Diagnostic accuracy of the Salzburg EEG criteria for non-convulsive status epilepticus: a retrospective study

Background Several EEG criteria have been proposed for diagnosis of non-convulsive status epilepticus (NCSE), but none have been clinically validated. We aimed to assess the diagnostic accuracy of the EEG criteria proposed by a panel of experts at the fourth London-Innsbruck Colloquium on Status Epilepticus in Salzburg, 2013 (henceforth called the Salzburg criteria). Methods We did a retrospective, diagnostic accuracy study using EEG recordings from patients admitted for neurological symptoms or signs to three centres in two countries (Danish Epilepsy Centre, Dianalund, Denmark; Aarhus University Hospital, Aarhus, Denmark; and Paracelsus Medical University, Salzburg, Austria). Participants were included from the Danish centres if they were aged 4 months or older, and from the Austrian centre if aged 18 years or older. Participants were sorted into two groups: consecutive patients under clinical suspicion of having NCSE (the clinical validation group) or consecutive patients with abnormal EEG findings but no clinical suspicion of NCSE (the control group). Two raters blinded to all other patient data retrospectively analysed the EEG recordings and, using the Salzburg criteria, categorised patients as in NCSE or not in NCSE. By comparing with a reference standard inferred from all clinical and para-clinical data, therapeutic response, and the final outcome, we calculated sensitivity, specificity, overall diagnostic accuracy, positive and negative predictive values, and inter-rater agreement for the Salzburg criteria. The reference standard was inferred by two raters who were blinded to the scorings of the Salzburg criteria. Findings We retrospectively reviewed EEG data from 220 patients. EEGs in the clinical validation group were recorded in 120 patients between Jan 1, and Feb 28, 2014 (Austria), and Aug 1, 2014, and Jan 31, 2015 (Denmark). EEGs in the control group were recorded in 100 patients between Jan 13 and Jan 22, 2014 (Austria) and Jan 12 and Jan 26, 2015 (Denmark). According to the reference standard, 43 (36%) of the 120 patients in the validation group had NCSE. In the validation cohort sensitivity was 97.7% (95% CI 87.9-99.6) and specificity was 89.6% (80.8-94.6); overall accuracy was 92.5% (88.3-97.5). Positive predictive value was 84.0% (95% CI 74.1-91.5) and negative predictive value was 98.6% (94.4-100). Three people in the control group (n= 100) fulfilled the Salzburg criteria and were therefore false positives (specificity 97.0%, 95% CI 91.5-99.0; sensitivity not calculable). Inter-rater agreement was high for both the Salzburg criteria (k= 0.87) and for the reference standard (k= 0.95). Therapeutic changes occurred significantly more often in the group of patients fulfilling Salzburg criteria (42 [84%] of 50 patients) than in those who did not (11 [16%] of 70; p< 0.0001). Interpretation The Salzburg criteria for diagnosis of NCSE have high diagnostic accuracy and excellent inter-rater agreement, making them suitable for implementation in clinical practice.