Introduction to Nerve Conduction Studies

Introduction to Nerve Conduction Studies definition: analysis and interpretation of electrical signals produced by depolarization of excitable nerve and muscle cells - ANS clinical electrophysiology what terminology to use? - electrodiagnostic testing (EDx) - electroneuromyography (ENMG) - EMG/NCS - most common term people use - electrophysiologic testing - technically more correct term - ANS benefits of electrophysiologic testing: - correlating with physical exam & other studies - definitive diagnosis in nerve/muscle disorders - assist physician in diagnosis - localization of lesion - exactly where is the nerve injury or muscle disease? - determination of the physiological status of lesion - what is involved (myelin sheath, axon, motor fibers, sensory fibers, etc.)? - data for clinical/lab research - ANS neurogenic symptoms (more common indications for the need of this test): - pain - numbness - paresthesias - weakness - cramping - ANS differential diagnosis: - focal mononeuropathy (carpal tunnel syndrome) - cervical/lumbar radiculopathy - polyneuropathy - brachial plexopathy (brachial or lumbar plexus) - CNS disorder (exclusion) - clinical electrophysiologic testing is not a good test for CNS disorders, however, a patient with numbness, tingling, weakness may be referred to determine if this is CNS versus PNS problem - myopathy (muscular dystrophy, polymyositis) - any combination of above - ANS true or false: diagnosis should never be made just on the results of electrophysiologic test - ANS true - take a history and form a differential Dx - perform your clinical examination and begin to narrow the differential Dx - look at the data - base your diagnosis on your evaluation of data from the history, your clinical tests & measures, your impression of the data - evaluate electrophysiologic test in the same manner as a radiologic study - as a PT, most states do not allow you to make a diagnosis, so one has to be very careful in this regard - when you make your impression, be sure to make your impression in terms of pathophysiologic status of the neuromuscular problem based only on the EMG and nerve conduction tests - refer back to doctor for a medical diagnosis - ANS what are the two broad areas involved in electrophysiologic testing? - ANS - evoked potential recording (MNCS, SNCS, electronic reflex studies, centrally recorded evoked potentials) - EMG - resting and volitional potential recording a brief pulse of electrical current that stimulates a nerve which results in an evoked potential (motor or sensory) what part of a nerve conduction study? - ANS stimulus what are the salient features analyzed with nerve conduction studies? - ANS - amplitude - latency - duration - conduction velocity - waveform shape nerve conduction studies: - stimulus - recording of evoked potentials - analysis of salient features - ANS motor nerve conduction study (MNCS): - essentially we stimulate the nerve (eg, at the wrist) - this evokes a response (an action potential) in the nerve and the axons which is conducted along the nerve until it gets to the neuromuscular junction of the muscle (in this case, a muscle in the thenar eminence) that depolarizes the muscle - then the muscle contraction and that response is evoked - then we measure the features of the motor action potential - ANS what electrode serves as an electrical zero point? - ANS reference electrode upward deflection is what part of the waveform? - ANS negative onset latency is a parameter of what type of fiber? - ANS motor peak latency is a parameter of what type of fiber? - ANS sensory what type of latency is time between onset of stimulus and onset of response? - ANS onset latency (motor) what type of latency is time between onset of stimulus and peak of response? - ANS peak latency (sensory) what type of latency reflects the conduction of the induced impulse of the fastest and largest fibers? - ANS onset latency (motor) baseline-to-peak amplitude is a parameter of what type of fiber? - ANS motor peak-to-trough amplitude is a parameter of what type of fiber? - ANS sensory trough is typically after the peak, some instances before compound muscle or sensory nerve action potential (CMAP or SNAP) M/SNCS parameter? - ANS amplitude what M/SNCS parameter represents the total number of physiologically intact axons? - ANS amplitude - axon may be structurally intact but not working physiologically - for example, if amplitude is reduced by 50% then theoretically, about 50% of axons of a nerve may not be functioning true or false: typical CMAP amplitude is 2.5 mV (2,500 uV) up to 15-20 mV (15,000-20,000 uV) depending on nerve that is being tested - ANS true what M/SNCS parameter is the interval from first negative deflection to the return to baseline? - ANS duration - remember, negative is up - duration is typically described subjectively or qualitatively - duration is probably least important parameter what M/SNCS parameter is essentially a measure of time? - ANS latency how long does it take to get from the point of stimulus until response is measured in milliseconds what MNCS parameter is a measure of how fast the nerve is traveling in terms of meters per second? - ANS conduction velocity - latency is how long does it take you to drive to work (eg, 30 minutes) - conduction velocity is how fast you were driving or your rate of speed how is conduction velocity calculated? - ANS CV = distance / time = distance / (T2-T1) = distance / (prox. latency - dist. latency) proximal motor latency is the time it takes for the impulse to go from the proximal stimulation site to distal stimulation site and across the NMJ and make muscle contract true or false: distal motor latency standard distance is 8 cm - ANS true - this latency has normal values in people without pathology - proximal distance will vary considerably because of anatomical