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R.NCS.T practice questions

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Question 1Nerve Conduction Studies

Which nerve is most commonly studied to obtain a sensory nerve action potential in the upper limb for routine screening?

  1. Axillary nerve
  2. Median sensory nerve (index finger) Correct answer.
  3. Radial motor nerve
  4. Phrenic nerve

Explanation

Sensory studies require a nerve with accessible cutaneous fibers; the median sensory branch to the index finger is a standard, reliable screening site. Motor and proximal plexus nerves such as the axillary or phrenic are not routine sensory screening targets.

Question 2Nerve Conduction Studies

In a standard motor nerve conduction study, the active recording electrode is placed over which structure?

  1. The tendon of the muscle
  2. The distal interphalangeal joint
  3. The motor point of the muscle Correct answer.
  4. The skin over the antagonist muscle

Explanation

The active (G1) electrode is placed over the motor point, the region where the nerve enters the muscle, to record the largest compound muscle action potential. Reference electrodes are placed distally over tendon.

Question 3Nerve Conduction Studies

What does the term 'supramaximal stimulation' mean in nerve conduction studies?

  1. Stimulus intensity that elicits a muscle twitch
  2. Stimulus intensity that produces a maximal sensory response only
  3. Stimulus intensity that is just above sensory threshold
  4. Stimulus intensity increased until the response no longer grows Correct answer.

Explanation

Supramaximal stimulation means increasing current until the compound response amplitude plateaus, ensuring all fibers are activated. This is required for reproducible amplitude and latency measurements.

Question 4Nerve Conduction Studies

Which measurement represents the time from stimulus onset to the initial deflection of the compound muscle action potential?

  1. Duration
  2. Amplitude
  3. Conduction velocity
  4. Distal latency Correct answer.

Explanation

Distal latency is measured from the stimulus artifact to the onset of the evoked response. Conduction velocity is derived from latency differences between sites, while amplitude and duration describe waveform size and shape.

Question 5Nerve Conduction Studies

How is motor nerve conduction velocity calculated between two stimulation sites?

  1. Amplitude divided by distance
  2. Latency difference multiplied by distance
  3. Distance divided by the distal latency
  4. Distance divided by the difference in latency between sites Correct answer.

Explanation

Motor conduction velocity uses the distance between stimulation sites divided by the difference in latencies, which excludes the neuromuscular junction and distal conduction time. This yields velocity in meters per second after unit conversion.

Question 6Nerve Conduction Studies

Which finding is characteristic of a demyelinating neuropathy on nerve conduction studies?

  1. Marked reduction in conduction velocity with relatively preserved amplitude Correct answer.
  2. Normal velocity with reduced recruitment on needle EMG
  3. Prolonged distal latency with normal velocity
  4. Marked reduction in amplitude with normal conduction velocity

Explanation

Demyelination slows conduction, producing markedly reduced velocity, prolonged latencies, and conduction block with relative preservation of amplitude distal to the lesion. Axonal loss instead reduces amplitude.

Question 7Nerve Conduction Studies

What is the primary purpose of performing F-wave studies?

  1. To evaluate autonomic function
  2. To assess proximal motor nerve conduction Correct answer.
  3. To measure neuromuscular junction transmission
  4. To assess sensory nerve integrity

Explanation

F-waves result from antidromic activation of motor neurons and provide information about proximal segments of the motor nerve and root. They are especially useful when distal studies are normal but proximal pathology is suspected.

Question 8Nerve Conduction Studies

A prolonged F-wave minimum latency with normal distal motor studies most likely suggests involvement of which region?

  1. Distal nerve segment
  2. Muscle membrane
  3. Proximal nerve segment or root Correct answer.
  4. Neuromuscular junction

Explanation

Because the F-wave travels to the spinal cord and back, prolonged latency with normal distal studies localizes the abnormality to the proximal nerve or root. This pattern is seen in radiculopathy and proximal demyelination.

Question 9Nerve Conduction Studies

What does an absent H-reflex in the soleus indicate when the tibial motor response is normal?

  1. A lesion in the peroneal nerve
  2. A lesion in the S1 reflex arc Correct answer.
  3. A myopathic process
  4. A normal variant in all adults

Explanation

The H-reflex evaluates the afferent and efferent limbs of the S1 reflex arc. An absent response with a normal motor study suggests pathology within that arc, such as an S1 radiculopathy.

Question 10Nerve Conduction Studies

Which factor most affects the amplitude of a sensory nerve action potential?

