Electromyography

Electromyography M.Prasanna, M.P.T

Definition It refers to recording of action potentials of muscle fibers in single or groups near the electrode in a muscle. EMG tests the integrity of entire motor system which consists of UMN, LMN, NMJ and muscles.

Instrumentation: Input Phase: Electrodes

 1.

Surface electrode 

1.

The surface electrodes are used to monitor large superficial muscles or muscle groups

Needle electrode 

Needle electrodes are used to record individual motor unit potentials discharging within a narrow radius from the recording tip.

Processing phase Electrodes convert the bioelectric signal from muscle or nerve depolarization into an electrical potential, capable of being processed by an amplifier.

Amplifiers: The recording electrodes pick-up various interference signals, as well as the desired bioelectric activity. The bioelectric activity is generally enhanced by a preamplifier that is placed relatively close to the electrodes to reduce the likelihood of contamination by additional interference signals. The bioelectric potentials must be amplified further before they can be displayed or recorded satisfactorily.

Output phase:  Display system: The amplified signal may be displayed by an analog or digital system. In the former method, bioelectric signals are displayed with high resolution on a cathode ray oscilloscope where they can be viewed directly, but the signals cannot be manipulated to facilitate measurement or analysis, such as is possible with digital systems.

Components of Analog electro diagnostic system

Components of Digital electro diagnostic system

Recording techniques and normal findings:   

Insertional activity Spontaneous activity Motor unit action potentials

Insertional activity  Needle electrode is placed in the muscle and electrical activity associated with its insertion is evaluated, as the needle breaks through the muscle fiber.  Normally electrical activity is in the form of brief bursts of electrical activity. Duration of insertional activity is less than 300 msec.  It appears as positive or negative high frequency spike in a cluster, usually accompanied by a crisp static sound over the loudspeaker.

 This insertional activity is caused by mechanical stimulation or injury of muscle fibers.  The wave forms are categorized into normal, decreased and increased patterns.  Insertional activity may be increased in denervated muscles and myotonia. Insertional activity is decreased in periodic paralysis during the attack and myopathies when muscle is replaced by connective tissue or fat.

Spontaneous activity  The muscle is evaluated at rest, i.e. with needle stationary in a relaxed muscle. Normally, electrical silence exists. When the needle is in endplate small intramuscular terminals are irritated and potentials are produced.

Motor unit action potentials Factors determining motor unit profiles: 1. No. of muscle fibers innervated by the motor neuron i.e. innervation ratio. 2. No. of muscle fibers per given cross-sectional area i.e. fiber density 3. Velocity in the terminal axon . 4. Integrity of NMJ 5. Temperature (decrease in temperature increases amplitude and duration) 6. Age 7. Nature of contraction

Abnormal potentials:  During insertional activity: After cessation of needle movement, a briefly sustained run of positive waves lasting several secs to mins are evident called as insertional positive waves. E.g.: Polymyositis, Progressive muscle disorders.

Insertional activity  Insertional activity may be increased in denervated muscle and myotonia where as in reduced in periodic paralysis during the attack and myopathies when muscle are replaced by connective tissue or fat.

During spontaneous activity: 1. 2. 3. 4. 5. 6.

Fibrillation potentials Positive sharp waves Fasciculation potentials Complex repetitive discharges Myotonic discharges Cramp potentials

Fibrillation potentials  Fibrillation potentials arise spontaneously from either a single muscle fibre or a few muscle fibres.  Deflection: Initially positive  Waveform: 20 – 200 µv  Duration: 1 – 5 msec  Frequency: 1 – 30 Hz.  On the loud speaker crisp clicking noise like wrinkling of the tissue paper is heard.  Fibrillation potentials are found in PNI, polyneuropathy, radiculopathy

Fibrillation potentials:

2. Positive sharp waves:  Initial positive spike followed by a slow negative potential much lower in amplitude and longer in duration.  Positive spike resembling a saw tooth.  Positive sharp waves have the same clinical significance as fibrillations. They also found in certain myopathic disorders, such as polymyositis and myotonic dystrophy.

Fibrillation and Positive sharp wave Neurogenic  Anterior horn cell disease  Radiculopathy  Axonal neuropathy  Plexopathy Neuromuscular Junction  Botulism  Myasthenia Garavis Myogenic  Myosities  Muscular dystrophy  Muscle trauma

Difference between fibrillation/positive waves?  Both are ‘single muscle fiber AP’s  Both result from muscle cell instability  Positive waves are seen earlier in any unstable muscle cell membrane  Positive wave are an artifactual recording with the needle electrode disrupting the muscle cell membrane

Positive sharp wave

3. Fasciculation potentials  Fasciculation potentials result from spontaneous discharges of a group of muscle fibers representing either a whole or possibly part of a motor unit.  They are visible to naked eye as a twitch. Fasciculation in deep muscles are recognized with EMG.  They produce a sudden dull thudding sound over the loudspeaker. Sometimes fasciculation potentials may occur in normal muscle in association with cramps.

Fasciculation potentials

Fasciculation potentials Neurogenic disorder  Amyotrophic lateral sclerosis  SMA  Radiculopathy  Syringomyelia  PNL

Fasciculation potentials Metabolic disorder  Tetany  Thyrotoxicosis  Anticholinesterase drugs Normal  Muscle cramp  Benign fasciculation

4. Complex repetitive discharges  These discharges typically begin abruptly and maintain a constant rate of firing for a short period.  Their waveform is polyphasic, complex and remains uniform.  Over the loudspeaker, they mimic the sound of a machine gun.

Complex repetitive discharges

Complex repetitive discharges Myogenic  Polymyositis  Muscular dystrophy Neurogenic  Poliomyelitis  Amyotrophic lateral sclerosis  Spinal muscular atrophy  Chronic Radiculopathy  Chronic neuropathies

5. Myotonic discharges  They are high frequency discharges of action potentials that wax and wane in frequency and amplitude  Causing a sound resembling that of a dive bomber on the loudspeaker (or sound of accelerating and decelerating motorcycle)  The frequency of myotonic discharges ranges between 40Hz to 100Hz which is regular and rhythmic.

Myotonic discharges    

Myotonia dystrophica, Myotonia congenital, Polymyositis Periodic paralysis

Cramp potentials  Spontaneous discharge of potentials occur at 40-60Hz usually with an abrupt onset and cessation  The cramps occur in salt depletion, chronic neurogenic atrophy and some normal persons

Motor unit potential  MUP represents the sum of the muscle action potentials supplied by an anterior horn cell  MUP is characterized by its duration, phases, amplitude and turns

Motor unit potential

Voluntary muscle contraction    

Short duration & Low-amplitude MUPs Long duration & High-amplitude MUPs Polyphasic potentials Mixed pattern

Normal amplitude

Reduced amplitude

Increased amplitude

Duration normal

Duration increased

Long duration MUP  Motor neuron disease  Axonal neuropathies with collateral sprouting  Chronic Radiculopathy  Chronic myositis

Duration reduced

Short duration MUP   

Myopathies NMJ disease Early stage of reinnervation after nerve damage

Normal MUP

Low amplitude polyphasic

Short polyphasic  Myopathies

Long polyphasic Neurogenic disease

Recruitment of MUP  The firing rate of MUP for a muscle is constant  On initiation of voluntary contraction of muscle the motor units are recruited in an orderly fashion the smallest appearing first larger later and the largest still later (Henneman’s)  If there is loss of MUPs the rate of firing of individual potential during muscle contraction will be out of proportion to the number of firing and termed as reduced recruitment – Neurogenic  If more units are recruited at relatively low force called rapid recruitment - Myopathies