NMJ and Myopathy EDX Review Flashcards
How would you expect the MEPP amplitudes and frequency to change in MG vs LEMS?
MG: Autoimmune disorder that affects post-synaptic ACh receptors.
MEPP –> Reduced amplitudes, but normal frequency (quanta still released normally into cleft, just no receptor for them)
LEMS: Presynaptic disorder of voltage-gated Ca channels –> No quantal release –> Normal MEPP amplitudes but reduced frequency
What would you expect to find on SNAP/CMAP studies in MG and LEMS?
SNAP: Typically normal
CMAP: Normal or decreased amplitude. If low, this should be followed by a 10 second maximum voluntary contraction, followed by a single stimulation –> If amplitude > 100% compared to pre maximum contraction –> Think LEMS
What would you expect to find on needle EMG studies in MG and LEMS?
Can be normal or abnormal:
In both MG and LEMS
In severe cases: Abnormal spontaneous activity. Short-duration, low-amplitude MUAPs with early recruitment are associated with blocking. –>
- MUAPs become unstable (variable amplitudes and configurations);
- drop off occurs with sustained contraction
- > 10% decrement on low rate repetitive stimulation
When performing EDX evaluation on a patient with a suspected NMJ disorder, you perform low-rate RNS. What would indicate pathology on the RNS study?
A > 10% decrease in amplitude from the 1st - 5th waveform is significant for pathology.
RNS is performed at 2-3 Hz. Each stimulus causes EPP amplitude to drop. If the safety factor is decreased, the potential will fall below the threshold necessary for activation –> resulting in decrease of MUAP amplitude
You perform low-rate RNS study on a patient with a suspected NMJ disorder. You notice >10% decrement of CMAP amplitudes.
What is the next step?
A 30-60 second isometric contraction or tetany-producing stimulation (50 Hz) should be performed. This is called Postactive Facilitation (PAF). PAF demonstrates a repair in the CMAP amplitude with an immediate follow up LR-RNS because of an improvement in NMJ transmission
What is Postactive Facilitation?
Performed after abnormal LR-RNS study.
30-60 second isometric contraction or tetany-producing stimulation (50 Hz) should be performed. This is called Postactive Facilitation (PAF). PAF demonstrates a repair in the CMAP amplitude with an immediate follow up LR-RNS because of an improvement in NMJ transmission
What is Postactivation Exhaustion (PAE)?
This response is seen as a CMAP amplitude decrease. It occurs with a LRRS performed every minute for 5 minutes after an initial 30- to 60- second isometric contraction. The greatest drop off is between 2 and 4 minutes. This test should be used if a decrement not present with the initial LRRS, but a diagnosis of a NMJ disorder is suspected.
When is High-rate repetitive stimulation used? What would expect to see with HRRS in MG vs LEMS vs Botulinum?
Repetitive stim (10-50 Hz). This causes accumulation of calcium in the cell, which assists ACH release and repairs the waveforms.
HRRS is uncomfortable and is performed if a patient is unable to perform a 30- 60-sec max isometric contraction (ie unable to do Postactivation facilitation)
Steroid myopathy causes which of the following muscle abnormalities?
Steroid myopathy causes atrophy of type II muscle fibers (fast-twitch, glycolytic). Thus, EMG is largely normal in these patients, as only with maximal effort does the patient recruit type II muscle fibers, and at that point, the EMG screen is flooded with motor unit action potentials (MUAPs), rendering analysis of these MUAPs quite difficult.
What does a polyphasic potential represent during an EMG?
Active reinnervation
What is the main problem with performing the Phalen test with the elbows flexed instead of extended?
When performing the Phalen test (flexing the wrists, holding this posture, and eliciting hand numbness/tingling due to median nerve compression within the carpal tunnel), it is best performed with the patient’s elbows extended. Flexing the elbows stretches the ulnar nerve and may irritate an already irritated ulnar nerve, thus causing hand numbness and tingling NOT due to median nerve compression.
You ask your patient to abduct and externally rotate their shoulders to 90 degrees and then begin opening and closing their hands over a 3-minute period. This test is known by which name, and evaluates which condition?
This question describes the Roos test, which assesses neurogenic thoracic outlet syndrome, which itself is a lower trunk plexopathy. A positive test is indicated by reproduction of the patient’s symptoms in the upper limb (pain, numbness, tingling in a C8-T1 pattern).
When a muscle recruits its first motor unit during initial contraction, it tends to fire at _____ Hz, and subsequent motor units are recruited for every ______ Hz increase in firing rate.
(5,5)
The initial motor unit is recruited at 5 Hz; every 5 Hz an additional motor unit is added to the recruitment pattern. Thus, your EMG screen should at some point show you 4 motor units firing at once with gradually increased muscle contraction by the patient: these units will be firing at 20, 15, 10, 5 Hz respectively.
How can you differentiate between neuropraxia vs axonotmesis vs neurotmesis on EMG?
Neurapraxia can be differentiated from axonotmesis (axon loss due to crush/stretch injury) and neurotmesis (axon loss due to complete nerve transection) due to the normal CMAPs present below the level of injury at 2 weeks out from injury.
There is also the lack of active denervation (fibrillations and positive sharp waves) that suggests neurapraxia rather than axon loss.
Neurapraxia, you recall, manifests as a conduction block, as in this case (normal amplitude CMAP distal to the lesion, decreased amplitude CMAP proximal to the lesion).
Neurapraxia/conduction block represents a focal demyelination event; thus the axons are intact, and all that is required is remyelination for the CMAP, recruitment, and patient’s strength to recover.
If this were axon loss, the CMAP amplitude would also be decreased distally at this point (2 weeks; recall that within ~10 days Wallerian degeneration is complete for both sensory and motor fibers); thus, the intact distal CMAP suggests the axons are alive and healthy, and just awaiting remyelination; thus, this patient’s prognosis for spontaneous recovery is good.
Compression of what nerve can occur at the Arcade of Frohse?
PIN
