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CNIM practice questions

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Question 1Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

In a typical myelinated neuron, action potentials are normally initiated at which site?

  1. Distal dendrite
  2. Axon terminal
  3. Cell body (soma)
  4. Axon hillock Correct answer.

Explanation

The axon hillock and initial segment carry the highest density of voltage-gated sodium channels and have the lowest threshold for spike initiation, so synaptic potentials summed at the soma trigger the action potential there. Dendrites conduct graded, decremental potentials, and the soma integrates rather than initiates spikes.

Question 2Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which EEG frequency band is defined as 8 to 13 Hz and forms the posterior dominant rhythm of a relaxed, awake adult with eyes closed?

  1. Theta (4 to 7 Hz)
  2. Alpha (8 to 13 Hz) Correct answer.
  3. Beta (14 to 30 Hz)
  4. Delta (less than 4 Hz)

Explanation

Alpha activity occupies the 8 to 13 Hz range, is maximal over posterior head regions, and attenuates with eye opening or mental activation. The remaining bands are defined by different frequency ranges and behavioral states.

Question 3Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

In a differential amplifier, the high-frequency filter (low-pass filter) primarily determines which characteristic of the recording?

  1. The input impedance of the amplifier
  2. The common mode rejection ratio
  3. The upper limit of the frequency response Correct answer.
  4. The lower limit of the frequency response

Explanation

The high-frequency filter attenuates signals above its cutoff and therefore sets the upper end of the amplifier's passband. The low-frequency (high-pass) filter sets the lower end, while CMRR and input impedance are separate amplifier properties.

Question 4Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

In a bipolar montage, the signal displayed for each channel represents the potential difference between which pair of electrodes?

  1. An exploring electrode and an indifferent reference
  2. Two reference electrodes
  3. Two adjacent exploring electrodes Correct answer.
  4. An exploring electrode and the ground electrode

Explanation

A bipolar (chain) montage subtracts the potential at one exploring electrode from that at an adjacent exploring electrode, which enhances focal, localized gradients. Referential montages instead compare an exploring electrode with a common reference.

Question 5Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The isolated patient input stage of a neurophysiologic amplifier is used primarily to:

  1. Reduce 60-Hz interference
  2. Increase the common mode rejection ratio
  3. Match the impedances of recording electrodes
  4. Limit leakage current reaching the patient Correct answer.

Explanation

Isolation circuitry electrically separates the patient-connected leads from mains-powered equipment, capping the leakage current that could pass through the patient. Interference rejection and impedance matching are handled by different design features.

Question 6Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which cranial nerve innervates the muscles of facial expression and is monitored with free-running EMG during posterior fossa surgery?

  1. Hypoglossal nerve (CN XII)
  2. Facial nerve (CN VII) Correct answer.
  3. Glossopharyngeal nerve (CN IX)
  4. Trigeminal nerve (CN V)

Explanation

The facial nerve supplies the muscles of facial expression and is at risk during cerebellopontine angle and other posterior fossa procedures, so it is monitored with free-running and stimulated EMG. The other listed nerves subdivide to different muscle groups and functions.

Question 7Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The dorsal columns of the spinal cord carry which type of information?

  1. Sympathetic outflow
  2. Proprioception and vibration sense Correct answer.
  3. Voluntary motor commands
  4. Pain and temperature sensation

Explanation

The dorsal columns convey fine touch, vibration, and conscious proprioception through the ipsilateral fasciculus gracilis and cuneatus, which is why they are the substrate for posterior tibial and median nerve somatosensory evoked potentials. Nociceptive and thermal traffic travels in the spinothalamic tract.

Question 8Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which electrode impedance is generally targeted before beginning an evoked potential or EEG recording?

  1. 50 to 100 kΩ
  2. Less than 5 kΩ Correct answer.
  3. Greater than 10 MΩ
  4. 1 to 5 MΩ

Explanation

Low and balanced impedances, typically under about 5 kΩ, minimize thermal noise, reduce susceptibility to 60-Hz interference, and preserve common mode rejection. Very high impedances degrade signal quality and increase artifact.

Question 9Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Motor evoked potentials are best preserved during surgery when anesthesia is maintained with which regimen?

  1. Propofol-ketamine total intravenous anesthesia Correct answer.
  2. Nitrous oxide plus isoflurane
  3. Sevoflurane at 1.5 MAC
  4. Isoflurane at 1.0 MAC

Explanation

Volatile anesthetics suppress cortical excitability and abolish myogenic motor evoked potentials in a dose-dependent manner, whereas ketamine preserves or enhances excitability, making a propofol-ketamine intravenous technique favorable when MEPs are required.

