sharps, spikes & slow waves

Any epileptiform discharge is a disruption of the usual functioning of the brain, and sharps and spikes are perhaps the most classic type. A sharp is a single epileptiform discharge defined by its duration lasting between 70-200ms, and by its disruption of the EEG background. A spike is very similar to a sharp but faster, with a duration from 20-70ms. Realistically, the cutoff between them is not very important clinically, as one isn't known to be any more "severe" than the other.

After a spike or sharp, there is typically a slow wave, which represents the refractory period of the affected neuron population after the large and synchronized EPSPs that led to the spike or sharp itself. Slow waves always immediately follow, and are often higher amplitude than, their precedent spikes or sharps.

Whether an interictal has an appreciable slow wave or not, it should always disrupt the background and have a field; this is to say, you should see "ripples" of a discharge in the surrounding EEG electrodes (as discussed in the technical section). The left temporal discharge above, for example, has a good field throughout the left temporal region, with the surrounding tracings "pointing to" the discharge site, as expected in bipolar montages. If you see a single discharge without a field, its good to make note of it but you probably shouldn't call it an interictal discharge (do, however, look for more of them and remain suspicious).

localizing the discharge

Generally speaking, the location of an epileptiform (or interictal) discharge suggests cortical irritability of that region. For example, a discharge at T4 suggests right mid-temporal hyperexcitability / epileptogenic potential. However, there are a few fine details or caveats to this rule.

First of all, generalized discharges cannot be localized. Typically found with primary or symptomatic generalized epilepsies, generalized discharges begin in such a widespread fashion that they effectively involve the entire cortex simultaneously. It is possible that deeper structures, such as the thalamus, may be involved with generalized discharges.

Note that on EEG generalized discharges do not have to be completely the same in every single lead; there is often an anterior predominance to them, for example, but as long as the morphology of the discharge remains throughout all the leads (even if some are less well formed or lower amplitude) and the time of onset is the same in all the leads, you should consider it a generalized discharge.

You may also see discharges that appear generalized but are actually focal with rapid bisynchrony, in which they actually arise from a single location but the networks involved propagate the signal too quickly to trace that location, and they appear generalized. Sometimes with rapid bisynchrony, with careful review you can find a small precedent change before the discharge, such as very low amplitude fast activity, to suggest a lateralized onset.
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The frontal lobes can be a confusing place for interictal discharges. Deep frontal discharges can be missed on scalp EEG entirely, and mesial frontal discharges can appear on EEG as if they are from the contralateral frontal lobe due to the direction of the discharges’ dipole. This isn’t always the case, but something to keep in mind when viewing frontal discharges.

Furthermore, while anterior temporal interictals are perhaps the most common focal epileptiform discharges, because of the proximity of the F7 and F8 electrodes to the inferior frontal region, what appear to be anterior temporal discharges can, at times, actually be from the inferior frontal region.

The central region is less complicated: discharges here are usually normal in the asleep state (ex. vertex waves from the central region) but basically never normal in the awake state. The one caveat is that very mesial discharges from the frontal or parietal lobes can sometimes be seen on the midline EEG electrodes, so if you see midline discharges in the asleep state that are not as symmetric as vertex waves, or with an atypical formation or field for vertex waves, you should be suspicious.

In the occipital region, remember that the O1 and O2 electrodes, similar to Fp1 and Fp2, suffer from the end of chain issue in bipolar montages. That is to say, because there is no electrode behind them to compare their voltage to, there is no phase reversal to stand out on EEG. So, if you see a positive discharge in the occipital regions that is not consistent with a lambda wave or POST, check a circumferential or referential montage to clarify if you’re seeing a true interictal discharge.

Periodic patterns

Generally speaking, periodicity on EEG is almost always suspicious. The background should be a little chaotic looking, because all the neurons are going about their own business within their respective networks. When the background becomes too organized--including via periodicity--this raises suspicion for abnormal synchronous firing of neurons.

