Sleep spindle asymmetry in epileptic patients

Objective: To investigate sleep spindle asymmetry in patients with idiopathic generalized epilepsy (IGE), cryptogenic partial epilepsy (CPE), and symptomatic partial epilepsy (SPE). Methods: Post-sleep deprivation EEG records of the patients were investigated. Fast Fourier transform was applied to 200 s of stage-2 activity (without spikes, spike-waves and artefacts). Individual spindle frequency and topography was identified in each patient. Spindle intensity was computed and expressed in microvolts. In order to calculate spindle asymmetry the site of maximum spindle intensity within one hemisphere was compared to the mirror site in the other hemisphere. Asymmetry was expressed by the asymmetry index (AI). In the PE group AI was defined as [AI=(P−N)/(P+N)×100], where P and N were spindle intensity in the pathological and normal hemisphere, respectively. In the IGE group P and N were substituted for L and R (spindle intensity in the left and right hemisphere, respectively). Results: In IGE patients spindle asymmetry was in the range of −13 to +12%. No significant lateralization to the left or right hemisphere was observed. In PE patients, spindle asymmetry ranged from −23 to+29%. As compared to spindle intensity in the unaffected hemisphere, spindling was enhanced in the ‘epileptic’ hemisphere in most PE patients (15/20 in the CPE and 5/7 in the SPE group). Conclusion: In IGEs (where the epileptic condition involved both hemispheres to the same extent) spindle intensity did not show significant asymmetry. In PE patients (where the epileptic process was clearly lateralized to one hemisphere while the other hemisphere was unaffected) enhanced spindling usually was related to the side of the epileptic process. The generally held notion that the pathological hemisphere displays less spindling than the healthy one might be misleading in cases of focal epilepsy. The results suggest that epileptogenic cortex slightly facilitates spindle generation.