For patients with known epileptic encephalopathy, there should be an increase in prominence or frequency of epileptiform discharges from baseline as well as electrographic and clinical improvement with IV ASDs. Broadly, the diagnosis of nonconvulsive seizures or NCSE is given in patients without known epileptic encephalopathy whose EEG shows epileptiform discharges greater than 2.5 Hz, or if less than or equal to 2.5 Hz, also demonstrate EEG and clinical improvement after intravenous (IV) antiseizure drugs (ASDs) or subtle clinical ictal phenomena or typical spatiotemporal evolution. At any frequency and with any morphology, the appearance of motor manifestations, including myoclonic or rhythmic muscle jerking in conjunction with GPDs, indicates that the pattern is ictal, and both clinical and electrographic manifestations should be treated as with any seizure. When GPDs are found, with or without triphasic morphology, it is always important to consider first whether or not they represent an ictal rhythm. However, this study utilized only two raters, where the prior IRA study for GPDs used 11 to rate EEGs this difference could have contributed to the disparity in the IRA. In a retrospective cohort study of 92 patients with GPDs, the IRA for ‘triphasic morphology’ was “substantial,” with a κ of 0.67. This was similar to a study examining the IRA for EEGs of comatose patients in general, as well as a study specifically examining GPDs that found that among 20 patients, the IRA for ‘triphasic morphology’ was only fair (κ of 0.33). However, the IRA for ‘triphasic morphology’ was only moderate (58%). Forty-nine raters showed almost perfect agreement for seizures, terms 1 and 2 (e.g., generalized and periodic), and modifiers describe sharpness, amplitude, frequency, and the number of phases. In 2014, interrater agreement (IRA) of the ACNS’ standardized terminology was examined and showed that for most terms, IRA was high. These guidelines have been helpful in providing more uniformity in EEG reporting, although there continues to be some variability in the interpretation and reporting of GPD+TW. They are commonly associated with toxic/metabolic encephalopathy, anoxia, hypothermia, infections, acute neurological injury, and nonconvulsive status epilepticus (NCSE). The presence of GPDs is highly suggestive of a global encephalopathy and is seen in approximately 4% of patients in the hospital or intensive care unit setting undergoing EEG monitoring. As a result of their frequent association with cardiac arrest and anoxic brain injury, it is thought that high-energy excitatory pyramidal cells may be more severely affected by hypoxic energy failure, resulting in disruption of feed forward inhibitory networks and propagation of GPDs.
A more recent theory is that they result from either a synaptic failure of interneurons or impaired excitation of inhibitory interneurons, resulting in disinhibition of excitatory pyramidal cells. Early work suggested that GPDs were due to widespread cortical destruction with relative sparing of white matter. There are different theories for their etiology and pathophysiology. By definition, they are repeated and generalized waveforms with relatively uniform morphology and duration, with a quantifiable interdischarge interval between consecutive waveforms, and recurrence of the waveform at nearly regular intervals ( Fig.
Generalized periodic discharges (GPDs) are electroencephalographic (EEG) waveforms that can be seen in a wide array of encephalopathies. Key Words: Triphasic waves Brain diseases Generalized periodic discharges Status epilepticus Further studies of nonsedating antiseizure drugs in patients with GPDs with triphasic morphology that incorporate continuous EEG monitoring will be useful in tailoring therapy to optimize long-term clinical outcomes and recovery. Recent work has advocated for a more proactive approach in evaluating GPDs with triphasic morphology. The most established literature suggests that GPDs, including those with triphasic morphology, are associated with the development of electrographic seizures, but that in the absence of clinical information, distinguishing waveforms based on morphology alone may not be clinically useful. The clinical significance of these waveforms and their relationship to seizures and prognosis has been debated, and differentiation between interictal patterns, patterns associated with seizures, and patterns representing nonconvulsive status epilepticus can at times be a challenge.
Generalized periodic discharges (GPDs) with triphasic morphology are an electroencephalographic (EEG) pattern traditionally associated with encephalopathy and coma, although they have been observed in a wide array of neurological disorders.
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