Electroneurography

Question 1

What is electroneurography?

Answer 1

• nerve conduction measurement

Question 2

Motor nerve conduction

Answer 2

• using same EMG and nerve stimulation equipment

- can quantify motor nerve conduction

Question 3

Use of electrodes

Answer 3

• most nerve conduction studies use surface electrodes
• occasionally monopolar needle electrodes are used
• stimulating cathode placed peripherally (closest to muscle being monitored)

Question 4

Basic measurements

Answer 4

• nerve typically stimulated at 2 sites down its length
• site nearest to muscle is called the distal site
• further is proximal site
• measurements made of muscle potential amplitude, duration of negative (upward) phase, distal and proximal latencies

Question 5

Decay

Answer 5

Decay = 100 x (Adist-Aprox)/Adist

• measure of decrease in size of the muscle potential when stimulated at the proximal compared with the distal site
• because of spread in nerve conduction velocities, muscle potentials arising from proximal stimulation may be more dispersed causing slight reduction in amplitude

Question 6

Dispersion

Answer 6

Dispersion = 100 X (DURprox - DURdist)/DURprox

• measures spread of potential when stimulated at the proximal compared with the distal site

Question 7

Conduction block

Answer 7

• > 20% amplitude or area decay less than 15% dispersion
• > 50% amplitude or area decay
• both criteria are equally sensitive, but latter is more specific
• decay will be increased
• spread of conduction velocities increasing dispersion will also affect amplitude causing increase in decay when there is no conduction block

Question 8

Simple conduction velocity measurement

Answer 8

• conduction velocity is temperature dependent, becoming faster as the muscle warms up
• for accurate measurements, temperature controller may be used

Question 9

Sensory nerve conduction

Answer 9

• no large muscle action potential to measure
• can’t stick needles into nerves
• surface electrodes can measure very small compound nerve action potentials, but these may be lost in the noise

Question 10

Averager

Answer 10

• adds together responses from repetitive stimulation
• responses sum linearly, whereas noise sums in proportion to the square root of the number of stimuli
• stimuli should not be synchronised with the mains frequency

Question 11

Ring electrodes for sensory stimulation

Answer 11

• for motor nerve stimulation
• electrodes designed for minimal current density in the skin = minimising discomfort
• sensory stimulation electrodes are designed to stimulate nerve endings
• spring electrodes can be slipped over fingers and tightened

Question 12

Averager response

Answer 12

• with single stimulus, response can be seen
• can be easily mistaken for a noise peak
• some averagers designed to deliver stimuli at different points of the mains cycle to eliminate this type of interference

Question 13

Sensory nerve conduction velocity measurement

Answer 13

• sensory stimulating electrodes placed away from motor end plate zone of any muscles innervated by the nerve
• avoids stimulating motor nerves

Question 14

Hopf collision technique

Answer 14

• method of obtaining complete distribution of motor nerve velocities, rather than just measuring the fastest
• proximal and distal stimuli applied (distal first) with variable interval in between (DPSI)
• because of refractory period = when antidromic pulse from distal site collides with orthodromic potential from proximal site, propagation ceases in both directions
• if DPSI is long enough, distal pulse will pass proximal site allowing time for refractory period to end before proximal stimulus is given
• proximal potential can then propagate to muscle and induce a muscle action potential

Question 15

Increasing distal-proximal stimulus interval

Answer 15

• gradual increase in size of second proximally stimulated muscle action potential as DPSI is increased
• initially, no second pulse, then muscle fibres innervated by faster nerves create a potential and finally the slower ones
• each DPSI value corresponds to specific nerve conduction velocity
• amplitude or area of second muscle potential corresponds with cumulative distribution of the motor nerve conduction velocities
• SEE SLIDE

Question 16

Nerve conduction in the CNS

Answer 16

• motor cortex can be stimulated in the same way as peripheral
• larger surface electrodes and higher voltage
• alternatively nerves stimulated with eddy currents and induced by fast electromagnetic pulse
• safely used to stimulate motor cortex in brain or spinal cord
• also to stimulate visual cortex