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pulsation in response to stimulation diminished after the skin flap was retracted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.
