Answers

C

In this question, the eye lead electrodes are very helpful. By convention the LOC electrode is placed on the left lower outer canthus and the ROC electrode is placed on the right upper outer canthus. With eye closure, the corneas (positively charged) deflect upwards, making the LOC electrode more electronegative at the same moment that the ROC becomes more electropositive. Thus, with eye movements, the eye leads are mirror images of each other. In frontally predominant GRDA, both eye leads record essentially the same activity from the frontal lobe so the deflections are synchronous.

A is incorrect because in frontal slowing both sets of eye leads would be synchronous. B is incorrect because with eye movements both sets of eye leads would be mirror images. Of note, frontally predominant GRDA often shows some deflection in all electrodes, maximal anteriorly, as is evident here.

D

The generator for the EEG is thought to be the summation of IPSPs and EPSPs at any point in time for a given electrode. IPSPs and EPSPs are potentials generated on the cell body or the dendrite of a neuron. In an EPSP, an excitatory neurotransmitter is released, causing the influx of Na ²⁺ , which makes the extracellular matrix more negative and intracellular matrix more positive. As the EEG measures the extracellular matrix, synchronized EPSPs will be electronegative on EEG. In this EEG, there is negative phase reversal at the F8 electrode, so we would say that the electronegative potential is maximal at F8.

Action potentials induce a brief (10 ms or less) local current in the axon with a very limited potential field. They are not the generators of the EEG. For a sharp wave to be apparent on the EEG, at least 6 cm ² of cortex must be involved.

B

In this diagram, F8, T8, P8 are involved in a potential that is −100 µV, compared to Fp2 and O2 which are overlying cortex that is −20 µV. F8, T8, P8 are more electronegative than Fp2 and O2. However, since the negative field is large and involving 3 electrodes equally, the 2nd and 3rd channels in the diagram do not show a potential difference. The astute electroencephalographer will pause because of the downward deflection in channel 1 and the upward deflection in channel 4. To investigate this further the EEG should be placed in a referential montage in which all electrodes are compared to some relatively neutral reference. This will show upward deflection of equal amplitude in the F8, T8, and P8 channels, confirming the existence of the negative field involving these electrodes.

A is incorrect; this diagram is designed to teach the student that lack of potential difference does not necessarily mean that there is nothing of interest happening in that channel. There is a large electronegative field here, but Fp2 and O2 are more electropositive than the other depicted electrodes. D is incorrect as this is a bipolar recording and the principle of localization in a bipolar recording is phase reversal, not amplitude. In a reference montage, the principle of localization is amplitude.

D

The brain waves depicted are normal. However, the deflection generated by a right eye blink are absent. This could be secondary to a third nerve palsy, a right eye prosthesis, or any condition that could cause right eye ophthalomoplegia. This patient had a right eye prosthesis. While the patient can blink both eyelids, he only has the dipole of the cornea and retina on the left, so there is no deflection on the right.

In alpha coma, there is no posterior dominant rhythm which attenuates with eye opening and comes back with eye closure as noted in this EEG. C is incorrect because the left eye blink deflections are normal.

A

This EEG shows right beating lateral nystagmus. Any occasion when there are potentials which are out of phase in the frontotemporal derivations should raise the suspicion for lateral eye movements. In lateral eye movements, the frontotemporal derivations are mirror images because when the eyes deviate to the right, the right cornea makes the F8 electrode electropositive and the left retina makes the F7 electrode electronegative. On the right, there is a more rapid rise followed by a gradual fall which is the corrective movement. The steeper positive phase reversal, seen here on the right, indicates the direction of the fast component of nystagmus. Similarly, the eye leads, located on the right and left outer canthus, are out of phase with lateral eye movements. This supports the eyes as the generator of the waveform. Note: The eye leads will show mirror images with both vertical and horizontal eye movements.

Bilateral independent frontal RDA or status epilepticus would not spare the frontocentral region. Furthermore, the finding does not evolve to meet the electrographic criterion for status epilepticus. Electrode pop can cause adjacent channels to appear like mirror images, but it is virtually inconceivable that the F8 and F7 electrodes would pop at exactly the same rate and in the same relation to each other.

