CNIM Practice Questions – Comprehensive
Certification Exam Preparation
What time constant would be the most effective for cutting out low frequency noise from your SSEP
cortical responses?
Answer: 10 mS
SSEP cortical responses are in the typical frequency range of 100-200 Hz. That being said, let's see what
time constant would give us a good Low Frequency Filter. Remember LFF filters out everything below
the cut off frequency and passes higher frequencies.
Formula to know: Cut off Frequency = 1 / Time Constant (tc)
1/.01 s = 100 Hz
10 ms = .01 s
This answer would filter out everything below 100 Hz so it would eliminate all the frequencies lower
than 100 Hz and pass desired response and all higher frequencies.
In order to achieve asynchronous stimulation you need to:
delay the second stimulator, so the stim goes "left, right, left, right"
What is the active electrode for recording popliteal fossa sciatic nerve response after PTN stimulation at
the ankle?
PF distal:
appropriate montage for recording pop fossa response is Pf (distal) - Pf (proximal). The distal electrode is
placed directly behind the knee in the leg crease, and the proximal electrode is placed 2-4 cm proximal.
The response recorded is the N9 propagated afferent volley
,CNIM Practice Questions – Comprehensive
Certification Exam Preparation
A negative electrical field is recorded by the negative input of the amplifier channel, while a reference
electrode in the positive input records no activity of interest. What is true about the output?
An upward deflection represents a negative polarity waveform.
By convention, activity of interest is applied to the negative input of the amplifier. At a distinct moment
in time, the differential amplifier subtracts the negative input from the positive input, yielding the
"difference" between the two inputs. If the output is a positive number, the waveform will point
upward, and this upward deflection represents the fact that the information input was MORE NEGATIVE
than the information in the positive input.
Requirements for classifying SSEP data as "replicating" and therefore monitorable prior to surgical
manipulation
Replicating amplitude within 15%
Minimum of 2 trials (300-500 averages each) per limb
Replicating morphology of the waveform
Replicating latency within 1%
Appropriate SSEP replication criteria
1% latency, 15% amplitude
Which contact lies on the precentral gyrus?
4
precentral gyrus = motor cortex
Image: Which contact lies on the precentral gyrus?
,CNIM Practice Questions – Comprehensive
Certification Exam Preparation
How can the polarity of waveform B be described?
Negative in the referential channel, cancelled in the bipolar channel
First look at the montage to determine the obligate waveform of interest. B is the N34 waveform. We
know this because it is recorded by a scalp electrode that is referenced to a non-cephalic electrode.
Since the generator of the N34 is the brainstem, we know that a scalp-scalp (bipolar) channel will cancel
the waveform.
Image: How can the polarity of waveform B be described?
Most common complication of TcMEPs
tongue lacerations
If you run a TcMEP with the anode at C4 and the cathode at C3
the left AH response should be larger
remember: unlike other modalities, the action potentials fire under the anode (+) for TcMEPs. The
TcMEP pathway decussates in the medulla (around 85-90% of fibers cross over before descending down
the corticospinal tract). Ipsilateral muscles (same side as anode) will likely record small CMAPs (or
absent but not necessarily). Contralateral responses should be larger.
During resection of an intracranial tumor that is located in midline and near the central sulcus, which of
the following will be the most beneficial modality?
PTN SSEPs
Phase reversal is of course beneficial when working near the central sulcus. However, most sources
indicate that phase reversal SSEPs are only most effectively performed following upper limb stimulation,
not PTN stimulation. Therefore, PTN SSEPs will be the more critical modality.
, CNIM Practice Questions – Comprehensive
Certification Exam Preparation
Following TcMEP stimulation at C1-C2, you successfully elicit responses from the upper extremities but
not from the lower extremities. What corrective action might be helpful in troubleshooting?
Move stimulating electrodes more anterior.
Keep in mind that C1 and C2 are situated directly over the central sulcus. MacDonald recommends
placing electrodes 1-2 cm anterior to central sulcus.
Benefit of using corkscrew electrodes for TcMEP stimulation at the scalp
they have lower impedances and they are less likely to become dislodged
2016 ACNS guidelines describe corkscrews as optimal choice of stimulation electrode for MEPs.
Apraxia definition
A motor disorder caused by damage to the brain in which the individual has difficulty performing tasks
or movements when asked. The nature of the brain damage determines the severity, and the absence of
sensory loss or paralysis helps to explain the level of difficulty. Muscles of the face and extremities may
be affected.
Axonotmesis
when the nerve sheath remains intact but the axons may be divided following traction or crush injury.
The condition can result in complete loss of muscle (motor) function, sensations and autonomic
functions transmitted by the affected nerve. The recovery usually takes several months to years.
