EMG/NCS Nerve Studies Flashcards
what does electrophysiological testng asses?
the stx integrity of peripheral nervous system
- its an extension of the clinical exam
= includes that NCS and EMG and specail tests (NM jxn)
What kind of info does EMG and NCS provide ?
•Localization of lesion in the peripheral nervous system
•Type of nerve fibers involved: motor, sensory or mixed involvement
•Components of nerve fibers injured: Axonal, demyelinating injury or both
•Severity of injury can be assessed
•Prognosis
- myopathy and neuromuscular junction disorders can also be ID’d
Is emg/ncs sensitive or specific ?
Highly sensitive indicator of early nerve injury
Why are nerve studies performed?
•Detects dynamic and functional injury missed by MRI
•Provides information regarding chronicity of nerve injury
•Highly localizing
- can classify is global issue with focal onset
can all nerve fibers be assessed?
•NCS- Only large myelinated nerve fibers (IA) assessed : sensory and motor fibers
- EMG:•Only Type I muscle fibers assessed with needle EMG
•Does not assess CNS
where are motor cell bodies located ?
located in anterior horn
where are sensory cell bodies located ?
located in dorsal root ganglion
What is a spinal nerve
exits the spinal canal through foramina and split into anterior and posterior rami
what do posterior rami innervate?
innervate capsule, facets, paraspinal muscles and skin
what do anterior rami innervate?
-cervical, upper thoracic, lumbar and sacral and thoracic spine
what is a motor point and unit?
- Motor point = site where the nerve enters the muscle, often the center of the muscle, often theres more than 1
- Motor unit = one nerve fiber (axon) and all the muscle fibers it innervates- assessed with electromyography
what pathology could be associated with Anterior horn cell?
Amyotrophic lateral sclerosis (ALS)
what pathology could be associated with a nerve root?
Radiculopathy
what pathology could be associated with a plexus?
Plexopathy
what pathology could be associated with a Peripheral nerve?
Mononeuropathy or polyneuropathy
what pathology could be associated with Neuromuscular junction?
Myasthenia gravis
what pathology could be associated with a muscle ?
myopathy
what does a NCS study?
- Motor and sensory fibers assessed for a motor or nerve conduction study
- Recording /measurement of a nerve and muscle action potential in response to a single supramaximal electrical stimulus
- Nerve is artificially stimulated with an electrical current which generates an action potential
What is an Evoked Potential?
in a NCS, a response from the sensory or motor nerve produces a electrical event as it passes by the recording and reference electrode to make a SNAP (sensory) or (CMAP) reading
- sensory nerve action potential (SNAP)
- compound muscle action potential (CMAP)
what are Antidromic and Orthodromic conductions?
Sensory N :
•Orthodromic: AP propagated in the same direction as physiologic conduction
•Antidromic: AP propagated in opposite direction to physiologic conduction
What is latency in a NCS study ?
time lapse from the stimulation to the response
•Motor = distal motor latency = DML
•Sensory = distal sensory latency = DSL
What is Amplitude in a NCS study ?
- sum total of all the fibers (nerve or muscle) recorded
- measure from baseline to peak
** gives info on # of axon working**
Motor: millivolts
sensory : microvolts \
What is Duration in a NCS study ? Does demylination increase or decrease duration?
- measured in milliseconds
- initial deflection of the negative phase to the return of the positive phase
- of some axons have undergone demylination duration would increase
What is Velocity in a NCS study ?
- speed at which an impulse (action potential) is conducted along a nerve
- Nerve conduction velocity (NCV) for the chemical event of a motor nerve cannot be calculated across the NMJ
- therefore NVC is calulated proximal to the NMJ (above wrist for median N)
- tiem to cross NMJ is about .5-1 msec
how is motor conduction velocity conducted?
distance divided by (latency 1 - latency 2)
what are NCS Variables?
- temp
Cold slows NCV, warm increases NCV
toes higher than 29c fingers higher than 31c - age
slower for <5 yrs
> 40 mild slowing
>70 sig slowing - proximal vs. distal
faster in UE
in UE nerves are shorter and larger diameter and hotter
what is a EMG study ?
recording of insertional and spontaneous and voluntary muscle activity with a needle
when/what activities are evaluated with EMG?
insertion, rest and contraction (max and min) of muscle
what determines which and how many muscles are studied?
the suspected condition and findings as the test progresses.
- EMG is the most important test for radic
- look at the same myotome but different peripheral nerves
- true weakness on MMT is the biggest predictor of abnormalities
what should the muscle activity be at rest?
quiet/electrically silent
- if upon needle insertion, theres activity, then there’s membrane instability
what are types of membrane instability, when are they seen?
•Fibrillation potentials
•Positive sharp waves
- seen at rest
what does membrane instability indicate ?
axonal pathology
-acute denervation with ongoing axonal injury
what is a MUAP : Motor Unit Action Potential
Waveform produced by motor units with muscle activation
- can help determine chronicity and if nerve is reinnervating based on morphology
what results will you see on EMG if there is ongoing axonal injury
- increased insertional activity (Ins Act)
- fibrillation potentials (Fibs)
- positive sharp waves (PSW)
what are technical Sources of Error in emg ?
- Equipment
- Environment: temperature, electrical sources of interference
- Patient characteristics: morphology, anomalous innervation, open wounds, hardware, ability to achieve muscle membrane resting state, co-operation
- Provider:
how long can it take for Denervation potentials on needle EMG to develop? is this a limitation?
- can take 1-4 weeks to develop
- yes, limitation
what does Prolonged distal latency indicate in a NCS?
- focal demyelination, maybe axon involvement
what does Decreased amplitude indicate in a NCS ?
SNAP or CMAP decrease Suggestive of axonal injury or conduction block
what does Slowed NCV indicate ?
neuropathy or poly
what does Increased insertional activity indicate in a EMG ?
ongoing axonal degeneration
what does Decreased recruitment indicate in a EMG ?
axonal injury where motor units have been lost
- decrease Strength
what does Polyphasic MUAP indicate in a EMG ?
- older axonal injury
- some reinnervation or collateral sprouting
when should you Request EMG/NCS?
- unclear diagnosis
- When pathology is worth revealing and evidence dictates a different management strategy
when should NOT you Request EMG/NCS?
•NOT when the diagnosis is obvious unless:
•You are concerned about prognosis
•You desire to better identify a process and/or it’s severity -such as a neuropathy
-
•NOT unless a good history and clinical exam has been completed
•NOT unless the results are going to change your management
•NOT for screening purposes
what can you use the results for?
- Clarify diagnostic picture
- Healing nerves need TLC
- Remember that nerves can be angry and irritated without being electrophysiologically damaged: neuritis vs neuropathy
