Discussion
Among postoperative complications in parotid surgery, there is no doubt
that facial nerve palsy is the most important. In a meta-analysis by
Witt et al. 10), temporary and permanent postoperative facial nerve
palsy reportedly occurred in 59.8% and 4.0% of cases, respectively, in
total parotidectomy, and in 17.9% and 0.2% of cases, respectively, in
partial parotidectomy. In our facility, a review of 902 patients who
underwent surgery for benign parotid tumors revealed that 19.3% of
patients had temporary palsy and 0.5% had permanent palsy; these
results are in line with previous reports 3). Partial parotid lobectomy
requires confirmation of the facial nerve, so even if the surgeon
successfully preserves the facial nerve, transient facial paralysis may
occur. Therefore, we examined whether it is possible to predict the
incidence of postoperative facial nerve palsy, as well as the time
required for recovery, by measuring the responses of the nerve to
stimulation during surgery.
When measuring electrical potential in response to facial nerve trunk
stimulation before and after tumor resection, significantly higher
responses were observed after tumor resection than before for all
branches in group A. We suspected the effect of muscle relaxant
administered at the start of anesthesia remained before tumor resection.
The median time from muscle relaxant administration at the time of
anesthesia introduction to electrical potential measurement was 69
minutes before tumor resection and 117 minutes after resection. It was
previously reported that in parotid surgery for which muscle relaxants
were administered at the start of general anesthesia, changes in the
numerical values of the muscle relaxant monitor differed from patient to
patient, and as a result, differences in FNM response were also noted
among patients 11). In addition, with muscle relaxant monitors,
measurements are obtained at the ulnar nerve, and the ulnar nerve and
facial nerve respond differently to muscle relaxants 12). Therefore, in
this study, based on the values of the muscle relaxation monitor before
tumor resection, it was assumed that there was no effect of muscle
relaxants, but in fact, in some cases, the effect of muscle relaxation
on the facial nerve may not have completely disappeared. In other words,
we considered it likely that the electrical potentials in response to
stimulation measured before tumor resection were lower than the true
values. In group C, a decrease in electrical potential was observed for
all branches after resection compared to before resection; in
particular, there was a statistically significant difference for MB.
This decrease in electrical potential is likely due to damage to the
facial nerve during resection, but actual difference might have been
even larger when considering the effect of the muscle relaxant before
resection. On the other hand, in group B, no significant difference in
electrical potential was observed before and after resection. In group
B, we did not note any occurrence of muscle paralysis corresponding to
the measured branches, but we presumed there was minor damage to the
nerve because muscle paralysis corresponding to other branches occurred.
Accordingly, we considered that the reason we observed no significant
difference in electrical potential in response to stimulation before and
after resection in group B was because the decrease in electrical
potential after resection was offset by the effect of the muscle
relaxant before resection.
In contrast, the measurements of electrical potential in response to
nerve trunk and branch stimulation after tumor resection were performed
when enough time had passed after administration of the muscle relaxant.
It is therefore likely that the effect of the muscle relaxant had
already disappeared. Given that there was no damage to the non-exposed
part of the nerve, the electrical potentials in response to branch
stimulation were considered normal. In contrast, the electrical
potentials in response to trunk stimulation (based on the results of
examinations before and after resection) were estimated to be normal in
group A, slightly decreased in group B, and decreased in group C. In
fact, the electrical potentials in response to trunk stimulation in
group B were slightly lower than those of group A, although no
significant differences were observed. In group C, responses to trunk
stimulation decreased for all branches. Among the branches, a
significant difference was observed only for BB and MB, but it is highly
likely that other branches would also show a significant difference if
the number of patients were increased.
To estimate the degree of postoperative nerve damage and the resulting
facial nerve palsy, it was considered more appropriate to use the
response ratio, rather than absolute values. We calculated the ratio of
electrical potential in response to trunk stimulation before and after
tumor resection (ARR1) and in response to trunk stimulation and branch
stimulation after tumor resection (ARR2). ARR1 is the ratio that relates
to the electrical potential in response to stimulation before tumor
resection, whereas ARR2 relates to the electrical potential in response
to branch stimulation. Considering that there might have been the
residual effect of the muscle relaxant on stimulation before tumor
resection, ARR2 is probably more reliable than ARR1. In fact, compared
to ARR1, the number of outliers was clearly lower for AAR2. When we
constructed ROC curves for the relationship between ARR and the presence
or absence of postoperative facial nerve palsy, the cut-off values were
0.63 for ARR1 and 0.55 for ARR2. Using these cut-off values, the
sensitivity, specificity, and accuracy were determined to be 92.7%,
77.8%, and 91.7%, respectively, for ARR1, and 96.5%, 93.1%, and
96.0%, respectively, for ARR2. We found that ARR2 can reduce false
positives better than ARR1, indicating that ARR2 is the more reliable
measure.
Together, these data show that by obtaining ARR during surgery, we can
predict facial nerve palsy intraoperatively, and ARR can serve as an
index for administering drugs such as steroids immediately
postoperatively. A limitation of this method is that it is not
applicable in cases where the tumor is resected without identifying the
facial nerve, or where damage to the nerve involves an area more central
than the identified facial nerve trunk, or more peripheral than the
facial nerve branches that were traced.