The cumulative yield of any epileptiform abnormality through the third EEG study is about 70% for both incident epilepsy and a single unprovoked seizure, results from a novel analysis demonstrated.
"The yield of epileptiform abnormalities in serial EEG recordings has not been addressed in a population-based setting for subjects with incident epilepsy or a single unprovoked seizure," researchers led by neurologist Elisa Baldin, a postdoctoral research scientist at Columbia University, New York, wrote July 9 in Epilepsia. "In addition, there are no studies of the predictors of epileptiform abnormalities on clinical EEG studies in an unselected population followed after a single unprovoked seizure or incident epilepsy. Such data are critical for evaluating these issues, because all patients, regardless of their later prognosis, are included, and selection bias related to factors other than the presence of epilepsy is minimized. Thus, this design is crucial for obtaining valid results that form the basis for planning cost-effective diagnostic workups for patients with incident epilepsy or a single unprovoked seizure."
In an effort to assess the yield and predictors of epileptiform abnormalities for the first and subsequent EEG studies, the researchers evaluated comprehensive medical data from the Rochester Epidemiology Project to identify residents of Rochester, Minn., who were born in 1920 or later and received a diagnosis of incident epilepsy or a single unprovoked seizure between 1960 and 1994, and who had at least one EEG study. The investigators defined an unprovoked seizure as one occurring in the absence of an identified proximate precipitating factor, which excluded seizures that were associated only with an acute insult to the central nervous system or a generalized systemic metabolic disturbance. They defined a single unprovoked seizure as one that occurred between 1960 and 1994 and did not recur during the follow-up period, which extended to 2008 (Epilepsia 2014 July 9 [doi:10.1111/epi.12720]).
The study population included 478 patients with a diagnosis of incident epilepsy and 141 with a diagnosis of a single unprovoked seizure. Their mean ages at diagnosis were 19.6 and 17.3 years, respectively, and the entire study population was evenly divided between men and women. Among patients with a diagnosis of incident epilepsy, the cumulative yields of epileptiform abnormalities after the first and third EEG studies were 53% and 72%, respectively. "Subjects aged 20 years and older were less likely than younger subjects to have an epileptiform abnormality," Dr. Baldin and her associates wrote. "By the third EEG study, the cumulative yield was 82.4% among subjects aged 1-19 years and 78.0% among those younger than 1 year, compared to 58.4% for those 20 years or older (P = less than .0001)."
Among patients with a single unprovoked seizure, the cumulative yield of epileptiform abnormalities after the first and third EEG studies were 39% and 68%.
When all EEG studies were considered, the adjusted risk of finding any epileptiform abnormality was higher in subjects aged 1-19 years at diagnosis, compared with those aged 20 years or older (hazard ratio, 1.80). Patients who had epilepsies of unknown origin had a lower risk of any epileptiform abnormality than did those with idiopathic epilepsies (HR, 0.7).
"This is the first population-based study of the risk of finding epileptiform abnormalities over multiple EEG recordings in people with incident epilepsy or a single unprovoked seizure," the researchers wrote. "Clinically it may be worthwhile to consider the low probability of finding an epileptiform abnormality after the third nonepileptiform EEG. This is most evident in incident epilepsy, and specifically, in younger subjects. The utility of ordering multiple EEG recordings over time should be considered carefully, to avoid potentially unnecessary testing."
They acknowledged certain limitations of the study, including the fact that no data on the use of antiepileptic drugs were collected during the follow-up period. "Antiepileptic drugs could affect the detection of an epileptiform abnormality on the EEG, either increasing or decreasing their presence, depending on the specific drugs used, although this has not been shown consistently," they wrote.
The study was supported by a grant from the National Institute of Neurological Disorders and Stroke. Dr. Baldin stated that she had no relevant financial conflicts to disclose. Other authors disclosed financial relationships with manufacturers of antiepileptic drugs as well as receiving research support from various institutions, organizations, and associations.