Question 10Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which filter settings are appropriate for recording a median nerve somatosensory evoked potential?

  1. 1 to 30 Hz
  2. 10 to 3000 Hz Correct answer.
  3. 100 to 1000 Hz
  4. 0.1 to 100 Hz

Explanation

Short-latency SSEPs contain rapid, sharp components best captured with a low-frequency filter near 10 to 30 Hz and a high-frequency filter near 3 kHz. This passband rejects slow baseline sway while preserving the fast cortical and subcortical peaks.

Question 11Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

During pedicle screw placement, a sustained train of high-frequency motor unit potentials on free-running EMG most likely indicates:

  1. A normal finding during neuromuscular blockade
  2. Ischemic injury to the muscle
  3. Mechanical irritation of a nerve root Correct answer.
  4. Complete transection of the nerve

Explanation

Neurotonic trains reflect repetitive firing of motor units in response to mechanical or thermal irritation of a nerve, alerting the team before permanent injury occurs. A complete transection would instead produce a brief injury discharge followed by electrical silence.

Question 12Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

In a motor nerve conduction study, the compound muscle action potential amplitude is conventionally measured from:

  1. The onset latency
  2. Baseline to the negative peak Correct answer.
  3. Negative peak to positive peak
  4. The total waveform duration

Explanation

Amplitude is measured from the isoelectric baseline to the first negative peak, reflecting the number of excitable muscle fibers depolarized. Latency and duration are separate measures of timing and waveform dispersion.

Question 13Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which EEG pattern characterizes burst suppression?

  1. Generalized 3-Hz spike-and-wave discharges
  2. A continuous posterior alpha rhythm
  3. High-voltage activity interrupted by periods of isoelectric suppression Correct answer.
  4. Low-voltage fast activity without slow waves

Explanation

Burst suppression consists of bursts of higher-voltage mixed-frequency activity separated by periods of near-flat, isoelectric background, and is seen with deep anesthesia, hypothermia, or severe encephalopathy. It reflects marked global depression of cortical activity.

Question 14Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Wave V of the brainstem auditory evoked potential is generated primarily by which structure?

  1. Superior olivary complex
  2. Inferior colliculus Correct answer.
  3. Cochlear nucleus
  4. Auditory nerve (CN VIII)

Explanation

Wave V is generated in the region of the inferior colliculus and lateral lemniscus and is the most robust peak used for latency and threshold determination. Earlier waves correspond to progressively more caudal stations of the auditory pathway.

Question 15Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

A high common mode rejection ratio is desirable in evoked potential recording because it:

  1. Increases the input impedance of the amplifier
  2. Amplifies the biological signal of interest
  3. Eliminates the need for a ground electrode
  4. Rejects interference that appears equally at both inputs Correct answer.

Explanation

Biological signals appear as a difference between the two active inputs, whereas 60-Hz interference typically appears equally at both and is therefore a common mode signal. A high CMRR allows the amplifier to reject that shared interference while amplifying the differential signal.

Question 16Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The P100 component of the pattern-reversal visual evoked potential is maximal at which electrode site?

  1. Oz Correct answer.
  2. Fz
  3. Cz
  4. T4

Explanation

The visual evoked potential is generated in the occipital cortex, so the P100 is recorded with maximal amplitude at Oz referenced to Fz or a linked-ear reference. Frontal and temporal placements are far from the generator.

Question 17Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The artery of Adamkiewicz, which supplies the lower thoracic and lumbar spinal cord, most commonly arises from which segmental level?

  1. S1 to S3
  2. T8 to L1 Correct answer.
  3. L4 to L5
  4. C3 to C5

Explanation

The great anterior radiculomedullary artery typically arises from a left-sided intercostal or lumbar artery between about T8 and L1, most often in the lower thoracic region. Its location is a key reason thoracic aortic surgery carries a risk of cord ischemia.

Question 18Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

A reproducible decrement of more than 10 percent on 2 to 3 Hz repetitive nerve stimulation is characteristic of:

  1. Lambert-Eaton myasthenic syndrome
  2. Axonal polyneuropathy
  3. Myasthenia gravis Correct answer.
  4. Myotonic dystrophy

Explanation

In myasthenia gravis, postsynaptic acetylcholine receptor loss causes a progressive fall in the compound muscle action potential with slow repetitive stimulation, typically exceeding 10 percent. Presynaptic disorders such as Lambert-Eaton show the opposite pattern, an increment.

