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 conc erned about the possible risk to the temporal lube caused by the retraction of the lobe when exposing the internal acoustic meatus. EEG recordings wer e therefore obtained prospectively before (21 patients) and after MF tumor removal (all 23 patients operated from 1989 to 1997). Only three patients h ad 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 transl abyrinthine (TL) also had pre- and postoperative EEG and served as a contro l 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 mainta ined in all patients. Postoperatively, 10 patients (44%) had useful hearing (hearing class A and B) on the operated side. Four patients had anacusis a nd an additional 3 patients were reduced to hearing class D with very low P TA and SDS. In total, 9 patients (39%) retained their preoperative hearing class, while 14 patients (61%) had impairment in their hearing class. In co nclusion, EEG changes (low frequency activity and IEA) may be provoked or w orsened as part of the middle cranial fossa procedure. The mechanism is not fully known, but may reflect peroperative pressure on the temporal lobe. E EG changes are Fewer and lighter in translabyrinthine-operated patients. Th e practical clinical implications of the possibility of developing EEG chan ges, even without clinical signs, are potentially serious, and must be incl uded in the information given to the patients before surgery.