  1. Stimulus duration
  2. Number of functioning sensory axons Correct answer.
  3. Skin temperature
  4. Distance between recording electrodes

Explanation

Sensory amplitude reflects the number of viable sensory axons contributing to the response. Temperature and technical factors influence latency and waveform but amplitude chiefly tracks axonal population.

Question 11Nerve Conduction Studies

Cooling of the limb during nerve conduction studies typically produces which change?

  1. No effect on any parameter
  2. Decreased latency and increased velocity
  3. Increased latency and decreased velocity Correct answer.
  4. Increased amplitude and decreased duration

Explanation

Lower temperature slows ion channel kinetics and nerve conduction, prolonging latency and reducing velocity. Warming the limb to near 32°C is standard to avoid misinterpretation.

Question 12Nerve Conduction Studies

What is temporal dispersion on a motor nerve conduction study?

  1. Loss of amplitude with increased duration of the compound muscle action potential Correct answer.
  2. A stable waveform across all stimulation sites
  3. Complete loss of the response
  4. Shortening of the waveform with increased amplitude

Explanation

Temporal dispersion occurs when fibers conduct at different velocities, spreading out the compound muscle action potential and reducing amplitude while increasing duration. It is distinct from conduction block.

Question 13Nerve Conduction Studies

How is conduction block distinguished from temporal dispersion on a motor study?

  1. Temporal dispersion shows no amplitude change
  2. Conduction block shows amplitude drop without proportional duration increase Correct answer.
  3. Conduction block shows proportional amplitude drop with duration increase
  4. Conduction block occurs only in sensory nerves

Explanation

Conduction block reduces amplitude without proportional widening because fibers are blocked rather than desynchronized. Temporal dispersion reduces amplitude with a proportional increase in duration.

Question 14Nerve Conduction Studies

Which nerve conduction finding is expected in a severe axonal neuropathy?

  1. Reduced compound muscle action potential amplitude with mildly reduced velocity Correct answer.
  2. Prolonged distal latency with normal velocity
  3. Normal amplitude with conduction block
  4. Markedly reduced velocity with normal amplitude

Explanation

Axonal loss reduces the number of excitable fibers, lowering amplitude, while surviving fibers conduct at near-normal velocity. Velocity may fall mildly due to loss of fast fibers.

Question 15Nerve Conduction Studies

What is the purpose of averaging in sensory nerve conduction studies?

  1. To increase the stimulus intensity
  2. To eliminate the need for supramaximal stimulation
  3. To shorten the latency measurement
  4. To improve the signal-to-noise ratio of small responses Correct answer.

Explanation

Averaging multiple responses cancels random noise and enhances small sensory potentials. It does not alter the underlying physiology or replace proper stimulation technique.

Question 16Nerve Conduction Studies

Which electrode serves as the reference in a standard sensory nerve conduction study?

  1. The stimulator cathode
  2. The active electrode over the nerve
  3. The ground electrode
  4. The electrode placed distally over the digit Correct answer.

Explanation

Sensory studies typically use a distal reference electrode, often over the digit, paired with an active electrode over the nerve. The ground electrode reduces artifact but is not the reference.

Question 17Nerve Conduction Studies

Which parameter is used to calculate sensory nerve conduction velocity?

  1. Stimulus duration and intensity
  2. Area under the waveform
  3. Peak latency and distance between stimulation and recording sites Correct answer.
  4. Amplitude and duration

Explanation

Sensory conduction velocity uses the distance between the stimulation and recording sites divided by the onset or peak latency. Amplitude and duration do not determine velocity.

Question 18Nerve Conduction Studies

In a motor study, what does a reduced compound muscle action potential amplitude with normal distal latency suggest?

  1. A proximal conduction block only
  2. A normal variant requiring no interpretation
  3. Demyelination of the distal nerve
  4. Loss of motor axons or muscle fibers Correct answer.

Explanation

Amplitude reflects the number of excitable motor units, so a reduction with normal timing suggests axonal loss or a myopathic process. Demyelination would prolong latency or slow velocity.

Question 19Nerve Conduction Studies

Which nerve is assessed when recording the compound muscle action potential from the abductor pollicis brevis?

  1. Ulnar nerve
  2. Median nerve Correct answer.
  3. Musculocutaneous nerve
  4. Radial nerve

Explanation

The abductor pollicis brevis is innervated by the recurrent branch of the median nerve, making it a standard recording site for median motor studies. Ulnar studies use the abductor digiti minimi.

