Neurodiagnostics, Pt. 3

Question 1

What are electrodiagnostics based on?

Answer 1

electrical properties of living tissues

• not performed often
• EEG, EMG = spontaneous potentials

Question 2

What is responsible for membrane excitability of neurons?

Answer 2

Na/K ATPas

• 3 Na out, 2 K in = negative membrane potential of neurons

Question 3

What is Nernst potential?

Answer 3

equilibrium potential - chemical and electrical gradients are equal and opposite in direction

Question 4

What is the Goldman-Hodgkin-Katz equation?

Answer 4

describes ionic flux across a cell membrane as a function of the transmembrane potential and the concentrations of the ion inside and outside of the cell

Question 5

What kind of response is an action potential? How does an action potential occur?

Answer 5

all or none

• local current
• propagation along axon
• refractory period
• re-establishment of resting potential

Question 6

What electrodes are used to record extracellular waveforms?

Answer 6

active electrode (G1) is the recording electrode that sees the depolarization in context of the inactive/reference electrode (G2)

Question 7

What electrodiagnostic tests are used in veterinary medicine?

Answer 7

• EEG
• EMG
• brainstem auditory evoked response (BAER)
• motor nerve conduction velocity (MNCV)
• F and H waves
• repetitive nerve stimulation (RNS)
• sensory nerve conduction velocity (SNCV)
• somatosensory evoked potentials (SSEP)
• magnetic motor evoked potentials (MEP)
• single-fiber electromyography (SF-EMG)

Question 8

What is electromyography (EMG) used for? How are results read?

Answer 8

evaluates integrity of muscle fibers

• hyperexcitable with denervation of myopathies
• normal muscle is silent

(sensitive, but not specific)

Question 9

What noise is considered normal in EMGs? What do fibrillation/positive sharp waves, bizarre high frequency potentials, and myotonic potentials indicate?

Answer 9

endplate noise - tip of the EMG needle is within a neuromuscular junction/close to a motor end plate

action potentials of single muscle fibers that fire spontaneously in the absence of innervation

myotonia = divebomber sound

Question 10

Myotonia, EMG:

Answer 10

Question 11

What does motor nerve conduction velocity (MNCV) record? What nerves are most commonly used?

Answer 11

compound muscle action potential (CMAP) or M waves after nerve stimulation

minimum of 2 sites, usually sciatic or peroneal nerves

Question 12

What 3 things are measured by motor nerve conduction velocity (MNCV)? What is it primarily used for?

Answer 12

1. latency measured by computer
2. distance between sites measured by operator
3. conduction velocity is calculated

suspected neuropathies

Question 13

What are F and H waves used to investigate? What to they measure?

Answer 13

nerve root pathologies

smaller M waves at longer latencies
(normal F waves have been published for dogs)

Question 14

What is repetitive nerve stimulation (RNS) used for? What is set up like?

Answer 14

measures neuromuscular transmission

same as MNCV, but not measuring conduction time

Question 15

What is repetitive nerve stimulation especially used to diagnose? What is supportive?

Answer 15

acquired myasthenia gravis

decrementing response

Question 16

What is single-fiber electromyography?

Answer 16

similar to repetitive nerve stimulation, but with a specialized recording electrode that measures a single fiber and calculates “jitter”

• very specific for MG in people, reported in dogs and cats

Question 17

What does sensory nerve conduction velocity measure? How is it performed?

Answer 17

depolarization of the nerve (CAP) vs. muscle

• stimulate distal nerve branches
• measure proximal to the stimulation site

Question 18

How does somatosensory evoked potentials (SSEP) work?

Answer 18

• recording electrodes are placed over the interacruate spaces along the spine
• field potentials are then generated by incoming afferent axons
  (same setup as SNCV)

aka cord dorsum potentials

Question 19

What does brainstem auditory evoked response utilize?

Answer 19

hearing pathways - evaluates hearing and brainstem integrity using far-field potentials time-locked to stimulus

Question 20

How is the brainstem auditory evoked response test performed? What response is expected?

Answer 20

• scalp electrodes are placed
• sound stimulus or masking noise delivered
• VM and VT1 leads measure response

symmetry

Question 21

What are 4 uses for brainstem auditory evoked response tests?

Answer 21

1. congenital/acquired deafness
2. suspected brainstem lesions - peripheral vs central vestibular
3. possible screening tool for chiari-like malformations
4. combined with EEG in cases of expected brain death

Question 22

What does EEGs record? How is it recorded?

Answer 22

spontaneous electrical activity of the cerebral cortext

scalp electrodes

Question 23

What 2 things does EEGs normally record? What indicated seizures?

Answer 23

background rhythm and phases of alertness

spike/spike-wave activity

Question 24

How are EEGs used today?

Answer 24

• wireless recording electrode for continuous monitoring
• spike detection software
• quantitative/surgical strip EEG
• localizing seizure foci for location-related epilepsy

Question 25

What is the purpose of magnetic motor evoked potentials (MEPs)? How does it work? In what patients is it most commonly used?

Answer 25

evaluated descending motor pathways

it stimulates the cerebral motor cortex and records M waves from limb muscles

horses - little utility in dogs and cats