Brief potentially ictal rhythmic discharges and paroxysmal fast activity as scalp electroencephalographic biomarkers of seizure activity and seizure onset zone.

Abstract

The electroencephalographic (EEG) terms "brief potentially ictal rhythmic discharges" (BIRDs) and "paroxysmal fast activity" (PFA) are considered distinct entities; however, their definitions overlap, and they may have similar clinical significance. We investigated their clinical significance and their association with seizures and the seizure onset zone (SOZ). We retrospectively identified an adult cohort (July 2015 to March 2018) whose long-term (>12h) EEGs in any setting reported BIRDs (>4Hz, lasting .5-10s) and/or PFA. Different frequency cutoffs for PFA (>13Hz or ≥8Hz) were tested to compare their clinical significance. Patient demographics, clinical history, and EEG features were recorded. We identified 94 patients with BIRDs/PFA out of 3520 patients (3%); 36 were critically ill (12 with epilepsy), and 58 were noncritically ill (all with epilepsy). The frequency of BIRDs/PFA was largely dependent on EEG background: it tended to be slower (theta) in the absence of a posterior dominant rhythm or in the presence of continuous focal slowing in the same region (p=.01). Sixty-two of 94 patients (66%; 32/36 [89%] critically ill, 30/58 [52%] noncritically ill) had electrographic seizures during the recording. The scalp EEG SOZ colocalized with BIRDs/PFA in all cases. BIRDs with faster frequency (also qualifying as PFA by definition) had similar seizure risk to that of slower BIRDs (62%-71%), regardless of frequency cutoff used to define PFA. In addition, 30 of 30 (100%) patients with evolving BIRDs/PFA (which lasted a median of 6s, range = 2-9.5s) had electrographic seizures (>10s), compared to 32 of 64 (50%) with nonevolving BIRDs (median = 1s, range = .5-3.5s; p<.01). A high proportion of patients with BIRDs/PFA had seizures on EEG, regardless of their frequency (i.e., whether they also qualified as PFA), and their location colocalized with scalp SOZ in all cases. BIRDs appear to be a scalp EEG biomarker of uncontrolled seizure activity and a reliable localizing sign of the SOZ.