Electrodiagnostics Flashcards
How do newborn nerves compare to adults? At what age do they equal adults?
Newborns nerves are half as fast as adults. They equal adults at 3-5 years.
Describe the Sunderland Classification of Nerve Injury
- Neurapraxia (conduction block)
- Axonotmesis (Axon damage(
- Axon and Endoneurium injury
- Axon, Endoneurium, and Peineurium injury (only Epineurium intact)
- Neurotmesis (complete nerve transection)
How does axonal degeneration differ from Wallerian degeneration?
Axonal degeneration starts distally and ascends whereas with Wallerian the nerve degenerates distally from the site of the lesion.
How does nerve conduction velocity change with age?
decreases by 2 m/s per decade past age 50
How does putting electrodes closer than 4cm affect NCS parameters?
decreases peak latency, duration, amplitude, and rise time
How does a decrease in temperature affect sodium channels?
colder temperature causes a delay in opening and closing of the sodium channel gates which can increase amplitude and prolong conduction
How long does it take axons to regrow if connective tissue remains intact?
1mm/day or 1 inch/month
How long does it take median nerve stimulation at the wrist to reach Erb’s point? How long to reach the cortex?
9 msec and 20 msec respectively
How will lowering the high frequency filter affect the waveform?
It will decrease amplitude but prolong latency
How would a 5 deg C drop in temperature affect conduction velocity and distal latency?
It would increase distal latency by 1 msec and decrease conduction velocity by 10 m/s
If you suspect a low amplitude median CMAP is due to a Riche-Cannieu anastomosis, what should you do to confirm?
stimulate the ulnar nerve at the wrist and elbow
In general, what is the high-frequency filter set at?
10kHz
In order to increase the signal-to-noise ratio by a factor 3, how many averages need to be preformed?
9
S:N = (signal amplitude x square rt of # of averages preformed)/noise amplitude
name two causes of myokymic discharges
radiation plexopathy, rattlesnake venom
Normal duration, amplitude, and phases of a motor unit action potential?
- Duration: 5 - 15 ms
- Amplitude: up to 2 mV
- Phases: 2-4 (equal to # of baseline crossings + 1)
On EMG, what types of abnormal sponteneous activity are generated by muscle fibers?
Myotonia, CRDs, FIBs, PSWs
Please describe the A band, H Zone, and I band
- A band: Myosin
- H zone: Myosin not overlapped with Actin
- I band: Actin not overlapped with Myosin
What allows for muscle fiber relaxation?
ATP powering Ca++ being pumped back into the sarcoplasmic reticulum allowing tropomyosin to block actin’s active sites
What are normal conduction velocities for upper and lower limbs?
- Upper: 50 m/s
- Lower: 40 m/s
What are three NCS findings that could indicate a Martin-Gruber anastomosis?
- decreased distal amplitude of median nerve CMAP compared to proximal
- initial positive deflection of median nerve CMAP
- artifically fast CV
What diagnosis would you suspect if you see conduction block at non-entrapment sites?
GBS
What disorders are associated with myotonic discharges?
- Myotonic conditions
- Polymyositis
- Acid-maltase deficiency
- Hyperkalemic periodic paralysis
What does an A- (Axon) Wave represent?
collateral sprouting following nerve damage
What is Ohm’s Law?
V = I x R (voltage = current x resistance)
What is the definition of innervation ratio?
number of muscle fibers innervated per alpha motor unit (huge muscles have huge innervation ratio)
What is the normal latency for R1 and R2 in a blink reflex?
- R1 < 13
- ipsilateral R2 < 41 msec
- contralateral R2 < 44 msec
Source: Preston and Shapiro
What is the shortcoming of using needle electrodes with NCS?
CV cannot be assessed as only a few fibers are sampled
What makes up a motor unit?
anterior horn cell, axon, NMJ, and all muscle fibers innervated by that axon
What median muscles are affected with entrapment at the Ligament of Struthers (present in 1% of population)?
all of them can be affected
What muscle can be used for H-reflex in the upper extremity? Which pathway is tested?
Flexor carpi radialis for C7 pathway
What nerve fibers are stimulated to trigger the H-reflex?
Ia sensory afferent nerves
What nerve innervates the adductor magnus?
obturator and sciatic (tibial portion)
What nerves are most commonly used for SSEP studies?
median and tibial
What nerve innervates the brachialis muscle?
musculocuteneous and radial (dual innervation)
What temperatures should be limbs be kept at for NCS?
- Upper limbs: above 32 C
- Lower limbs: above 30 C
What type of nerve fibers do EDX studies evaluate?
Ia (large, myelinated)
What type of nerves take longer to undergo Wallerian degeneration?
Sensory (11 days vs the 7 days with motor axons)
Will changing the low-frequency filter affect the onset latency?
No
With blink reflex study, how is R2 response generated?
CN V sensory to Vs nucles in the medulla to bilateral VII nuclei in the pons to bilateral orbicularis oculi muscles via CN VII nerves.
With blink reflex study, how is the R1 response generated?
CN V sensory to Vm nucleus in the pons to ipsilateral VII nucleus in the pons to ipsilateral orbicularis oculi via CN VII
What type of neuropathy does diabetes cause?
axonal
What is seen on NCS studies in GBS?
conduction block and temporal dispersion at non-entrapment sites
What differentiates critical illness neuropathy from critical illness myopathy?
numbness
Genetic cause of CMT1A
PMP-22 duplication
What is the genetic cause of HNPP?
PMM-22 deletion
What does CMT2 cause?
axonal polyneuropathy
What surgery can improve weakness in myasthenia gravis?
thymectomy
How does NCS differ between myasthenia gravis and Lamber-Eaton?
Routine NCS studies are normal in MG but Lambert Eaton has low amplitude CMAPs
How do you detect NMJ disease with RNS?
NMJ disease will show a greater than 10% decrement in amplitude between the 1st and 4th waveforms with low rate RNS (2-3 Hz)
What is seen with high rate (10 - 50 Hz) RNS in NMJ disease?
In general, CMAP repair but huge increase (300%) if Lambert Eaton
What is Post-Exercise Facilitation?
Same as high rate RNS, see repair after maximal muscle contraction for 60 seconds.
What is post-exercise exhaustion?
Continue doing low rate RNS q 5 minutes to try to bring out the decrement
What is Werdnig-Hoffman Disease?
SMA 1: floppy infant that never sits and dies early of respiratory failure
What is Kugelberg-Welander Disease?
SMA3: can walk independently, normal life expectancy
What is the genetic cause of SMA?
mutation of the SMN1 (Survival Motor Neuron) gene
Pharmcologic treatment for SMA?
nusinersen
What clinical findings are notably absent in ALS?
bowel and bladder abnormalities
What type of muscle fibers are affected in steroid myopathy?
type II
What type of dermatomyositis is due to cancer?
type III
How long does it normally take the Tibial SSEP to reach the cortex?
45 msec
Carpal tunnel syndrome symptoms of what duration indicate a poor prognosis?
greater than 10 months
What amino acid is a norepinephrine precursor?
tyrosine
Three causes of axonal sensorimotor neuropathies
alcohol, amyloidosis, sarcoidosis
what is a normal amount of jitter in single fiber EMG?
less than 60 usec (micro second)
what myopathy is associated with malignant hyperthermia?
central core disease
what muscle most commonly shows EMG abnormalities in polymyositis and dermatomyositis?
paraspinals
