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Flashcards in NMJ Disorders Deck (15)
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1
Q

Describe the firing of action potentials during normal muscle contraction at LMNs.

A

AP reaches the motor neuron terminal -> Ca2+ influx facilitates presynaptic ACh release from vesicles -> ACh binds at nicotinic receptors in the end plate of the post-synaptoc muscle membrane -> activates ion channels and creates an end plate potential (EPP) -> contraction

2
Q

What limits the effect of ACh and EPP?

A

Synaptic acetylcholinesterase, which breaks down ACh

3
Q

What happens in MG?

A

AChR degraded or blocked, less ACh is able to bind, produces EPPs which are either initially below the threshold for generating APs or which become too low during repeated contractions; occurs at the post-synaptic NMJ

4
Q

What occurs in LEMS?

A

Voltage-gated calcium channel at the pre-synaptic membrane is targeted

5
Q

Presentation of MG?

A

Any age (childhood to late adult life)
Only motor symptoms -> weakness and fatigue of skeletal muscles during non-strenuous activity (chewing, climb stairs, etc.)
Most commonly: ptosis, diplopia, dysarthria, dysphagia, some with fatiguability

6
Q

What is ocular myasthenia?

A

Patients who have only visual symptoms (ptosis, diplopia)

7
Q

What is neonatal myasthenia?

A

Healthy newborns of mothers with MG who have symptoms for a few days before maternal antibodies “wash out”

8
Q

What is a myasthenic crisis?

A

Profound weakness that may cause quadriplegia and an inability to speak, walk, or breath; may be triggered by a serious infection or other systemic illness; may be unpredictable

9
Q

DDx - acute paralysis of speech, chewing, swallowing, limb, and respiratory muscles, as in MG

A

Extensive brain stem infarction (hyperreflexia, MRI confirms)
GBS (areflexia and sensory impairment with weakness, EMG confirms, expect elevated CSF protein)
Spinal cord lesion (spares CNs, localizing sensory level, may have neck or back pain, MRI)

10
Q

Dx MG?

A

Edrophonium test (AChE inhibitor) -> marked improvement of weakness

EMG -> repeated nerve stimulation shows NMJ abnormalities

Most specific test: presence of serum AChR Ab

11
Q

Rx MG?

A

Oral anticholinesterase drugs (pryidostigmine)

Some may need immunosupression (steroids, azathioprine, MMF, cyclosporine)

Thymectomy in some

Myasthetnic crisis - IVIg or plasmapheresis

12
Q

Presentation of LEMS?

A

Proximal muscle fatigue and weakness (shoulders, hips, trunk)

Do not typically involve the eyes, swallowing, or speech

Possible decreased reflexes that improve after a brief period of isometric exercise

May have autonomic symptoms (dry mouth, orthostatic hypotension, ED)

13
Q

Common cause of LEMS?

A

Small cell carcinoma

14
Q

Dx LEMS?

A

EMG -> pre-synaptic NMJ abnormalities

Ab to the voltage-gated Ca2+ channel

Search for small cell carcinoma

15
Q

Rx LEMS?

A

Search for small cell lung cancer and treat

Guanidine or 3,4-diaminopyridine (enhance ACh release)