We explored the frequency with which EEG ictal events shifted laterali
ty during a complex partial seizure (CPS), a phenomenon termed ping-po
ng seizures. Coded and shuffled 5-s segments from records of 38 scalp
EEG monitored patients and 2 patients with implanted depth electrodes
were categorized as left, right, or unclear laterality. Thirteen patie
nts (33%) showed a ping-pong effect. Mean duration of time to switch l
aterality was 10.3 +/- 7.2 s. Two patients with depth wires also demon
strated ping-pong seizures. No patient reversed laterality twice. If t
he beginning of a seizure is obscure or missed, the apparent lateraliz
ation may be incorrect.