Electromyography Actually Refers to the Needle Electrode Examination of Specific Muscles.
Electrodiagnosis (Edx) in Strict Terms Refers to Nerve Conduction Studies These Are Usually Performed With Surface Recording Electrodes Placed Over Specific Muscles or Nerves. The Nerve Is Then Stimulated at Some Distance From the Recording Electrodes. The Waveforms Thus Recorded Represent Compound Nerve or Muscle Action Potentials (CNAP, Cmap).
The Term Electrodiagnosis Is Correctly Applied in a Broader Sense to Designate Both the Nerve Conduction Studies As Well As the Needle Electrode Study or Emg
Recorded Potentials Surface Electrodes Used. Placed Along the Course of a Nerve With a Galvanometer Between Them. As Action Potential Approaches the First Electrode, Causes a Reversal in the Membrane Polarity. The First Electrode "Sees" a Negative Charge As the Action Potential Passes Beneath (Active) Electrode While the Second (Reference) Electrode Still "Sees" Outer Positive Charge Under It. A Flow of Electrons From the Active Electrode to the Reference Electrode Causing a Negative Deflection in the Galvanometer. As the Action Potential Reaches the Reference Electrode, the Negative Charge Is Now Under It and the Galvanometer Deflects in the Positive Direction.
Compound Action Potentials Single Axons or Muscle Cells Are Rarely Recorded. Action Potentials in Vivo Are Recorded As of Compound Action Potentials. The Action Potential Is Recorded As a Summated Response From Many Axons Simultaneously. Change Is Noted As an Overall Increase in Amplitude (Direct Summation). Initial Negative Deflection Is Due to the Fastest Conduction Fibers. Largest Diameter Fibers Result in the Largest Action Potentials. Difference Noted in Duration of the Compound Action Potential, Because Not All Travel at Exactly the Same Velocity (Desynchronization - Dispersion).
Stimulation: Electrical Vs Synaptic Nerves Are Normally Stimulated Chemically by Neurotransmitters at the Synapse. With Synaptic Stimulation Smaller Postsynaptic Neurons Reach Threshold Before Larger Neurons. Motoneurons Innervating Type I Muscle Fibers Will Be Recruited First. With Electrical Stimulation, the Largest Diameter Axons Are Recruited First. Exactly Opposite of Normal Physiological Recruitment.
When a Neuron Is Stimulated Electrically at Mid-axon, the Action Potential Generated Will Be Conducted in Both Directions Simultaneously. The Action Potential Which Travels in the Normal Direction Is Said to Be Conducted Orthodromically. Conversely, Antidromic Conduction Is in the Reverse of Normal Conduction.
Equipment Basics The Clinical EMG Equipment Is Much More Complicated That Standard ECG Equipment. The Basics of Traditional Emg and Paraspinal Emg Equipment Are the Same. It Is a Composite of Many Different Components, All Integrated Together.
Amplifier Most Modern EMG Units Contain Both a Preamplifier and an Amplifier. Some EMG Potentials Recorded May Be Less Than 1 mv in Amplitude. Sensitivity Control. Filter Selections.
Oscilloscope Analog Cathode Ray Screen Where the Recorded Action Potentials Are Displayed. More Modern EMG Equipment Use a Digital Monitor System to Display Potentials. Sweep May Be Set to Free Running or to Triggered Sweep for Electrical Stimulation. Storage Oscilloscope Features Are Included Which Allow the Recorded Potentials to Remain on Screen for Quantification or Closer Examination, and to Be Erased at the Operators Discretion.
Audio-amplifier and Speaker Recorded Potentials Are Also Listened to by the Electromyographer. With Experience, More Information Is Gleaned From the Audio Portion Then From the Video. Many of the EMG Potentials Produce Characteristic Sounds Which Readily Identify Their Presence. Useful in All Portions of the Needle Examination.
Stimulator The Stimulator Is Used for Nerve Conduction Studies and Stimulated Single Fiber Studies. Generates a Direct Current Square-wave With Active Depolarization at the Cathode There Are Three Control Features to the Duration or Rate. Intensity Can Be Varied From 0 to 500 V (O to 80 mA). Duration Stimulus Frequency (Rate). Short Duration Low Intensity and Slow Frequency Stimulation Is More Comfortable to the Patient.
Many Modern EMG Machines Are Now Equipped With Signal Averaging Features Which Will Average Out the Responses From the Desired Number of Stimuli. This Feature Is Commonly Employed for Low Amplitude Responses. The Number of Responses Averaged Varies From 8 to 10 in Sensory Nerve Studies up to 1,000 in Somatosensory Evoked Potentials.
Recording Apparatus All EMG Units Contain Some Means of Permanently Recording Potentials Polaroid Camera Paper Recording Magnetic Tape Computer Disc
Electrodes Three Types of Electrodes Used in EMG. Ground Electrode. Recording Electrode. Stimulating Electrodes.
