ELECTRODIAGNOSTIC TESTS

Group members:•Kanchan Sharma

•(BPT 4TH YEAR, AMITY UNIVERSITY)

INTRODUCTION Electrodiagnosis is the field of study that, by employing the

science of electrophysiology, uses electrical technology to study human neurophysiology.

Information needed to answer any questions regarding the nerve injury, muscle injury, muscle disease, localization, and prognosis can be thus obtained through the help of electrodiagnostic testings.

The information collected through the testing helps to focus treatment on the exact site of injury.

In addition to using a patient's history, physical examination and imaging results, the clinician can obtain information about the functional status of various parts of the nervous system by monitoring its electrical activity. This is accomplished via a variety of Electrodiagnostic tests like:

Electromyography (EMG) Nerve conduction velocity (NCV) Evoke potentials (EP)

NEUROPHYSIOLOGY One should be well aware of the normal

neurophysiologic function of the nervous system. Electrical signals are generated in the brain, pass

through the spinal cord and travel into the peripheral nervous system.

These signals are carried down the nerve to the synaptic cleft, where a chemical release of acetylcholine crosses the synaptic cleft to create an electrical discharge in the muscle. This electrical signal causes the muscle to contract. Neurodiagnostic testing bypasses the brain by delivering an electrical charge to the patient.

The equipment then is used to measure several aspects of the body's response to that signal to determine whether it is functioning properly.

Location and degree of the injury, acuity, prognosis, and (in some cases) specific diagnosis can also be determined by the electrodiagnostic testing.

ELECTOMYOGRAPHY Electromyography is the study of a motor unit activity. A technique by which the action potentials of contracting

muscle fibres and motor units are recorded and displayed. Recording the EMG requires a three phase system: An input phase: when the electrodes pick up electrical

potential from contracting muscle A processor phase: In this the small electrical signal is

amplified An output phase: electrical signal is converted to visual or

audible signals so that data can be displayed and analysed.

An electrode is a transducer, a device for converting one form of energy into another.

The distance of recording electrodes from the muscle fibre determines the rise time and fall time of muscle fibres.

The electrodes are of the following types: Surface electrodes Needle electrodes Finewire indwelling electrodes Intracellular electrodes Multilead electrode etc.

Technique of EMG recording: It includes the following steps:1. Patient is asked to relax and the needle is inserted inside the

muscle, simultaneously spontaneous burst of potential is observed.

2. The insertion activity is observed when the needle breaks the fibre membrane. The equipment of EMG recording is set up at sweep speed 5-10 ms/div; amplification 50 mv/divison for studying spontaneous activity and 200 microvolt for motor unit potentials and filter selting 20-10000 Hz.

3. The duration of MUP’s should be measured at a gain of 100microvolt/division and sweep speed of 5 ms/div and low filter at 2-3 Hz

MOTOR UNIT ACTION POTENTIAL:

It is the sum of electrical potential of the muscle fibres present in the single motor unit, having the capabilities of being recorded by he electrodes.

The normal motor unit action potential depends upon on the given five factors that is amplitude, duration, shape, sound and frequency.

In the normal muscle, the amplitude of a single motor unit action may range from 300 mV to 5mV from peak to peak. The total duration measured from initial baseline will normally range 3 to 16 m-sec.

The duration of the motor unit action potential normally varies from 5 to 15 ms depending upon the age of the patients, muscles examine and temperature.

Rise time of MUAP: duration from the initial positive two subsequent negative peak. It is an indicator of the distance of needle electrode from the muscle fiber. A greater rise time is attributed to the resistance and capacitance of the intervening tissue. 

Amplitude of MUAP: It is measured from peak to peak . It depends upon size and density of muscle fiber, synchrony of firing, proximity of needle to muscle fiber, age of subject, muscle examine and muscle temperature. Decrease in the muscle temperature results in the higher amplitude and longer duration of MUAP.

Phase of MUAP: portion of MUAP between departure and return to the base line. It can be monophasic, diphasic or polyphasic depending on the number of phases obtained.

Artifacts: Any unwanted electrical activity that arises outside of the tissues being examined. These artifacts can be of sufficient voltage to distort the output signal markedly.They are of two types:1: Movement Artifacts2: Power Line Interference

Cross-Talk: Another important consideration is the ability of electrodes to record activity selectively from a single muscle. Because of volume conduction, electrical activity from nearby contracting muscles, other than the muscle of interest, may reach the electrodes and be processed simultaneously.There is no way to distinguish this activity by looking at the output signal.Careful electrode placement and spacing and choice of size and type of electrode will help control such cross-talk or electrical overflow.

Amplifier System: Before motor unit potential can be visualized, it is necessary to amplify small myoelectric signals. An amplifier converts the electric signal large enough to be displayed. Differential amplifier: The electric potential is composed of the EMG signal from muscle contraction and unwanted noise from the static electricity in the air and power lines. To control for the unwanted part of the signal , the differential amplifier is used, as noise is transmitted to the amplifier as a common mode signal when the difference of potential is reduced at both the ends, the noise being cancelled out both the ends of amplifier. Common mode rejection ratio: CMRR is a measure of how much the desired signal voltage is amplified relative to the unwanted signal.A CMRR of 1000:1 indicates that the wanted signal is amplified 1000 times more than the noise. It can also be expressed 100000:1. The higher is this value , the better it is.

