nonepileptiform abnormalities


slowing suggests dysfunction

Outside of classic epileptiform activity, EEG can show a broad array of abnormalities due to underlying focal or generalized cerebral dysfunction. Most common among these is slowing, such as from a tumor or bleed. Slowing is categorized in several ways, including generalized vs focal, continuous vs intermittent, and polymorphic vs monomorphic/rhythmic. Other abnormalities discussed here include discontinuity, asymmetry, attenuation, and breach.

generalized slowing

Recall that when reading the background, you should look for a clear PDR, AP gradient, synchrony, symmetry, continuity, reactivity, and clear wave morphology. If any of these things are lacking in a tracing, it is said to be disorganized. Disorganization goes in hand with generalized slowing, a nonspecific finding suggestive of diffuse cerebral dysfunction / encephalopathy as can be seen with severe infections, medication or substance abuse, or neurodegenerative disease. Generalized slowing can be categorized into mild, moderate and severe. The exact delineation from one to the next is somewhat subjective, but a few rules of thumb may help.

Mild generalized slowing is typically marked by the presence of a slowed PDR and a poor AP gradient, often with excess theta admixed with the normal alpha activity throughout; reactivity, variability and the other markers of a normal awake EEG remain. With moderate generalized slowing the PDR is very fragmented or gone entirely, and the record is often predominantly theta into delta activity with much less alpha. Severe generalized slowing is highly disorganized and often discontinuous without reactivity or appreciable architecture such as an AP gradient or PDR, and is usually predominated by low amplitude delta activity.

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Mild generalized slowing

This tracing is marked by a mildly slow PDR of 6-7 Hz, along with a notably less organized AP gradient. Much of the usual architecture of a normal study is here though, including eye blinks and variability across the page. We see some excess theta admixed into the normal and expected alpha activity, but no major delta activity. The area marked by a purple arrow may appear to be frontal delta activity, but these are in fact epileptiform spike and slow waves (note the notch-like spike before each large slow wave), discussed in the epileptiform activity section.

Focal slowing

While generalized slowing suggests diffuse brain dysfunction, focal slowing is typically evidence of a structural abnormality involving the slowed area, particularly if the slowing is mostly delta.

Slowing is categorized in several ways. First, it can be persistent/continuous or intermittent. Continuous slowing is suspicious for a larger lesion such as a tumor, bleed, ischemic infarct or demyelinating lesion. Intermittent slowing can also be any of these things, but more often suggests a smaller lesion such as focal cortical dysplasia or the beginnings of a small tumor, and thus is more likely seen on EEG with certain state changes, overlying illness, or medication effect (this is not a hard and fast rule).

When considering any slowing, break it down into being either Slowing can be either continuous or intermittent and either polymorphic or monomorphic/rhythmic. Polymorphic slowing is most commonly seen and is thought to arise from a mix of white and grey matter injury but is generally nonspecific in etiology, while monomorphic/rhythmic slowing is thought to localize more to grey matter but, more importantly, is often concerning for epileptiform activity.

Continuous/persistent focal slowing is suggestive of a significant structural abnormality such as tumor, bleed, ischemic infarct or demyelinating lesion. While slower frequencies (delta vs theta, for instance) don't necessarily suggest a worse underlying abnormality, continuous slowing is worse than intermittent, and a rule of thumb is that the less reactive and variable a slowed region is, the worse the underlying cerebral dysfunction.

Intermittent focal slowing can also arise due to a structural abnormality, but such abnormalities tend to be smaller in nature, such as a focal cortical dysplasia or the beginnings of a small tumor, and thus are more likely seen on EEG with certain state changes, overlying illness, or medication effect (this is not a hard and fast rule).

On the example below, note how the left hemisphere (maximal temporally) continuously has a slower to at times absent PDR compared to the right, and is marked by theta to delta activity while the right hemisphere has normal alpha and beta activity.

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Left Hemispheric Polymorphic Delta Slowing

This tracing shows persistent high amplitude delta slowing over the left hemisphere. Given the high amplitude particularly over the parasagittal chain, this may represent not just slowing but breach, which is seen after brain surgery and discussed more below. Note also the periods of mu rhythm--the resting alpha frequency of the sensorimotor cortex that is arch-like in morphology, over the right parasagittal chain; mu rhythm goes away with thoughts of or initiation of movement.

The "irdas"

When focal slowing is seen as intermittent rhythmic delta activity, its significance varies by the region of the brain involved, with some locations suggesting epileptic potential and others just nonspecific encephalopathy. In order to be categorized as rhythmic activity, there must be 6 cycles of the activity; for example, one wave per second for 6 seconds, or two waves per second for 3 seconds. This is also discussed in the rhythmic activity section of the epileptiform activity page.

