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NCS and EMG Basics

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by

David Avila

on 4 January 2016

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Transcript of NCS and EMG Basics

What do we measure?
What do the measurements mean?
How are these measurements reported?
How do diseases
affect these measurements?


NCS and EMG Basics
David Avila, MD
AKA "La Mano"


Nerve
conduction studies
(NCS)
ELECTRODIAGNOSTIC
STUDIES
(EDX)
Needle electromyography
(EMG)
Motor
Sensory
Mixed
Insertional activity
Spontaneous activity
Special tests
Late responses:
F waves
H reflex
Repetitive stimulation

Volitional
activity
Blink reflex
NCS and EMG are an extension of the neurologic exam
NCS only test large myelinated fibers
Sensory NCS only test nerve distal to dorsal root ganglia
Motor NCS
Stimulate nerve at 2 sites
Record on muscle belly
Orthodromic
Measured in mV
Response: Compound muscle action potential (CMAP)
Routine: Median-APB, ulnar-ADM, peroneal-EDB, tibial-AH
Sensory NCS
Stimulate nerve
Record on nerve
Antidromic
Measured in uV
Response: Sensory nerve action potential (SNAP)
Routine: Median-Digit II, ulnar-digit V, radial-anatomic snuffbox, sural-ankle, superficial peroneal-dorsum of foot
CMAP
SNAP
Insertional activity
Muscle fiber discharges induced by needle being moved in resting muscle
Spontaneous activity
Discharges that occur without being triggered by needle movement in resting muscle
Fibrillation potentials

Positive sharp waves
Volitional activity
Motor unit action potential (MUAP)
Recruitment
Sequential activation of motor units to increase muscle power
5:1 recruitment ratio: For every 5Hz increase in firing frequency, an additional MUAP is recruited
Motor NCS
Amplitude:
TOTAL number of muscle fiber AP-> Proportional to number of conducting motor fibers
Latency:
EARLIEST muscle fiber AP-> FASTEST conducting motor fibers
Motor NCS
Conduction velocity:
FASTEST conducting fibers
Proximal latency - distal latency
Sensory NCS
Amplitude:
TOTAL number of conducting sensory fibers
Peak latency:
FASTEST conducting sensory fibers-Easier to identify than onset latency
Conduction velocity:
FASTEST conducting fibers
Distance / onset latency
NEUROPATHIC
Axon loss
Demyelination
Motor NCS: CMAP amplitude decreased
Sensory NCS: SNAP amplitude decreased
Latency and CV: Normal
Amplitude: Normal
Slowing of CV
Prolonged distal latencies and late responses
Conduction block
ABNORMAL NCS =
ABNORMAL EMG
Neurogenic
Myopathic
Spontaneous activity: Fibrillation potentials, positive sharp waves
Volitional activity: MUAPs of increased amplitude and duration, decreased recruitment
Spontaneous activity: Fibrillation potentials, myotonic potentials
Volitional activity: MUAPs of small amplitude and short duration, increased recruitment
Full transcript