Determining motor threshold Franziska Plessow, Ph.D. Intensive - - PowerPoint PPT Presentation

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Berenson-Allen Center for Noninvasive Brain Stimulation Department of Neurology Beth Israel Deaconess Medical Center Harvard Medical School Determining motor threshold Franziska Plessow, Ph.D. Intensive Course in Transcranial Magnetic

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Intensive Course in Transcranial Magnetic Stimulation, 10/28/2014

Determining motor threshold

Franziska Plessow, Ph.D.

Berenson-Allen Center for Noninvasive Brain Stimulation Department of Neurology Beth Israel Deaconess Medical Center Harvard Medical School

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What are our learning objectives?

1. What is motor threshold (MT)?
2. Why determine MT?
3. What types of MT can be determined?
4. What are the available methods to determine MT?
5. What are the key steps for determining resting MT with

electromyography?

Determining motor threshold

à Hands-on practice

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What is motor threshold (MT)?

§ The minimum amount of machine output necessary to elicit a motor response in a participant or patient in at least 50% of all attempts § Represents membrane-related cortical excitability of cortical axons § Depends on:

Inter- and intraindividual variance
Machine being used
Type of MT
Method of determination
Hemisphere stimulated

Determining motor threshold

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Why determine MT?

§ Easy to observe § Objective (phosphene threshold) § Indicator of relative cortical excitability § A way of calibrating and normalizing TMS coil output energy for inter- and intra-individual physiologic variability in experimental designs and therapeutic applications § Determines dosage and safety limits

Determining motor threshold

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What types of MT can be determined?

Resting motor threshold (RMT) Active motor threshold (AMT)

Determining motor threshold

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What are the available methods to determine MT?

Visual inspection Electromyography (EMG)

Determining motor threshold

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RMT with EMG

§ The minimum amount of machine output necessary to elicit a motor response in a participant or patient in at least 50% of all attempts à Minimum single-pulse stimulator output intensity resulting in motor evoked potentials (MEPs) of at least 50 µV peak-to-peak amplitude in ≥50% of pursued trials (≥5/10; Rossini-Rothwell method)

µV s s µV

Determining motor threshold

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What are the key steps for determining RMT with EMG?

1. Choosing an output target
2. Setting up and safety
3. Finding the “hot spot”
4. Finding the MT

Determining motor threshold

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1. Choosing an output target

Determining motor threshold

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1. Choosing an output target

Determining motor threshold

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2. Setting up and safety

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3. Finding the “hot spot”

(Jaspers, 1958)

≈5 cm lateral from vertex

Determining motor threshold

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3. Finding the “hot spot”

(Jaspers, 1958)

≈5 cm lateral from vertex

Determining motor threshold Preaurical point

à Draw grid around average hot spot

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3. Finding the “hot spot”

Determining motor threshold

à Use grid to find individual hot spot and mark it

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4. Finding the MT
1. Set intensity to 35% and deliver a single pulse
2. Go up in steps of 5% until an MEP is observed and deliver

several pulses (to ensure a consistent response is evident)

3. Record 10 MEPs
4. Progressively lower intensity in steps of 1% until ≥5/10

runs show an MEP of ≥50 µV § Alternatives:

≥3/6
Parameter estimation by sequential testing with the TMS Motor

Threshold Assessment Tool (clinicalresearcher.org)

Determining motor threshold

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Thank you for your active participation and good luck for the rest of the week. For further questions at any time: Franziska Plessow fplessow@bidmc.harvard.edu