The significance of the phi rhythm

We reviewed two series of patients with the phi rhythm (posterior rhythmic slow waves occurring after eye closure) to determine its characteristics and clinical significance. Phi rhythm was defined as a minimum of three consecutive monomorphic posterior delta waves occurring within 2 sec of eye closure on at least two occasions during electroencephalography. Group 1 consisted of 30 patients (16 male and 14 female) with a mean age of 11.6 +/- 8.4 years (range, 3 to 46 years) who were evaluated between 1978 and 1993. Phi rhythm most commonly occurred when the patient was alert and after concentrated visual attention, such as reading or picture or pattern scanning. Seventeen of these patients had epilepsy (11 generalized, 3 focal, 1 both, and 2 unclassified). The frequency of epilepsy was not significantly different from that of a control group of 60 patients matched for age and sex; however, generalized epilepsies were more common in the phi group (P = 0.008). Group 2 consisted of a previously unreported series of 121 patients evaluated between 1960 and 1962. A diverse range of underlying clinical diagnoses was evident in both groups. The mechanism of the phi rhythm is unknown, but the findings suggest that the origin is subcortical and that the presence of this activity should not be considered diagnostic of a seizure disorder.