Question 19Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Eye blink artifact is most prominent in which electrode locations?

  1. Occipital leads
  2. Temporal leads
  3. Central leads
  4. Frontal leads Correct answer.

Explanation

The blink artifact arises from the corneoretinal dipole and is maximal in the frontal leads (Fp1 and Fp2), where it appears as a synchronous, high-voltage slow deflection. Its anterior maximum helps distinguish it from cerebral slowing.

Question 20Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

What is the primary function of a differential amplifier in neurophysiologic recording?

  1. It removes high-frequency noise from the signal
  2. It amplifies each input separately and sums them
  3. It converts analog signals into digital form
  4. It amplifies the difference between two inputs while rejecting signals common to both Correct answer.

Explanation

A differential amplifier subtracts one input from the other, so the biological gradient between the two active electrodes is amplified while shared signals such as 60-Hz interference are largely cancelled. This is the basis of common mode rejection in evoked potential and EEG recording.

Question 21Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The N20 component of the median nerve somatosensory evoked potential is generated primarily by which structure?

  1. Cervical dorsal horn
  2. Postcentral gyrus Correct answer.
  3. Ventral posterolateral thalamus
  4. Brachial plexus

Explanation

N20 reflects the arrival of the somatosensory volley in the primary somatosensory cortex of the postcentral gyrus after relay through the thalamus. It is the cortical component used to confirm integrity of the entire ascending pathway.

Question 22Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The F-wave recorded during motor nerve conduction studies represents which phenomenon?

  1. A monosynaptic reflex mediated by Ia afferents
  2. A cortical response to peripheral stimulation
  3. A sensory nerve action potential from the skin
  4. Antidromic activation of anterior horn cells Correct answer.

Explanation

Supramaximal stimulation antidromically activates motor axons, causing anterior horn cells to backfire and produce a small late motor response. Because the impulse travels proximally and then distally, the F-wave assesses the proximal segment of the motor nerve.

Question 23Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which needle EMG finding best characterizes chronic reinnervation of muscle after a nerve injury?

  1. Reduced recruitment without spontaneous activity
  2. Small-amplitude, short-duration, polyphasic potentials
  3. Large-amplitude, long-duration, polyphasic potentials Correct answer.
  4. Fibrillation potentials with normal motor unit potentials

Explanation

Collateral sprouting after denervation enlarges each motor unit's territory and adds fibers with slightly different conduction times, producing motor unit potentials that are larger in amplitude, longer in duration, and more polyphasic. This is the hallmark of chronic reinnervation.

Question 24Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which stimulation configuration is used with transcranial electrical stimulation to activate the motor cortex for motor evoked potential monitoring?

  1. Bipolar stimulation of the occipital cortex
  2. Anodal stimulation of the cervical spinal cord
  3. Anodal stimulation of the motor cortex Correct answer.
  4. Cathodal stimulation of the motor cortex

Explanation

With transcranial electrical stimulation, anodal current over the motor cortex depolarizes corticospinal neurons most effectively, eliciting a descending volley recorded as a myogenic response. This polarity dependence is exploited when placing the stimulating electrodes at the vertex and over the motor region.

Question 25Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The EEG becomes approximately isoelectric when cerebral blood flow falls to which level?

  1. 50 mL/100 g/min
  2. 15 mL/100 g/min Correct answer.
  3. 25 mL/100 g/min
  4. 35 mL/100 g/min

Explanation

Normal cortical blood flow is roughly 50 mL/100 g/min; EEG slowing and attenuation appear as flow falls through the twenties, and the tracing becomes isoelectric at about 15 mL/100 g/min. Below that, membrane failure and irreversible injury occur if flow is not restored.

Question 26Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

At which level of the neuraxis do the majority of corticospinal tract fibers decussate?

  1. Pons
  2. Medulla Correct answer.
  3. Midbrain
  4. Cervical spinal cord

Explanation

Most corticospinal axons cross in the pyramidal decussation at the spinomedullary junction, forming the lateral corticospinal tract. A small uncrossed fraction continues as the anterior corticospinal tract. Lesions above the decussation therefore produce contralateral weakness.

Question 27Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The resting membrane potential of a typical neuron is determined primarily by the transmembrane gradient and permeability of which ion?