Ground Electrodes Fairy Large and Are Usually Made of Lead or Stainless Steel. Purpose Is for Patient Safety. Prevent the Passage of Electricity Through the Heart. Always Located on the Extremity Being Tested.
Recording Electrodes Pickup the Action Potentials of Interest. Two General Kinds. Surface Electrodes. Needle Electrodes.
Surface Electrodes Placed Over the Nerve or Muscle Being Recorded. Cathode Is Usually the Active Electrode and Is Placed Over the Area to Be Recorded. With CMAP’s (Compound Muscle Action Potentials), 6-8 Mm Disc Electrodes Are Commonly Used With the Active Electrode Over the Motor Point and the Reference Electrode Over the Tendonus Insertion So As to Yield an Initial Negative Deflection. With Snaps (Sensory Nerve Action Potentials) or Mixed Nerve Action Potentials Either Disc, Bar or Ring Electrodes Are Used. Bar Electrodes Are Very Convenient to Use "Over Nerve" Recordings. Ring Electrodes Are Convenient for Digital Nerve Studies.
Needle Electrodes For EMG Examination of Muscles. Sometimes for Conduction Studies of Difficult to Record Sensory Nerves. Needle Electrodes Are of Three Basic Types. Monopolar Electrode. Bipolar Electrode. Concentric Needle Electrode.
Stimulating Electrodes Prong Type. Come With the Stimulator. Conducting Paste Is Applied Directly to These Prongs. Surface, Bar or Ring Electrodes May Be Used As Stimulators. Occasionally Needle Electrodes May Be Used to Stimulate Deep Nerves or Even Nerve Roots.
Nerve Conduction Studies (Edx)
Compound Muscle Action Potentials Compound Muscle Action Potentials (CMAP) Are Made With Recording Electrodes Overlying Muscle. Latency Is Measured From the Time of Stimulation to the Point of Initial Negative Deflection and Therefore Represents the Conduction in the Fastest Motor Axons. Latency Is Comprised of the Conduction Velocity of the Motor Axon, Myoneural Junction Synaptic Transmission and Conduction of the Cmap Itself. Amplitude Is Measured From Peak-to-peak. Duration Is Measured From the First Departure From Baseline (Latency Point) to Its Final Return to Baseline.
Motor Conduction Studies Multiple Points Along the Motor Nerve Are Stimulated. The Latency in the Most Distal Segment of the Motor Nerve Is Called the Distal or Terminal Latency. It Cannot Be Used to Calculate a Conduction Velocity Since Time Is Dependent Upon Synaptic Delay and Cmap Spread in Addition to Conduct Time in the Fastest Motor Axons. Conduction Velocity Is Calculated for Motor Nerve Segments Between Two Points of Stimulation. This Is Done by Finding the Difference Between the Two Latencies and Dividing by the Distance Between These Two Points. Expressed in Meter/second, but Is Calculated in Millimeters/millisecond. Visually Compare Cmap Morphology and Amplitude Between Stimulus Points As Any Changes in These May Raise a Suspicion of Conduction Block or Anomalous Innervation Which Must Be Pursued Further.
Sensory Nerve and Mixed Nerve Action Potentials Sensory and Mixed Nerve Conduction Studies Are Essentially the Same. Sensory or Mixed Compound Nerve Action Potential Are Similar to Those Already Described for the CMAP. Amplitude Is Measured Peak to Peak and Provides an Estimate of the Number of Axons. Duration Is Measured Baseline to Baseline and Provides an Estimate of the Fastest to Slowest Conducting Fibers of the Largest Diameter Fibers. Direct Calculation of the CNAP Conduction Velocity Is Possible Since No Synaptic Transmission Occurs.
Late Responses F-waves and H-reflexes Are " Late" Responses Since They Are Observed Later in Time Than the M-wave (CMAP). They Must Ascend to the Spinal Cord Before They Return to Generate Their Muscle Response. These Responses Utilize More Proximal Portions of the Peripheral Nervous System. Provide Neurophysiological Information on Otherwise Inaccessible Portions by Standard Nerve Conduction Studies.
The H-reflex True Monosynaptic Reflex. Typically Recorded From the Soleus Muscle, Where It Is the Electrodiagnostic Equivalent of the Ankle Jerk Muscle Stretch Reflex.
The H-reflex Large Diameter Ia Sensory Axons Are Recruited First. Ascend to Reach the Soleus Ventral Horn Cells Where Synaptic Recruitment Occurs. Smallest Cell Bodies Reach Threshold First. As Stimulus Intensity Increases, More Ia Fibers Are Recruited, Which in Turn Recruit Progressively Higher Threshold Motor Units Until Hmax Amplitude Is Reached.
The H-reflex At Hmax, Alpha Motor Axons Are Directly Recruited and a Small Amplitude M-wave Appears Earlier in the Sweep. Where the Antidromic Motor Action Potential Meets the Orthodromic Action Potential, the Two Will Cancel, Blocking That Portion of the H-reflex. As Stimulus Intensity Increases, the M- wave Amplitude Increases and the H-wave Amplitude Decreases, Until Mmax Is Reached and All Fibers Contributing to the H-reflex Are Blocked.
