The probability of seizures during EEG monitoring in critically ill adults

Abstract

Objective: To characterize the risk for seizures over time in relation to EEG findings in hospitalized adults undergoing continuous EEG monitoring (cEEG).

Methods: Retrospective analysis of cEEG data and medical records from 625 consecutive adult inpatients monitored at a tertiary medical center. Using survival analysis methods, we estimated the time-dependent probability that a seizure will occur within the next 72-h, if no seizure has occurred yet, as a function of EEG abnormalities detected so far.

Results: Seizures occurred in 27% (168/625). The first seizure occurred early (<30min of monitoring) in 58% (98/168). In 527 patients without early seizures, 159 (30%) had early epileptiform abnormalities, versus 368 (70%) without. Seizures were eventually detected in 25% of patients with early epileptiform discharges, versus 8% without early discharges. The 72-h risk of seizures declined below 5% if no epileptiform abnormalities were present in the first two hours, whereas 16h of monitoring were required when epileptiform discharges were present. 20% (74/388) of patients without early epileptiform abnormalities later developed them; 23% (17/74) of these ultimately had seizures. Only 4% (12/294) experienced a seizure without preceding epileptiform abnormalities.

Conclusions: Seizure risk in acute neurological illness decays rapidly, at a rate dependent on abnormalities detected early during monitoring. This study demonstrates that substantial risk stratification is possible based on early EEG abnormalities.

Significance: These findings have implications for patient-specific determination of the required duration of cEEG monitoring in hospitalized patients.

Keywords: Continuous electroencephalography; ICU EEG monitoring; Nonconvulsive seizures.

Fig. 1

Seizure detection statistics. Numbers and percentages of patients in whom the most severe abnormality detected so far after 30 min of cEEG monitoring is slowing or no abnormality, epileptiform discharges (spikes/sharps or PEDs), or electrographic seizures (second row of boxes); and numbers within each category who eventually develop seizures.

Fig. 2

Time to event plots and monitoring durations. (A) The time elapsed since beginning cEEG monitoring until detection of epileptiform abnormalities, seizures, or the end of monitoring for all 625 patients, up until 48 h; data beyond 48 h is not shown. Event lines for subjects with no epileptiform abnormalities detected so far are colored blue. The development of epileptiform discharges is indicated by line color: a change to orange indicates the appearance of spikes or sharp waves; to red, the appearance of lateralized or generalized periodic epileptiform discharges (PEDs); to black, the detection of electrographic seizures. Termination of a line into the light gray zone indicates the time that cEEG monitoring was discontinued. In the analysis, patients whose monitoring ended without having a seizure are ‘right censored’ at the time cEEG monitoring is discontinued, meaning that it is unknown whether or not they went on to have subsequent seizures. Note that line color encodes the category of the most severe abnormality detected so far, but does not necessarily imply that the abnormality is continuously present, i.e. lines are not permitted to change back to a “less severe” color. (B) Distribution of cEEG monitoring durations. By design, no study briefer than 18 h in duration was included. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Estimated cumulative distribution curves (ECDFs). Curves showing the estimated proportion of subjects at risk for developing seizures over the first 48 h of monitoring, counting from the beginning of cEEG monitoring (A), and from t = 30 min after dividing patients into those with and without epileptiform abnormalities (but no seizures) within the first 30 min of cEEG monitoring (B). The ECDF for developing spikes/sharps or PEDs in subjects without epileptiform discharges in the first 30 min is also shown (C). ECDFs are estimated from the data in Fig. 2 while statistically correcting for early discontinuation of some studies (data censoring).

Fig. 4

Future seizure probability curves. (A) Estimated probability that a seizure will occur within the next 72 h if none have occurred so far is shown for the entire cohort, counting from t = 0, and (B) for subgroups with (red curve) and without (blue curve) epileptiform abnormalities after the initial 30 min of observation. Times at which the estimated 72-h seizure probability decays to 10% and 5% are marked. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)