Tailored Traction and Neurophysiology Monitoring Help Preserving the Frontotemporal Branches of the Facial Nerve during Cranial Base Approaches

Introduction: Preserving the frontotemporal branches of the facial nerve (FTB) is crucial for achieving good cosmetic and functional outcomes following anterolateral cranial base approaches. Experience shows frontalis muscle palsies can occur even when the FTB are anatomically preserved, signaling excessive traction of the skin flap could be responsible for nerve damage. Here, we describe how neurophysiology monitoring, combined with quantification of the traction force elicited by retraction hooks, can be use to preserve frontalis muscle function after surgery.

Methods: This single-institution case series involves patients who underwent anterolateral skull base approaches (supraorbital craniotomies, bifrontal, pterional, and orbitozygomatic approaches) in which the FTB was identified with neurophysiology and hook traction was quantified in order to preserve nerve and muscle function ([Fig. 1]). First, mechanical testing using a tensile tension machine was used to determine the Young's modulus of rubber tubing used in conventional neurosurgical hook retractors ([Fig. 2]), then monopolar stimulation at 5 mA of the FTB during the surgical opening was used to find the nerve and tailor the amount of traction so frontalis pulsations were visible in response to stimulation.

Fig. 1

Fig. 2

Results: A total of 11 patients were enrolled in this study, with a total of 13 mapped sides ([Table 1]). The mean patient age was 55.8 years, and 6 of the patients were female (54.5%). Pathologies included 9 meningiomas (81.8%), 1 NF1-associated dysplasia (9.1%), and 1 metastastatic lesion (9.1%). None of the patients presented with facial nerve palsy 2 weeks after surgery. Mechanical testing of surgical retraction hooks revealed that for a 200-mm-long tubing, the force (N) was = 63.8 × strain. Strain is the percentage elongated, calculated as displacement/length. For example, a 10-cm rope pulled to 11 cm (1-cm displacement) has a strain of 1/10 = 0.1 strain. Therefore, we made a 1-cm-long mark on the hook’s tubing; if the mark elongates to 2 cm long (1 strain), there will be 63.8 N force in the tubing ([Fig. 3]). Monopolar stimulation in all patients within the intefascial trajectory of the nerve resulted in visible stimulation of the frontalis muscle. We measured the length of the tubing after the hook was placed and determined that the visible frontalis pulsation in response to stimulation diminished after the skin flap was retracted more than 63.8 N thus if that force was surpassed, the incision was lengthened to avoid further tension.

Fig. 3

Table 1

Conclusion: None of the patients in this series had postoperative frontalis palsy. The study shows that monopolar stimulation mapping to identify the FTB, combined to tailored tension application on the retraction hooks, can help in reducing the risk of facial nerve damage during anterolateral skull base approaches. The adoption of this technique will be used in the future to compare the relative incidence of frontalis disfunction with a prior cohort, which will serve as control.

Publikationsverlauf

Artikel online veröffentlicht:
07. Februar 2025