Neuromuscular Disease Symposium: Localization and DDx

Question 1

How can electromyography (EMG) show the difference between axonal and demyelinating neuropathy?

Answer 1

Demyelinating: slower conduction speed
Axonal: lower amplitude of the compound muscle action potential (CMAP)

Question 2

How does one control for speed of transmission through the neuromuscular junction in EMG?

Answer 2

Record transmission time at two different positions, proximal and distal. Distance between sites / time difference = conduction velocity.

Question 3

How do a uniform demyelinating neuropathy and a focal demyelinating neuropathy look different on an EMG?

Answer 3

Uniform won’t have a change in CMAP amplitude between the proximal and distal sites, but a focal demyelinating neuropathy will (and the wave will be segmented - some signals arrive faster than others). See slide 17.

Question 4

What are 3 genes in which mutation can cause demyelinating neuropathy?

Answer 4

PMP22, Myelin P Zero, Connexin 32

Question 5

What do diseases of anterior horn cells tend to spare? (this threw us off twice in the ARS questions)

Answer 5

Anterior horn lesions tend to spare the face / eyes. (which kind of makes sense, given they’re from cranial nerves, not spinal nerves)

Question 6

Review: In needle EMG, what does a smaller than normal response tell you? What do bigger peaks tell you?

Answer 6

Smaller: loss of muscle cells
Bigger: loss of motor neurons has cause other neurons to take over via sprouting -> larger motor units

Question 7

Will repetitive nerve stimulation studies show a decrement due to muscle fatigue? (a good question from the audience)

Answer 7

Nope.

Question 8

In general, decrements in repetitive stimulation mean what? How about increments?

Answer 8

Decrements: Problem is post-synaptic
Increment: Problem is pre-synaptic

Question 9

What’s the treatment for symptoms myasthenia gravis? MoA?

Answer 9

-stigmine drugs (such as pyridostigmine). Cholinesterase inhibitors.

Question 10

What’s the treatment for the underlying cause of myasthenia gravis?

Answer 10

Immunosuppresion: prednisone, azathioprine (inhibits purine synth)

Question 11

Does hypotonia always mean weakness?

Answer 11

No.