Nerve Conduction

Question 1

Generally, what is a nerve conduction study? What info can I get from it?

Answer 1

recording and measurement of nerve and muscle AP generated by stimulation of the nerve with a supramaximal stimulus.
measure terminal latency, amplitude, duration, and conduction velocity.

Question 2

What is the difference in the direction of nerve conduction studied for sensory vs. motor nerves?

Answer 2

motor nerves studied orthodromically: in the same direction as physiological conduction
sensory nerves are studied antidromically: in the direction opposite physiological conduction. Can also be studied orthodromically,b but this is more technically challenging.

Question 3

How is a motor nerve conduction study conducted?

Answer 3

place a surface electrode on the belly of a muscle innervated by the nerve you want to test
place an indifferent electrode distal to a distal joint
apply a supramaximal stimulus to the nerve at a place that is a known distances proximal to the surface electrode
recorde the compound motor action potential (CMAP) that results.

Question 4

How is a sensory nerve conduction study conducted?

Answer 4

place surface skin electrodes over the skin that is innervated by the nerve in question. ring electrodes used on fingers.
indifferent electrode is placed distal to the active electrode
stimulate the nerve in a predetermined proximal location (antidromic) with a supramaximal potential
record the compound sensory nerve action potential (SNAP) that results

Question 5

What is the amplitude parameter of nerve conduction studies? What information does it tell us? What kinds of pathology causes a change here?

Answer 5

maximum voltage difference btw 2 points measured peak to peak. measured for compound muscle APs and sensory nerve APs. proportional to the number of available axons that are stimulated. Amplitudes are reduced in axonal neuropathies

Question 6

What is the terminal latency parameter of nerve conduction studies? What information does it tell us? What kinds of pathology cause changes?

Answer 6

interval between the time of stimulation and the onset of the resultant compound muscle AP or sensory nerve AP.
for sensory, this just involves conduction in the nerve; for motor, this parameter measures time in nerve, NMJ, and muscle. prolonged terminal latencies are seen in distal demyelinating lesions and in distal compressive neuropathies, like carpal tunnel syndrome

Question 7

What is the conduction velocity parameter? What information does it tell us? What kinds of pathology cause changes?

Answer 7

speed of propagation of an AP along a nerve: calculate it from the latencies of the evoked potentials (at supramaximal stimulation levels) at 2 different points along the nerve. This represents the conduction velocities of the fastest, largest, myelinated fibers ONLY. It is slowed in demyelinating neuropathies like Guillain-Barre.

Question 8

List 4 factors that can affect normal nerve conduction studies.

Answer 8

1. technique
2. limb temperature
3. portion of nerve stimulated
4. patient age

Question 9

How does technique affect normal nerve conduction studies?

Answer 9

technique can affect nerve conduction studies based on precision in measurement of distances btw stimulation pts, electrode placement, etc. Can result in falsely slowed nerve conduction velocity.

Question 10

How does limb temperature affect normal nerve conduction?

Answer 10

Nerve conduction at colder temps is slower (slowed by 5% for every degree C below 34). make sure limbs are warm enough

Question 11

How does the portion of the nerve stimulated affect normal nerve conduction?

Answer 11

proximal segments faster than distal ones- they are wider. velocities in the arms faster than in the legs- temperature differences.

Question 12

How does patient age affect normal nerve conduction?

Answer 12

infants are slower than adults by about 50%.

Question 13

What nerve conduction results would I expect to see with axonal neuropathies?

Answer 13

reduced amplitude of compound motor or sensory nerve APs

Question 14

What nerve conduction results would I expect to see with demyelinating neuropathies?

Answer 14

slowed conduction velocity and prolonged terminal latencies. conduction velocity proportional to the velocity of the largest diameter, myelinated fibers

Question 15

What nerve conduction results would I expect to see with conduction block?

Answer 15

conduction block = failure of an AP to be conducted past a particular pt in the nerve. documented with a reduction in the amplitude of an evoked potential at 2 different stimulation pts on a nerve trunk. often seen with severe focal compression.

Question 16

What nerves are most frequently studied by nerve conduction? (5)

Answer 16

median motor and sensory
ulnar motor and sensory
peroneal/fibular motor
tibial motor
sural sensory

Question 17

When are nerve conduction studies most helpful?

Answer 17

polyneuropathies and compression neuropathies

Question 18

Describe polyneuropathies.

Answer 18

may be axonal or demyelinating, and usually involve distal nerves in a symmetrical pattern. Most polyneuropathies are mixed: and have both axonal and demyelinating features. paradoxically, severe axonal neuropathies can result in slowed conduction velocities, and severe demyelinating neuropathies can result in reduced amplitude

Question 19

What nerve conduction results would I expect to see with compression neuropathies?

Answer 19

slowed conduction times and evidence for conduction block across the site of compression.

Question 20

What are the 4 most common compression neuropathies?

Answer 20

carpal tunnel (median nerve at the wrist)
tardy ulnar palsy (ulnar nerve at the elbow)
peroneal nerve palsy (peroneal/fibular nerve at the fibular head)
tarsal tunnel syndrome: tibial nerve at the ankle

Question 21

How are F waves and H reflexes used?

Answer 21

to look at conduction parameters for proximal portions of nerves, including nerve roots

Question 22

What is the F wave? (latency, configuration, amplitude, stimulus, presence pathwa, distribution)

Answer 22

late compound muscle ap evoked intermittently from a muscle by a SUPRAMAXIMAL electrical stimulus to the nerve. this is a result of antidromic activation (backfiring) of alpha motor neurons. Variable latency and configuration, small in amplitude, only present intermittently, requires supramaximal stimulus. seen in ALL alpha motor neurons

Question 23

What is the H reflex (latency, configuration, amplitude, stimulus, presence, pathway, distribution)

Answer 23

late compound motor AP that has a consistent latency and is evoked regularly from a muscle by a submaximal nerve stimulus. spinal reflex. result of stimulation of Ia afferent fibers.
large amplidute.
only routinely observed in calf muscles by stim of the tibial nerve in the popliteal fossa.