ELECTRODIAGNOSIS
Electrodiagnosis
• Medical diagnostic procedures that obtains information about a disease/injury
by passively recording the electrical activity of affected body parts AND by
measuring their response to external electrical stimuli.
• The most widely used methods of recording spontaneous electrical activity are
various forms of electrodiagnostic testing (electrography) such as
electrocardiography (ECG), electroencephalography (EEG), and
electromyography (EMG).
• Electrodiagnostic physicians apply electrophysiologic techniques, including
needle electromyography and nerve conduction studies to diagnose, evaluate,
and treat people with impairments of the neurologic, neuromuscular, and
muscular systems.
• Faradic or interrupted direct currents are used for electrodiagnosis in
neuromuscular disorders.
• The mechanism underlying normal electrical activity of muscle and nerves
when stimulated with electrical current form the basis of electrodiagnosis.
• When there is disease or injury of motor nerves or muscles, alterations are
liable to occur in response to electrical stimulation.
• Altered electrical reactions may be of considerable assistance in diagnosing
the type & extent of the lesion.
• Reduction or loss of voluntary power of muscle can be due to:
1. Lesion of Upper Motor Neuron (stroke, multiple sclerosis, cerebral palsy)
2. Lesion of Lower Motor Neuron (spinal cord injury, peripheral nerve damage,
or diseases affecting the lower motor neurons, such as amyotrophic lateral
sclerosis (ALS) or polio)
3. Damage of muscle itself
4. Fault at neuromuscular junction (Myasthenia gravis)
STRENGTH DURATION CURVE
• Excitability of nerve is studied by two properties
Chronaxie
Rheobase
which is determined by plotting strength duration curve.
• Strength duration curve is a graphical representation of the relationship
between the intensity of electrical stimulus at the motor point of a muscle
and the length of time taken to elicit a minimal contraction in that muscle.
• For plotting this curve, we need to stimulate the nerve using electrical
stimulus.
• When stimulus is of threshold intensity, nerve responds by generation of
action potential which can be recorded.
• Now when we deliver a stimulus, charge will change the potentials and if
charge quantity is enough, potential change will reach the threshold. This is
known as threshold stimulus.
• Electrical stimulus has two features:
• Stimulus strength
• Stimulus duration
• Strength refers to the stimulus intensity on the vertical axis
• Duration refers to the pulse duration on the horizontal axis
• Representation of this stimulus with this diagram