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30 Cards in this Set
- Front
- Back
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What voltage range are motor studies in? Sensory?
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millivolts. Microvolts.
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What part of the stimulator is placed closest to the active electrode?
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The cathode.
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What is the compound muscle action potential?
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The summation of all underlying muscle fiber action potentials.
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How much supramaximal stimulation should you go?
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20%
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What three processes does CMAP latency represent?
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conduction velocity along the nerve, NMJ transmission, muscle depolarization.
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When is CMAP duration usually present (pathologically)?
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In demyelinating lesions, due to loss of synchony.
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What is the typical amplitude range of sensory studies (SNAPs)?
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1-50 microA
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What are some of the advantages of measuring a SNAP peak latency? What's the deal with this and CV?
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It's less affected by artifact and noise. It doesn't represent the fastest fibers thus cannot be used to find CV.
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What are the typical durations of CMAPs and SNAPs?
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1.5 ms, 5-6 ms
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What is the main advantage of antidromic NCS?
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Usually, when done, the antidromic recording electrodes are closer to the nerve.
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What is the main disadvantage with antidromic SNAPs?
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Sometimes the CMAP is volume conducted along with it, causing a larger negative deflection to follow (and sometimes overlap).
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If you are unsure if you're looking at a CMAP or SNAP, what is the best way to differentiate?
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Look at the duration (SNAP 1.5 ms, CMAP 6 ms).
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Re: CMAPs, SNAPs, and MUAPs (EMG), what will a lesion proximal to the DRG frequently show?
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Decreased amplitude on CMAPs, signs of denervation on EMG, normal SNAPs.
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Describe the differing effects of phase cancellation and temporal dispersion with sensory and motor NCS.
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Little effect on motor b/t distal and proximal stim. With sensory, increased duration, decreased area and amplitude.
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Describe temporal dispersion and phase cancellation
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Basically the same thing: change in the summated waveform over distance when there is a variability in conduction velocity among the components of the SNAP. Phase cancellation occurs with temporal dispersion when the positive deflection of a wave subtracts from the total area of the waveform.
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Why is there not much temporal dispersion with motor NCS?
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Pretty much all MUAPs conduct at the same speed, with the same duration.
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What are the three most common mixed nerve conduction studies?
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Median nerve at the wrist, ulnar at the elbow, tibial at the tarsal tunnel.
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What types of nerves are recorded with mixed nerve action potentials?
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type Ia.
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What is the typical NCS pattern seen with axon loss?
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Decreased amplitude with preserved CV and DL.
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When does mild slowing occur with pure axonal loss? Major slowing?
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with axonal loss of the fastest axons. Significant slowing should not occur.
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What's the cutoff of CV and DL drop for purely axonal lesions with NCS?
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never less than 75% of the lower limit of normal for CV and never more than 130% of the upper limit of normal for DL.
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Timewise, how is Wallerian degeneration different for motor versus sensory nerves?
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motor degrades faster (3-5 days) than sensory (6-10)
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What conduction velocities denote unequivocal demyelination?
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Less than 35 m/s in the UEs or 30 m/s in the LEs.
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What is the rare exception when markedly slow CVs don't denote demyelination?
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Nerve regrowth/resprouting following transection.
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What class of pathology does conduction block most typically occur?
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With acquired demyelination.
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What is the CMAP amplitude drop cutoff usually used to denote conduction block?
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20% drop (especially when associated with slowing)
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Why is the presence of demyelination with polyneuropathy a significant finding?
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Because few PNs are demyelinating.
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With demyelinating polyneuropathies, when would you see a diffuse pattern versus a focal pattern?
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Diffuse pattern typically seen more with inherited pathologies (e.g. CMT type 1), focal pattern seen with acquired (AIDP, CIDP).
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What do CMAPs look like with myopathies?
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Typically normal, but sometimes distal CMAP amplitudes may be low (rare myopathies and critical illness myopathy).
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How does the CMAP differ in presynaptic and postsynaptic NMJ disorders?
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Normal in postsynaptic. In presynaptic, amplitude is low at rest and is improved with RNS.
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