Periodicity, as defined by the ACNS criteria, requires the relevant discharges to occur for at least 6 cycles. So, if the discharges are periodic at 1 per second, they must go on for 6 seconds; if they are periodic at 2 discharges per second, they must go on for at least 3 seconds; if they are periodic at 3 discharges per second, they must go on for at least 2 seconds, and so on.

If the interval between each discharge in a periodic set of discharges varies by 25-50%, it is termed quasi-periodic. If the variance between each one is more than 50%, it isn't actually periodic at all. Full details of the ACNS approved documentation of periodic and rhythmic patterns on EEG are available here.

Below is an example of lateralized periodic discharges (LPDs), also termed periodic lateral epileptiform discharges (PLEDs). This example may be borderline between periodic and quasi periodic, as several of the discharges have longer intervals between them bordering on a 25% difference discharge to discharge.

While the above example is lateralized, you may also see generalized periodic discharges (GPDs). These are also concerning but lateralized activity is more likely to be associated with seizure activity. Note that periodic patterns can also be part of a seizure, but seizures require temporal and/or spatial evolution. However, if a periodic pattern arises and reaches 2.5 Hz for at least 10 seconds, that should be considered a seizure and merits treatment.

rhythmic patterns

Recall that some rhythmic activity, such as frontal intermittent rhythmic delta activity (FIRDA) or generalized intermittent rhythmic delta activity (GIRDA), is not necessarily epileptogenic. When you see a consistently lateralized rhythmic pattern (such as theta or delta), however, this should raise suspicion for focal cortical hyperexcitability.

Rhythmic patterns carry similar requirements to periodic patterns, needing at least 6 cycles to be considered a rhythmic pattern. While periodic patterns, however, have discrete intervals between each discharge, rhythmic patterns do not--one waveform flows directly into the next.
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Temporal intermittent rhythmic delta activity (TIRDA), in particular, is an epileptogenic pattern often associated with an underlying focal temporal lesion such as a tumor or cortical dysplasia.

Occipital intermittent rhythmic delta activity (OIRDA) is more common in children and usually also considered epileptogenic, although perhaps not as universally as TIRDA.

paroxysmal fast activity

When you see diffuse fast activity, its often in the setting of excess beta activity (usually a benzodiazepine effect) and is benign, while very fast activity over the frontal regions is usually muscle artifact. However, paroxysmal runs of fast activity can be epileptiform, and are most commonly seen in patients with generalized epilepsy, Lennox-Gastaut Syndrome, and tonic seizures.

Paroxysmal fast activity can be localized, as in the example below, or generalized. When you see burst of generalized paroxysmal fast activity, ensure it is not actually a tonic seizure, of which fast activity is classically a key part.

brief potentially ictal rhythmic discharges (birds)

If you look up in the sky you may see birds, but you can also see them on EEG tracings. Brief potentially ictal rhythmic discharges, or B(I)RDs, are ictal appearing rhythms that evolve but do not meet the minimum 10 second criteria to be considered a seizure. They can be seen in a broad range of patients, but most commonly in critically ill patients (especially those with newfound tumors and strokes), neonates, and sometimes those with NMDA receptor encephalitis. 

While B(I)RDs are not technically seizures, they are associated with a similarly poor outcome and thus merit treatment; in fact, over 90% of patients with B(I)RDs on EEG will go on to have seizures. In the example below, we see an ~8 second run of evolving left temporal spike wave activity consistent with B(I)RDs, and note the field into the left parasagittal and subtemporal regions.

photoparoxysmal Response

The photoparoxysmal response, while perhaps one of the most well known facets of epilepsy, is in actuality quite rare. It describes the emergence of interictal activity as a result of photic stimulation. Typically, this interictal activity is generalized or focal occipital, arises during but outlasts the photic stimulation itself, and is always reproducible at the same frequency of light flashes. Note that photic driving does not confer an increased risk for a photoparoxysmal response or photosensitive seizures.