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Flashcards in Short Answer Physics Test Deck (34):
1

Electrodiagnostics

  • it is a method of obtaining information about diseases by passively recording the electrical activity of body parts or by measuring their response to external electrical stimulus.
  • is a branch of medical practice in which electrical tests are performed to evaluate excitability of nerves and muscles using DC and PC.
  • The aim of electrodiagnositcs is to demonstrate the changes in excitability, occuring in the neuromuscular system which is sick

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Electrodiagnostic testing 

  • Is a powerful tool for diagnosing and developing treatment plans for patients with diseases of the peripheral nervous system and muscles

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The Electrodiagnostic examinations

  • can provide essential information in cases of suspected peripheral nervous system disorders or injury

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Electrical stimulation should not be used under 3 conditions

  • with patients having a pacemaker
  • over the carotid sinus
  • over the abdominal area during pregnancy

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Qualitative techniques

  • involve observation of a type of muscle contraction in response to specific electrical pulses

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Quantitative Techniques

  • are based on a quantitative determination of physical quantities, which are measurement of muscle excitability. Examples: strength-duration curve, chronaxie measurements

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Motor point

  • a small area overlying muscle where aslight visible contraction is most easily elicited with a  minimal amplitude(intensity) electrical stimulus

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A motor point of a nerve

  • corresponds to a point on the skin, at which a nerve lies closest to skin surface

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A motor point of a muscle

  • a point where a nerve enters into a muscle

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Traditional clinical electrical evaluation tests

  • Reaction of degeneration test(RD)
  • Strength-duration curve and chronaxie test
  • nerve conducting tests
  • clinical electromygraphy(EMG)

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Reaction of degeneration test(RD)

  • This tests is a useful screening procedure for assessment of problems that may involve lower motor neurons
  • Reaction of normally innervated muscle=it will respond with a brisk twitch when stimulated with a short-duration pulse lasting less than 1 ms and also when stimulated with longer pulse durations (for example 100 ms)
  • Reaction of normally innervated muscle- it will respond with sustained or tetanic contraction if the pulses are applied in rapid succession
  • Reaction of a muscle with peripheral denervation - it will not respond to a stimulus of 1 ms or shorter but will contract in a sluggish manner when the longer pulse duration stimulus is applied

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RD Test - using electrodes

  • The electrode is used to search as precisely as possiible or the motor point of the muscle of interest
  • The negative(cathode) electrode- is used as the active stimulating electrode over the motor point

13

The first part of Classical RD test

  • the motor point area is stimulated with a series of short-duration(less than 1ms) pulses
  • a frequency of the stimulus-greater than 20Hz (a tetanic or sustained contraction)
  • a waveform- monophasic or biphasic(assymmetrical)

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Reaction of a muscle in the first part of the classical RD test

  • a tetanic contraction- a muscle is normally innervated
  • a sluggish response or no response- peripheral denervation is likely to occur

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The second part of the classical RD test

  • the involved muscle is stimulated with a long-duration pulse
  • a monophasic pulse of atleast 100 ms duration or preferably longer

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Reaction of a muscle in the second part of the classical RD test

  • a slow or sluggish response-contractile muscle tissue is present but the muscle is either partially or completely denervated

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Reaction of degeneration test(RD)-Part 2

  • The RD test is usually not done until atleast 10 days after onset of the problem, so that the process of neural degenration can progress to a stage in which electrical changes would appear
  • An abbreviated form of the test for reaction of degeneration may be used as a quick screening test for differentiating a muscle with normal peripheral innervation from a muscle with peripheral denervation
  • The RD test is only a gross screening procedure and should not be expected to differentiate or precisely identify the location of pathology. The test may be indicated in conditions of unexplained paralysis

18

Strength - Duration Curve and Chronaxie test

Q image thumb

  • 1930-1960 - strength - duration curves and chronaxie measurements were widely used for electrodiagnosis of peripheral nervous system disorders in these years
  • Their frequency of use sharply declined with the development of nerve conduction testing and electromyography
  • Three tests provide a reliable means of assessing the locatio,n, severity and progress of peripheral nerve - nerve degeneration and regeneration

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Chronaxie

  • a minimal pulse duration of stimulus of twice rheobase strength that will cause the excitable cell membrane to discharge

