Myopathies and their Electrodiagnosis1

Randall L. Braddom, M.D., M.S. Clinical Professor Robert Wood Johnson Medical School and the New Jersey Medical School [email protected]

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

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

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

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

THE FIVE STEPS OF EMG

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

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 MUAP’s analyzed will be Type I Use wide band width filter: (at least 10 10,000 Hz)

MUAP ANALYSIS

 Amplitude (peak to peak)  Duration  Phases  # of baseline crossings plus one  5 or more phases is polyphasic  10-32% of MUAP’s are polyphasic

Motor Unit Action Potential

MUAP ANALYSIS

 

Rise Time

 Dumitru recommends 0.5 msec. or less

Turns

 Change in direction without baseline crossing

MUAP ANALYSIS

 Satellite Potentials       Time locked (usually a few msec later) Normal: 10% of MUAP’s Myopathy: 45% of MUAP’s Neuropathic: Slightly more than normal Don’t include in MUAP duration Probably due to fiber splitting and slow conduction along immature terminal sprouts

MEAN MUAP DURATIONS

MUAP ANALYSIS



Myoneural Junction problems can cause



Variability in amplitude



Drop-out of MUAP’s

Five Steps of EMG



Step IV



Recruitment



Interference Pattern

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

RECRUITMENT

 Experienced EMG’ers 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 MUAP’s seen

Recruitment Examples

 Normal  Fastest potential is 20 Hz, 4 MUAP’s present  Recruitment ratio is 5  MYOPATHIC  Ratio Low: 4 or less  NEUROPATHIC  Ratio High: Usually 10 or more

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

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)

FASCICULATIONS



Spontaneous firing of all or part of a motor unit

 Denny-Brown and Pennypacker 1938

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

FIBRILLATIONS



Spontaneous firing of a single muscle fiber

 Best seen Step I  1-50 Hz  Regular pattern

FIBRILLATIONS

 Spontaneous oscillations in membrane potential of a denervated or injured muscle fiber  Diphasic or triphasic with

initial positivity

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

FIBRILLATIONS

 Most important factor 

Regular rhythm

 This separates them from end plate spikes and voluntary motor unit action potentials and fasciculations

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

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

POSITIVE WAVES

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

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

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

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

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

INFLAMMATORY MYOPATHIES

 Muscle membrane irritability  Usually involve myositis  Examples:  Dermatomyositis  Polymyositis  Trichinosis

NON-INFLAMMATORY MYOPATHIES

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

MYOTONIC MYOPATHIES

 All have the myotonic phenomenon  Examples  Myotonic dystrophy  Myotonia congenita

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

INFLAMMATORY MYOPATHIES

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

5 STEPS OF EMG



Slowly progressive myopathy

 I: Normal   II Normal III Reduced amplitude, duration of MUAP’s   IV Increased # of motor units firing per strength of contraction  Normal interference pattern NCV’s usually normal

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 NCV’s typically normal except for reduced amplitude of the evoked potential in motor studies

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

Myopathy’s Recruitment Change

 Remember the “

Rule of Fives

”   Determine frequency of fastest firing MUAP Divide by number of MUAP’s seen  If fastest is at 20 Hertz and four are present, recruitment ratio is 5  Myopathy typically has recruitment ratio of 4 or less

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 MUAP’s are weak, the patient will tend to fire more than one at a time  Hard for the patient to fire only one MUAP

Remember



STEROIDS QUIET MUSCLE MEMBRANES



EMG PIN CAN CAUSE PROBLEMS WITH MUSCLE BIOPSY