Myopathies and their Electrodiagnosis1

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Myopathies and their Electrodiagnosis1

• Randall L. Braddom, M.D., M.S.
• Clinical Professor
• Robert Wood Johnson Medical School and the New
  Jersey Medical School
• rlbraddom_at_comcast.net

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The Five Steps of EMG

• First published by Johnson and Melvin in 1971.
• Johnson EW, Melvin JL. Value of electromyography
  in lumbar radiculopathy. Arch Phys Med Rehabil
  (June) 1971. 52 239-243

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THE FIVE STEPS OF EMG

• STEP I (muscle at rest)
• Put the pin in the muscle
• Sit back, relax
• Watch for spontaneous potentials
• Fibrillations, Fasciculations, Complex Repetitive
  Discharges, Myotonic Potentials

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THE FIVE STEPS OF EMG

• STEP II (insertional activity)
• Insert the pin with quick, one mm. movements
• Baseline should be quiet when needle stops
• Insertional Activity lasts about 50 msec.
• After the movement noise, watch for
• Positive Waves
• All Spontaneous Potentials

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GRADING INSERTIONAL ACTIVITY

• NORMAL
• Easy to find, lasts 50 msec.
• DECREASED
• Hard to find, usually due to muscle loss
• INCREASED
• Easy to find, lasts longer than 50 msec.
• Some include positive waves, fibs, etc

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THE FIVE STEPS OF EMG

• STEP III
• Isolate one motor unit, focus on it
• Study the MUAP
• Frequency of firing
• Amplitude
• Duration
• Phases

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MUAP ANALYSIS

• Only a few fibers near the pin produce the
  amplitude (2-12 fibers within 0.5 mm)
• The fibers distant from the pin produce the early
  and late components of the duration
• Due to Henneman size principle, almost all MUAPs
  analyzed will be Type I
• Use wide band width filter (at least 10-10,000
  Hz)

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MUAP ANALYSIS

• Amplitude (peak to peak)
• Duration
• Phases
• of baseline crossings plus one
• 5 or more phases is polyphasic
• 10-32 of MUAPs are polyphasic

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Motor Unit Action Potential
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MUAP ANALYSIS

• Rise Time
• Dumitru recommends 0.5 msec. or less
• Turns
• Change in direction without baseline crossing

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MUAP ANALYSIS

• Satellite Potentials
• Time locked (usually a few msec later)
• Normal 10 of MUAPs
• Myopathy 45 of MUAPs
• Neuropathic Slightly more than normal
• Dont include in MUAP duration
• Probably due to fiber splitting and slow
  conduction along immature terminal sprouts

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MEAN MUAP DURATIONS
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MUAP ANALYSIS

• Myoneural Junction problems can cause
• Variability in amplitude
• Drop-out of MUAPs

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Five Steps of EMG

• Step IV
• Recruitment
• Interference Pattern

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RECRUITMENT Rule of Fives

• Isolate one MUAP
• Begins at 3 Hz and can fire in stable manner at
  5-7 Hz
• Note firing frequency of first potential when
  second one begins...usually 10 Hz
• Freeze the screen and measure time between the
  two potentials (recruitment interval) (usually
  100 msec, corresponding to 10 Hz)
• The second potential appears firing at 5 Hz
• Third potential appears when first is at 15 Hz
  and second at 10 Hz

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RECRUITMENT

• Experienced EMGers can hear this
• Easy quantitative method
• Freeze the screen with a number of motor units
  firing
• Determine frequency of fastest firing MUAP
• Divide by the number of MUAPs seen

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Recruitment Examples

• Normal
• Fastest potential is 20 Hz, 4 MUAPs present
• Recruitment ratio is 5
• MYOPATHIC
• Ratio Low 4 or less
• NEUROPATHIC
• Ratio High Usually 10 or more

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INTERFERENCE PATTERN

• Ask the patient to maximally recruit the muscle
• Watch for
• Holes in the Interference Pattern
• Amplitude of the Interference Pattern
• Observe the frequency of firing to make sure you
  are seeing a maximal effort by the patient

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THE FIVE STEPS OF EMG

• STEP V The Cerebral Step
• Put together all the steps to reach conclusions
• Do the findings support your clinical hypothesis?
• Determine how to proceed from this point in the
  study (Dynamic rather than protocol approach)

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FASCICULATIONS

• Spontaneous firing of all or part of a motor unit
• Denny-Brown and Pennypacker 1938

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FASCICULATIONS

• Step I (Muscle at Rest)
• Irregularly Irregular
• Origin anywhere in the lower motor neuron
• Usually look like polyphasic MUAP, but can be any
  size and shape
• Often normal, but also common in anterior horn
  cell disease