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Strength - Duration Curve

  • is obtained by imposition of values of the time and values of the intensity which produce the minimal muscle contraction in the coordinate system
  • Plotting an S-D curve requires pulse duration ranging from 0.01 ms to 1000ms
  • To plot the strength - duration curve rectangular pulses are use

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Rheobase

  • a minimal intensity of stimulus amplitude(strength) required to eleicit a minimal visually perceptible muscle contraction (for 1000 ms impulse)

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Strength - Duration Curve Uses

Can be used for:

- evolution of variety of lower motor neuron pathologies

- Assessment of peripheral nerve injuries

These tests can provide reliable and accurate information on the status of peripheral innervation and denervation

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The intensity of the Stimulus (criteria of the intensity (amplitude) or strength and pulse duration or time of the stimulus)

must be strong to depolarize the membrane to its threshold level for excitability 

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Time of the stimulus ( criteria of the intensity(amplitude) or strength and pulse duration or time of the stimulus) 

the stimulus be of suffiecient duration to overcome the capacitance of the membrane

Q= i• t= constant

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To plot the strength- duration curve

  • triangular pulses can be used
  • using progressively shorter pulse durations , the values of stimulus amplitude which produce a minimal muscle contraction are recorded and then plotted on the graph
  • chronaxie and rheobase can be determined from S-D curve
  • Chronaxie
    • 0.1-1ms (a muscle is normally innervated)
    • 30-50ms ( fully denervated muscle)

A image thumb
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Strength- Duration Curve and Chronaxie Testing:

- Are as objective as nerve conducting testing and electromyography

- are non-invasive

- can provide valuable information on the status and progress peripheral nerve injuries

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Strength- Duration Curve and Chronaxie Testing, other applications:

- Can be for evaluation of peripheral neuritis, other peripheral nerve disease that may involve axonal degeneration and motor-neuron disease conditions

- the tests may be used to complement other evaluative procedures in differentiating between normal nerve tissue and neuropathology

28

Electromygraphy (EMG)

  • an assessment which provides a means of monitoring and evaluating electrical activity of muscle directly-without artificial stimulation
  • advantage: characteristics of muscle can be studied during relaxation and voluntary contraction
  • useful in evaluating electrical activity of lower motor neurons and muscle fibers, because electromygraphy helps in identifying electrical changes consistent with pathologic processes in these anatomic areas
  • instrumentation required is similar to what is used in nerve conduction testing, but an electrical stimulator is not needed
  • a sterile needle electrode is inserted directly into the muscle
  • endogenous electrophysiologic activity produced by depolarization and repolarization of the muscle cell memebrane is transduced from the electrode and displayed on oscilloscope
  • monopolar and concentric needle electrodes are most commonly used in routine EMG  

29

Electromygraphy (Electromygraphic activity is studied under the following conditions) 

  • while the muscle is at rest and is completely relaxed
  • during a mild contraction, just strong enough to produce individual motor unit action potentials
  • during a very strong contraction, held with enough force to recruit as many motor units as possible 

30

Nerve Conducting Tests: Purpose

- The purpose of these tests is to assess the time and quality of the conduction of neural impulses in peripheral motor and sensory nerves

- A controlled monophasic pulsed electrical stimulus is applied to the skin overlying a nerve

31

Nerve Conducting Tests: Instrumentation

- a differential amplifier capable of detecting and accurately amplifying signals in a range from 2 mV to 50 mV

- an electrical stimulator that provides square wave monophasic pulsed stimulus from 0.05 to 1 ms, and output amplitude up to 500 V or 100 mA

- an oscilloscope

32

Nerve Conducting Tests: Responses

- the stimulus frequency capability should be variable from signal to pulse trains of 50 Hz

- The responses are recorded from the specific sites of interest, which may be from:

  >Muscle in the motor nerve conduction tests

  >peripheral sensory nerves

  >the scalp in evokes potential studies

33

Nerve Conducting Tests: Tests

The following tests are distinguished:

-motor nerve conduction

-f wave nerve conduction

-sensory nerve conduction

-h reflex response

These tests are useful in establishing or rulingout the presence of a peripheral neuropathy and determining and localizing a peripheral nerve entrapment or a plexopathy

34

Nerve Conducting Tests: Another Feature of Assessment

Another Feature of the assessment of nerve conduction test results is differentiation, when possible between nerve conduction changes consistent with a demyelinating process and those seen with an axonal disorder.