  1. Potassium Correct answer.
  2. Sodium
  3. Chloride
  4. Calcium

Explanation

At rest the membrane is far more permeable to potassium than to other ions because of open leak channels, so the potential sits near the potassium equilibrium potential. The sodium-potassium pump maintains the gradients that drive this relationship.

Question 28Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

The primary auditory cortex is located in which region of the cerebral hemisphere?

  1. Superior temporal gyrus Correct answer.
  2. Frontal precentral gyrus
  3. Parietal postcentral gyrus
  4. Medial occipital cortex

Explanation

Primary auditory cortex corresponds to Brodmann area 41 in Heschl's gyrus on the superior surface of the temporal lobe. It receives thalamic input from the medial geniculate nucleus. The surrounding association cortex supports auditory discrimination.

Question 29Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which neurotransmitter mediates fast excitatory transmission at the skeletal neuromuscular junction?

  1. Dopamine
  2. GABA
  3. Glycine
  4. Acetylcholine Correct answer.

Explanation

Motor neuron terminals release acetylcholine, which binds nicotinic receptors on the motor end plate to produce a fast depolarizing end plate potential. Acetylcholinesterase then terminates the signal. This synapse is the target of neuromuscular blocking agents used during surgery.

Question 30Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

What is the primary function of the ground electrode in a clinical neurophysiology recording?

  1. It serves as the active site for recording cortical potentials
  2. It balances the impedances of the two active electrodes
  3. It establishes the zero-voltage baseline for the differential pair
  4. It shunts common-mode interference away from the amplifier inputs Correct answer.

Explanation

The ground provides a low-impedance return path for common-mode current, which reduces 60 Hz interference in the recording. It is not one of the two inputs whose difference constitutes the signal. The differential pair itself defines the recorded voltage.

Question 31Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which thalamic nucleus relays somatosensory information from the body to the primary somatosensory cortex?

  1. Medial geniculate nucleus
  2. Lateral geniculate nucleus
  3. Ventroposterolateral nucleus Correct answer.
  4. Ventrolateral nucleus

Explanation

The ventroposterolateral nucleus receives the medial lemniscus and spinothalamic input from the trunk and limbs and projects to the postcentral gyrus. Facial sensation relays in the adjacent ventroposteromedial nucleus. Each thalamic nucleus is specialized for a particular sensory or motor pathway.

Question 32Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Which best characterizes the normal posterior dominant alpha rhythm in an awake adult?

  1. 4 to 7 Hz, blocked by drowsiness
  2. Less than 4 Hz, reactive to hyperventilation
  3. 14 to 30 Hz, maximal frontally
  4. 8 to 13 Hz, attenuated by eye opening Correct answer.

Explanation

The alpha rhythm is an 8 to 13 Hz posterior rhythm seen in relaxed wakefulness with eyes closed and is attenuated by eye opening or mental alertness. It is the hallmark of a normal awake background. Frequencies outside this band define other named rhythms.

Question 33Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Silver-silver chloride electrodes are widely used for recording because they have which property?

  1. Selective sensitivity above 100 Hz
  2. Complete freedom from maintenance
  3. A stable, low offset potential Correct answer.
  4. No requirement for conductive paste

Explanation

The Ag/AgCl half-cell has a low and stable offset potential, which minimizes DC drift and polarization at the electrode-skin interface. This makes it suitable for both EEG and evoked potential work. It still requires proper skin preparation and gel, and surfaces can degrade over time.

Question 34Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Hyperventilation typically alters the EEG of a healthy adult by producing which change?

  1. Continuous spike-and-wave discharges
  2. Diffuse faster frequencies with alpha augmentation
  3. Diffuse slowing with reduced amplitude Correct answer.
  4. Isolated loss of posterior alpha

Explanation

Hyperventilation lowers arterial carbon dioxide, causing cerebral vasoconstriction and reduced cerebral blood flow. The resulting EEG change is generalized slowing with lower amplitude, sometimes with increased delta. This response is the basis of the activation procedure.

Question 35Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

A patient with a unilateral cerebellar hemisphere lesion is most likely to show which abnormality?

  1. Spastic hemiparesis with hyperreflexia
  2. Resting tremor with cogwheel rigidity
  3. Intention tremor and ipsilateral dysmetria Correct answer.
  4. Loss of vibration sense in the feet

Explanation

The cerebellar hemispheres coordinate ongoing movement, so their damage produces ipsilateral limb ataxia with errors in trajectory and terminal tremor. Signs are ipsilateral because cerebellar output pathways decussate below the level of the lesion. Strength and sensation are typically preserved.

