Abstracts

Rationale: The double distance reduced EEG montage is commonly implemented in patients with structural barriers to the placement of a standard 10-20 system array. Despite widespread use across institutions, particularly in post-operative neurosurgical patients, its sensitivity has not been adequately addressed. At Johns Hopkins Hospital, over 20% of the continuous EEGs performed in the neurocritical care unit are in a 9 electrode reduced montage. We evaluated the sensitivity and specificity of this montage for seizure detection. Methods: All continuous EEGs completed in the neurocritical care unit using a full montage between 2013-2016 were evaluated for seizure activity. 155 continuous EEGs with unequivocal seizures or interictal abnormalities were selected, comprising 73 ictal and 82 non-ictal EEGs. EEGs were randomized, reformatted to the reduced montage (Figure 1), and then reviewed independently by two experienced epileptologists who documented the presence of seizures and background abnormalities. Findings were compared to the official EEG report. Results: Overall, 20.3% of the full montage continuous EEGs completed in the neurocritical care unit had evidence of seizure, and 7.8% had evidence of status epilepticus. The average sensitivity and specificity of the reduced montage for seizure detection was 80.7% and 92.1%, with a kappa of 0.71, demonstrating substantial inter-rater agreement. The sensitivity for status epilepticus was lower at 68.9% but specificity remained high at 96.8% with a kappa of 0.67. Several of the EEGs mis-categorized as non-ictal were labeled as rather having rhythmic activity or periodic discharges (Figure 2). There was no clear association between the location of the seizure focus and correct detection of seizure activity. Evaluation of various background patterns on the ictal interictal continuum demonstrated sensitivities ranging from 67.5% to 83.3%. Extrapolating from our data, we predicted that in 2016, 2 of the 54 patients in the neurocritical care unit who underwent a reduced montage continuous EEG may have been mis-labeled as non-ictal and 3 patients over-diagnosed as having seizures. Conclusions: The sensitivity and specificity of the double distance montage widely used in post-operative neurosurgical patients is comparable to prior studies that have evaluated reduced arrays that cover the scalp,1, 2 but is suboptimal. While the specificity of the reduced array is good, given the reduced sensitivity of this array, epileptologists should remain vigilant when monitoring patients using this montage. Prolonged monitoring when rhythms on the ictal-interictal continuum are present could increase the sensitivity, as better sensitivity on a full montage has been reported with longer recording periods. Our study suggests that a full array should be used whenever possible; therefore, employing assistance from ICU nurses to help EEG technicians move head bandages and drains may help decrease the number of missed seizures.1. Rubin MN, Jeffery OJ, Fugate JE, et al. Efficacy of a reduced electroencephalography electrode array for detection of seizures. Neurohospitalist 2014;4:6-8.2. Herta J, Koren J, Furbass F, et al. Reduced electrode arrays for the automated detection of rhythmic and periodic patterns in the intensive care unit: Frequently tried, frequently failed? Clin Neurophysiol 2017. Funding: None