A

HFF filters are typically set at 70 Hz and are useful for attenuating the amplitude of high frequency oscillations like muscle which are not generated by the brain. In a HFF the input signal is placed across a resistor and capacitor in series and the output signal is measured across the capacitor alone. At low frequencies the impedance of a capacitor is very high and at high frequencies the impedance of a capacitor is very low. Hence, if we are measuring our output over the capacitor alone, higher frequencies will attenuate to near zero as there is less potential difference across the capacitor. At lower frequency, the impedance is very high, so the potential difference across the capacitor is high, and hence the voltage of the input signal will be maintained in the output signal. In a low frequency filter, the input signal is placed across a capacitor and a resistor in series and the output signal is measured across the resistor alone. Again, the impedance of any capacitor is very high with low frequencies. In this arrangement, low frequencies are essentially blocked by the capacitor.

The input and output signal is measured in voltage. The output signal is typically displayed as voltage over time. Both LFF and HFF simply attenuate the amplitude of certain frequencies, they do not transform the frequency into another frequency. Hence B and D are incorrect.

D

Changing the sensitivity helps the electroencephalographer properly examine the EEG. Sensitivity is defined as the ratio of input voltage to pen deflection. Gain, an older term, is the ratio of the output voltage to the input voltage. An increase in the gain from 7 µV/mm to 15 µV/mm will lower the amplitude, decreasing the sensitivity. This change is often made when reading the EEGs of children as their brain waves are higher in amplitude and may interfere with one another when looking at a sensitivity of 7 µV/mm. So students heed this: When you lower the amplitude of the EEG you lower the sensitivity. This entails increasing the gain. The higher the gain, the flatter the EEG.

C

This EEG shows slow wave sleep. In slow wave sleep the EEG has at least 20% diffuse delta activity. The spindles and K-complexes of stage II sleep become rare. There is moderate muscle tone and there are no rapid eye movements. The parasomnias associated with slow wave sleep are night terrors, sleep walking, and bedwetting.

Frontal lobe seizures are often nocturnal and typically arise from stage II sleep, not slow wave sleep. This EEG is too slow to represent REM sleep. Furthermore the eye leads are synchronous and would be mirror images in REM sleep. Sleep paralysis and hypnogogic hallucinations occur typically in the transitions from wake to sleep or sleep to wake. During sleep paralysis, the individual experiences a complete inability to move often accompanied by an urgent need to flee from an intruder or respond to a pressing situation. Sleep paralysis can occur alone or with hypnogogic hallucinations, narcolepsy and/or cataplexy.

C

Mu rhythm is normal and found in the central derivations (C3/C4) over the motor strip. It can be bilateral or unilateral. It attenuates with movement or even the thought of movement of the contralateral upper extremity.

The PDR attenuates with eye opening. Wicket spikes are found in the mid-temporal electrodes, not the central derivations and are unrelated to arm movements. BIRDs (brief potentially ictal rhythmic discharges) are very brief (less than 10 seconds, typically 0.5–4 seconds) runs of rhythmic activity greater than 4Hz without evolution, which are associated with seizures and correlated with the seizure focus.

A

The EEG shows diffuse high voltage theta activity which is a normal response to hyperventilation and only occurs with good effort. Adults are less likely than children to have high amplitude theta and delta activity with hyperventilation.

This EEG lacks the 3 Hz spike and wave of childhood absence epilepsy. It is not appropriate to evaluate for diffuse slowing during hyperventilation. There are no features here to suggest a toxic metabolic disturbance.

D

The EEG shows a photomyogenic response which is somewhat rare but perfectly normal. Myogenic potentials (EMG artifacts) are seen in the frontal derivations, time locked to the flash frequency (arrows).

Seizures can occur in response to a specific stimulation such as photic stimulation, reading, thinking, or even hearing a particular note of music. Of these photosensitivity is the most common, frequently seen in idiopathic generalized epilepsy, particularly juvenile myoclonic epilepsy. In reflex epilepsy, an individual only has seizures in response to the stimulus. In a typical photoparoxysmal response, an individual will have high voltage generalized spike/polyspike wave discharge in response to photic stimulation. Photosensitivity is often maximal at 14–16 flashes per second. If the technician encounters a photoparoxysmal response the photic stimulation should be stopped to prevent a GTCC. Typically, the evoked discharges outlast cessation of the flash stimulus by a second or so.

Photic stimulation can evoke a rhythmic frequency in the occipital derivations which is at same frequency (the fundamental), a harmonic (twice the flash frequency) and/or a subharmonic (half the flash frequency). This is termed photic driving and it is perfectly normal. It is not seen here.