When reading any EEG you start with the background, which reflects the overall health of a person's brain and can be affected by many factors including acute illness, medications, degenerative disease and normal state changes.
The organization of an EEG tracing refers, broadly, to how the waveforms appear across the entirety of the page, and includes continuity, symmetry, and the anterior posterior gradient.
Continuity refers to the waveforms being uninterrupted by periods of flat or very attenuated activity. Healthy children and adults should always have a continuous record, but early neonatal tracings can show periods of discontinuity (this is discussed further in the neonatal and pediatric section). Below are examples of both a continuous and discontinuous record.
The next component of organization is symmetry, in which both the left and right sides appear, largely, the same in terms of both amplitude and frequency. Healthy EEGs should always be symmetric, and intermittent or persistent asymmetries can arise from structural entities such as tumors or bleeds. Changes in symmetry can be subtle, but note how on the asymmetric example below how the left hemisphere has higher amplitude, slower delta activity compared to the right side.
In addition to symmetry and continuity, consider the anterior posterior gradient, in which faster, lower amplitude frequencies are present towards the front of the brain while slower, higher amplitude frequencies are found in the back of the brain. The AP gradient leads into the last component of organization, the posterior dominant rhythm (PDR), discussed in the next section.
The posterior dominant rhythm (PDR) is the resting frequency of the occipital region when eyes are closed and the patient is resting quietly. It is a vital part of a normal EEG and among the first things you should look for; the PDR used to be called the alpha rhythm because the normal PDR (8.5-12 Hz ) is in the alpha range (7-13 Hz).
The PDR should be symmetric in both frequency and amplitude; if there is a more than 50% difference in amplitude or a more than 1 Hz difference in frequency between sides, this is abnormal. Of note, it is normal for the PDR on the left to be slightly attenuated compared to the right, thought to reflect a thicker skull on the left side in most people.
To determine the PDR, wait till the eyes are closed and then count the number of waves per second in the occipital region. It is helpful to check at more than one period, as the PDR can fluctuate mildly and you want to give a patient their best possible PDR. Of note, up to 5% of normal people have no PDR at all.
Note that the PDR emerges right after the patient closes their eyes (the eye closure is seen as the large positive wave right before the blue box -- we'll discuss why that is in the Artifact section). In adults, the normal PDR should be between 8.5 and 12 Hz and symmetric, but in children a normal PDR depends on the age, as discussed in the Pediatric EEG section. When the PDR is slower than 8.5 Hz, there may be generalized slowing present, as discussed in the Abnormal EEG section.
Of note, when finding the right PDR, be careful of two things in particular: alpha squeak and drowsiness. Alpha squeak describes a transient quickening of the PDR immediately after eye closure, and is named because, when EEG was still traced out on paper, the pen would squeak from moving so quickly. If you choose a PDR based on an area of alpha squeak you'll think it is faster than it actually is. On the other hand, if you choose the PDR when a patient is very drowsy or entering stage I sleep, even though it may be more apparent than when they're more awake, it might look a little slower than it actually is.
In this normal EEG tracing, note first that the anterior-posterior gradient is intact and normal, with faster, lower amplitude frequencies seen over the frontal regions and slower, higher amplitude frequencies seen over the posterior regions. Furthermore, the PDR, best seen after the eye closure (the large frontal positive wave right before the blue box, due to Bell's Phenomenon), is a crisp and symmetric 10 Hz. Note that the PDR recedes upon eye opening (the large frontal negative wave) several seconds later, as expected.
Another facet of a normal awake EEG is the presence of variability and reactivity. Variability refers to the presence of shifts in the waveforms across the span of a tracing. A normal brain should have regular fluctuations in the waveforms from second to second. Reactivity is simply the presence of shifts in frequency according to external stimuli; for example, the tracing of a drowsy patient whose background is mostly theta may become again mixed with faster frequencies if they hear a noise or other stimulus.
In patients who are in altered states of consciousness, such as under sedation for hypothermia protocol or refractory status epilepticus, reactivity and variability may be temporarily absent or reduced; in brain dead patients, the tracing is irreversibly neither reactive nor variable.
This is not a trick question, just an extreme example. This tracing of an unfortunately brain dead patient has no background activity or variability. Note the cardiac tracing does still have activity, but shows multiple abnormalities including profound bradycardia. Compare this flat EEG tracing to the other examples on this page to get a sense of how the EEG tracing should change second to second.
The last part of reading the background for a tracing is determining the patient's state, or whether they're awake, drowsy, or asleep. An awake adult EEG is marked by a plethora of findings including a symmetric PDR with predominant alpha and beta activity (there should be no delta activity in a healthy adult background), and the presence of many artifact types includine eye blinks, movement artifact (usually seen as very high amplitude, chaotic appearing changes in the background), myogenic artifact (seen as high frequency, low amplitude activity usually maximal over the frontal regions, due to the forehead's movement), and even chewing artifact. These artifacts will be further discussed in the Artifacts section.