Question 36Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

What is the main purpose of patient isolation circuitry in a clinical evoked potential system?

  1. To increase the amplitude of the recorded response
  2. To limit fault current that could reach the patient Correct answer.
  3. To synchronize the stimulator with the averaging sweep
  4. To remove 60 Hz interference before averaging

Explanation

Isolation circuitry, usually transformer or optical, prevents dangerous current from flowing through the patient in the event of an internal fault or a competing ground path. It does not shape the physiologic signal. Safe design keeps patient leakage current well below the levels that can trigger arrhythmia or injury.

Question 37Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

Sleep spindles occur in which sleep stage and at what frequency?

  1. Stage N2; 12 to 14 Hz Correct answer.
  2. Stage REM; 8 to 12 Hz
  3. Stage N3; 0.5 to 2 Hz
  4. Stage N1; 4 to 7 Hz

Explanation

Sleep spindles are brief 12 to 14 Hz bursts seen in stage N2 sleep, often over central regions, along with K-complexes. They are a defining feature of N2 and help identify sleep stage during long-term monitoring. Slower delta activity defines N3.

Question 38Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

An ischemic stroke involving the left anterior cerebral artery most commonly produces weakness that is greatest in which distribution?

  1. Ipsilateral arm
  2. Bilateral extraocular muscles
  3. Contralateral leg Correct answer.
  4. Contralateral face and hand

Explanation

The anterior cerebral artery supplies the medial frontal and parietal cortex, where the leg is represented in the motor homunculus. Occlusion therefore produces contralateral leg-predominant weakness, sometimes with urinary incontinence and abulia. Face and arm are supplied mainly by the middle cerebral artery.

Question 39Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

A 12-bit analog-to-digital converter has an input range of plus or minus 5 volts. What voltage does one least-significant bit represent?

  1. 2.44 mV Correct answer.
  2. 1.22 mV
  3. 0.24 mV
  4. 4.88 mV

Explanation

A 12-bit converter has 2^12, or 4096, levels across the full span of 10 volts. Dividing 10 volts by 4096 gives about 2.44 millivolts per least-significant bit. Resolution improves with more bits or a narrower input range.

Question 40Basic Science: Neuroanatomy, Neurophysiology, and Instrumentation

According to the size principle of motor unit recruitment, which motor units are activated first as voluntary force increases?

  1. Large, fast-fatigable units with high thresholds
  2. Units with the largest innervation ratios
  3. Small, slow units with low thresholds Correct answer.
  4. Units with the highest conduction velocities

Explanation

The size principle states that motor neurons are recruited in order of increasing size, so small, low-threshold, fatigue-resistant units fire first. Larger fast units are added as force demand rises. This orderly recruitment underlies the graded increase in muscle force.

Question 41Anesthesia and Pharmacology Affecting Monitoring

Volatile anesthetics affect cortical somatosensory evoked potentials in which characteristic pattern?

  1. Increases latency and decreases amplitude in a dose-dependent manner Correct answer.
  2. Increases amplitude without changing peak latency
  3. Has no measurable effect at clinical end-tidal concentrations
  4. Decreases latency and increases amplitude in a dose-dependent manner

Explanation

Halogenated agents produce dose-dependent depression of cortical SSEPs, seen as prolonged peak latencies and falling amplitudes as end-tidal concentration rises. The effect is progressive and becomes marked above roughly 0.5 to 1.0 MAC.

Question 42Anesthesia and Pharmacology Affecting Monitoring

Which intravenous induction agent characteristically increases the amplitude of cortical somatosensory evoked potentials?

  1. Etomidate Correct answer.
  2. Thiopental
  3. Propofol
  4. Isoflurane

Explanation

Etomidate enhances cortical SSEP amplitude, an effect sometimes used to unmask marginal responses, though it also produces myoclonus and adrenal suppression. Propofol, barbiturates, and volatile agents all depress cortical responses.

Question 43Anesthesia and Pharmacology Affecting Monitoring

During transcranial motor evoked potential monitoring, administration of a nondepolarizing neuromuscular blocking agent primarily alters the recording by:

  1. Reducing or abolishing the myogenic response Correct answer.
  2. Increasing cortical MEP amplitude
  3. Changing spinal cord conduction velocity
  4. Shortening MEP onset latency

Explanation

Motor evoked potentials recorded from muscle depend on an intact neuromuscular junction, so blocking agents attenuate or eliminate the response at the muscle. The descending volleys themselves are unaffected.