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FIBRILLATIONS

• Spontaneous firing of a single muscle fiber
• Best seen Step I
• 1-50 Hz
• Regular pattern

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FIBRILLATIONS

• Spontaneous oscillations in membrane potential of
  a denervated or injured muscle fiber
• Diphasic or triphasic with initial positivity

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FIBRILLATIONS

• 1-5 msec duration
• 20-1000 uV amplitude
• Can be initially negative near end plates
• Usually indicate denervation, but occur with any
  muscle membrane irritability
• Tick or sound like rain on a tin roof

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FIBRILLATIONS

• Most important factor
• Regular rhythm
• This separates them from end plate spikes and
  voluntary motor unit action potentials and
  fasciculations

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GRADING FIBRILLATIONS

• 0 None (or isolated fib)
• 1 Found in at least two muscle regions
• 2 Moderate found in three muscle regions
• 3 Many in all muscle areas tested
• 4 Baseline obliterated by fibrillations

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POSITIVE WAVES

• Same as Fibrillation
• Recorded by pin next to muscle fiber
• Seen best in Step II
• Due to muscle membrane irritability, often
  because of denervation

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POSITIVE WAVES

• Frequency 1-50 Hz
• Regular
• Sharp positive deflection, then long negative
  phase
• Duration 1-5 msec
• Amplitude to 1 mV
• Thumping sound

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Positive Waves and Fibs

• New Trend in nomenclature
• They are both the same thing
• The shape is the only difference
• Some now calling both fibrillations
• Positive waves now called fibrillations with
  positive wave shape

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MYOPATHIC CHANGES

• Decreased amplitude
• Decreased duration
• Increased number of phases
• Increased of motor units firing per strength of
  contraction
• Some refer to this as increased recruitment

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MYOTONIC DISCHARGES

• Wax and Wane in frequency and amplitude
• 20-80 Hz
• Two types can resemble fibrillations or positive
  waves
• Due to repetitive discharges of single muscle
  fibers

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EMG Separates Myopathies into Three Groups

• Inflammatory
• Muscle membrane irritability
• Rapid destruction of muscle fibers
• Non-Inflammatory
• Little muscle membrane irritability
• Slow destruction of muscle fibers
• Myotonic

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INFLAMMATORY MYOPATHIES

• Muscle membrane irritability
• Usually involve myositis
• Examples
• Dermatomyositis
• Polymyositis
• Trichinosis

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NON-INFLAMMATORY MYOPATHIES

• Few or no positive waves/fibrillations
• Often only slowly progressive
• Examples
• FSH-MD
• Steroid Myopathy

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MYOTONIC MYOPATHIES

• All have the myotonic phenomenon
• Examples
• Myotonic dystrophy
• Myotonia congenita

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NON-INFLAMMATORY MYOPATHIES

• From an EMG standpoint, non-inflammatory merely
  means that the electrical membranes are
  sufficiently stable that there are no
• Fibrillations
• Positive Waves
• Motor units will look myopathic

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INFLAMMATORY MYOPATHIES

• These typically have
• High sedimentation rate
• High muscle enzymes
• Relatively acute history
• Rapid onset of weakness
• Toxic symptoms

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5 STEPS OF EMG

• Slowly progressive myopathy
• I Normal
• II Normal
• III Reduced amplitude, duration of
  MUAPs
• IV Increased of motor units firing per strength
  of contraction
• Normal interference pattern
• NCVs usually normal

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5 STEPS OF EMG

• Rapidly progressive myopathy
• Step I Reduced and Fibrillations
• Step II Positive Waves, Fibrillations
• Step III Reduced amplitude, duration
• Step IV Increased of motor units firing per
  strength of contraction
• NCVs typically normal except for reduced
  amplitude of the evoked potential in motor
  studies

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MUAP CHANGES IN MYOPATHY

• Shorter duration
• Due to less contribution from distant fibers of
  same motor unit
• Lower amplitude
• Less contribution from fibers close to the pin
• Polyphasic
• Less integrated potential due to drop out of some
  fibers

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Myopathys Recruitment Change

• Remember the Rule of Fives
• Determine frequency of fastest firing MUAP
• Divide by number of MUAPs seen
• If fastest is at 20 Hertz and four are present,
  recruitment ratio is 5
• Myopathy typically has recruitment ratio of 4 or
  less

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Step III Problem in Myopathy

• Major Clue that myopathy might be present
• When doing Step III, it will be difficult to
  isolate a single MUAP
• Since all are MUAPs are weak, the patient will
  tend to fire more than one at a time
• Hard for the patient to fire only one MUAP

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Remember

• STEROIDS QUIET MUSCLE MEMBRANES
• EMG PIN CAN CAUSE PROBLEMS WITH MUSCLE BIOPSY