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Central Sulcus Mapping Simplified: (CLTM Board Prep Knowledge)

The CLTM board exam has many questions related to brain mapping, which can be intimidating to some REEGTs. I found it difficult to find resource material online while studying for my board exam and I hope this provides help to a fellow REEGT. You can learn mapping without having the hands on experience simply by learning the basics. To start you need some basic background knowledge. Take some time to learn the Brodmann's areas, basic anatomy, and the homunculus. It is also recommended you brush up on your SSEPs of upper extremities. Once you have these basics you can better understand brain mapping concepts.

What is mapping and why is it done?

Brain mapping essentially is a subdural grid or strip placed directly on the cortex in the OR and then stimulation is applied to the contralateral median nerve to locate the central sulcus. This is most similar to SSEPs. Typically TIVA is the best anesthetic of choice for this procedure. The procedure is done prior to any parietal or frontal lobe epilepsy surgical procedure to reduce the risk of neurological damage to the brain.

What is the central sulcus and why do we need to find it?

This is the area of the brain where motor and sensory meet. It is identified through stimulation on either side of the grid or strip placed on the cortex. The central sulcus is not easy to identify visually, but must be identified before epilepsy surgical procedures involving the frontal or parietal lobes so that motor and sensory functions can remain unaffected during surgery. Both motor mapping and sensory mapping are performed.

How is the procedure displayed?

Once you understand your anatomy and the goals of brain mapping you can begin to understand the concepts required to identify the motor and sensory pathways. Much like an evoked potential (SSEP) study, the results of stimulation (or waveforms) are displayed in a grid format. In this grid there will be 4-32 channels depending on the grid or strip applied. The channels will display varying waveforms. For the novice tech the most important waves to identify would be the P20 and the N20, knowing this going into your board exam will be essential.

What are the waveforms associated with mapping and how do I identify them?

The P20 is the motor response. It is of positive polarity, hence the “P” in the name of the waveform. It has a typical latency of 20 ms hence the “20” in P20. The P20 is generated in the frontal lobe, and we know our anatomy and know the motor strip is in the frontal lobe. So a P20 waveform is a motor response. The N20 follows the same rules. It is of negative polarity hence the “N” in the name and has the same typical latency of 20 ms, hence the “20” in N20. The N20 is generated in the parietal lobe, and we know our anatomy and know the parietal lobe holds the sensory strip. So a N20 waveform is a sensory response. Given this information we can look at a grid and identify the P20 and N20 phase reversal and know that between these two electrodes lies the central sulcus. We also know that both the P20 and N20 are near field responses.

What will this look like on the board exam?

It won’t always be laid out in this simple format as seen above on the board exam. You will need to use what you’ve learned and understand to apply these concepts to your board exam. We know that we are trying to look for the highest amplitude or best formed waveform on an EP study and the same applies here. In the sample above we can see that the first 2 channels have the best formed waveforms. We see a clear phase reversal between P20 and N20 clearly identifying that the central sulcus is between these two electrodes. The waveforms are the best formed because they are the ones generating the motor and sensory response, they are less well formed in other channels because the stimulation is further away. Remember it is like throwing a pebble in a pond, the ripples are the biggest at the site where it hits the water and become less as it ripples further away.

The above picture is an example of a mapping grid sample on the ASAP board prep. In this sample it is not as crystal clear where the N20 and P20 are located like it was in the previous picture. The waveforms are not as well formed, however we do see a difference between waveforms generated between G4 and G5 channels and if we apply the ripples in the pond theory we know that the G4 channel is definitely one of the waveforms we want to take a closer look at. We do see a phase reversal between G4 and G5 and we do know that is what we are looking for, using the power of critical thinking and deduction we can assume the central sulcus is between G4 and G5 channels. For further deduction we know that the P20 is a positive waveform and positive waveforms have a downward deflection, so the P20 is in G4 generated from the frontal lobe motor strip. The N20 is negative and has an upward deflection so the N20 is in the G5 channel and is generated in the parietal lobe sensory strip. So now we can answer any questions related to this example on the exam.

The example above is the most similar to what you will see on the CLTM board exam. There will often be a brain with a grid on it and you may be asked to identify the electrodes over the central sulcus. Use the knowledge you have gained and the labels applied to the grid to identify the electrodes. Make sure to double check your work!

How do I find the latency of a waveform?

The last concept that we need to know in order to feel confident in our knowledge and ability on board exams is latency. If you do not regularly perform EPs or participate in mapping this may be a far gone concept for you. If you are asked about the latency of a waveform the basic information you need to remember is that latency is measured from stimulation to peak. In the sample above the stim and peak are labeled. Unlike in EEG, where we measure from baseline to baseline for durations; the latency of a waveform starts at the stimulation and ends at the peak of a wave. The grid is typically a 50ms grid with the gridlines labeled at 7.5ms/div.

I hope this helps someone out there gain the confidence they need to learn a new concept and apply it to achieve their CLTM. Good Luck!

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