feature, so we measure true or false: cannot calculate NCV from distal stimulation site to recording electrodes - ANS true 1) terminal nerve starts to arborize (branch freely and repeatedly) 2) conduction slows as diameter decreases 3) distance of terminal branches not known 4) time for neuromuscular transmission is unknown 27 centimeters is equal to how many millimeters? - ANS 270 millimeters what is the purpose of sensory nerve conduction studies? - ANS to assess the sensory component of a mixed peripheral nerve - some nerves are sensory only (eg, superficial radial) mixed peripheral nerve - efferent-action-motor - afferent-feedback-sensory sensory nerve conduction studies measure what sensory component? - ANS touch - large axons--we are just measuring large axons - heavy myelin--no concentric rings - dermis of skin--receptor organ--dendrites true or false: sensory nerve conduction studies measure conduction speed of axons that carry pain information - ANS false; SNCS are NOT able to measure conduction speed of axons that carry pain information important to understand we are just measuring large axons SNCS examine what sensory modality? - ANS light touch true or false: SNCS record distal latency--no calculation of velocity - ANS true velocity is not commonly done for sensory conduction - uniform nerve diameter - low threshold to electrical stimulation - antidromic techniques physiologic considerations for what type of NCS? - ANS sensory conduction - uniform nerve diameter if were just talking about touch, but not as uniform as motor fibers - it does not take as much current to stimulate sensory axons compared to motor what is the gain/sensitivity for SNCS? - ANS 10 to 20 microvolts small evoked response amplitude for motor conduction is measured in? - ANS millivolts - normal motor amplitude is 2.5 mV to more than 20 mV - normal sensory amplitude is 5 uV to 100 uV amplitude for sensory conduction is measured in? - ANS microvolts gain/sensitivity is 10 to 20 microvolts true or false: sensory responses are much, much smaller compared to motor - ANS true can range from 5 uV to 100 uV average responses--sometimes useful if we have very small responses but not routinely done technical consideration for what type of conduction study? - ANS sensory conduction - sensory responses can be so small that sometimes just the electrical noise in the room may be just as big as the biologic response - evoked response is very small needs enhancement - replicate evoked response when needed amplitude is measured peak to peak or peak to trough for what type of conduction study? - ANS sensory lowest peak to highest peak latency is measured to the negative peak for what type of conduction study? - ANS sensory we do not use onset latency for sensory conduction - sweep speed-------------------- 1 or 2 millisecond - gain/sensitivity-------------------- 20 microvolts - high frequency-------------------- 3000 Hz - low frequency-------------------- 20 Hz - intensity level-------------------- low instrumentation requirements for what type of nerve conduction study? - ANS sensory - standard distance - SNAP, DSL - can calculate a velocity, but routinely don't what type of nerve conduction study? - ANS sensory nerve conduction - SNAP (sensory nerve action potential) usually refers to a SNAP amplitude or how high is the reponse - DSL (distal sensory latency) refers to the time it takes - most of the time just do a distal stimulation true or false: we can do proximal stim w/ sensory, but usually don't - ANS true - most of the time just do a distal stimulation - sensory axons are different in terms of conduction speed - as the distance between the stimulus and recording electrode becomes greater and greater, the waveform tends to spread out more and more (becomes smaller) - analogy--football team--100 yard dash versus one mile--players more spread out what type of nerve conduction study is criterion to differentiate preganglionic root avulsion and plexopathy? - ANS sensory nerve conduction impulse is traveling in the same direction physiologically of how are sensory system work what type of sensory conduction study? - ANS orthodromic - we are stimulating distally and recording proximally - sensory information goes from distal to proximal physiologically - ortho means "same" impulse is traveling the opposite direction physiologically of how are sensory system works what type of sensory conduction study? - ANS antidromic - antidromic studies usually get a better amplitude - we are stimulating proximally and recording distally true or false: amplitude response is much larger in motor than sensory - ANS true what type of nerve conduction abnormality results in reduced amplitude? - ANS axonal damage/dysfunction both when we stimulate distally and when we stimulate proximally what type of nerve conduction abnormality results in prolonged latency, decreased conduction velocity or increased temporal dispersion? - ANS demyelination - temporal dispersion - some axons may be more effected or more slow than others - myelin helps speed up nerve conduction velocity because we get node to node conduction - so if we lose myelin, the nerve conduction is slower true or false: most injuries are a combination of axonal loss and demyelination - ANS true - typically in a more mild type of nerve injury we tend to see focal demyelination first - if it progresses and gets worse, we see demyelination plus axonal loss true or false: rarely do we see axonal loss without demyelination, but there are some specific diseases such as ALS - ANS true in general, what effect would demyelination have on latency, conduction velocity or temporal dispersion? - ANS prolonged latency, decreased conduction velocity or increased temporal dispersion in general, what effect would axonal damage/dysfunction have on amplitude? - ANS reduced amplitude conduction block is