Background
The Bilateral Bispectral index (BIS) was designed to allow the user to record and display four channels of EEG; two from each side of the brain. This monitor equally shows changes in the power spectrum distribution through the Density Spectral Array (DSA)1. Functional hemispherectomy (FH) is a surgical technique used in pediatric population to treat drug-resistant epilepsy caused by extensive hemispheric lesions. There are few cases in the literature about FH in adults but there are not about the use of DSA in this surgery. 
We present a case where we observed changes in DSA during a FH.
Case report
A 40-year-old woman with epilepsy caused by cortical focal dystrophy was underwent a mesial frontal lobe section 25 years ago. However, she continued with epileptic crisis and daily disconnections and presented mild mental retardation, memory problems and left hemiparesis with slow and spastic gait. She was scheduled for a right FH to treat epilepsy. On entering the operating room, the electrocardiogram, non invasive blood pressure and the percentage oxygen saturation were monitored. Bilateral BIS electrode strip was placed on the front temporal position according to the International 10-20 system of electrode placement. Anaesthesia was maintained with propofol TCI (4.7 mg/kg/h) and infusion of remifentanil (0.1 mcg/kg/min) to keep BIS values within 45-60 range. Rocuronium was also administered (0.3 mg/kg/h). At the beginning of surgery, we observed a power increase in low frequency (0.1-4 Hz) and alpha bands (8-12 Hz) in the right hemisphere, where the epileptogenic focus was (fig. 2). After the frontal disconnection there was a marked decrease of power in low frequency and alpha bands in the right side (fig. 3). During the disconnection of the other cerebral areas there were no changes in the DSA (fig. 4). Unlike DSA, the BIS trend did not reflect differences between two hemispheres.
The patient was extubated in the operating room and transferred to the recovery room. There were not  differences in neurological examination.

Discussion
In our case the DSA provides information about the depth of anaesthesia and sedation. One possible limitation of DSA is the location of epileptogenic focus. In our patient, it was very useful because the epileptogenic focus was in the frontal area2. In this case it was helpful to monitor the effectiveness of the surgery too.

Learning points:
DSA is useful in epilepsy surgery for detecting the side of epileptogenic focus.
In future surgeries there won’t be differences in DSA between two hemispheres.

Density Spectral Array of BIS VISTA Monitoring System in a functional hemispherectomy