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Triphasic Waves: Swinging the Pendulum Back in this Diagnostic Dilemma

Commentary The nature, origin, and etiology behind the generation of triphasic waves (TW) have occupied neurophysiologists for decades. This seemingly clear and self-descriptive—yet elusive— term has given us reassurance and uncertainty intermittently over the years as it has evolved from being associated with very specific metabolic conditions to being widely associated (in a less specific manner) to a diverse group of conditions ranging from metabolic, autoimmune, toxic or degenerative encephalopathies, postictal states, and active ictal manifestations. TW are defined by their typical morphology, characterized by a predominant surface positive wave, preceded and followed by smaller surface negative deflections, conferring them a triphasic appearance. The contour of the waves is blunt and broad, not easily mistaken for epileptiform interictal discharges. Other characteristics of the pattern include their periodic presentation (1–2.5 Hz) and generalized distribution, usually showing a geographic time lapse from anterior to posterior in bipolar longitudinal montages. TW are often overriding abnormal and slow EEG background activity. They are seen only in adult humans, notoriously absent in children and animal models. TW were first described in 1955 by Brickford and Butt (1), and the controversy about their meaning keeps going to this day. Their initial and best known association is with hepatic encephalopathy. TW were not only considered a phenomenon pathognomonic to the condition but were associated with a very specific stage within the progression of the disease and closely related to elevated ammonia levels (2). Later studies have highlighted the likely coexistence of structurally diffuse white matter lesions in combination with metabolic abnormalities of diverse etiology that results in this very specific pattern (3). With the ubiquitous use of continuous video EEG (CVEEG) within the intensive care setting, we have also become proResponse Rates to Anticonvulsant Trials in Patients with Triphasic-Wave EEG Patterns of Uncertain Significance.