Ch 5 - EDX: NCS Flashcards
What does onset latency reflect in sensory fibers?
Fastest fibers
What does the peak latency represent?
Initiation of conduction along the majority of axons. Recorded from stimulus to peak waveform
What is conduction velocity?
Speed an impulse travels along a nerve dependent on myelin sheath
What are the age variations in conduction velocity?
Newborns 50% of an adult
1 yo 80% of adults
3-5 yo equal to adults
Dec 1-2 m/s per decade >50yo
What is considered a normal limb temperature for NCS?
Upper: 32 deg C
Lower: 30 deg C
How does temperature affect CV?
Dec 2.4 m/s per 1 deg C drop
5% dec for each 1 deg C below 29 deg C
What is amplitude?
Max voltage diff b/w two points reflecting # of nerve fibers activated and synchronicity of firing
How is amplitude measured?
Peak-to-peak or baseline-to-peak
How is duration measured?
Initial deflection from baseline to return
What is the duration?
Summation and firing rate of numerous axons
What is latency of activation?
Time b/w initiation of electrical stimulus and beginning of saltatory conduction
<0.1 ms
What is conduction?
Saltatory conduction of an AP along myelinated axons to terminal branches, unmyelinated twigs and NMJ
What is Synaptic transmission?
Chemical transmission of signal across the NMJ to initiate a single fiber AP
0.2-1.0 ms
What are normal values for CV?
Upper: >50 m/s
Lower: >40 m/s
What is the area in NCS?
Both amp and duration of the waveform
What is temporal dispersion?
Range of CV of fastest and slowest nerve fibers
How does temporal dispersion change with proximal to distal stimulation?
Waveform spreads out with proximal compare to distal stim
What is phase cancellation?
Comparing proximal to distal stim, drop in amp and inc duration occurs
When is phase cancellation most notable?
SNAP due to short duration
When accounting for phase cancellation what is a normal drop of SNAP when moving proximal?
50%
When accounting for phase cancellation what is a normal drop of CMAP when moving proximal?
15%
What is a SNAP?
Sensory nerve study representing conduction of an impulse along sensory nerve fibers
What is SNAP useful for localizing a lesion to?
DRG
What happens to SNAP with lesions proximal to the DRG?
SNAP is preserved as axonal transport from cell body to peripheral axon intact
What are SNAPs more sensitive than CMAP in detection of?
Incomplete nerve injury
What are advantages of antidromic studies over orthodromic studies?
Easier to record
More comfortable
Require less stim intensity
Larger amp due to nerve more superficial at distal recording sites
What happens to motor and sensory responses with Postganglionic injuries?
CMAP and SNAP diminished or absent
What happens to motor and sensory responses with Preganglionic injuries?
CMAP diminished or absent
SNAP normal
What happens to the waveform when reference and recording electrodes are <4 cm apart?
Decreased peak latency, amp, duration and rise time
What do CMAPS or M waves represent?
Conduction of an impulse along motor nerve fibers of a motor unit
When can CMAP be ABN with a normal SNAP?
Lesion proximal to the DRG
Lesion affecting a purely motor nerve
Describe the typical waveform of a CMAP.
Biphasic with initial negative deflection
Why would a CMAP have an initial positive deflection?
Inappropriate placement of active electrode to motor point
Volume conduction from other muscles or nerves
Anomalous innervation
What can contribute to a falsely decreased amp and inaccurate latency for CMAP?
Act and Ref electrodes too close
Submaximal stim
Stim over callous or edema
What is an H-reflex?
Electrically evoked analogue to a monosynaptic reflex (NCS late response)
ex: Achilles reflex
How is an H-reflex stimulated?
Submaximal stim for 1.0ms activates IA afferent nerve fibers causing an orthodromic sensory response to the SC and then orthodromic motor resonse back to recording electrode
What is H-reflex used to monitor?
C7 or S1 radiculopathy
What locations are stim to find the H-reflex?
FCR: Median nerve: C7
Soleus: Tibial nerve: S1
How can H-reflex be potentiated and inhibited by muscle contraction?
Potentiated: agonist muscle
Inhibited: antagonist muscle
What are limitations of the H-reflex?
Eval long neural pathway so hinder specificity of lesion
Can be normal with incomplete lesions
Doesn’t distinguish b/w acute and chronic
Once ABN, always ABN
What is a F-wave?