Question 20Nerve Conduction Studies

A patient has normal median sensory studies but prolonged median distal motor latency to the abductor pollicis brevis. Which condition is most consistent with this pattern?

  1. Cervical radiculopathy at C6
  2. Ulnar neuropathy at the elbow
  3. Carpal tunnel syndrome affecting motor fibers Correct answer.
  4. Myasthenia gravis

Explanation

Prolonged distal motor latency across the carpal tunnel with normal sensory studies suggests motor-predominant median nerve compression. Carpal tunnel syndrome can affect motor fibers with or without sensory changes.

Question 21Nerve Conduction Studies

What is the typical effect of limb warming on sensory nerve action potential amplitude?

  1. Amplitude is unaffected by temperature
  2. Amplitude decreases with warming
  3. Amplitude increases slightly with warming Correct answer.
  4. Amplitude becomes unmeasurable with warming

Explanation

Warming to physiologic temperature optimizes ion channel function and can slightly increase sensory amplitude while shortening latency. Cold limbs produce lower amplitude and prolonged latency.

Question 22Nerve Conduction Studies

Which finding indicates a conduction block across a segment during motor nerve conduction studies?

  1. A prolonged distal latency with normal proximal amplitude
  2. A fall in amplitude with proportional increase in duration
  3. A rise in amplitude with proximal stimulation
  4. A fall in amplitude of more than 50% between proximal and distal stimulation with little change in duration Correct answer.

Explanation

Conduction block is defined by a significant amplitude drop between stimulation sites without proportional temporal dispersion. A proportional amplitude drop with duration increase suggests dispersion instead.

Question 23Nerve Conduction Studies

Which structure is stimulated when performing an H-reflex study of the soleus?

  1. The tibial nerve in the popliteal fossa Correct answer.
  2. The peroneal nerve at the fibular head
  3. The sural nerve at the ankle
  4. The femoral nerve in the groin

Explanation

The soleus H-reflex is elicited by stimulating the tibial nerve, which activates Ia afferents that synapse on alpha motor neurons. Recording is from the soleus muscle.

Question 24Nerve Conduction Studies

What is the primary difference between the F-wave and the H-reflex?

  1. Both are purely sensory responses
  2. The F-wave requires no stimulation
  3. The H-reflex is a motor response, while the F-wave is a sensory response
  4. The F-wave is a motor response from antidromic activation, while the H-reflex is a monosynaptic reflex Correct answer.

Explanation

F-waves arise from antidromic activation of motor axons and are not true reflexes, whereas the H-reflex is a monosynaptic stretch reflex arc. This distinction affects how each is interpreted.

Question 25Nerve Conduction Studies

Which technical factor can falsely prolong a measured distal latency?

  1. Supramaximal stimulation
  2. Correct electrode placement over the motor point
  3. Adequate limb warming
  4. Excessive stimulus artifact obscuring the response onset Correct answer.

Explanation

A large stimulus artifact can obscure the true onset of the response, leading to an erroneously prolonged latency. Careful electrode placement and artifact reduction are essential for accurate measurement.

Question 26Nerve Conduction Studies

Which measurement on a motor nerve conduction study is used to calculate conduction velocity between two stimulation sites?

  1. The distance between the active and reference recording electrodes divided by the distal latency
  2. The amplitude ratio between proximal and distal responses divided by the segment length
  3. The distance between the cathodal stimulation points divided by the latency difference Correct answer.
  4. The total duration of the negative peak divided by the interstimulus interval

Explanation

Motor conduction velocity across a segment is derived from the distance between the two stimulation sites divided by the difference in onset latencies of the two responses. This cancels out the distal latency and neuromuscular junction time, isolating the segment under study. Amplitude and duration measures do not enter the velocity calculation.

Question 27Nerve Conduction Studies

During a routine motor nerve conduction study, supramaximal stimulation is achieved when which condition is met?

  1. The stimulus intensity is set to the patient's sensory threshold
  2. The stimulus duration is doubled while intensity is held constant
  3. The stimulus intensity is increased until the CMAP amplitude no longer increases Correct answer.
  4. The stimulus intensity is reduced until an F-wave first appears

Explanation

Supramaximal stimulation means all motor axons in the nerve are depolarized, which is confirmed when further increases in stimulus intensity produce no additional rise in CMAP amplitude. This ensures amplitude and latency measurements reflect the whole nerve rather than a fraction of it. Submaximal responses underestimate amplitude and can distort latency comparisons.

