Normal/Abnormal EMG

EMG: Normal time for electrical signal to stop with insertional activity

50 msec

What does it mean when you hear a dull sound on EMG (should be crisp)

Needle is not close enough to the motor unit

What is interpreted/assessed in EMG

SAD: shape, amplitude, and duration

Normal range for amplitude on EMG

300-8000 microV

First few motor units recruited with submaximal isometric contraction should show motor units with amplitudes no higher than __

1500 microV

Typical duration (onset to return to baseline aka width) for a motor unit (contraction force is irrelevant)

7-12 sec (longer than 1 box = abnormal)

Abnormal vs normal for # of phases (# of times the motor unit crosses the baseline)

normal = <4 phases

abnormal = >4 (polyphasic)

How is amplitude measured on EMG

negative to positive peak

We need to assess roughly __ different motor units by moving needle to different depth/quadrant

12-20

Max # of motor units that should be seen on the screen and why

3-4

>3-4 = motor units may appear to be polyphasic

EMG parameter: interference

means to which you can evaluate the # of MUs and the muscle force exerted with maximal voluntary contraction/effort

Full pattern for interference parameter? Indicates __ disorder

Baseline is obscured by the MUPs with max effort; neurogenic

Discrete interference pattern? Indicates ___ disorder

IIIndividual MUPs are recognizable with max effort; myopathic disorder

How many different depth’s / quadrants should a muscle be assessed in EMG

3 different depths at 4 different quadrants

EMG 4 conditions to look at muscle

1) insertional activity (100)

2) activity at rest (100)

3) submaximal activity (200) - slight contraction

4) recruitment from slight to maximal contraction (400) - diamond

interference too? (maximal)

What is associated with increased/decreased insertional activity?

Increased: spontaneous potentials - activity after 50 msec

Decreased: Reduced # of healthy muscle fibers (fibrotic - bad prognosis) - no potentials with movement of the needle

Only exception to electrical silence at rest + significance

end plate spikes - needle is too close to NMJ (accompanied by discomfort)

Fibs and positive sharp waves (thought to be physiologically the same) are generated by ___ and signify __

a single muscle fiber; muscle fiber denervation/axonal injury

Difference in shape between fibs and PSWs?

fibs - initial SMALL positive deflection followed by negative deflection (biphasic)

PSWs - positive LARGE deflection followed by a ROUNDED/small negative deflection (biphasic)

Size of fibrillation potentials/PSWs that indicate acute/chronic deenervation

acute - 100-1000 microvolts

chronic - <20-50 microvolts

How do fibs differ from motor units (they look similar)

Fibs are irregular and dependent upon needle movement

Grading scale for PSWs/fibrillation potentials (0, 1+, 2+, 3 +, 4+)

0: No PSWs/FIbs

1: persistent/unsustained single trains in at least two muscle regions

2+: moderate numbers in 3 or more muscle

3+: many in all muscle regions

4+: baseline obliterated with potentials in all areas of muscles examined

Complex repetitive discharges are generated by __ and signifies __

generated by ephatic muscle fiber conduction

signifies chronic neuropathies (more common) or myopathies

Distinct sound of CRD

abrupt start/stop of “machine gun”

Myotonic discharges are generated by __ and signifies __

generated by muscle fibers

signifies chronic myopathies (more common) or neuropathies

Fasciculation potentials are generated by __ and signifies __

motor unit; chronic neurogenic disorders *especially LMN* (ALS, Jacob-Cruetzfeld, chronic root compression)

Why are fasciculations characterized by the “company they keep”

fasciculations are only abnormal if multiple different muscles have fasciculations (could be normal if a single muscle is twitching due to fatigue/spasm)

Myokymic discharge

2-10 “bursts” of fasciculations - VERY rare (only seen in chronic nerve Dz)

Abnormal motor units typically indicate ___

reinnervation/chronic injury

dire

large Amp, long duration, polyphasic motor units

Type of MUP seen in direct regeneration, early collateral sprouting, or myopathic disorders

small Amp, short duration, polyphasic motor units

Recruitment pattern type: rate of individual MUs firing will be out of proportion to the total number that are firing

Neurogenic = reduced recruitment (fewer units available and the remaining ones increase their rate to increase force) *ALL MU’s MUST HAVE SAME SHAPE*

Neuropathic recruitment (EMG) indicates ___

chronic neuropathy

Recruitment pattern type: More MUs activated than suspected force of contraction

Myopathic - rapid recruitment

Myopathic or neuropathic disorder?

Small amp., short duration (<6 msec), polyphasic MUP

rapid recruitment pattern

full interference pattern

myopathic disorder

Myopathic or neuropathic disorder?

Reduced recruitment pattern

discrete interference pattern

Neurogenic disorder

Fibs, PSWs, and insertional activity will all __ as neurogenic disorders become more chronic

decrease/go away