Triphasic Pattern

Description

The term “triphasic waves” is a commonly used name for a specific pattern that denotes more than a triphasic waveform. Moreover, this pattern is not considered an epileptiform pattern despite the triphasic waveform, which is the classic waveform for epileptiform discharges. As such, the EEG interpretation “triphasic waves” has sometimes produced confusion or miscommunication among those without EEG expertise. To help improve communication accuracy among EEG researchers, an American Clinical Neurophysiology Society subcommittee has proposed the term “generalized periodic discharges” with specification of the recurrence frequency when referring to what would be called “triphasic waves” in clinical EEG (Hirsch et al., 2005). This step forward in clarity is based on “triphasic waves” and periodic discharges having similarities in waveform and both also recurring within a recording, and the proposal tacitly raises the question of whether “triphasic waves” should exist as an EEG pattern that is separate from periodic discharges. As clinical EEG interpretation continues to use the term “triphasic waves,” the pattern has not been included in the periodic discharges chapter; however, the term “triphasic pattern” is used here instead of “triphasic waves” to help reduce the confusion between a waveform description and a named pattern with particular clinical significance.

Triphasic pattern appears as runs of bisynchronous and symmetric waves with a triphasic waveform and occurs in adults with one of many pathologic conditions (Bickford and Butt, 1955). It rarely occurs in those younger than 20 years. Blunt spike wave is a less commonly used alternative name for triphasic pattern that provides an appropriate description for the waveform. The blunting is evident in triphasic pattern’s classically lower amplitude, less sharply contoured initiating spike when compared to the initiating spike of an epileptiform discharge with triphasic waveform. The three components of the triphasic pattern’s overall waveform is a complex of waves with a sequence of the sharpest wave, followed by the tallest wave, and then the longest wave. The middle wave, the most prominent of the three, has an amplitude that typically is 200 to 300 μV (Markand, 1984). It also has a positive (downward) deflection in common reference montages, and the other two waves are negative (upward). The three components also may be characterized as demonstrating progressively increasing durations through the complex.

The anterior head region usually manifests the most prominent triphasic pattern with waves that have higher amplitude and a more clearly defined waveform, but a posterior predominance also may occur. Source analysis of typically anterior triphasic pattern has identified a single dipole generator in the medial frontal regions, which is plausible when considering the field (Kwon et al., 2007). Regardless of whether the triphasic pattern has an anterior or posterior predominance, a phase lag often is present with the predominant region temporally leading the less predominant one by 25 to 140 milliseconds (Bickford and Butt, 1955). When channels are organized in the anterior-to-posterior (AP) direction and the triphasic pattern is anterior, the lag is evident as a minor, incremental displacement of the triphasic pattern’s waves to the right with each successive channel. The absence of an AP lag is an accepted variation in the manifestation of the triphasic pattern.

The repetition pattern of triphasic pattern’s complexes is another characteristic feature. Although the complexes within the pattern occasionally occur individually, they usually occur in runs of successive complexes without intervening background activity. These trains typically have complexes repeating at a frequency of 1 to 3 per second with the train as a whole lasting about 1 to 5 seconds. The trains start and stop abruptly without changes to the complex’s waveform or repetition frequency or to the surrounding background activity. The intermittent occurrence of each run is without a pattern or periodicity.

Distinguishing Features

• Compared to Bilateral Periodic Epileptiform Discharges

Although bilateral periodic epileptiform discharges (BiPEDs) are often generalized, they usually have a frontal predominance that is similar to the frontal predominance of triphasic pattern. Nevertheless, the waveform and the recurrence pattern are two principal differentiating features. As epileptiform discharges (EDs), BiPEDs are triphasic in waveform; however, their waveform is more similar to epileptic discharges than is the waveform of triphasic pattern. As such, BiPEDs have a more sharply contoured initiating spike or sharp wave and an overall shorter complex duration. They also differ from triphasic pattern by recurring over extended portions of the EEG recording with brief intervals of background activity separating the individual complexes. Triphasic pattern has runs of relatively shorter duration that occur intermittently and often without background activity between the complexes. Furthermore, the intervals between the individual complexes that constitute BiPEDs tend to be similar in duration. This gives BiPEDs their periodicity. Although the complexes within a triphasic pattern run are separated by similar intervals, the time intervals between the runs are highly variable. Triphasic pattern’s usual occurrence with an AP lag is another differentiating feature. This is usually manifested by a slightly earlier occurrence of each wave in the more anterior channels.

• Compared to Frontal Intermittent Rhythmic Delta Activity

Frontal intermittent rhythmic delta activity (FIRDA) and triphasic pattern have similar distributions and repetition patterns. Both recur in trains that typically last 1 to several seconds. Differentiation between the patterns rests essentially on waveform. FIRDA is composed of monophasic slow waves, and triphasic pattern complexes clearly include multiple phases. The distinction may be more difficult if FIRDA has superimposed faster frequencies that give the appearance of notched slow waves because subtle, initiating spikes or sharps of triphasic pattern also may cause notching of a slow wave. Distinguishing notched FIRDA due to a mixture of frequencies from triphasic pattern relies on identifying a consistent location of the notch along the slow wave’s slope. A mixture of frequencies will produce notching that is more random in its location. A subtle spike or sharp wave always will be present at the same location with regard to the slow wave’s apex.

• Compared to Ictal or Interictal Epileptiform Discharges

Generalized ictal or interictal EDs have a triphasic waveform and often occur in repetitions of several complexes. Especially when generalized EDs occur as the slow spike and (slow) wave manifestation, their repetition rate may be similar to the complexes of triphasic pattern. Waveform differences provide one means for differentiation. EDs usually are centered at the frontal midline and have a more sharply contoured initiating spike or sharp wave. However, the initiating discharge of slow spike and wave may be similar to that of triphasic pattern. The occurrence of frequent trains can be helpful because an abundance of waves is uncommon for interictal EDs. However, frequent waves may occur with either triphasic pattern or the ictal EDs of nonconvulsive status epilepticus due to symptomatic generalized epilepsy (Sheridan and Sato, 1986). In practice, the patient’s history can help to determine if the discharges are triphasic pattern or slow EDs, but waveform, repetition frequency, and background activity also are useful. Triphasic pattern more often repeats at less than 2 Hz, has a less sharp initiating wave, and occurs in the context of significant generalized slowing, especially when it is due to metabolic disease (Boulanger et al., 2006; Fisch and Klass, 1988). The background activity for generalized epilepsies due to structural, metabolic, or other causes usually is only moderately slowed. Resolution of the pattern with the administration of a benzodiazepine is not an indicator of whether the finding is triphasic pattern or EDs because both may respond electrographically to this treatment (Fountain and Waldman, 2001). Improvement in mentation following antiepileptic treatment is a more specific means to differentiate the two (Nowack and King, 1992

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