EMG

Introduction

Recording of electrical activity of muscle

Skin surface

referenceActiveActive

reference

Uses of EMG

Localization of lesion Specific diagnostic information Severity of lesion In treatment

Source generators Source Motor neuron

Terminal axon

NMjMuscle

fiber

Multiple muscle fiber

Insertional activity

Brief burst for several milliseconds Important for confirmation Increased when

Last longer than 300ms. Other than end plate potential

Decreased when muscle replaced by fat and fibrous tissue

Spontaneous activity

Spontaneous activity

Morphology

Stability

Firing charecteristics

•Miniature end plate potential•Muscle fiber•Multiple muscle fiber•Motor unit

Spontaneous activity

Morphology

Stability

Firing charecteristics

•Stable•unstable

Spontaneous activity

Morphology

Stability

Firing charecteristics •Firing rate•Firing pattern

NORMAL ACTIVITY

End plate noise

Miniature end plate potentials Result from spontaneous exocytosis of AcH across

NMJ Low amplitude, monophasic, negative potential Irregular at 20-40 Hz Seashell sound

End plate spikes

Nerve potentials or muscle fiber action potential (MFAP)

due to irritation of terminal nerve leading MFAP Biphasic (triphasic) with initial negative deflection Duration 1-5 ms with low amplitude Irregular at 50 Hz

ABNORMAL ACTIVITY

Fibrillation potential

Extracellular record of single muscle fiber Spontaneous depolarization of muscle fiber Marker of active denervation Duration- 1-5ms Low amplitude- 10-100µV (chronic <10µV) Regular at 0.5- 10 Hz Sounds like rain on roofDifferentiate from end plate spikes

Positive sharp waves

Due to mechanical deformation of muscle fiber and membrane inexcitability.

Long duration Amplitude 10-100µV Regular at 0.5-10 Hz Sound like dull pop

Differentiate from voluntary contraction

0- none present +1 - persistent single trains of potential in at least 2

areas +2 - moderate potential in 3 or more areas +3 - many potential in all areas +4 – full interference pattern of potential +4 seen in nerve lacerations, vasculitis where all

fibers denervate simultaneously

Complex repetitive discharge

Depolarization of single muscle fiber followed by spread to adjacent denervated muscle fiber i.e. muscle membrane to membrane

High frequency 20-150 Hz Perfectly repetitive in nature Machine like sound Common in myopathies along with denervation

Myotonic discharge

Firing rate is 20-150 Hz Waxing and waning of both amplitude and

frequency Positive wave or brief spike morphology Revving engine sound Common in myopathies

Differentiate from positive wave and fibrillation potential

Fasciculation potentials

Single, spontaneous, involuntary discharge of individual motor unit

Fire slow and irregularly at1-2 Hz Generated from motor neuron or axon Corn popping sound Complex and large in case of reinnervation

Difeerentiate in voluntary action and BENIGN potential

Doublets, triplets, multiplets

MUAP fire in group of two, three or multiples Seen with fibrillation potential Seen in neuropathic conditions, spasms

Myokymic discharge

Grouped, repetitive, rhythmic discharge of same motor unit

Firing at 5-60 Hz Due to spontaneous depolarisation along the

demylinated segments of nerve Common in radiation injuries (plexopathies),

brainstem lesions (MS)

Cramps

Painful, involuntary contraction of muscle Repetitive and irregular firing at 40-75 Hz Associated with benign conditions, neuropathic,

metabolic, endocrinologic conditions.

Neuromyotonic discharges

Decrementing, repetitive discharge of single motor unit

High frequency- 150-250 Hz Pinging sound Generated by peripheral motor axons

Differentiate from myotonia

Tremors

Bursting of MUAP’s seperated by silence MUAP’s difficult to assess in tremors

Differentiate from myokymia

Physiology of motor unit

Extracellular compound potential of the muscle fiber of motor unit

Size principle1. Size of axon2. Thickness of myelin sheath3. Conduction velocity of axon4. Threshold to depolarization5. Type of muscle fiber

Morphology

MUAP showes normal variability MUAP duration is shorter in proximal muscles Larger in adults Loss of 1% year after third decade

Duration

Time for initial deflection from baseline to final return to baseline.

Between 5-15 ms Indicate number of muscle fibers in motor unit and

dispersion of their depolarization varies directly with age Inversely with temperature Long duration- dull and thuddy Short duration- crisp like

Polyphasia

Measure of synchrony 5-10% normal (deltoid – 25%) High frequency clicking sound Serrations indicate less

synchronus firing

Satellite potential

Linked or parasite potential Seen in early reinnervation Small, unmylinated

Collateral sprouts having slow conduction time and increased distance

After reinnervation it becomes phase of main complex

Amplitude

Greater than 100 µV Reflect fibers nearest to the

needle (2-12) Factors influencing amplitude1. Proximity of needle2. Increased number of muscle

fiber3. Increased diameter4. synchronisation

Major spike

Largest positive to negative component of MUAP Affected by surrounding tissue MUAP parameter should measured only when

needle is very close to motor unit

Stability

MUAP’s are usually are stable Unstable MUAP result in NMJ disorder Change in amplitude and number of phases During reinnervation unstable MUAP’s are formed

due to newly matured, imature NMJ

Firing pattern

MUAP fire in semi rhythmic pattern

Type Firing patternMUAP Semi rhythmic pattern

Positive sharp wavesFibrillation potential

Regular

Complex repetative discharge

Perfectly regular

Myotonic discharge Waxing and waningFasciculations irregular

Interference pattern

Muscle force is increased by Increasing firing rate Addition of motor units

Ratio of firing frequency to MUAP is 5:1 Maximal firing frequency is 30-50 Hz

Activation refers to ability to increase firing rate Central process Due to CNS lesions, pain, poor cooperation

Recruitment refers to ability to add motor units as the firing increases Seen in neuropathic conditions, end stage myopathies

Early recruitment

Due to drop out of muscle fiber from motor unit Motor units become small generating lesser force Hence many motor units fire to generate smaller

force.

Neuropathic disorder

Spontaneous activity

MUAP morphology MUAP interference

Duration Amplitude Phases Activation Recruitment

Acute axonal

N N N N N ↓

Chronic axonal

Fibr potn+ wave

↑ ↑ ↑ N ↓

CV slowing N N N N N NConduction block

N N N N N ↓

Early innervation

Fibr potn+ wave

↓ ↓ ↑ N ↓↓

CNS N N N N ↓↓ N

Myopathic disorders

Spontaneous activity

MUAP morphology MUAP interferenceDuration Amplitude Phases Activation Recruitment

Acute •N•Fibr potn•myotonia

↓ ↓ ↑ N N/ early

Chronic •N•Fibr potn•CRD

↓/↑ ↓/↑ ↑ N N/ early

End stage •N•Fibr potn•CRD

↓/↑ ↓/↑ ↑ N ↓↓

NMJ disordes

Spontaneous activity

MUAP morphology MUAP interferenceDuration Amplitude Phases Activation Recruitment

Increased jitter

•N•Fibr potn•+ waves

N N N N N

Intermittant block

N/↓ U N/↓ U N/↑ U N N/ early

Severe block

↓ ↓ ↑ N ↓↓

Thank you!