identified by marked reduction of more than what percentage in CMAP amplitude across portion of the nerve? - ANS >20% in CMAP amplitude - temporary and potentially reversible--generally carries a good prognosis - intervention is removal of offending agent and protection of injured nerve (eg, tight ligament or muscle constricting nerve, trauma) - may lead to wallerian degeneration if no intervention provided--axon dies from the point of injury distal motor conduction studies: - the shape of the waveform should be consistent and the amplitude of the waveform should be consistent at all stimulation sites for that nerve (eg, median nerve at wrist, elbow, and axilla) - the amplitude should not drop any more than 20% for most nerves - ANS if onset-to-peak amplitude (O-P amp) at above elbow stimulation site (eg, ulnar nerve) has more than 20% drop, then the injury is likely where? - ANS between the above elbow stimulation site and the below elbow stimulation site identification of conduction block helps localize the injury what type of nerve conduction abnormality can cause conduction block or temporal dispersion? - ANS demyelination - typically there's a combination of both - waveform is more spread out (temporal dispersion) - conduction is getting through but slowly--it's hitting a speed bump instead of a roadblock prolonged latency with dispersed waveform is seen in what type of nerve conduction abnormality? - ANS demyelination - waveform is more spread out (temporal dispersion) - conduction is getting through but slowly--it's hitting a speed bump instead of a roadblock normal latency with decreased and dispersed CMAP is seen in what type of nerve conduction abnormality? - ANS axonal degeneration - waveform is smaller but typically the same shape--not temporally dispersed - we don't typically see slowing when it's purely axonal degeneration - we may seen slowing but it's not significant conduction failure distal to nerve section is seen in muscle how many days post-injury? - ANS 5-9 days post-injury ex. median nerve is lacerated at the elbow: - if we stimulate distal at the wrist, we may still get conduction for 5-9 days, because where nerve is injured at the elbow, it takes time for axons to degenerate from point of injury distal - if nerve was completely lacerated and we stimulated proximal to the elbow, even on the day of injury, it's not going to conduct across injured nerve conduction failure distal to nerve section is seen in neural action potential how many days post-injury? - ANS 7-11 days post-injury muscle activation failure distal to nerve section occurs how many days post-injury? - ANS 5-9 days neural action potential failure distal to nerve section occurs how many days post-injury? - ANS 7-11 days longer time for nerve itself to fail compared to muscle what electrophysiologic test is excellent for identifying demyelination? - ANS NCS what electrophysiologic test identifies entrapment neuropathies? - ANS NCS what electrophysiologic test is the only technique for sensory study? - ANS NCS what electrophysiologic test is +/- identifying axonal injury? - ANS NCS what electrophysiologic test has limited use for radiculopathies or myopathies? - ANS NCS what electrophysiologic test is not helpful for timing? - ANS NCS what electrophysiologic test is the gold standard to identify axonal injury? - ANS EMG what electrophysiologic test identifies radiculopathies and myopathies? - ANS EMG what electrophysiologic test is helpful for classifying acuteness? - ANS EMG what electrophysiologic test is not helpful to identify demyelination? - ANS EMG what electrophysiologic test only evaluates motor nerves or muscle? - ANS EMG does not evaluate sensory nerves electromyography records from intramuscular needle electrode under what four conditions? - ANS - with needle movement - insertional activity - at rest - spontaneous potentials - nothing should be happening in normal muscle at rest - with minimal voluntary contraction - motor unit size/shape - with contraction increasing to maximum - recruitment pitfalls with NCS/EMG - anatomic variations - reference values - temperature: cold hand = prolonged latency, decreased amplitude - excessive stimulus intensity - increased NCV, volume conduction to adjacent nerve - inadequate study: "R/O CTS" request conveys larger responsibility than answering the question - nerve conduction test is part of clinical exam, you can't make a diagnosis based on this test alone - examiner dependent (widely varied quality based on training/technique) - high specificity, moderate sensitivity - the more tests performed, the more chance for false positive (clinical correlation required) - routine EMG/NCS does not assess the CNS--centrally evoked potentials have only limited value for this - ANS cold hand has what effect on latency and/or amplitude? - ANS prolonged latency, increased amplitude true or false: routine EMG/NCS do not assess the CNS - ANS true centrally evoked potentials have only limited value for this - bleeding risk: patient with coagulopathy - typically still safe - infection risk: particularly immune compromised - pneumothorax or peritoneal insertion (certain sites) - discomfort precautions/contraindications of what electrophysiologic test? - ANS EMG - avoid stimulation near pacemakers - generally, can do with pacemaker but avoid proximal stimulation (eg, axilla area) - always use grounded equipment - never remove ground prong from plug - discomfort precautions/contraindications of what electrophysiologic test? - ANS NCS nerve conduction velocity is calculated by? - ANS distance divided by time MNCS latency is measured to what part of the waveform? - ANS onset SNCS latency is measured to what part of the waveform? - ANS negative peak how is MNCS amplitude measured on waveform? - ANS baseline to negative peak how is SNCS amplitude measured on waveform? - ANS peak to trough