Josef Bednařík
Department of Neurology, University Hospital Brno and
Faculty of Medicine, Masaryk University Brno
8.11.2019
Electromyography, evoked potentials
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.
NEEDLE EMG I
1. Insertion activity:
Example of abnormal insertion activity: myotonic
discharges
NEEDLE EMG II
2. Abnormal spontaneous activity: end-plate activity
 („end-plate noise“)
 („end-plate spikes“)
NEEDLE EMG III
2. Abnormal spontaneous
activity:
fibrillation potentials and
positive sharp waves
NEEDLE EMG IV
2. Abnormal
spontanneous
activity:
tetanic
discharges
(dublets,
triplets,
multiplets)
NEEDLE EMG V
2. Abnormal spontaneous activity:
neuromyotonic discharges
NEEDLE EMG VI
3. Quantification of
parameters of motor unit
potentials (MUPs) –
indicator of
microarchitecture of motor
unit
 Signs of new re-innervation
NEEDLE EMG VII
3. Quantification of
parameters of
motor unit
potentials (MUPs)
– indicator of
microarchitecture
of motor unit
 Signs of chronic
re-innervation
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)
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)
NEEDLE EMG X
4. Assessment of recruitment of motor units and
interference pattern
CONDUCTION STUDIES I
1. Motor conduction studies: diffuse conduction
slowing
CONDUCTION STUDIES II
1. Motor conduction studies : focal conduction
slowing + conduction block
CONDUCTION STUDIES III
1. Motor conduction studies : focal conduction
slowing + axonal loss
CONDUCTION STUDIES IV
1. Motor conduction studies : focal conduction
slowing („inching“ technique)
CONDUCTION STUDIES V
1. Motor conduction studies : focal parcial
conduction block
CONDUCTION STUDIES VI
1. Motor conduction studies : multifocal conduction
slowing + conduction block
CONDUCTION STUDIES VII
1. Motor conduction studies : repetitive stimulation
of motor nerve
CONDUCTION STUDIES VIII
2. Sensory conduction studies:
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
elektrodiagnostic 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.
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).
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
VISUAL EVOKED POTENTIALS
BRAINSTEM AUDITORY EVOKED POTENTIALS
SOMATOSENSORY EVOKED POTENTIALS
 Electrical stimulation of the median nerve at
the wrist
 Used montage:
 Cortical pariental wave N 20 (from contralateral
parietal scalp C3/4’ - Parc – referred to
contralateral pariental scalp C3/4’ - Pari);
 Cervico-medullar wave P14 (from ipsilateral
parienta scalp C3/4’ - Pari - 2 cm behind vertex
and 7 cm laterally referred to non-cephalic
reference);
 Segmental medullar wave N13 from posterior
horns (from processus spinosus C VI referred to
ventral neck electrode),
 Brachial plexus wave N9 (from ipsilateral Erb‘s
point to reference electrode at Fz).
Cortex
Cervico-medullar junction
Cervical posterior horns
Erb‘s point
SOMATOSENSORY EVOKED POTENTIALS
 Electrical stimulation of the median nerve at
the wrist
 Used montage:
 Cortical parietal wave P40 (from Cz’ 2 cm
behind vertex to cephalic reference Fpz);
 Cervico-medullar wave P30 (from ipsilateral
parietal C3/4’ - Pari - 2 cm behind vertex and 7
cm laterally to non-cephalic reference);
 Segmental medullar wave N22 from posterior
horns (from processus spinosus L I to reference
electrode from contralateral spina iliaca),
 Wave N9 from tibial nerve (from ipsilateral
fossa poplitea to reference electrode from
patella).
Cortex
Cervico-medular junction
Lumbar medullar posterior horns
Fossa poplitea
MOTOR EVOKED POTENTIALS
RL
RL
CL
CL
RL
CL
RL
CL
ADQ
muscle
AH
muscle
R
L
R
L
Transcranial and root magnetic stimulation
Recording from m. abductor digiti quinti
(ADQ) and abductor hallucis (AH) bilaterally;
Measured parameters:
•Shortest motor latencies after cortical and
root stimulation;
•Difference between cortical and root
latencies: central motor conduction time;
•Largest motor amplitudes after cortical
stimulation and ratio of cortical and
peripheral (nerve) motor amplitudes:
MEP/CMAP