Neurodiagnostics, Pt. 3 Flashcards
What are electrodiagnostics based on?
electrical properties of living tissues
- not performed often
- EEG, EMG = spontaneous potentials
What is responsible for membrane excitability of neurons?
Na/K ATPas
- 3 Na out, 2 K in = negative membrane potential of neurons
What is Nernst potential?
equilibrium potential - chemical and electrical gradients are equal and opposite in direction
What is the Goldman-Hodgkin-Katz equation?
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
What kind of response is an action potential? How does an action potential occur?
all or none
- local current
- propagation along axon
- refractory period
- re-establishment of resting potential
What electrodes are used to record extracellular waveforms?
active electrode (G1) is the recording electrode that sees the depolarization in context of the inactive/reference electrode (G2)
What electrodiagnostic tests are used in veterinary medicine?
- 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)
What is electromyography (EMG) used for? How are results read?
evaluates integrity of muscle fibers
- hyperexcitable with denervation of myopathies
- normal muscle is silent
(sensitive, but not specific)
What noise is considered normal in EMGs? What do fibrillation/positive sharp waves, bizarre high frequency potentials, and myotonic potentials indicate?
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
Myotonia, EMG:
What does motor nerve conduction velocity (MNCV) record? What nerves are most commonly used?
compound muscle action potential (CMAP) or M waves after nerve stimulation
minimum of 2 sites, usually sciatic or peroneal nerves
What 3 things are measured by motor nerve conduction velocity (MNCV)? What is it primarily used for?
- latency measured by computer
- distance between sites measured by operator
- conduction velocity is calculated
suspected neuropathies
What are F and H waves used to investigate? What to they measure?
nerve root pathologies
smaller M waves at longer latencies
(normal F waves have been published for dogs)
What is repetitive nerve stimulation (RNS) used for? What is set up like?
measures neuromuscular transmission
same as MNCV, but not measuring conduction time
What is repetitive nerve stimulation especially used to diagnose? What is supportive?
acquired myasthenia gravis
decrementing response
What is single-fiber electromyography?
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
What does sensory nerve conduction velocity measure? How is it performed?
depolarization of the nerve (CAP) vs. muscle
- stimulate distal nerve branches
- measure proximal to the stimulation site
How does somatosensory evoked potentials (SSEP) work?
- 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
What does brainstem auditory evoked response utilize?
hearing pathways - evaluates hearing and brainstem integrity using far-field potentials time-locked to stimulus
How is the brainstem auditory evoked response test performed? What response is expected?
- scalp electrodes are placed
- sound stimulus or masking noise delivered
- VM and VT1 leads measure response
symmetry
What are 4 uses for brainstem auditory evoked response tests?
- congenital/acquired deafness
- suspected brainstem lesions - peripheral vs central vestibular
- possible screening tool for chiari-like malformations
- combined with EEG in cases of expected brain death
What does EEGs record? How is it recorded?
spontaneous electrical activity of the cerebral cortext
scalp electrodes
What 2 things does EEGs normally record? What indicated seizures?
background rhythm and phases of alertness
spike/spike-wave activity
How are EEGs used today?
- wireless recording electrode for continuous monitoring
- spike detection software
- quantitative/surgical strip EEG
- localizing seizure foci for location-related epilepsy
What is the purpose of magnetic motor evoked potentials (MEPs)? How does it work? In what patients is it most commonly used?
evaluated descending motor pathways
it stimulates the cerebral motor cortex and records M waves from limb muscles
horses - little utility in dogs and cats
