A Newly Described Ictal Pattern: The Initial Ictal Slow Shift

This report deals with a newly described ictal pattern, called the initial ictal slow shift (IS) ² . This pattern may be seen in subdural records as the first sign of an ictal event, occurring before the later typical rhythms of a seizure state appear. A positive shift, very similar in appearance from one seizure to another, usually lasted for 1–2 sec, followed by a negativity for 7–9 sec that included the typical rhythmical discharges. At times, a negative shift occurred first, seen up to 15 mV very high in amplitude, for a few seconds before the typical ictal rhythms were seen. Scalp records may also demonstrate slow shifts, and examples are shown of the typical 3/sec bilateral spike and wave (S+W) complexes of absence seizures. A slow shift occasionally appeared 1 sec before the onset of these complexes, but more often a few seconds after the onset. Finally, after the end of the S+W complexes, a positive shift, for as long as 5–6 sec may occur, up to 600 μV in amplitude. These shifts could relate to data showing that patients are not really back to a normal responsiveness for at least 5 sec after the end of the S+W complexes (see Discussion).