gms | German Medical Science

Objective: Brainstem surgery carries the risk of damage to the corticospinal tract. Intraoperative motor evoked potentials (MEP) are used to monitor motor function in order to detect damage at a reversible stage and thus impede permanent neurological deficits. However, warning criteria in this context are not generally agreed upon.

Methods: MEP were recorded during 104 consecutive brainstem surgeries. MEP were recorded after transcranial electrical stimulation (TES). To obtain the initial TES intensity threshold, we started with TES at 25 mA and then increased in 5 mA steps until the target muscle responded reliably, or until a self-imposed limit of up to 220 mA was reached. Testing was performed continuously when the greatest risk to brainstem function occurred. Surgeons were warned whenever stimulation intensity increased by > 20 mA or response fell by 50%. Motor grade was documented prior to and after surgery, early postoperatively and at discharge.

Results: Immediately postoperatively, 16 patients experienced a new motor deficit. In 9/16 patients, stimulation was increased by > 20 mA; in this group the paresis persisted until discharge in 3/9 patients. In 5/16 patients MEP amplitude was reduced by 50% at the stimulation limit, three of whom showed persisting paresis. In the remaining 2/16 patients, intraoperative MEP were stable but new paresis appeared due to postoperative bleeding. In 10/15 children MEP were unstable, but only one of them showed a new motor deficit at discharge. Of all 104 patients, 7 have deteriorated in motor grade at discharge, 92 remained unchanged and 5 patients have improved.

Conclusions: We propose MEP stimulation intensity increase by > 20 mA or 50% response reduction as a reliable criterion to identify unstable MEP. Even small MEP changes may predict new motor deficits in brainstem surgery.