Frontal intermittent rhythmic delta activity (FIRDA) is prognostically similar to generalized slowing in that it suggests cerebral dysfunction of nonspecific etiology, and is commonly seen in very sick and encephalopathic patients, and those with dementia. On the example below note that the bifrontal delta activity does not travel back to the posterior regions; if it did, it would not be frontal but rather generalized rhythmic delta activity (GIRDA).

Frontal intermittent rhythmic delta activity (FIRDA)

While FIRDA is nonspecific, temporal intermittent rhythmic delta activity (TIRDA) is considered an epileptiform abnormality with high risk for seizures from the affected temporal lobe(s); see the epileptiform activity section for details.

Left temporal intermittent rhythmic delta activity (TIRDA)

Somewhere in the middle, prognostically speaking, of TIRDA and FIRDA is occipital intermittent rhythmic delta activity (OIRDA), which is far, far more common in children and while often seen in kids with epilepsy can also be found in those with encephalopathy of varying etiologies. In the example below note that the OIRDA has some embedded spikes (see epileptiform activity section); in cases such as this, you should definitely consider this pattern to be epileptogenic.

Occipital intermittent rhythmic delta activity (OIRDA)
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Left temporal intermittent rhythmic delta activity

This tracing shows intermittent runs of approximately 3 Hz delta activity over the left temporal region, more prominent in the anterior than posterior temporal region. Because these intervals of 3Hz delta last at least 2 seconds, they complete the 6 cycle requirement for rhythmic delta activity. Note that the delta is very apparent in the lateral temporal leads below the traditional parasagittal and temporal chains--a reminder to always keep an eye on all of your available chains on EEG.

breach activity

Breach activity describes higher amplitude, often spiky and irregular appearing activity over a region of the skull that has been previously opened due to brain surgery. It is so named because it arises due to the breach in the skull.

Often, breach activity coincides with slowing because the region of brain underneath, if it needed surgery, likely had something wrong with it in the first place. It is important not to mistake the spiky appearance of breach activity for epileptiform discharges, although it is common that epileptiform activity is embedded within breach (again, abnormal brain that led to the necessary surgery may be irritable and thus with epileptiform activity).

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Moderate Generalized Slowing

Recall that when assessing background you should look for a PDR, AP gradient, synchrony, symmetry, continuity, reactivity, and clear wave morphology. On this page, we have a continuous tracing that is synchronous and symmetric. However, there are only fragments of a PDR on both sides, and when present it is no faster than 5 Hz. Thus we have at least mild generalized slowing here. However, there is also no AP gradient--realistically, the page would look much the same if we flipped it upside down. Furthermore, we don't have much alpha activity, with the page mostly theta and delta activity that is relatively monotonous in appearance from beginning to end. Overall, then, this would be considered moderate generalized slowing.

Attenuation & Discontinuity

While breach activity is seen as higher than expected amplitude and spiky morphology, attenuation describes lower amplitude activity of varying causes. Persistent attenuation over a region can arise from something sitting between the brain and the skull, such as a hematoma or hygroma. A sudden regional attenuation may be evidence of a stroke. Intermittent attenuation may even be interictal or ictal, such as tonic seizures that classically involve a sudden diffuse attenuation with overriding fast activity.

Discontinuity describes intermittent periods of attenuation, and outside of the neonatal stage of life is always highly abnormal and a marker of severe generalized slowing. You may see it in comatose patients and those with severe brain injury; in cases of refractory status epilepticus unresponsive to multiple antiseizure drugs and infusions, extreme sedation leading to an intentional discontinuity called burst suppression (which requires at least 50% of the record to be suppressed) is used.

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Persistent Right Hemispheric Polymorphic Delta Slowing on a background of Mild Generalized Slowing

This is a complicated tracing so let's look at one side at a time. First note that on the left side you have a clear PDR of 8-8.5 Hz, which is in the normal range. However, the AP gradient on the left is poor, with excess slow activity in the frontal regions. Note also that the F7 electrode is bad (note how Fp1-F7 and F7-T3 move in exact opposition to each other, a sign of a bad F7 electrode) and Fp1 and F3 are too close to each other (they've formed a salt bridge, seeing the same voltage as each other and thus cancelling each other out on bipolar, leading to a flat line; on an average montage, they'd have identical appearing lines).

Moving to the right side, we see a persistent / continuous, somewhat low amplitude delta activity in the frontal regions, and fragments of a PDR in the occipital leads. There is even less of an AP gradient and normal morphology than on the left side. As such, we say this tracing has persistent right hemispheric polymorphic delta slowing on a background of mild generalized slowing.

