EMG/NCS Flashcards
Different names for clinical electrophysiology test
- Electromyography (EMG)
- Nerve conduction studies (NCS)
- Nerve conduction velocity (NCV)
- Electrodiagnostic tests
- Electrophysiologic tests
Neurogenic Symptoms
- Pain
- Numbness
- Paresthesias
- Dysesthesias
- Weakness
- Cramping
- Allodynia
Differential diagnoses that clinical electrophysiologic tests can detect?
- Focal mononeuropathy
- Cervical/Lumbar radiculopathy
- Polyneuropathy
- Brachial plexopathy
- CNS disorder (exclusion bc it is not a good test for this)
- Myopathy
- Any combination of above
How Would You Evaluate the Results of a Radiologic Study?
- Take a history and form a differential Dx
- Perform your clinical examination and begin to narrow the differential Dx
- You would look at the image (i.e. the data)
- You would read the radiologist’s report
- You would base your diagnosis on your evaluation of data from the history, your
clinical tests & measures, your impression
of the image and the radiologist’s report
Is electrophysiologic data alone sufficient for diagnosis?
no
gotta correlate the findings with other clinical evidence
Outcome of EPT is to classify/clarify….
– Nerve(s)
– Location
– Motor vs. sensory vs. both
– Axonal vs. demyelinating vs. both
– Timing (chronic, acute, etc)
– Re-innervation?
– What its not
– Recommendations
5 parts of The Clinical Electrophysiologic Examination
- Patient history
- Clarifying examination
- Nerve conduction studies
- Electromyography
- Interpretation
5 salient features analyzed in nerve conduction studies
– Amplitude
– Latency
– Duration
– Conduction velocity
– Waveform shape
Parts of a motor nerve conduction study
- Stimulating sites (anode and cathode)
- ground electrode
- active electrode
- reference electrode
What is latency
Time between onset of stimulus and onset or peak of response
What does onset latency reflect?
the conduction of the induced impulse of the fastest and largest fibers
Amplitude
- Baseline -to- peak (motor) or peak-to-trough (sensory) amplitudes
- Compound Muscle or Sensory Nerve Action Potential (CMAP or SNAP)
- Represents the total number of physiologically intact axons
What is duration
interval from first negative deflection to the return to baseline
How to calculate conduction velocity ?
distance/time OR
distance / (T2 - T1)
Can you calculate NCV from distal stimulation site to recording electrodes for motor study?
- no
1) terminal nerve starts to arborize, 2) conduction slows as diameter decreases, 3) distance of terminal branches not known, 4) time for neuromuscular transmission is unknown
Normal velocity for UE
> 50 m/sec
Orthodromic sensory nerve conduction study
normal direction
stimulating distally and recording proximal
antidromic sensory nerve conduction
stimulate at wrist and record at fingers
Axonal damage/dysfunction =
decreased amplitude proximal and distal
demyelination =
increased latency, decreased conduction velocity, decreased amplitude proximal to injury
entrapment neuropathies:
mild cases = demyelination
severe cases = axonal loss and demyelination
Does demyelination or axonal degeneration recover faster?
demyelination
NCS benefits
– Excellent for identifying demyelination
– Entrapment neuropathies
– Only technique for sensory study
– +/- identifying axonal injury
– Limited use for radiculopathies or myopathies
– Not helpful for timing
EMG Beenfits
– Gold standard to identify axonal injury
– Identifies radiculopathies and myopathies
– Helpful for classifying acuteness
– Not helpful to identify demyelination
– Only evaluates motor nerves or muscle
4 conditions that electromyography records?
– with needle movement – insertional activity
– at rest – spontaneous potentials
– with minimal voluntary contraction – motor
unit size/shape
– with contraction increasing to maximum - recruitment
What is insertional activity?
