1: Introduction to Electrodiagnostics
BASIC PRINCIPLES OF ELECTRODIAGNOSTICS
The human body constantly generates electrical energy. Specifically, the muscle and nerve cells constantly use electric discharges to communicate among different parts of the body.
These electric discharges can be recorded, displayed, measured, and interpreted by using specialized equipment.
In the presence of disease or injury, the architecture and normal processes of nerves and muscles are altered. Recognizing these changes can be useful for diagnosis, monitoring disease progression, and assessing treatments.
Electrodiagnostic (
EDX ) medicine is the process of observing and interpreting neuromuscular electrical discharges for clinical purposes. We use “EDX ” for the remainder of this book to represent this testing, which includes nerve conduction studies (NCS ) and electromyography (EMG ).
BASIC LOGISTICAL DETAILS FOR YOUR FIRST DAY PERFORMING ELECTRODIAGNOSTICS
The novice
EDX consultant (from here on referred to as “you”) is faced with learning many new concepts in parallel and at once.These include the associated basic science concepts (Table 1.1), hands-on
EDX procedures, and analysis of theEDX results. The second basic concept—the hands-onEDX procedure—is the emphasis of this book.At first, you will just perform the procedures of an algorithm accurately and efficiently under the guidance of your instructor.
Once your hands are moving easily from one procedure to the next, you should be able to spontaneously analyze on the fly and mentally adjust a dynamic algorithm.
Membrane Physiology
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Volume Conduction Theory
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Neuromuscular Physiology
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Body’s Response to Injury
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Anatomic Innervations
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BASIC CONSTRUCTION OF AN ELECTRODIAGNOSTIC CONSULT
Consult Request
You will be consulted by a clinician who will send all types of chief complaints to “rule out” things. These usually include complaints of numbness, tingling, or weakness. You should view the referral simply as a guideline or starting point for the
EDX visit.The
EDX encounter is different from a regular consult because the emphasis is placed on you as a technician. Therefore, you may want to be more focused and direct without sacrificing professionalism, compassion, or congeniality. Look for possible contraindications (see Box 3.3 in Chapter 3) prior to performing the procedure. Point out reasonable expectations of the test to your patients and warn them that it is uncomfortable but well tolerated by most people.
History
This is the time to establish a broad differential diagnosis. Try to figure out which body system is involved. Is the etiology neurological? Musculoskeletal? Psychological? Bilateral? Proximal versus distal?
Physical Examination
At this time, you will essentially confirm your thoughts using the history. This will inform and direct your
EDX testing, which should never be set in stone.
Nerve Conduction Studies
Typically, this is the first part of the
EDX examination. You will stimulate nerves by delivering an electrical current usually via a stimulator wand. The axons of the stimulated nerve generate action potentials that propagate both proximally and distally from the site of stimulation. Electrodes placed over the nerve or a relevant muscle “pick up” the action potential as it propagates under them. The electrical signal that is picked up by the electrodes is amplified, filtered, and processed by the equipment.The signal is converted to a waveform (aka evoked response) that is displayed on a screen. The x-axis represents time (usually in milliseconds) and the y-axis represents voltage (millivolts or microvolts). Considering units of measure is very important prior to analysis of the waveform. Properties of the nerves and muscles can be extrapolated and analyzed thereafter. Certain changes in specific attributes of the waveform can reveal the presence or absence of pathology.
Sensory Nerve Conduction Studies
Pick-up electrodes are placed over sensory nerves. Stimulation of the corresponding nerve (typically proximally) results in a sensory nerve action potential (aka
SNAP ) on the screen. TheSNAP is the electrical signal summation of discharges by the sensory neuron’s depolarizing membranes (i.e., axolemma) beneath the pick-up electrodes (Table 1.2).
Latency
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Amplitude
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Duration
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Area
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Motor Nerve Conduction Studies
Pick-up electrodes are placed over a muscle belly. Proximal stimulation of the corresponding nerve produces a compound muscle action potential (aka
CMAP ), which is the summation of electrical signals discharged by the muscle fibers’ depolarizing membranes (i.e., sarcolemma) under the pick-up electrodes.
Late Responses
These studies are called “late” because of the prolonged time required for the distal stimulus to travel proximally and return back to the periphery. They are often used to evaluate the “proximal” portions of the peripheral nerves. Some late responses include the H-reflex, F-wave, and A-wave.
Evoked Potentials
Stimulation occurs at the periphery and pick-up electrodes are placed proximally over the scalp and other areas. Sometimes they are used intraoperatively for monitoring. These include:
Auditory-evoked potentials, usually recorded from the scalp but originating at the brainstem level (auditory brainstem response, brainstem auditory-evoked response, brainstem-evoked response, brainstem-evoked potential [
BSEP ])Visual-evoked potentials
Somatosensory-evoked potentials
Peripheral sensory nerve stimulation
Motor-evoked potentials
Peripheral motor nerve stimulation
Electromyography
Typically, this is the last part of the
Involuntary Attributes
Insertional activity: Bursts of electrical discharge directly correlated with irritation by the needle electrode
Normal and abnormal spontaneous discharges (e.g., endplate spikes, positive sharp waves, fibrillation potentials, fasciculation potentials, complex repetitive discharges, myotonic discharges, myokymic discharges, etc.)
Voluntary Attributes
Motor unit action potential (
MUAP ) morphologyMUAP activationMUAP recruitment
EXPLANATION OF FINDINGS
Although the final report will not yet be complete, preliminary findings should be discussed with the patient after the encounter.
ADDITIONAL READINGS
- Dumitru D, Amato AA, Zwarts MJ. Electrodiagnostic Medicine. 2002.
- Wilbourn AJ. Nerve conduction studies. Types, components, abnormalities, and value in localization. Neurol Clin. 2002;20(2):305–338, v.