Question 28Nerve Conduction Studies

What is the standard landmark for stimulating the median nerve at the wrist during a routine motor study?

  1. At the midpoint of the dorsal forearm
  2. Over the ulnar artery at the volar wrist crease
  3. Two centimeters proximal to the medial epicondyle
  4. Midway between the palmaris longus and flexor carpi radialis tendons Correct answer.

Explanation

The median nerve at the wrist is stimulated between the tendons of palmaris longus and flexor carpi radialis at the distal wrist crease. This position overlies the nerve as it passes superficial to the flexor retinaculum. The other landmarks correspond to different nerves or segments.

Question 29Nerve Conduction Studies

Which recording site is standard for the motor study of the ulnar nerve?

  1. Extensor digitorum communis
  2. Abductor pollicis brevis
  3. Abductor digiti minimi Correct answer.
  4. First dorsal interosseous of the foot

Explanation

The ulnar nerve motor response is conventionally recorded over the abductor digiti minimi, which is a reliable distal ulnar-innervated muscle. The abductor pollicis brevis is used for the median nerve. Recording from the wrong muscle confuses which nerve is being assessed.

Question 30Nerve Conduction Studies

A technologist measures a motor latency to the onset of the first negative deflection of the CMAP. What is the rationale for using onset rather than peak?

  1. Onset reflects the fastest conducting motor fibers and is less affected by waveform dispersion Correct answer.
  2. Peak latency is unaffected by amplifier settings and is preferred for velocity
  3. Onset latency varies with stimulus duration and is easier to reproduce
  4. Onset reflects the slowest fibers and therefore best represents the whole nerve

Explanation

Onset latency marks arrival of the fastest conducting fibers at the recording electrode, so it is relatively stable even when the waveform disperses. Peak latency depends on the shape and synchrony of the whole response and shifts with dispersion. Using onset allows consistent velocity calculation across segments.

Question 31Nerve Conduction Studies

A motor study shows a normal distal CMAP amplitude but a 60% drop in amplitude when stimulating proximally, with only mild waveform widening. What is the most appropriate interpretation?

  1. An axonal loss affecting the distal nerve only
  2. Temporal dispersion from unequal fiber conduction
  3. Conduction block in the segment between the stimulation sites Correct answer.
  4. A normal variant caused by deep proximal stimulation

Explanation

A large amplitude drop between distal and proximal stimulation with minimal widening indicates that many axons fail to conduct through the segment, which is conduction block. Temporal dispersion would produce more prominent waveform widening and duration increase. Amplitude preservation distally confirms the axons are intact below the lesion.

Question 32Nerve Conduction Studies

Which nerve conduction abnormality best localizes a lesion to the dorsal root ganglion rather than the peripheral nerve?

  1. An absent sensory nerve action potential with normal motor studies
  2. A normal sensory nerve action potential with an abnormal somatosensory evoked potential Correct answer.
  3. A reduced CMAP amplitude with normal sensory studies
  4. A prolonged F-wave latency with normal distal latency

Explanation

Lesions proximal to the dorsal root ganglion spare the peripheral sensory axon, so the SNAP remains normal while central conduction (SEP) is delayed. A lesion of the peripheral nerve itself abolishes or reduces the SNAP. This distinction helps separate radiculopathy from plexopathy or neuropathy.

Question 33Nerve Conduction Studies

Which factor most directly determines the latency of the H-reflex recorded from the soleus?

  1. The number of motor units in the soleus muscle
  2. The thickness of the subcutaneous tissue over the tibial nerve
  3. The distance between the active and reference recording electrodes
  4. Conduction time along the afferent and efferent limbs of the reflex arc Correct answer.

Explanation

H-reflex latency equals the time for the Ia afferent volley to reach the spinal cord plus the efferent conduction time back to the muscle. It therefore reflects the conduction properties of the whole reflex arc. Muscle bulk and electrode spacing influence amplitude rather than latency.

Question 34Nerve Conduction Studies

When performing a sensory study, what effect does increasing the interelectrode distance between the active and reference recording electrodes have on the recorded SNAP?

  1. It shortens the onset latency of the response
  2. It increases the amplitude of the response Correct answer.
  3. It decreases the amplitude of the response
  4. It eliminates the need for signal averaging

Explanation

A larger spacing between active and reference electrodes captures a greater potential difference along the nerve, which increases SNAP amplitude within limits. Excessive spacing can introduce noise or pick up distant activity, so standard distances are used. Latency is determined by stimulation-to-recording distance, not electrode spacing.

