What are focal EEG waveform abnormalities?

Updated: Oct 09, 2019
  • Author: Alexis D Boro, MD; Chief Editor: Selim R Benbadis, MD  more...
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The role of EEG, and in particular the focus on focal abnormalities, has evolved over time. In the past, the identification of focal EEG abnormalities often played a key role in the diagnosis of superficial cerebral mass lesions. For example, if the EEG of a comatose patient revealed decreased amplitude over 1 cerebral hemisphere, subdural hematoma was strongly suspected. Today, with the availability and routine use of detailed imaging techniques such as CT and MRI, the EEG no longer plays this role, although it still has a central place in the diagnosis and management of patients with seizures, epilepsy, and altered mental status. [1] While the indications for EEG have evolved, the EEGs recorded today still exhibit the same variety of focal abnormalities as in the past and a framework for analyzing these abnormalities is still necessary to interpret and articulate the clinical implications of a recording.

Focal EEG abnormalities may be categorized as epileptiform or nonepileptiform. Epileptiform transients such as spikes and sharp waves are the interictal marker of a patient with epilepsy and are the EEG signature of a seizure focus. [2] Nonepileptiform abnormalities are characterized by alterations in normal rhythms or by the appearance of abnormal ones. While these 2 types of nonepileptiform abnormalities often coexist, especially when clinically significant, distinguishing them is conceptually useful and aids in their recognition and description.

Nonepileptiform EEG abnormalities are associated with focal cerebral dysfunction, often due to a demonstrable structural lesion. By contrast, more widespread central nervous system (CNS) derangements, such as those due to metabolic disturbances, usually produce generalized EEG abnormalities. On occasion, widespread CNS derangements do result in the appearance of focal EEG abnormalities; this often occurs in patients with preexisting structural abnormalities.

The discussion that follows assumes familiarity with the normal EEG. For an excellent introduction, see Normal EEG Waveforms.

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