Progression of convulsive and nonconvulsive seizures during epileptogenesis after pilocarpine-induced status epilepticus.

Although convulsive seizures occurring during pilocarpine-induced epileptogenesis have received considerable attention, nonconvulsive seizures have not been closely examined, even though they may reflect the earliest signs of epileptogenesis and potentially guide research on antiepileptogenic interventions. The definition of nonconvulsive seizures based on brain electrical activity alone has been controversial. Here we define and quantify electrographic properties of convulsive and nonconvulsive seizures in the context of the acquired epileptogenesis that occurs after pilocarpine-induced status epilepticus (SE). Lithium-pilocarpine was used to induce the prolonged repetitive seizures characteristic of SE; when SE was terminated with paraldehyde, seizures returned during the 2-day period after pilocarpine treatment. A distinct latent period ranging from several days to >2 wk was then measured with continuous, long-term video-EEG. Nonconvulsive seizures dominated the onset of epileptogenesis and consistently preceded the first convulsive seizures but were still present later. Convulsive and nonconvulsive seizures had similar durations. Postictal depression (background suppression of the EEG) lasted for >100 s after both convulsive and nonconvulsive seizures. Principal component analysis was used to quantify the spectral evolution of electrical activity that characterized both types of spontaneous recurrent seizures. These studies demonstrate that spontaneous nonconvulsive seizures have electrographic properties similar to convulsive seizures and confirm that nonconvulsive seizures link the latent period and the onset of convulsive seizures during post-SE epileptogenesis in an animal model. Nonconvulsive seizures may also reflect the earliest signs of epileptogenesis in human acquired epilepsy, when intervention could be most effective. NEW & NOTEWORTHY Nonconvulsive seizures usually represent the first bona fide seizure following a latent period, dominate the early stages of epileptogenesis, and change in severity in a manner consistent with the progressive nature of epileptogenesis. This analysis demonstrates that nonconvulsive and convulsive seizures have different behavioral outcomes but similar electrographic signatures. Alternatively, epileptiform spike-wave discharges fail to recapitulate several key seizure features and represent a category of electrical activity separate from nonconvulsive seizures in this model.