Drowsiness is seen as a mild and diffuse slowing with decreased frequency of eye blinks and roving eye movements marked by very slow waveforms in the bilateral frontal regions. This arises because the corneas are positively charged, and when the eyes look to the right the F8 electrode sees the right cornea's positive charge while the F7 electrode sees a relatively negative charge. As the eyes move slowly back and forth when drowsy, this leads to the slow, undulating, opposing frontal waveforms that are classic for the drowsy state as seen below.
The transition from drowsiness to stage I sleep is subtle, and marked mostly by the emergence of POSTS (posterior occipital sharp transients of sleep) and vertex waves, but the asleep EEG is a topic all its own, and will be discussed in the next section.
Interpret the background of the tracing below
(AP gradient, PDR, variability/reactivity, state of consciousness)
The key to any EEG interpretation is a consistent approach. Look for a good anterior-posterior gradient, which this tracing shows with faster, lower amplitude beta activity in the frontal regions and slower, higher amplitude alpha activity in the occipital regions. Next, find the PDR and ensure it is symmetric in both frequency and amplitude, which this one is. If the PDR is present, the patient is awake, but eye blinks can help to confirm this. Variability and reactivity are almost always present if all of the above factors are present and normal.
As part of many EEG studies, provocation is done to better assess any underlying risk for seizures. The two main types of provocation are photic stimulation and hyperventilation.
In photic stimulation, a light is flashed in trains of increasing frequencies to look for photic driving, in which the background rhythm becomes time locked and in sync with each light flash. It is a normal response but many patients will not have it on EEG, and that is also normal. A driving response should be largely symmetric in terms of amplitude, and any major asymmetry may be suggestive of underlying dysfunction of the occipital / posterior brain regions. There is a form of driving called harmonic driving in which the background becomes time locked to some multiple of the light flashes; for instance, 5 Hz light flashes lead to a 10 Hz PDR or 6 Hz flashes to a 12 Hz PDR.
In those with a history of seizures, driving can very rarely can give rise to a photoparoxysmal response with epileptiform activity, which is further discussed in the epileptiform abnormalities section. Below is an example of a patient with photic driving from 6 Hz all the way up to 30 Hz; on this tracing, each flash of light is marked with a red line at the bottom of the screen.
The other major type of provocation is hyperventilation, which should not be done in patients over 65 years of age, those with chronic respiratory issues, or those with a recent stroke or myocardial infarction. The prototypical response to hyperventilation is diffuse slowing, but this is not always seen. The classic case for use of hyperventilation in epilepsy is with absence seizures, in which blowing on a pinwheel or other mode of causing hyperventilation can often cause brief absence seizures marked by generalized 2.5 Hz spike and wave activity (discussed further in the seizures section).
Photic stimulation marks each flash of light on EEG with a line at the bottom of the screen. This tracing shows a portion of a 5 Hz driving period in the first few seconds, then several seconds of no stimulation followed by a period of 8 Hz stimulation. Both the 5 Hz and 8 Hz period show driving, in which the posterior dominant rhythm becomes time locked to the frequency of the photic stimulation. Note the abrupt stop of this synchronization on cessation of the stimulation.
This tracing shows a clear, crisp 10 Hz PDR, best seen in the middle section upon eye closure (eye closure is the large positive deflection in the 6th second of the page)
Note the excellent anterior-posterior gradient with faster, lower amplitude frequencies in the front and slower, higher amplitude frequencies in the back, and the clear PDR on eye closure
This shows a good anterior-posterior gradient, eye blinks, and PDR emergence with eye closure and recession with eye opening. There is also some myogenic artifact over the frontal regions
Another example of frontal myogenic artifact, seen in the first half of the tracing, and PDR with eye closure. Lateral eye movements are seen as opposing waveforms in the bilateral frontopolar electrodes
Notice the diffuse frontal fast activity in all the chains, indicative of myogenic artifact. Otherwise, this is a normal page with a clear 7-8 Hz PDR. Recall that a normal PDR is 7.5-12 Hz
This is a good example of several key features of a normal awake background: a normal AP gradient, good PDR that emerges with eye closure, multiple eye blinks, and movement/myogenic artifact.
Eye blinks appear due to Bell's Phenomenon: eyes roll back when you blink and, because the retina is negatively charged and the cornea is relatively positive, this eye roll moves the retina down and away from frontal electrodes, which thus see a positive deflection
Note the very high frequency myogenic artifact overlying T2 and T3. Myogenic artifact is often maximal over the front, from the forehead and is a very common finding in the awake state
Drowsiness is marked by a diffuse slowing and attenuation of activity, with fragmentation of the PDR and roving lateral eye movements seen over frontal leads
Opposing waveforms in frontal leads when drowsy arise because the cornea is positively charged. When you look to the right, the right cornea gets closer to F8, which thus sees a positive deflection, while the left cornea moves away from F7 and it sees a negative one
Photic driving is when the PDR becomes time locked to the frequency of the light flashes (which are each marked with a red line at the bottom of the tracing). It is a normal but not requisite response
Lack of driving is not considered abnormal, as is the case here where we see a train of photic stimulation without any corresponding time lock of the background activity