What are important caveats in interpreting focal interictal epileptiform discharges (IEDs) on EEG?

Updated: Oct 09, 2019
  • Author: Alexis D Boro, MD; Chief Editor: Selim R Benbadis, MD  more...
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Important caveats are as follows.

  • Most blind patients with rare occipital spikes do not have seizures.

  • When the general population is screened, the presence of IEDs characteristic of the idiopathic or primary epilepsy syndromes frequently occur in patients who do not have seizures.

    • This is particularly true of centrotemporal spikes, which are associated with the syndrome of benign childhood epilepsy with centrotemporal spikes (BECTS). This syndrome presents in preadolescent children and usually resolves in the mid teens. The EEG signature of the syndrome is stereotyped, usually bilateral, broad diphasic centrotemporal spikes that always become more frequent in sleep and often display a characteristic transverse dipole that is negative in the centrotemporal area and positive frontally. In one population study, up to 40% of patients with centrotemporal spikes did not have seizures. [8]

    • Seizures in BECTS are usually nocturnal, typically characterized by drooling and rhythmic contraction of 1 side of the face and arm and may secondarily generalize. The presence of the typical discharge secures the diagnosis.

    • If a patient has a substantially different seizure type, the possibility that the discharges are an incidental finding must be carefully considered, along with the possibility that the patient suffers from a localization-related epilepsy with an electroclinical signature that mimics BECTS.

  • Mistakes in EEG interpretation are an important source of false-positive results. Familiarity with the common normal EEG variants that resemble epileptogenic discharges is necessary to avoid characterizing them as IEDs. Commonly encountered examples include posterior slow waves of youth and positive occipital sharp transients of sleep. This family of patterns is discussed in Normal EEG Variants. Artifacts produced by electrode dysfunction can also resemble epileptiform discharges. Features of the normal EEG background such as the mu rhythm, and vertex waves (K-complexes of drowsiness and sleep) can be sharply contoured and resemble spikes and sharp waves (see Normal EEG Waveforms). The most common mistake, however, may not be to interpret a well-described normal variant as epileptiform, but rather to over-read sharply contoured fluctuations of background in the temporal regions as temporal sharp waves. [9]

  • While the presence of IEDs in a routine EEG is often helpful in clinical decision making, the converse is not necessarily the case. The absence of IEDs in a routine EEG certainly does not prove that a paroxysmal event was not a seizure or that a patient does not have epilepsy. Routine EEGs are normal in about 50% of patients with a clinical diagnosis of seizures, although the yield improves with multiple or more prolonged EEGs. As a limiting case, in 2 series of patients studied in epilepsy monitoring units, IEDs were never identified in 4-19% of those in whom epileptic seizures were recorded. [10, 11] In patients with partial epilepsy, this is more likely to occur with extratemporal foci. The absence of IEDs in patients with generalized onset seizures is exceptional in children and unusual in adults.

  • In patients with established seizures, IEDs often shed light on the underlying epilepsy syndrome. For example, in a neurologically normal adolescent patient with normal imaging who presents with a history of convulsions, whether the patient has a generalized or localization-related seizure disorder may be uncertain. The presence of 3-Hz generalized spike-wave abnormalities on EEG would suggest a primary generalized seizure disorder. By contrast, a normal EEG or one with nonepileptic abnormalities would be more consistent with a localization-related epilepsy syndrome and the presence of temporal spikes would suggest the diagnosis of temporal lobe epilepsy. 

  • Left anterior temporal spike. This EEG was recorde Left anterior temporal spike. This EEG was recorded in the drowsy state (note the absence of a posterior dominant rhythm and the presence of slow horizontal roving eye movements). The patient was a 53-year-old man with a history of monthly episodes of behavioral arrest and automatic behavior consistent with complex partial seizures arising from the temporal lobe.
  • Right posterior temporal spike. This EEG is from a Right posterior temporal spike. This EEG is from a 15-year-old boy with a history of complex partial seizures. The spike phase reverses at T6. The field of the discharge extends into the parietal and occipital regions.
  • An EEG typical of the syndrome of benign childhood An EEG typical of the syndrome of benign childhood epilepsy with centrotemporal spikes (BECTS). This EEG was recorded from an otherwise neurologically normal 9-year-old boy with nocturnal seizures that consisted of drooling and clonic contractions of 1 side of the face and arm. There are frequent right-sided central mid temporal spikes. These stereotyped discharges exhibit a positive phase-reversal over the frontal region (asterisks). A longitudinally oriented horizontal dipole is frequently seen in patients with BECTS, although it is not specific for this syndrome.

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