Y P O Berenson-Allen Center for Noninvasive Brain Stimulation - - PowerPoint PPT Presentation

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Y P O Berenson-Allen Center for Noninvasive Brain Stimulation - - PowerPoint PPT Presentation

Nov 27, 2022 218 likes •435 views

Y P O Berenson-Allen Center for Noninvasive Brain Stimulation Department of Neurology C Beth Israel Deaconess Medical Center Harvard Medical School T O Determining motor threshold N O Franziska Plessow, Ph.D. D Paula Davila-Prez,

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Intensive Course in Transcranial Magnetic Stimulation, 06/18/2019

Determining motor threshold

Franziska Plessow, Ph.D. Paula Davila-Pérez, M.D., Ph. D. Ali Jannati, M.D., Ph.D. Peter J. Fried, Ph.D.

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

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Safety First!!!

Determining motor threshold

  • Screen for contraindications and side-effects
  • Earplugs must be worn by subject and operator

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Motor threshold (MT) definition

  • The minimum intensity (% of maximum machine output)

to elicit a motor response in at least 50% of trials

Determining motor threshold

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Role of MT

  • Objective measure of relative cortical excitability/reactivity
  • Reflects voltage-dependent ion channel function
  • Highest test-retest reliability of any TMS measure
  • Used to individualize intensity of further TMS
  • Consistent with safety limits (Rossini et al., 2009)

Determining motor threshold

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Types of MT

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

Determining motor threshold

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Factors that influence MT

  • Subject factors:
  • Inter- and intra-individual variance
  • Activity of brain/muscle
  • Coil-to-cortex distance
  • CNS drugs
  • Physical Parameters:
  • Device (stimulator and coil)
  • Pulse waveform/shape/direction
  • Navigated vs. Non-navigated TMS
  • Method of determination (visible/EMG)

Determining motor threshold

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Methods of determining MT

Visual inspection Electromyography (EMG)

Determining motor threshold

MEP latency Peak-to-peak MEP Amplitude TMS Pulse

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RMT with EMG

Rossini-Rothwell method:

  • Minimum intensity to elicit motor evoked potentials (MEPs)
  • f ≥ 50 μV peak-to-peak amplitude

in ≥50% of consecutive trials (typically 10)

μV s s μV

Determining motor threshold

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Target muscle

Determining motor threshold

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Penfield‘s motor homonculus

Determining motor threshold

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Attaching EMG electrodes

Identify Clean Attach

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Finding the “hot spot” with neuronavigation

Determining motor threshold

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Finding the “hot spot” without neuronavigation

(Jaspers, 1958)

≈5 cm lateral from the vertex

Determining motor threshold

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Finding a starting location

5cm 5cm 7cm

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Finding the “hot spot”

  • 1. Set intensity to 30% and deliver a couple of pulses
  • 2. Go up in steps of 5-10% until MEPs are observed
  • 3. Deliver several pulses to ensure a consistent response is

evident (suprathreshold)

  • 4. Test four spots around the location of the MEP

(north, east, south, west)

  • 5. Repeat Step 4 until the individual’s

“hot spot” is identified

Determining motor threshold

Whatever you do, do it consistently.

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Finding the MT

  • 1. Record 10 MEPs
  • 2. Progressively lower intensity (1-2%) until ≥5/10 trials elicit

an MEP of ≥50 μV (or visible twitch)

  • 3. Always check 1 intensity lower

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Finding the MT

  • Alternatives under time constraints:
  • ≥3/6
  • Adaptive MT determination/Parameter estimation by sequential

testing (PEST) with the TMS Motor Threshold Assessment Tool (clinicalresearcher.org)

  • Trouble shooting:
  • No MEP detected (relaxation, AMT, silent period)
  • MEP latencies = 20-30 ms

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Phosphenes

  • Visual percept not triggered by phototransduction (i.e.,

rubbing your eyes, blow to the head, TMS to visual cortex)

  • Means to probe excitability of visual cortex (like MEPs)
  • Unlike MEPs, phosphenes are subjective

Eyes closed: phosphene Eyes open: scotoma Determining phosphene threshold

P L E A S E D O N O T C O P Y

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Phosphene hotspot/threshold

  • Measure 2cm dorsal and 2cm lateral from the inion
  • Center the coil with handle pointing away from midline
  • Phosphenes should appear in contralateral visual field
  • Increase intensity or move coil until phosphenes are reported
  • Assess phosphene threshold same as motor (i.e., 5 of 10)

Determining phosphene threshold

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Safety First!!!

Determining motor threshold

  • Screen for contraindications and side-effects
  • Earplugs must be worn by subject and operator

P L E A S E D O N O T C O P Y