routinely used. Sleep is considered by some to be an activation method, but is considered separately, above, since it is a normal state change rather than an applied method.
Photic stimulation
Photic stimulation is repetitive flashes of light delivered at different rates. The flash stimulus can evoke epileptiform discharges at certain flash rates. Photic
stimulation is more likely to evoke epileptiform discharges in patients with generalized epilepsies than in patients with focal epilepsies.
Methods
The stimulating protocols are pre-programmed into most EEG machines. General guidelines for performing photic stimulation include the following:
Figure 4-11: Photic evoked potential
Left medial portion of the longitudinal bipolar montage, with channel 1 showing the stimuli. Flash at 5/sec produces an evoked potential in the fourth channel, due to activity in the occcipital lead. The upgoing potential in this bipolar montage indicates positivity at the O1 electrode. The positivity is delayed from the stimulus by about 100 msec, indicating that this is an evoked potential rather than a photic response.
• Train duration of 10 sec • Trains delivered every 20 sec • Initial flash rate of 3/sec
• Higher rates for successive trains. We use 3, 5, 7, 9, 11, 13, 15, 18, 20, 24, and 30 flashes per second.
If the discharge is activated as a specified frequency, the technician should repeat that
frequency at the
completion of the photic stimulation routine. Normal responses to photic stimulation include the visual evoked
response, the driving response, and the
photomyoclonic response. Normal photic response Visual evoked response: A visual evoked response can be seen in occipital leans at flash frequencies of less than 7/sec. This response is the same as the flash-induced visual evoked potential discussed in Chapter 14, but is much more variable because the response is not averaged.
Driving response: A driving response is seen at flash frequencies of 7/sec and greater. The two responses look alike but are distinguished by their temporal relation to the stimulus. The visual evoked response occurs approximately 100 ms after the stimulus, and the driving response is exactly time-locked to the stimulus.
The absence of a visual evoked response or a driving response is not abnormal unless it is well developed on one side and absent on the other. Such asymmetry suggests an
abnormality affecting either the projections from the lateral geniculate to the cortex or the calcarine cortex, itself.
Photomyoclonic response
Figure 4-12: Photic driving response
Photic stimulation at 10/sec produces a fast response which is time- locked to the stimulus, differentiating it from the evoked potential. Frequencies which produce a driving response are the faster than those which produce an evoked response. Left medial portion of the LB montage.
The photomyoclonic response is caused by repeated contraction of frontal muscles that are time locked to the flash stimulus with a delay of 50-60 ms. The potentials are
suppressed by eye opening and disappear with neuromuscular blockade. Muscle activity that may have the appearance of seizure discharges appears in the anterior leads. Several factors help to distinguish between a photomyoclonic response and a photoconvulsive response:
• The photomyoclonic response is anterior whereas the photoconvulsive responses are posterior or generalized.
• The photomyoclonic response stops promptly at the end of the stimulus train, whereas the photoconvulsive response typically outlasts the stimulus.
• The spikes that make up the photomyoclonic response are mush faster than those of cerebral origin. The synchrony of muscle fiber discharges is much greater than that of neuronal discharges.
• The photomyoclonic response has the same frequency as the flash, whereas photoconvulsive discharges are often slower, in the range of 3/sec.
Photoconvulsive response
The photoconvulsive response is characterized by spike-wave complexes during photic stimulation. The discharge is usually activated only by a few specific flash frequencies. It never begins with the first flash and usually ends before the flash ends. The correlation of a photoconvulsive discharge with seizures is greatest if the discharge continues after the end of the flash train.
Hyperventilation
Hyperventilation is usually used to activate the three-per-second spike and wave discharge of primary generalized epilepsy. In some patients, discharges are seen only during
hyperventilation. The patient is asked to mouth-breathe deeply for approximately 3 minutes. If there is suspicion of absence seizures, the patient should hyperventilate for 5 minutes. The normal response to hyperventilation is generalized slowing of the background activity in the theta range in both hemispheres. Absence of slowing is nor abnormal and depends on effort, age, and time from last meal, Children show more slowing than adults with hyperventilation. Hypoglycemia may augment slowing.
Figure 4-13: Photoconvulsive response
Photoconvulsive response, usually produces by a speccific range of photicc frequencies. The discharge is not time- locked to the stimulus and typically last longer than the stimulus. Left medial portion of the LB montage.
Movement artifact may contaminate the record, especially in the posterior leads, as a result of head movement with chest excursions. Normal low activity may have a notched appearance and should be interpreted as epileptiform. The epileptiform discharges activates by hyperventilation are usually not subtle. Hyperventilation should not be
performed in patients with
cerebrovascular disease or intracranial hemorrhage. Hypocapnia and alkalosis may cause vasospasm and impair cerebral perfusion.
Figure 4-14: Hyperventilation
Top: Normal waking EEG. Left medial portion of the LB montage.
Bottom: Slowing in the theta and delta range with hyperventilation. There is also muscle artifact.