Neurophys - EMG

• Tempetature
• Age (CV 1/2 normal at birth while Amp dec c/ age)
• Length of segment and height
• Gender

• Response elicited from any motor nerve with supramaximal stimulation
• Created by antidromic conduction which triggers a back-firing response
• Does not involve a synapse
• Abnormalities are seen as an early pathologic finding in demyelinating neuropathies like AIDP / CIDP

• Can only be elicited in the tibial nerve in response to a submaximal stimulus
• It is the electrical equivalent of the ankle jerk reflex (S1) and measures the integrity of the entire reflex arc
• Monosynaptic in the cord

• Axonal
• Demyelinating

• Reduced CMAP (motor) and SNAP (sensory) amplitudes
• Mildy Reduced CV due to a loww of the fastest conducting fibers (not below 75% of normal)

There is no change in the distal motor latency, F-wave, conduction block, or abnormal temporal dispersion.

• Axonal Neuropathies
• Nerve Trauma
• Motor Neuron Diseases
• Radiculopathies (CMAPs only)
• Secondary axonal loss in a primarily demyelinating process

• Substantial increases in CV (conduction velocity) which may be noted as:
• - Decreased CV in motor units (<70% nl)
• - Prolonged distal motor latencies (>130% nl)
• - Prolonged F-waves (>130% nl)

Conduction Block / Abnormal Temporal Dispersion

• A decrease in the amplitude of a proximal response relative to that of the distal response without an increase in the response duration.
• Reflects the block of conduction through a subset of motor axons in the nerve resulting from segmental demyelination.
• Must be at least a 30% decrement but often exceeds 50%

Conduction Block

There is a reponse to distal but not proximal stimulation indicating a segmental, demyelinating process.

• An abnormal prolongation of the motor response.
• Reflects the desynchronization of components of the response due to different conduction velocities along the length of the nerve
• >30% increase in dispersion is typically abnormal

Temporal Dispersion

CMAP

• Normal for 2-3 days
• Decline over 3-6 days
• Disappear by 7 days

10 days

• Maxiaml decline in CMAPs are seen at 5-7 days
• Maximal decline in SNAPs are seen at 7-10 days

• Conduction Block across the site of injury (T0)
• No conduction across the site of injury (T0)
• Reduced Amplitudes on distal stimulation (T+7-10)
• Acute Denrvation (fibrillation portentials - T+14-35)
• Renervation (T+70-84)

It is the motor respnse generated by passing a needle through the muscle and typically last <20 ms

Acute Denervation

• Inexcitable Tissue (fibrosis, fatty replacement, ischemia)
• Physiologic Contractures (McArdle's, PFK)
• During an attack of periodic paralysis

• From Muscle:
• Fibrillation Potentials
• Positive Sharp Waves
• Complex Repetative Discharges
• Myotnic Discharges

• From Motor Neuron:
• Fasciculation Potentials
• Myokymia
• Neuromyotonia
• Tremor
• Cramp Discharges

• Spontaneous firing of a single muscle action potential
• Diphasic with initial positivity (downward)
• Appear 2-4 weeks following:
• - Denervation
• - Neuropathy
• - Myopathy

Fibrillation Potentials

• A train of grouped muscle fiber action potentials
• Fire in a regular, lock-step, drumming fashion
• Begin and end abruptly
• Seen in any muscle disease

• These are repetative discharges provoked by passage of the needle through the muscle or percussion
• Train of muscle fiber action potentials with a waxing and waning frequency / amplitude ("dive bomber")
• Seen in hereditary myotonias, periodic paralysis, and some toxic / metabolic myopathies

Complex Repetative Discharges

Myotonic Discharges

Spontaneous motor neuron discharge leading to the appearance of an action potential (MUAP) but slower

• Many causes:
• - Neurogenic Diseases
• - Cholinesterase Inhibitors
• - Benign Fascics / Cramps-Fasciculation Syndrome

• Repeating discharges of groups of MUAP’s which have the appearance of grouped fasciculations.
• Bursts repeat at regular or semiregular intervals
• Variety of causes:
• - Facial: MS, Brainstem Lesions
• - Limb: radiation plexopathies
• - General: Isaac's SYndrome

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