Middle fossa approach in vestibular schwannoma surgery. Postoperative hearing preservation and EEG changes.

When the middle fossa (MF) approach was introduced in Denmark, we were concerned about the possible risk to the temporal lobe caused by the retraction of the lobe when exposing the internal acoustic meatus. EEG recordings were therefore obtained prospectively before (21 patients) and after MF tumor removal (all 23 patients operated from 1989 to 1997). Only three patients had normal EEG recordings before and after surgery, while 86% had induction, or worsening, of focal and paroxystic activity, even at the last follow-up (median 3.5 years). Sixteen patients operated prospectively via the translabyrinthine (TL) also had pre- and postoperative EEG and served as a control group. Only minor EEG changes were found in this group. In neither group did the patients display any clinical neurological signs (seizures). At the latest evaluation the facial function was reduced in 8 patients (35%) with 6 patients going 1 step up the scale, 1 patient 2 steps up and 1 patient 3 steps up (from HB-1 to HB-4). The integrity of the facial nerve was maintained in all patients. Postoperatively, 10 patients (44%) had useful hearing (hearing class A and B) on the operated side. Four patients had anacusis and an additional 4 patients were reduced to hearing class D with very low PTA and SDS. In total, 9 patients (39%) retained their preoperative hearing class, while 14 patients (61%) had impairment in their hearing class. In conclusion, EEG changes (low frequency activity and IEA) may be provoked or worsened as part of the middle cranial fossa procedure. The mechanism is not fully known, but may reflect peroperative pressure on the temporal lobe. EEG changes are fewer and lighter in translabyrinthine-operated patients. The practical clinical implications of the possibility of developing EEG changes, even without clinical signs, are potentially serious, and must be included in the information given to the patients before surgery.