Superconditioning TMS for examining upper motor neuron function in MND

Motor potentials evoked by transcranial magnetic stimulation: interpreting a simple measure of a complex system

Transcranial magnetic stimulation (TMS) is a non-invasive technique that is increasingly used to study the human brain. One of the principal outcome measures is the motor-evoked potential (MEP) elicited in a muscle following TMS over the primary motor cortex (M1), where it is used to estimate changes in corticospinal excitability. However, multiple elements play a role in MEP generation, so even apparently simple measures such as peak-to-peak amplitude have a complex interpretation. Here, we summarize what is currently known regarding the neural pathways and circuits that contribute to the MEP and discuss the factors that should be considered when interpreting MEP amplitude measured at rest in the context of motor processing and patients with neurological conditions. In the last part of this work, we also discuss how emerging technological approaches can be combined with TMS to improve our understanding of neural substrates that can influence MEPs. Overall, this review aims to highlight the capabilities and limitations of TMS that are important to recognize when attempting to disentangle sources that contribute to the physiological state-related changes in corticomotor excitability.

Establishing the Minimum Intensity of Transcranial Magnetic Stimulation Superconditioning Pulses to Effect Inhibition and Facilitation of Motor Evoked Potentials

PURPOSE

Previously, we showed that a three-pulse train of weak transcranial magnetic stimulation (TMS) pulses-a superconditioning (SC) train-when followed by a stronger TMS pulse could enhance the inhibition or facilitation of the resultant motor evoked potential (MEP) compared with that seen with traditional dual-pulse inputs. The purpose of the present study was to establish the relative minimum intensity of SC pulses needed to influence MEP output and whether this differed for upper- versus lower-limb muscles.

METHODS

We examined 33 older adult subjects, targeting abductor pollicis brevis and tibialis anterior muscles. Older subjects were included in the anticipation of using findings from this study to guide further studies in persons with amyotrophic lateral sclerosis. Three-pulse trains of SC inputs of different intensities were delivered either 1 millisecond before (for inhibition) or 10 millisecond before (for facilitation) a stronger TMS test pulse. Motor evoked potential magnitudes for SC +test sets were normalized to test input responses and were compared within and between subjects.

RESULTS AND CONCLUSIONS

For inhibition, the minimum intensity of SC pulses needed to influence the follow-on MEP was found to be 60% of the target muscle's resting three-pulse MEP threshold for most abductor pollicis brevis and tibialis anterior muscles (2-millisecond interpulse intervals). For facilitation, somewhat higher intensities (70%) were typically needed to cause facilitation. Both values of SC pulses for inhibition/facilitation are considerably lower than the intensity of the conditioning pulse-often reported as 80% of the single-pulse threshold-typically used in dual-pulse TMS paradigms. This approach may allow testing of upper motor neuron function using weaker stimulus pulse intensities than are typically employed, improving testing compliance in persons whose thresholds are elevated because of injury or disease.

On the Use of TMS to Investigate the Pathophysiology of Neurodegenerative Diseases

Neurodegenerative diseases are a collection of disorders that result in the progressive degeneration and death of neurons. They are clinically heterogenous and can present as deficits in movement, cognition, executive function, memory, visuospatial awareness and language. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation tool that allows for the assessment of cortical function in vivo. We review how TMS has been used for the investigation of three neurodegenerative diseases that differ in their neuroanatomical axes: (1) Motor cortex-corticospinal tract (motor neuron diseases), (2) Non-motor cortical areas (dementias), and (3) Subcortical structures (parkinsonisms). We also make four recommendations that we hope will benefit the use of TMS in neurodegenerative diseases. Firstly, TMS has traditionally been limited by the lack of an objective output and so has been confined to stimulation of the motor cortex; this limitation can be overcome by the use of concurrent neuroimaging methods such as EEG. Given that neurodegenerative diseases progress over time, TMS measures should aim to track longitudinal changes, especially when the aim of the study is to look at disease progression and symptomatology. The lack of gold-standard diagnostic confirmation undermines the validity of findings in clinical populations. Consequently, diagnostic certainty should be maximized through a variety of methods including multiple, independent clinical assessments, imaging and fluids biomarkers, and post-mortem pathological confirmation where possible. There is great interest in understanding the mechanisms by which symptoms arise in neurodegenerative disorders. However, TMS assessments in patients are usually carried out during resting conditions, when the brain network engaged during these symptoms is not expressed. Rather, a context-appropriate form of TMS would be more suitable in probing the physiology driving clinical symptoms. In all, we hope that the recommendations made here will help to further understand the pathophysiology of neurodegenerative diseases.