Preoperative percutaneous mapping of the frontal branch of the facial nerve to assess the risk of frontalis muscle palsy after a supraorbital keyhole approach

OBJECT: Although a supraorbital keyhole approach utilizing an eyebrow incision and supraorbital minicraniotomy is one of the most commonly used keyhole approaches for treating cerebral aneurysms, the risk of frontalis muscle palsy due to an injury of the frontal branch of the facial nerve remains a serious drawback to a supraorbital keyhole approach as a minimally invasive surgical technique. Therefore, the authors attempted to evaluate the risk of frontalis muscle palsy by mapping the frontal nerve branch in the lower forehead using a nerve conduction study in individual patients. METHODS: Percutaneous mapping of the frontal nerve branch was performed preoperatively on 52 patients who underwent supraorbital keyhole approaches for aneurysmal clipping. The maximal compound muscle action potentials (CMAPs) in the lower forehead were observed at 5 points along a laterally inclined line angled 30° from the midpupillary line, in which the points were 1.0, 1.5, 2.0, 2.5, and 3.0 cm as measured from the supraorbital margin. ResULTS Severe frontalis muscle palsy was observed in 11 patients (21.2%), yet recovery occurred 2-5 months after surgery. No patients experienced permanent palsy. The incidence of severe palsy was 7.4% in those patients showing clear CMAPs with a high location (exclusively at 2.0, 2.5, or 3.0 cm), 14.3% in those with a bimodal distribution, 40.0% in those with a low location (exclusively at 1.5 cm), and 83.3% in those with an extremely low location (exclusively at 1.0 cm). CONCLUSIONS: Percutaneous mapping of the frontal branch of the facial nerve using a nerve conduction study can be used to assess the risk of postoperative frontalis muscle palsy following a supraorbital keyhole approach. The patients with the highest risk of postoperative palsy showed a clear CMAP exclusively at 1.0 cm along the inclined line measured from the supraorbital margin.