Chapter 3: EMG/NCS

Question 1

concentric vs monopolar EMG needles?

Answer 1

• monopolar needles give rise to increased variability of motor unit potentials ,
• monopolar record from larger areas of muscle and produce larger motor unit potentials.

For these reasons, most centers prefer to use concentric needles.

• monopolar needles may cause less pain however.

Question 2

normal CMAP and absent response with stimulation after trauma suggests?

Answer 2

neuropraxic injury > transection injury, as with transection injury you expect CMAP to be abnormal

Question 3

increment found with 20Hz fast stim is seen in ?

Answer 3

botulism (less than 100%) and LEMS (>100%)

Question 4

decrement found with slow stim?

Answer 4

myasthenia, botulism

Question 5

decreased interference pattern seen on EMG depicts?

Answer 5

neurogenic process

Question 6

CMT1 vs CMT2A

Answer 6

1 is primary demyelinating and 2A is primary axonal

Question 7

In a posterior cord lesion to the plexus, which reflexes are abnormal: Biceps, brachialis, triceps

Answer 7

triceps and brachialis, biceps is preserved

Question 8

most sensitive test to diagnose MG?

Answer 8

single fiber emg of the frontalis muscle

Question 9

hereditary vs acquired demyelinating polyneuropathies difference on NCS?

Answer 9

Both have prolonged distal latency and reduced conduction velocity, but only acquired have abnormal temporal dispersion.

Question 10

In patients with neurogenic TOS, which ares of the cord are primarily affected?

Answer 10

medial cord or lower trunk

Question 11

EMG sign of chronic denervation

Answer 11

MUP with large duration and amplitude

Question 12

myotonia congenita presents with

Answer 12

channelopathy that presents with myotonia, stiffness and weakness in cold temperatures

Question 13

What is the earliest diagnostic change in a patient with length dependent axonal polyneuropathy

Answer 13

loss of sural sensory response

Question 14

amyloid myopathy?

Answer 14

usually caused by hereditary amyloidosis or primary (not common in secondary)

patients can present with peripheral neuropathy and hypertrophic muscles (although still weak)

CK is elevated

EMG/NCS shows combination of neuropathic and myopathic changes

muscle biopsy reveals amyloid deposition

Question 15

myopathic EMG?

Answer 15

short duration, small amplitude motor units with reduced recruitment

Question 16

whats considered a normal change in amplitude after 15 seconds of exercise?

Answer 16

<30% inc

Question 17

repair vs facilitation after stim?

Answer 17

repair: improvement in the junction after brief exercise and therefore improvement in the decrement

Facilitation: significant increase in the CMAP with with exercise.

Question 18

expected EMG findings in patients with MGA and CTS?

Answer 18

delayed distal latency (because of CTS) + spuriously inc conduction velocity and positive dip on proximal stimulation

Question 19

How do differentiate a cervical root avulsion from a brachial plexopathy?

Answer 19

Sensory studies should be normal in a cervical root avulsion (only tests to the DRG)

Question 20

CMAP and conduction velocity in MFS?

Answer 20

normal, as the demyelination is occurring in specialized sensory fibers not tested on NCS

Question 21

you can have a significant drop in amplitude with antidromic sensory studies when you stimulate proximally and this is a normal response (I am not sure why)

Answer 21

just know

Question 22

When you have a sciatic neuropathy, how are tibial and common peroneal nerves affected on testing?

Answer 22

Both have dec amplitude, but common fib is more susceptible so will have worsened amplitude compared to tib

Question 23

Normal recruitment frequencies for a limb and cranial muscle are:

Answer 23

Limb: 7-10Hz
Cranial: 16 Hz

Question 24

Lactate and ammonia results during forearm exercising test

Answer 24

If LAC inc and ammonia doesnt: myoadenylate deaminase def
If LAC doesnt but ammonia does: glycogen storage disease/disorder of glycolysis
If both: normal
Neither: suboptimal effort

Question 25

Decrement with slow repetitive stimulation at 2-3 Hz can be seen in ?

Answer 25

NMJ disorders (botulism, MG, LEMS) paramyotonia congenita neomycin toxicity

Question 26

The duration of the CMAP is representative of

Answer 26

temporal dispersion: CMAP is the summation of the responses of from all axons with varying conduction velocities within the motor nerve.

Question 27

What is the gain for recording spontaneous activity?

Answer 27

50 uV

Question 28

Myopathic vs neurogenic EMG

Answer 28

Myopathic: short duration, small amp, early recruitment Neurogenic: long dur, large amp, dec recruitment

Question 29

What wave can occur between M waves and F waves?

Answer 29

A waves: due to the additional firing from reinnvervation fibers

Question 30

What insertional activity has a repetitive rhythm?

Answer 30

Fasics are irregular discharges PSWs, complex rep discharges, fibs, myokymia are regular discharges