Image: Axonotmesis
Friedrich's Ataxia
Certification Exam Preparation
What time constant would be the most effective for cutting out low frequency noise from your SSEP
cortical responses?
Answer: 10 mS
SSEP cortical responses are in the typical frequency range of 100-200 Hz. That being said, let's see what
time constant would give us a good Low Frequency Filter. Remember LFF filters out everything below
the cut off frequency and passes higher frequencies.
Formula to know: Cut off Frequency = 1 / Time Constant (tc)
1/.01 s = 100 Hz
10 ms = .01 s
This answer would filter out everything below 100 Hz so it would eliminate all the frequencies lower
than 100 Hz and pass desired response and all higher frequencies.
In order to achieve asynchronous stimulation you need to:
delay the second stimulator, so the stim goes "left, right, left, right"
What is the active electrode for recording popliteal fossa sciatic nerve response after PTN stimulation at
the ankle?
PF distal:
appropriate montage for recording pop fossa response is Pf (distal) - Pf (proximal). The distal electrode is
placed directly behind the knee in the leg crease, and the proximal electrode is placed 2-4 cm proximal.
The response recorded is the N9 propagated afferent volley
,CNIM Practice Questions – Comprehensive
Certification Exam Preparation
A negative electrical field is recorded by the negative input of the amplifier channel, while a reference
electrode in the positive input records no activity of interest. What is true about the output?
An upward deflection represents a negative polarity waveform.
By convention, activity of interest is applied to the negative input of the amplifier. At a distinct moment
in time, the differential amplifier subtracts the negative input from the positive input, yielding the
"difference" between the two inputs. If the output is a positive number, the waveform will point
upward, and this upward deflection represents the fact that the information input was MORE NEGATIVE
than the information in the positive input.
Requirements for classifying SSEP data as "replicating" and therefore monitorable prior to surgical
manipulation
Replicating amplitude within 15%
Minimum of 2 trials (300-500 averages each) per limb
Replicating morphology of the waveform
Replicating latency within 1%
Appropriate SSEP replication criteria
1% latency, 15% amplitude
Which contact lies on the precentral gyrus?
4
precentral gyrus = motor cortex
Image: Which contact lies on the precentral gyrus?
,CNIM Practice Questions – Comprehensive
Certification Exam Preparation
How can the polarity of waveform B be described?
Negative in the referential channel, cancelled in the bipolar channel
First look at the montage to determine the obligate waveform of interest. B is the N34 waveform. We
know this because it is recorded by a scalp electrode that is referenced to a non-cephalic electrode.
Since the generator of the N34 is the brainstem, we know that a scalp-scalp (bipolar) channel will cancel
the waveform.
Image: How can the polarity of waveform B be described?
Most common complication of TcMEPs
tongue lacerations
If you run a TcMEP with the anode at C4 and the cathode at C3
the left AH response should be larger
remember: unlike other modalities, the action potentials fire under the anode (+) for TcMEPs. The
TcMEP pathway decussates in the medulla (around 85-90% of fibers cross over before descending down
the corticospinal tract). Ipsilateral muscles (same side as anode) will likely record small CMAPs (or
absent but not necessarily). Contralateral responses should be larger.
During resection of an intracranial tumor that is located in midline and near the central sulcus, which of
the following will be the most beneficial modality?
PTN SSEPs
Phase reversal is of course beneficial when working near the central sulcus. However, most sources
indicate that phase reversal SSEPs are only most effectively performed following upper limb stimulation,
not PTN stimulation. Therefore, PTN SSEPs will be the more critical modality.
, CNIM Practice Questions – Comprehensive
Certification Exam Preparation
Following TcMEP stimulation at C1-C2, you successfully elicit responses from the upper extremities but
not from the lower extremities. What corrective action might be helpful in troubleshooting?
Move stimulating electrodes more anterior.
Keep in mind that C1 and C2 are situated directly over the central sulcus. MacDonald recommends
placing electrodes 1-2 cm anterior to central sulcus.
Benefit of using corkscrew electrodes for TcMEP stimulation at the scalp
they have lower impedances and they are less likely to become dislodged
2016 ACNS guidelines describe corkscrews as optimal choice of stimulation electrode for MEPs.
Apraxia definition
A motor disorder caused by damage to the brain in which the individual has difficulty performing tasks
or movements when asked. The nature of the brain damage determines the severity, and the absence of
sensory loss or paralysis helps to explain the level of difficulty. Muscles of the face and extremities may
be affected.
Axonotmesis
when the nerve sheath remains intact but the axons may be divided following traction or crush injury.
The condition can result in complete loss of muscle (motor) function, sensations and autonomic
functions transmitted by the affected nerve. The recovery usually takes several months to years.
Image: Axonotmesis
Friedrich's Ataxia
