Clinically significant acute pain disturbs motor cortex intracortical inhibition and facilitation in orthopedic trauma patients: A TMS study

ObjectivePrimary motor (M1) cortical excitability alterations are involved in the development and maintenance of chronic pain. Less is known about M1-cortical excitability implications in the acute phase of an orthopedic trauma. This study aims to assess acute M1-cortical excitability in patients with an isolated upper limb fracture (IULF) in relation to pain intensity.MethodsEighty-four (56 IULF patients <14 days post-trauma and 28 healthy controls) performed a single transcranial magnetic stimulation (TMS) session over M1 (resting motor threshold (rMT); short-intracortical inhibition (SICI); intracortical facilitation (ICF); long-interval cortical inhibition (LICI)). IULF patients were divided into two subgroups according to pain intensity (mild versus moderate to severe pain).ResultsReduced SICI and ICF were found in IULF patients with moderate to severe pain, whereas mild pain was not associated with M1 alterations. Age, sex, and time since the accident had no influence on TMS measures.DiscussionThese findings show altered M1 in the context of acute moderate to severe pain, suggesting early signs of altered GABAergic inhibitory and glutamatergic facilitatory activities.

Desynchronization does not contribute to intracortical inhibition and facilitation: a paired-pulse paradigm study combined with TST

The paired-pulse (PP) transcranial magnetic stimulation (TMS) paradigms allow the exploration of the motor cortex physiology. The triple stimulation technique (TST) improves conventional TMS by reducing effects of desynchronization of motor neuron discharges allowing a precise evaluation of the corticospinal conduction. The objective of our study was to explore PP TMS paradigms combined with the TST to study whether the desynchronization contributes to these phenomena and whether the combined TMS-TST protocol could improve the consistency of responses. We investigated the PP paradigms of short intracortical inhibition (SICI) with 2 ms interstimulus interval (ISI) and of intracortical facilitation (ICF) with 10 ms ISI in 22 healthy subjects applying either conventional TMS alone or combined with the TST protocol. The results of the PP paradigms combined with the TST of SICI and ICF do not differ from those with conventional TMS. However, combining the PP paradigm with the TST reduces their variability. These results speak against a contribution of the desynchronization of motor neuron discharges to the PP paradigms of SICI and ICF. Combining the PP TMS paradigm with the TST may improve their consistency, but the interindividual variability remains such that it precludes their utility for clinical practice. NEW & NOTEWORTHY Combining the triple stimulation technique with the paired-pulse stimulation paradigm improves the consistency of short intracortical inhibition and facilitation and could be useful in research, but the interindividual variability precludes their utility for clinical practice. Our findings do not suggest that desynchronization of descending discharges following transcranial magnetic stimulation contributes to short intracortical inhibition or intracortical facilitation.

Corticospinal excitability underlying digit force planning for grasping in humans

Control of digit forces for grasping relies on sensorimotor memory gained from prior experience with the same or similar objects and on online sensory feedback. However, little is known about neural mechanisms underlying digit force planning. We addressed this question by quantifying the temporal evolution of corticospinal excitability (CSE) using single-pulse transcranial magnetic stimulation (TMS) during two reach-to-grasp tasks. These tasks differed in terms of the magnitude of force exerted on the same points on the object to isolate digit force planning from reach and grasp planning. We also addressed the role of intracortical circuitry within primary motor cortex (M1) by quantifying the balance between short intracortical inhibition and facilitation using paired-pulse TMS on the same tasks. Eighteen right-handed subjects were visually cued to plan digit placement at predetermined locations on the object and subsequently to exert either negligible force (“low-force” task, LF) or 10% of their maximum pinch force (“high-force” task, HF) on the object. We found that the HF task elicited significantly smaller CSE than the LF task, but only when the TMS pulse coincided with the signal to initiate the reach. This force planning-related CSE modulation was specific to the muscles involved in the performance of both tasks. Interestingly, digit force planning did not result in modulation of M1 intracortical inhibitory and facilitatory circuitry. Our findings suggest that planning of digit forces reflected by CSE modulation starts well before object contact and appears to be driven by inputs from frontoparietal areas other than M1.