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ELECTROMYOGRAPHY
EVOKED POTENTIALS
Josef Bednařík, Department of Neurology, University Hospital Brno and Faculty of
Medicine, Masaryk University Brno
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DEFINITION OF ELECTROMYOGRAPHY
(EMG)
̶ EMG is an electrodiagnostic method aimed at the diagnosis of neuromuscular
disorders (i.e., involvement of peripheral motor, sensory, and autonomic
neurons, neuromuscular transmission and voluntary muscles).
̶ Methodologically it comprises two groups of techniques:
̶ Needle EMG using needle recording electrodes for registration of
bioelectrical potentials from voluntary muscles;
̶ Conduction studies using artificial electrical stimulation of nerves and
recording evoked responses from muscles or nerves with surface recording
electrodes
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NEEDLE EMG I
1. Insertion activity:
̶ Example of abnormal insertion activity: myotonic discharges
Definujte zápatí - název prezentace / pracoviště4
NEEDLE EMG II
̶ („end-plate noise“)
̶ („end-plate spikes“)
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NEEDLE EMG III
̶ 2. Abnormal spontaneous activity:
̶ fibrillation potentials and
positive sharp waves
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NEEDLE EMG IV
2. Abnormal
spontanneous
activity:
̶ tetanic discharges
(dublets, triplets,
multiplets)
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NEEDLE EMG V
2. Abnormal spontaneous activity: neuromyotonic discharges
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NEEDLE EMG VI
3. Physiological activity at rest
(no potentials)
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NEEDLE EMG VII
3. Quantification of
parameters of motor unit
potentials (MUPs)
– indicator of
microarchitecture of
motor unit
̶ → Signs of chronic re-
innervation
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NEEDLE EMG VIII
3. Quantification of
parameters of motor
unit potentials
(MUPs)
̶ indicator of
microarchitecture of
motor unit
̶ → Signs of
myogenic lesion
(decreased number
of muscle fibers)
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NEEDLE EMG IX
3. Quantification of parameters of motor unit potentials (MUPs) –
indicator of microarchitecture of motor unit
̶ → Signs of myogennic lesion (decreased number of muscle fibers)
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NEEDLE EMG X
4. Assessment of recruitment of motor units and interference pattern
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CONDUCTION STUDIES I
1. Motor conduction studies: diffuse conduction slowing
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CONDUCTION STUDIES II
1. Motor conduction studies : focal conduction slowing + conduction block
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CONDUCTION STUDIES III
1. Motor conduction studies : focal conduction slowing + axonal loss
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CONDUCTION STUDIES IV
1. Motor conduction studies : focal conduction slowing („inching“ technique)
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CONDUCTION STUDIES V
1. Motor conduction studies : focal parcial conduction block
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CONDUCTION STUDIES VI
1. Motor conduction studies : multifocal conduction slowing + conduction block
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CONDUCTION STUDIES VII
1. Motor conduction studies : repetitive stimulation of motor nerve
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CONDUCTION STUDIES VIII
2. Sensory conduction studies
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EVOKED POTENTIALS (EP): DEFINITION
̶ Evoked potentials represents bioelectrical response of the brain (or spinal cord
and peripheral nerves) to external stimuly (mostly of sensory character) –
sensory EP.
̶ Evoked potentials (as a diagnostic method or tool) are electrodiagnostic
methods that register and evaluate bioelectrical potentials triggered by visual
(VEP), auditory (BAEP) and somatosensory stimuli (SEP).
̶ Motor evoked potentials (MEP) use magnetic (originally electrical) stimulation
to excitate motor cortex (transcranially) and to register response from a muscle.
̶ Endogenous or congitive potentials are long-latency responses related to
cognitive processes or iniciation of voluntary movement; it is mostly research
tool.
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EVOKED POTENTIALS: TECHNICAL PRINCIPLE
Evoked potentials generated in the cortex or spinal cord and
recorded over the scalp or the spine have the magnitude in order
of microvolts (therefore lower than EEG or artifacts). Extraction of
these EP „burried“ in other electrical activity at the recording areas
is enabled by the „averaging“ method performed by a computer.
EP appears in a constant time interval from the stimulus (in
contrast to otherwise accidental other electrical activities).
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EVOKED POTENTIALS: CLINICAL IMPORTANCE
They:
̶ Objectify clinical data and offer quantitative information;
̶ Capture subclinical lesion or dysfunction
̶ Offer precision of localisation of the lesion;
̶ Could monitor function of the system or pathway during surgery
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VISUAL EVOKED POTENTIALS
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BRAINSTEM AUDITORY EVOKED POTENTIALS
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SOMATOSENSORY EVOKED POTENTIALS
Electrical stimulation of the median nerve
at the wrist
Cortex
Cervico-medullar junction
Cervical posterior horns
Erb‘s point
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SOMATOSENSORY EVOKED POTENTIALS
Electrical stimulation of the tibial nerve at behind
malleolus
Cortex
Cervico-medular junction
Lumbar medullar posterior horns
Fossa poplitea
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MOTOR EVOKED POTENTIALS
Magnetic
stimulation of the
cervical or lumbar
roots (RL) and
cortex (CL)