The H-reflex If the Latency Is Prolonged It Signifies a Proximal Lesion. Differentiation Between the Afferent and Efferent Arc Portions Is Not Possible. In Conduction Block, Hmax Is Expected to Be Decreased Relative to Mmax (Decreased Hmax/Mmax Ratio). Increased Hmax/Mmax Ratios May Objectify Low Back Pain. There Is Evidence That With Spinal Adjustment, the Hmax/Mmax Ratio Returns to Normal, and Parallels Subjective Improvements in Low Back Pain.
F-wave The F-wave Differs From the H-reflex in That It Is Not a True Reflex. The Alpha Motoneuron Serves As Both the Afferent and Efferent Limbs of the Response. The F-wave Is Currently Thought to Represent Conduction in the Fastest Conducting (Largest Diameter) Alpha Motoneurons. It Affords Information on the Proximal Segments of Motor Axons. F-wave Latency Is Most Useful in Evaluating Proximal Demyelinating and Diffuse Demyelinating Neuropathies.
The H-Reflex and F-Wave
Somatosensory Evoked Potentials (SEP’s) Similar to CNAP With Lower Amplitude Responses. This Requires an Averaging Technique Where From 250 to 1,000 Stimuli Are Averaged to Produce an Adequate Response. The SEP Is Time-locked to the Stimulus, Whereas the Random Noise Is a Not, So Averaging Increases the Signal/noise Ratio up to a Point.
Somatosensory Evoked Potentials (SEP) In Recording SEP’s, the Stimulating Electrode Is Placed Over a Distal Nerve in Either the Upper or Lower Extremity. In the Case of Dermatomal Evoked Potentials, the Stimulus Is Applied to a Specific Dermatome Rather Than Over a Peripheral Nerve. Recording Electrodes Are Usually Placed Over the Contralateral Primary Somatosensory Cortex and at the Spinal Entry Zone at a Minimum.
Somatosensory Evoked Potentials (Sep) The SEP Provides Information On: Peripheral Conduction Time. Total Transit Time. Central Transit Time (the Difference Between the Two). Sep Is Capable of Differentiating Between Peripheral and Central Sensory Conduction Delays.
Needle Electrode Electromyography (Emg) There Are Four Parts to the Needle Examination of Muscles Which Are Repeated for Each Muscle Sampled. Insertional Activity. Spontaneous Activity. Motor Unit Potentials. Interference Pattern.
When a Needle Is Inserted Into Muscle, the Muscle Fibers Irritated Produce a Burst of Electrical Activity. This Activity Is Short Lived, Begins With Needle Movement, and Does Not Outlast Cessation of Needle Movement. It Will Be Increased After an Acute Peripheral Nerve Lesion, but Decreased in the Chronic Stages or With Myopathy. Insertional Activity
Represents Electrical Activity in Muscle Which Is Not Under Voluntary Control. May Be in One of Two Forms. Those Induced by Needle Movement but Outlasting Insertional Activity. Those Which Are Truly Spontaneous and Continue or Occur in the Absence of Needle Movement. Spontaneous Activity
Positive Sharp Waves Not Considered Pathognomonic for Denervating (Lower Motoneuron) Lesions As They May Be Observed in Functionally Denervated Muscle Secondary to Myopathy or Even in Upper Motor Neuron Disease. Fibrillations Are Diphasic or Triphasic Potentials, They Are Not Pathognomonic for Denervation. Fasciculations Are Spontaneous Discharges of Groups of Muscle Fibers, and May Be Accompanied by a Visible Twitch of the Muscle If Located in a Superficial Region of the Muscle. Termed Malignant (Pathological) or Benign. They May Be in Evidence With Any Denervating Lesion. Bizarre-high Frequency Discharge Is Occasionally Observed in Denervating Lesions. They Have the Same Significance As Positive Sharp Waves.
Motor Unit Potentials The Morphology of the Motor Unit Potential (MUP) in a Muscle Also Provides Valuable Information About the Lesion. Normal Motor Units Are Biphasic or Triphasic With a Sharp Rise Time Provided the Needling Electrode Is Sufficiently Close to the MUP. Mups Are Recorded With Slight Contraction of the Muscle Under Study.
Interference Pattern During Low Level Voluntary Recruitment, One or Two Motor Unit Potentials May Be Observed Finding at a Low Rate. These Will Usually Be of the Small Amplitude, Type I Fiber Types. As Additional Strength of Contraction Is Produced, the Rate of Firing of the First Units Increase and a Second or Third MUP May Be Added at a Slower Rate of Firing. This Rate and Numbers Principal Continues Until the Full Interference Pattern Is Reached and the Screen Is Filled With Mups Obliterating the Baseline. In Pathologic States These Normal Patterns of Recruitment Are Altered.