Small later pure motor response occurring after the CMAP
How is a F-wave produced?
Short duration, supramaximal stim initiates an antidromic motor response to the anterior horn cells in the SC which produces an orthodromic motor response to the recording electrode
Why is a F-wave not a true reflex?
No synapse along the nerve pathway being stim
Why do configuration and latency of an F-wave change with each stim?
Due to activation of different groups of anterior horn cells with each stim
What can F-wave be useful for detecting?
Polyneuropathies and plexopathies
Not radiculopathies
Where can F-wave be obtained?
Any muscle
What are limitations of F-waves?
Eval long neural pathway and hinders specificity of injury
Only accesses motor fibers
What is a significant side to side difference in H-reflex latency?
0.5-1 ms
What is a significant side to side difference in F-wave latency?
Upper limb: 2.0 ms
Lower limb: 4.0 ms
How can an A-wave be obtained?
Evoked during CMAP with submaximal stim and abolished with supramaximal stim
Where does an A-wave typically occur?
b/w CMAP and F-wave at a constant latency
What does an A-wave represent?
Collateral sprouting following nerve damage
What is the blink reflex?
NCS is an electrically evoked analogue to the corneal reflex
How is a blink reflex initiated?
Stim supraorbital branch of the trigeminal nerve→ pons and branches to lateral medulla→ ipsi and contralateral orbicularis oculi via the facial nerve
What responses are evaluated during the blink reflex?
Ispilateral R1
Bilateral R2
Which response is the blink associated with?
R2 response
What is the afferent pathway of the blink reflex?
Sensory branches of CN V (trigeminal nerve)
What is the efferent pathway of the blink reflex?
Motor branches of CN VII (facial nerve)
What is the course of the R1 (early) response (blink reflex)?
Through the pons
What is the course of the R1 (late) response (blink reflex)?
Through the pons and lateral medulla
What lesions affect the R1 response (blink reflex)?
Trigeminal nerve
Pons
Facial nerve
What lesions affect the R2 response (blink reflex)?
Consciousness level Parkinson's disease Lateral medullary syndrome Contralateral hemisphere Valium Habituation
How is NCS of CN VII (facial nerve) performed?
Stim distal to the stylomastoid foramen at the angle of the mandible
Response recorded over nasalis muscle
How does a patient with peripheral facial nerve injury present?
Equal weakness in upper and lower facial muscles
How does a patient with a lesion rostral to the facial nucleus present?
Lower facial muscles more severely affected than upper
What is Synkinesis?
Aberrant regeneration of axons in facial nerve injuries leading to reinnervation of inappropriate muscles
How can Synkinesis present?
Lip twitching when closing an eye
Crocodile tears when chewing
What is facial NCS useful for monitoring injury for?
Bell's palsy Neoplasms Fractures Middle ear infection DM Mumps Lyme disease
What indicates outcomes of facial nerve injury?
Demylinating better than axonal injury
No evoked potential for 7 days indicates poor prognosis
What are treatments of facial nerve injury?
Prednisone
Massage
Electrical stim
What does a somatosensory evoked potential (SSEPs) evaluate?
Time-locked responses of the nervous system to an external stim
Describe the ascending sensory pathway of an afferent potential during SSEP.
Peripheral nerve→ plexus→ root→ SC (posterior column)→ contralateral medial lemniscus→ thalamus→ somatosensory cortex
How is SSEP initiated?
Repetitive submaximal stim of sensory nerve, mixed nerve or dermatome recorded from spine or scalp
What are the MC nerves used to test SSEP?
Median and tibial nerve
For what is SSEP useful for monitoring?
Peripheral nerve injury
CNS lesions (MS)
Intraoperative monitoring of spinal surgery
ABN SSEP in MS are most likely to be seen in____.
Lower limb prolonged interpeak latencies
What can indicate nerve injury during spinal cord surgery?
Loss of tibial nerve potentials with preservation of median nerve
What are advantages of SSEP?
Eval sensory PNS and CNS
Eval d/o CNS, dorsal nerve roots and peripheral nerves
ABN results present immediately
What are disavantages of SSEP?
Only eval fibers sensing vibration and proprioception
Limited in ability to localize lesion
Affected by sleep and anesthesia