Question 35Nerve Conduction Studies

A patient's median motor study shows a normal distal latency and normal CMAP amplitude, but the F-wave is absent from all ten trials while ulnar F-waves are present. What is the most likely explanation?

  1. Failure of recurrent motor axon firing in the median nerve Correct answer.
  2. A lesion of the dorsal root ganglion at C7
  3. A technical error caused by excessive averaging
  4. A sensory neuropathy affecting the median nerve only

Explanation

Absent F-waves with otherwise normal median motor studies suggest that the recurrent discharges are not being generated or recorded, which can occur with proximal conduction failure or technical factors such as submaximal stimulation. F-waves test the proximal motor segment, so an isolated absence warrants attention. The other options do not fit a preserved distal motor response.

Question 36Nerve Conduction Studies

Which change in the CMAP is most consistent with a demyelinating process affecting a proximal segment of a motor nerve?

  1. Decreased proximal amplitude with relatively preserved distal amplitude Correct answer.
  2. Increased amplitude of the proximal response
  3. Uniform reduction of amplitude at all stimulation sites
  4. Prolonged distal latency with normal proximal amplitude

Explanation

Demyelination in a proximal segment causes conduction block or dispersion that lowers the proximally evoked amplitude while the distal response remains relatively intact. Uniform amplitude reduction at all sites suggests axonal loss. Increased proximal amplitude is not a recognized pattern.

Question 37Nerve Conduction Studies

During a study of the tibial nerve, the CMAP recorded from the abductor hallucis is unexpectedly small. Which technical factor is most likely to explain this?

  1. Submaximal stimulation at the ankle Correct answer.
  2. Incorrect filter settings set too wide
  3. Use of a supramaximal stimulus
  4. Excessive averaging of the response

Explanation

A small CMAP often results from submaximal stimulation, which activates only part of the motor axon pool. Confirming supramaximal intensity by observing a plateau in amplitude corrects this. Filter settings and averaging affect waveform clarity rather than the fundamental size of a well-stimulated response.

Question 38Nerve Conduction Studies

A nerve conduction study shows a markedly prolonged distal motor latency with a normal CMAP amplitude and normal proximal conduction velocity. Which interpretation is most consistent with these findings?

  1. A sensory ganglionopathy
  2. Axonal loss affecting the distal nerve
  3. A proximal radicular lesion
  4. Focal demyelination in the distal nerve segment Correct answer.

Explanation

A prolonged distal latency with preserved amplitude and normal proximal velocity points to focal demyelination in the distal segment, as seen in entrapment at the wrist. Axonal loss would reduce amplitude, and proximal lesions would not prolong distal latency. Preserved amplitude argues against significant axonal degeneration.

Question 39Nerve Conduction Studies

Which combination of findings best supports a diagnosis of a sensorimotor demyelinating polyneuropathy rather than an axonal polyneuropathy?

  1. Absent sensory responses with normal motor studies
  2. Prolonged distal latencies, slowed conduction velocities, and conduction block with relatively preserved amplitudes Correct answer.
  3. Reduced SNAP and CMAP amplitudes with normal velocities and latencies
  4. Normal conduction velocities with reduced CMAP amplitudes only

Explanation

Demyelinating polyneuropathy is characterized by slowing, prolonged latencies, and conduction block, often with relatively preserved amplitudes until secondary axonal loss occurs. Axonal polyneuropathy produces amplitude reduction with relatively normal velocities and latencies. The pattern of slowing and block is the key discriminator.

Question 40Nerve Conduction Studies

A patient has a normal median sensory study but an absent ulnar sensory response, with normal ulnar motor studies. Which anatomic explanation best accounts for this pattern?

  1. A lesion of the ulnar motor fibers proximal to the anterior horn
  2. A median nerve entrapment at the wrist
  3. A generalized axonal polyneuropathy
  4. A lesion of the ulnar sensory fibers distal to the dorsal root ganglion Correct answer.

Explanation

An absent ulnar SNAP with normal ulnar motor studies indicates damage to the sensory axon distal to the dorsal root ganglion, such as a focal ulnar sensory neuropathy. Motor fibers are spared, so the CMAP remains normal. A proximal motor lesion or generalized neuropathy would affect motor studies as well.

Question 41Somatosensory Evoked Potentials

In upper extremity somatosensory evoked potential testing, which nerve is most commonly stimulated at the wrist?