Abnormal spontaneous potential: As a normal muscle at rest exhibits electrical silence, any

activity seen during the relax state is considered as abnormal. The common abnormal spontaneous activities are:

1.Fibrillation Potential 2. positive sharp waves 3. fasciculation potential 4. repetitive discharges

Fibrillation potential: fibrillations are spontaneous occurring action potential from single muscle fibre.

Fibrillation potential is seen in the denervated muscle as they give spontaneous dischages due to circulating acetyle choline. Fibrillation potential are classically indicative of LMN disorders such as peripheral lesion, anterior horn cell disease, radiculopathy and polyneuropathies with axonal degeneration.

Positive Sharp Waves: Found in denervated muscles at rest and accompanied by fibrillation potentials. These are recorded as a biphasic with a sharp initial positive deflection followed by slow negative phase. Positive sharp waves are seen in primary muscle disease like muscular dystrophy, polymyositis but sometimes also seen in UMN lesions.

Fasciculation potential: random twitching of muscle fiber or a group that may be visible through skin. These are spontaneous potentials (may be biphasic, triphasic or polyphasic) seen with irritation or degeneration anterior horn cell, nerve root compression and muscle spasm or cramps.

Repetitive discharges: also called “bizarre high frequency discharges”. These are characterized by an extended train of potentials of various forms. These are seen with lesions of the anterior horn cells, peripheral nerves and with the myopathies.

EVOKED POTENTIAL It is the application of electrodiagnostic testing to the

CNS. These tests are clinically useful means to do the following:

1. demonstrate abnormal sensory function when the neurologic examination results do not reveal abnormalities.

2.reveal clinically unsuspected pathology when demyelinating diseases are suggested.

3. objectively monitor a patient’s progress or deterioration over time.

The test includes a number of ways to perform Evoke potentials like:

1. somatosensory EP 2. visual EP 3. Brainstem auditory EP 4. Dermatomal and myotomal EP

Somato-sensory EP: Stimulation occurs at extremity, recorded on scalp near sensory cortex.Useful in localizing demyelinating diseases such as MS, determine level of coma and to evaluate for brain death.

Visual EP: A photoelectric checkboard-pattern flash is used to stimulate the optic nerve which is recorded on the cortex and is then arrived at occiput, near the visual centres.

Brainstem auditory EP: The auditory click is used to stimulate cochlear nerve and response is collected over the cortex area.

Dermatomal EP and Myotomal EP: The stimulation occurs along the dermatome or myotome with it’s recordings at the cortex.

ROLE OF Evoke Potentials IN SURGERIES: It helps in detecting any interruptions in the signals

across the spinal cord while the spinal surgeries (for eg; scoliosis) are progressed.

The electromyographer repeatedly performs the somato-sensory evoke potentials and informs the surgeon about the interruptions, so that needed interventions can be taken, thus, permanent injury can be prevented.

NERVE CONDUCTION VELOCITY

NCV test determines how quickly electrical signals move through a particular peripheral nerve. It also sometimes known as NERVE CONDUCTION STUDY and is used in the diagnosis of nerve damage or nerve dysfunction. Purpose of NCV test :• The peripheral nerves are the nerve outside the brain and the spinal cord. These nerves helps control the muscles and experience important senses. • Healthy nerves send electrical signals more quickly and with greater strength than damaged nerves. For this reason, NCV is helpful in determining the existence, type, and extent of nerve damage in a patient.• The NCV test allows the physician to tell the difference between an injury to the nerve axon and an injury to the myelin sheath- the protective covering surrounding the nerve.

It is also useful for telling the difference between a nerve disorder and a condition where nerve injury has affected the muscles.

Being able to make these distinctions is important for diagnosis and for determining an appropriate course of treatment.

When is this test necessary: This test is useful for diagnosing a variety of different

muscular and neuromuscular disorders. A doctor may use this test if they suspect a pinched

nerve. Alternately, they may use it if they wish to check for the presence of nerve disease.

The test is often performed with an electromyography (EMG), which is a test that records electrical signals moving through the muscles.

Preparation on NCV test: Going into the test, it is important that the patient

have a normal body temperature. This is because a low body temperature slows down nerve conduction.

If the weather is very cold, for instance, the doctor may ask the patient to sit in a warm room for a few minutes before the test is performed.

How a NCV test is performed: Flat, patch-style electrodes are placed on the skin at

intervals over the nerve that is being examined. These electrodes give off low- intensity electrical

impulses, which stimulate the nerve. This stimulation may feel like a slight electric shock,

though it is not particularly painful. This impulses produced by this electrical current are

viewed on an oscilloscope or computer screen. This monitoring system allows the physician to determine how fast the impulses are travelling through the nerves.

Understanding the test results: The test may have the following outcomes: If the response from the electrical current is slower than

normal, this is a likely sign of damage to the myelin sheath. If the response shows a decreased response but with a

normal speed, there is probably damage to the nerve fiber.

The result of this test, as well as the cause of the nerve damage, will help determine a proper course of treatment.

Possible causes for abnormal results on this test are: 1. axonopathy (damage to the nerve cell)

2. conduction block (obstacle to impulse within nerve)3. demyelination (damage to the myelin sheath)

THANK YOU