potential causes, including toxic, metabolic, degenerative, and multifocal vascular disease.
• Diffuse spikes or sharp waves – correlate with a generalized seizure disorder. Cannot rule-out secondary generalization.
Abnormal frequency composition
Interpretation of the EEG is a complex assessment of frequency composition and localization. Abnormal frequency composition can consist of any of the following: • Slowing of the background rhythms
• Excessive fast activity • Excessive theta activity
Slowing of the background rhythms is discussed in Slow activity. Excessive fast activity is usually seen in patients sedated with benzodiazepines – beta activity is prominent frontally. Theta activity is present in almost all recordings, and can be seen if the gain is high enough. Theta is not a prominent component of the background in waking adults, and when it stands out from the baseline is abnormal; there are several potential clinical correlations, discussed below.
Slow activity
Diffuse slowing
Diffuse slowing can have several presentations. The most common is slowing of the posterior dominant rhythm in the waking state. Occasionally, slow activity can be superimposed on an otherwise normal waking background. Identification of abnormal slow activity in sleeping records is especially challenging.
Slowing of the posterior dominant rhythm
The normal adult waking EEG consists of mainly fast rhythms. With eyes closed, rhythms in the alpha range are seen from the posterior regions and faster frequencies are
seen from the frontal regions. Slowing of the PDR to less than 8.5 Hz is always abnormal in adults. Slowing of the posterior dominant rhythm (PDR) is usually seen as the
posterior rhythm in the theta range, e.g. 6-7 Hz. The slow posterior dominant rhythm differs from the normal faster rhythm in a few ways:
• Slow PDR is less stereotyped than normal PDR, with bumps on the waves • Slow PDR is less reactive to eye opening than normal PDR, it does not show the
degree of attenuation of normal PDR
• Slow PDR is often associated with theta prominent more forward of the occipital regions than the normal PDR extending forward of the occipital regions.
Interpretation of the slow PDR
The slow PDR is interpreted as being abnormal, but is not specific. Possible causes include:
• Toxic-metabolic encephalopathy • Degenerative dementia
• Multifocal vascular disease
The impression when this is the only finding might be: “Abnormal study because of slowing of the posterior dominant rhythm. This is suggestive of a diffuse encephalopathy, although it is a nonspecific finding.” Comment might be made about metabolic, toxic, and dementing causes, depending on the specified clinical question.
Subharmonic PDR (Normal variant)
Occasionally, the PDR has the appearance of a subharmonic – where there may appear to be a 5-6 Hz PDR with otherwise normal frequency composition and appearance of the EEG. This is a normal variant, and should be interpreted as normal. The subharmonic PDR can be differentiated from slowing of the PDR in the following ways:
• Slowing of the PDR in the 5-6 Hz range should be associated with slowing seen anteriorally to the occipital lobes, whereas subharmonic PDR has otherwise normal frequency compositions.
• Slowing of the PDR in the 5-6 Hz range will usually not attenuate completely to eye opening, whereas subharmonic PDR completely attenuates.
• Slowing of the PDR in the 5-6 Hz range with have an irregular, polymorphic appearance, whereas subharmonic PDR is regular, and usually notched, so that the underlying 10 Hz rhythm can be seen.
Slow activity superimposed on the waking background
Theta and delta activity in waking records is usually abnormal. Diffuse slowing is usually polymorphic delta or irregular theta which is seen from both hemispheres. The slowing does not typically have the regional concentration of the normal PDR or frontal fast
activity. Most of the time, the PDR is slow when there is diffuse slowing, but not always. Diffuse theta with a temporal prominence can be associated with a PDR still in the alpha range.
Causes of slow activity superimposed on an otherwise normal waking background includes:
• Encephalopathy due to toxic or metabolic causes • Cerebrovascular disease which is multifocal or diffuse • Head injury
Generalized slowing in sleep recordings
Identification of abnormal slowing in a waking record is easy, but identification of abnormal slowing in a sleeping record is much more difficult. The sleep record consists of slow activity in the theta and delta range, and the exact pattern depends on sleep stage. For example, stage 3 and 4 sleep is composed of prominent delta activity with virtual abolition of the faster frequencies. These deeper stages of sleep could easily be
misinterpreted as encephalopathy if the electrophysiologist is more used to seeing waking records and stages 1 and 2 of sleep. Therefore, encephalopathy should be the
interpretation of a sleep record only if the slow activity is inconsistent with any stage of the sleep-wave cycle. Even then, a waking record should be examined, it at all possible. Conversely, normal sleeping record does not rule-out an encephalopathy. It is very common to have abnormal slowing during the wake state yet normal sleeping patterns. In this instance, the EEG report should reflect the limitations on interpretation of
encephalopathy in the sleeping state. The impression might read:
“Normal sleeping EEG. Encephalopathy is difficult to diagnose in the sleeping state. A waking record should be obtained, if clinically indicated”.