Excess beta

Excess beta activity, when diffuse or frontally predominant, is a commonly seen phenomenon most often arising as a medication effect in the setting of benzodiazepine or barbiturate use. It is marked, as expected, by low amplitude beta overriding the normal activity throughout the tracing. It can also be seen with anxiety and, somewhat counterintuitively, with drowsiness. While excess beta is an abnormality, it is essentially a benign one.

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Generalized Intermittent Rhythmic Delta
(right side predominant)

This tracing shows a burst of generalized high amplitude delta activity at about 3Hz, lasting several seconds and thus meeting the ACNS criteria for rhythmicity (aka requires 6 cycles). The delta activity is generalized but more prominent on the right side, particularly in the temporal chains. We would thus classify this as generalized intermittent rhythmic delta activity (GIRDA) with right-sided predominance. Recall that GIRDA is similar to FIRDA prognostically in that it suggests nonspecific and diffuse cerebral dysfunction.

triphasic waves

Triphasic waves are a nonspecific waveform most classically associated with metabolic (historically, hepatic) encephalopathy. Their name is descriptive, as they are comprised of three parts, with each part being slightly longer than the preceding one. They are most often generalized, with a subtle delay in the onset of each as you move from the anterior to posterior regions.

Sometimes, triphasics can be periodic, particularly in critically ill patients. In the example below there are several on the page, but the most well formed one is marked; note that this page also shows moderate generalized slowing.

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Left Temporal Polymorphic Theta / Delta Slowing

Here we see a good PDR throughout of 10 Hz, but note while the right temporal region is alpha and beta activity, the left temporal region has a kind of "sloppier" appearance with admixed slower frequencies from theta and delta. In a normal awake adult, you should basically never see theta or delta activity. The slowing here is not rhythmic and seems to run across the entire page, so we'd classify it as focal left temporal persistent polymorphic theta to delta slowing.

Generalized slowing is a sign of nonspecific cerebral dysfunction & encephalopathy

Mild generalized slowing is marked by a slow PDR and poor AP gradient; moderate gen slowing has minimial PDR and diffuse theta to delta activity; severe gen slowing is often unreactive, attenuated and discontinuous

Focal polymorphic slowing suggests the presence of an underlying abnormality such as tumor, bleed or stroke

The significance of focal rhythmic slowing depends on location: FIRDA suggests nonspecific and diffuse dysfunction, TIRDA is an epileptiform variant, and OIRDA can be either

Breach artifact is seen as a higher amplitude, spiky activity over a region of prior brain surgery, often with slowing

Attenuation has multiple etiologies including hematomas, strokes and even seizures or interictal activity

Discontinuity is seen as intermittent attenuation and is a sign of severe generalized slowing

Excess beta activity is a consequence of benzodiazepine or barbiturate use, and is abnormal but benign

Triphasic waves are generalized waveforms with a delay in each phase of the wave, as well as an AP gradient, which are indicative of metabolic encephalopathy

Mild Generalized Slowing with Focal Polymorphic Left Temporal Slowing

This tracing shows diffuse theta activity with a slow PDR and relatively poor AP gradient, consistent with mild generalized slowing. however, there is also a polymorphic focal delta slowing over the right temporal region. Especially when you're starting to read EEGs, this sort of slowing-on-slowing can be hard to see.

Mild Generalized Slowing with FIRDA

Starting with the background here, we see a relatively poor AP gradient and slow, often poorly formed PDR. This is consistent with mild or even the beginnings of moderate generalized slowing. We also see, in the first half of the page, higher amplitude and somewhat rhythmic delta activity in the frontal regions that goes on for at least 6 cycles, consistent with FIRDA albeit not the cleanest example as such.

Moderate Generalized Slowing

This tracing has no clear AP gradient, a PDR that is mostly absent and, when seen, is no faster than 5 Hz, and is predominated by theta to delta activity. All of this is consistent with moderate generalized slowing. Recall that mild generalized slowing retains a PDR and AP gradient but the former is slowed and the latter is suboptimal, and that severe generalized slowing usually has discontinuity, poor reactivity/variability, and mostly delta activity that is often low amplitude.

Right Temporal Breach

This tracing has a normal background with a PDR of about 12 Hz. The right temporal region, however, has higher and more sharply contoured morphology than does the left temporal or other regions, consistent with breach artifact as is seen after brain surgery due to the residual skull defects. Within the breach rhythm we see very nicely formed wickets, a benign finding in the temporal lobes that are marked by arch-like, phase reversing waves that come in runs. They should not be mistaken for epileptiform discharges, and are often more easily seen within breach.

right temporal breach with wickets
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generalized slowing

Focal slowing

intermittent rhythmic delta activity

attenuation & discontinuity

triphasic waves

excess beta activity

breach rhythm