Question 44Anesthesia and Pharmacology Affecting Monitoring

Which monitor is used to quantify the degree of neuromuscular blockade when motor evoked potentials must be preserved?

  1. Train-of-four peripheral nerve stimulation Correct answer.
  2. Near-infrared spectroscopy
  3. Bispectral index
  4. Processed EEG entropy

Explanation

Train-of-four stimulation of a peripheral nerve objectively grades the extent of neuromuscular blockade so that partial recovery can be confirmed before MEP stimulation. The other monitors assess depth of anesthesia or cerebral oxygenation, not endplate function.

Question 45Anesthesia and Pharmacology Affecting Monitoring

Nitrous oxide at a typical intraoperative concentration of 50 to 60 percent most commonly produces which change in cortical somatosensory evoked potentials?

  1. Abolition of Erb point potentials
  2. Amplitude reduction with little latency change Correct answer.
  3. Loss of the peripheral nerve response
  4. Marked amplitude increase

Explanation

Nitrous oxide chiefly depresses the cortical component, lowering amplitude with only modest latency prolongation. The effect is additive with that of volatile agents and can make cortical responses difficult to interpret when combined.

Question 46Anesthesia and Pharmacology Affecting Monitoring

Propofol administered at usual anesthetic doses affects transcranial motor evoked potentials by:

  1. Enhancing their amplitude
  2. Prolonging them without changing amplitude
  3. Suppressing them in a dose-dependent manner Correct answer.
  4. Having no effect at any clinically used dose

Explanation

Propofol depresses motor pathway excitability through GABA-mediated inhibition, so MEP amplitude falls as dose or effect-site concentration increases. This is why propofol-based TIVA is titrated to the lowest depth compatible with interpretable responses.

Question 47Anesthesia and Pharmacology Affecting Monitoring

Ketamine is generally avoided as the sole anesthetic during somatosensory evoked potential monitoring because it tends to:

  1. Abolish peripheral nerve conduction
  2. Eliminate cortical responses at subanesthetic doses
  3. Increase fast EEG activity and cortical amplitudes, complicating interpretation Correct answer.
  4. Markedly shorten interpeak latencies

Explanation

Ketamine activates cortical activity, raising amplitudes and producing fast EEG patterns, which can obscure or mimic changes caused by surgery. It also tends to preserve or enhance evoked responses rather than suppress them.

Question 48Anesthesia and Pharmacology Affecting Monitoring

Degradation of interpretable motor evoked potentials typically begins at approximately what end-tidal concentration of a volatile anesthetic?

  1. 2.0 MAC
  2. 0.5 MAC Correct answer.
  3. 1.5 MAC
  4. 2.5 MAC

Explanation

Motor responses begin to deteriorate at about 0.5 MAC and are frequently unobtainable near 1.0 MAC with halogenated agents. This steep dose dependence is why MEP cases are usually managed with TIVA or very low volatile concentrations.

Question 49Anesthesia and Pharmacology Affecting Monitoring

Which anesthetic regimen best preserves interpretable motor evoked potentials during spine surgery?

  1. Propofol-remifentanil TIVA Correct answer.
  2. Desflurane at 1.2 MAC
  3. Isoflurane at 1.0 MAC with nitrous oxide
  4. Sevoflurane at 1.5 MAC

Explanation

Propofol-based total intravenous anesthesia with a short-acting opioid minimizes depression of the motor pathways and permits reliable MEP recording. Volatile agents at or above roughly 1 MAC, especially combined with nitrous oxide, commonly abolish myogenic responses.

Question 50Anesthesia and Pharmacology Affecting Monitoring

A bolus of etomidate for induction may produce which finding that can be misinterpreted during neurophysiologic monitoring?

  1. Complete loss of cortical SSEPs for 30 minutes
  2. Persistent burst suppression
  3. Myoclonic jerks mimicking spontaneous EMG Correct answer.
  4. Profound neuromuscular blockade

Explanation

Etomidate commonly causes excitatory myoclonus during induction, and the resulting muscle activity can appear as spontaneous EMG or as artifact on motor recordings. Recognizing the timing relative to drug administration avoids a false alarm.