-stroke period, the balance of activity between CST and RST may matter more for the paretic phenotype than RST upregulation per se.

Keywords

stroke, upper limb, motor impairment, skill learning, corticospinal tract, reticulospinal tract

Introduction

Motor impairment after stroke is closely associated with ipsilesional corticospinal tract (CST) damage.^1-4 In addition, recent data suggest that arm flexor synergies, finger enslaving on the paretic side and mirror movements on the non-paretic hand after stroke are all attributable to an increased influence of the reticulospinal tract (RST) after damage to the CST.^5-11 Studies in primates have shown that 6 months after a lesion in the pyramidal tract,¹² there is upregulation of the RST. In patients with chronic stroke, the incidence of contralesional connectivity to the ipsilateral paretic limb is increased, particularly in patients with moderate to severe paresis,^13,14 suggesting a similar upregulation of RST activity during recovery.¹⁵ An unanswered question is the impact of this RST upregulation after the initial plegic stage³; does it contribute to, or impede recovery, or is it an epiphenomenon of recovery, neither good nor bad.⁷ Furthermore, it is unclear whether unwanted muscle synergies result from actual upregulation of pre-existing cortico-reticulospinal descending pathways or can be attributed instead to a relative imbalance between them (in the absence of upregulation) and the CST.⁶

Using transcranial magnetic stimulation (TMS), we sought to determine the degree of ipsilesional and contralesional cortical connectivity to paretic arm muscles in a group of patients with moderate to severe stroke in the early sub-acute period. TMS of the human motor cortex in one hemisphere can evoke responses in ipsilateral muscles with characteristics compatible with activation of oligosynaptic cortico-bulbospinal pathways,¹⁶ most likely representing cortico-reticulo-spinal connection.^13,14,17-19 This provides an indirect method of assessing the excitability of the RST in stroke survivors.^11,20-22 We further investigated the effect of these two forms of connectivity on strength, synergies, planar reaching accuracy and capacity for skill-acquisition. We examined inputs to proximal muscles involved in planar reaching movements since these are thought to receive greater reticulospinal inputs than distal arm muscles.^16,23

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