  1. Ulnar nerve
  2. Radial nerve
  3. Median nerve Correct answer.
  4. Musculocutaneous nerve

Explanation

The median nerve is the standard stimulation site at the wrist for upper limb SEPs because of its consistent anatomy and large somatosensory cortical representation. Ulnar stimulation is used as an alternative or complementary study, but not as the default.

Question 42Somatosensory Evoked Potentials

For lower extremity SEPs, stimulation of the posterior tibial nerve is typically performed at which location?

  1. Over the popliteal fossa
  2. Over the dorsum of the foot
  3. Behind the medial malleolus at the ankle Correct answer.
  4. At the fibular head

Explanation

The posterior tibial nerve is most accessible just posterior to the medial malleolus at the ankle, where it lies superficially. This is the standard distal stimulation site for lower limb SEPs.

Question 43Somatosensory Evoked Potentials

Which cortical waveform is the most robust and commonly analyzed component of the median nerve SEP?

  1. P9
  2. N20 Correct answer.
  3. N13
  4. P40

Explanation

The N20 potential is a negative deflection recorded over the contralateral somatosensory cortex at approximately 20 ms and is the most reliable cortical component of median SEPs. The other listed waveforms belong to different nerve or spinal levels.

Question 44Somatosensory Evoked Potentials

The Erb's point potential in a median nerve SEP is generated by which structure?

  1. Primary somatosensory cortex
  2. Brachial plexus Correct answer.
  3. Cervical dorsal horn
  4. Thalamus

Explanation

The Erb's point potential (typically labeled N9 or P9) reflects the compound action potential of the brachial plexus as fibers pass through the supraclavicular region. It serves as a peripheral reference for comparison with more central components.

Question 45Somatosensory Evoked Potentials

What is the standard stimulation rate used for routine clinical SEP studies?

  1. 100 Hz
  2. 0.5 Hz
  3. 20 to 50 Hz
  4. 2 to 5 Hz Correct answer.

Explanation

Rates of approximately 2 to 5 Hz provide an optimal balance between acquisition speed and waveform clarity for routine SEPs. Higher rates may attenuate later cortical components, while very low rates unnecessarily prolong the study.

Question 46Somatosensory Evoked Potentials

Which montage is typically used to record the cortical N20 potential of the median nerve SEP?

  1. Contralateral scalp (C3' or C4') referenced to a noncephalic or ear reference Correct answer.
  2. Over the cervical spine referenced to the sternum
  3. Ipsilateral scalp referenced to the contralateral scalp
  4. Over the popliteal fossa referenced to the ankle

Explanation

The N20 is recorded over the contralateral somatosensory cortex (C3' for right median stimulation, C4' for left) referenced to a relatively inactive site such as an ear or noncephalic reference. This montage maximizes the cortical signal.

Question 47Somatosensory Evoked Potentials

Which patient factor most strongly affects SEP waveform latency and amplitude?

  1. Dietary intake
  2. Limb temperature Correct answer.
  3. Time of day
  4. Skin pigmentation

Explanation

Limb temperature directly influences peripheral conduction velocity; cooling prolongs latencies and can reduce amplitudes. This is why limb warming is recommended before SEP testing.

Question 48Somatosensory Evoked Potentials

The typical latency of the N20 potential following median nerve stimulation is approximately which value?

  1. 60 ms
  2. 20 ms Correct answer.
  3. 40 ms
  4. 10 ms

Explanation

The N20 is named for its typical latency of approximately 20 ms in healthy adults after median nerve stimulation at the wrist. The other values correspond to different components or to lower limb SEPs.

Question 49Somatosensory Evoked Potentials

A technologist records a median nerve SEP with the active electrode placed over the cervical spine at Cv7 and a reference at Fz. Which component is primarily recorded by this derivation?

  1. N20
  2. P40
  3. N9
  4. N13 Correct answer.

Explanation

A Cv7-to-Fz derivation is used to record the cervical spinal N13 potential, generated at the dorsal horn level. The N20 cortical response requires a scalp-to-reference derivation.

Question 50Somatosensory Evoked Potentials

A patient has a normal Erb's point potential but absent N13 and N20 potentials after median nerve stimulation. This pattern most likely indicates which type of lesion?

  1. Peripheral nerve lesion at the wrist
  2. Cortical lesion only
  3. Brachial plexus lesion
  4. Central conduction abnormality proximal to the brachial plexus Correct answer.

Explanation

A preserved Erb's point response with absent central components indicates that conduction through the peripheral nerve and plexus is intact but central pathways are disrupted. This pattern localizes the abnormality proximal to the brachial plexus.