- EMG parameter
- Electrical activity due to insertion or movement of a needle in muscle
– Normal, increased, decreased, or prolonged
What is increased insertional activity associated with?
spontaneous potentials
What does decreased insertional activity indicate?
reduced number of healthy muscle fibers
- due to chronic denervation (fibrosis) or muscle necrosis = poor prognosis
Abnormal Findings at rest
– Fibrillation Potentials (Fibs)
– Positive Sharp Waves (PSWs)
– Complex Repetitive Discharge (CRD)
– Myotonic Discharges
– Fasciculation Potentials
What is myotonia
hereditary muscle disease that causes cramps
What causes fibrillation potentials and positive sharp waves
- due to either muscle fiber denervation or muscle fiber splitting (no longer receiving acetylcholine so they become extremely sensitive)
- severity is graded from 1+ to 4+ to grossly reflect amount of denervation
amplitude of fibrillation potentials and positive sharp waves
Acute = 600 mV (first 2 mo)
Chronic = 100 mV (1 year; muscle atrophy)
Grading of fibrillation potentials and positive sharp waves
0: No PSWs or Fibs
1+ Persistent/unsustained single trains in at least two muscle regions
2+ Moderate numbers in 3 or more muscle
areas
3+ Many in all muscle regions
4+ Baseline obliterated with potentials in all
areas of muscle examined
EMG Parameter: Complex Repetitive Discharge
- Occur in a Wide Variety of Chronic,
Progressive Neurogenic and Myopathic
Disorders
– To Include some Chronic Denervating
Conditions
Fasciculation Potential
discharge of entire motor unit
characterized in the company they keep
Polyphasic or Large Amplitude Motor Units
- Evidence of Motor Unit Reorganization;
Reinnervation - Two Means of Reinnervation:
– Collateral sprouting
– Direct regeneration of the original nerve
terminal
Collateral Sprouting
Terminal branches of an intact axon sprout to reinnervate motor fibers of another degenerated axon/motor unit
– polyphasic motor unit potential with larger amplitude and duration (means at least subacute)
Direct Reinnervation
Axons of a degenerated motor unit regenerate to directly reinnervate its own motor fibers
- polyphasic motor unit potential with smaller amplitude and short duration
EMG Parameter: Neurogenic recruitment
- Reduced Recruitment
– Rate of Individual MUs Firing will be out of
proportion to the total number that are firing
– Smaller units drop out and the remaining ones increase their rate to increase force
– Seen in disorders of axonal destruction/block – May be only sign in neurapraxi
EMG Parameter: Myopathic recruitment
- Rapid Recruitment
– More motor units activated than expected for
the force of contraction
– Seen when the motor fibers can not generate
normal force (i.e. in myopathic disorders)
– Rate firing is normal but amplitude is small
Abnormal EMG findings - Acute
- increased insertional activity, positive sharp waves and fibrillation potentials
- decreased recruitment
- normal motor units
Abnormal EMG findings - Subacute
- increased insertional activity, positive sharp waves and fibrillation potentials
- decreased recruitment
- polyphasic motor unit or normal amplitude
abnormal EMG findings - chronic
- decreased insertional activity
- non or very small amplitude PSWs and Fibs
- decreased recruitment
- large amplitude motor units
Clinical Electrophysiologic Findings of Carpal Tunnel Syndrome
- 1st neural defect in early CTS is demyelination of sensory fibers
- increased median DSL (more important)
- decreased median SNAP amplitude (less important)
- increased median DML (less sensitive, appears later)
- decreased median CMAP amplitude (less sensitive, appears later)
- normal NCV forearm segment (usually but not always)
- EMG abnormalities in hand intrinsic Mm. supplied by median N. (thenar Mm. & lumbricals I-II)
– least sensitive, appear in advanced cases
– PSWs, Fibs, increased insertional activity, decreased recruitment
Mild Classification of CTS
- prolonged DSL +/- decreased SNAP amplitude
Moderate Classification of CTS
prolonged DSL +/- decreased SNAP amplitude plus prolonged DML
Severe Classification of CTS
prolonged DSL, prolonged DML, decreased CMAP amplitude, EMG abnormalities
Clinical Electrophysiologic findings of cubital tunnel syndrome
– decreased ulnar sensory and motor NCV across elbow
– absent DUC SNAP in moderate to severe lesions
– decreased ulnar SNAP amplitude (possibly)
– > 20% CMAP amp. decreased with stim. above elbow (conduction block)
– > 0.4 ms latency difference in sequential 1-cm “inching” technique
– EMG abnormalities all ulnar Mm. except FCU
* least sensitive, appear only in severe cases
* PSWs, Fibs, increased insertional activity, decreased recruitment
Pitfalls of NCS/EMG
- Anatomic variations *
Reference values - Temperature: cold hand = increased latency, increased amplitude
- Excessive stimulus intensity - increased NCV, volume conduction to adjacent nerve
