Surgical Procedure
All patients underwent partial parotidectomy. The operative field was
prepared by S-shaped incision from the front part of the ear. For nerve
treatment, the trunk of the facial nerve was identified first, then the
branches, which were identified and dissected within the range required
for tumor resection 9). For deep lobe tumors, the superficial lobe
tissue was dissected to identify nerve branches, and the superficial
lobe was returned to its original position after resection of the deep
lobe tumor. In all cases, 0.6 mg/kg rocuronium was administered
intravenously as a muscle relaxant at the time of introduction of
anesthesia. After this, no additional muscle relaxants or antagonists
were administered. Neuromuscular monitoring was set up in the ulnar
nerve region after anesthesia introduction to monitor the contraction of
the adductor pollicis muscle. No drugs such as steroids were
administered to patients who developed postoperative facial nerve palsy.
Measurement of reaction potential of FNM (Fig. 1)
FNM was performed using NIM Response® (Medtronic). Under general
anesthesia, needle electrodes were inserted into the dominant regions of
the temporal branch (TB), zygomatic branch (ZB), buccal branch (BB), and
marginal mandibular branches (MB) via four channels. The stimulation
current was set constant at 1 mA. In the measurement before tumor
resection, we stimulated the trunk at the time of its discovery and
measured the electrical potential evoked in response to the stimulation.
In the measurement after tumor resection, we stimulated the exposed
trunk (△) and each of the four branches (▽) in the operative field and
measured the electrical potential evoked in response to stimulation
(Fig. 1).
Postoperative facial nerve dysfunction (Table 2)
Facial nerve palsy was observed in 20 of the 113 patients (8 of the 48
male patients; 12 of the 65 female patients). All cases of facial nerve
palsy were temporary; no patients developed permanent paralysis. Nine of
67 patients with superficial lobe tumors, 7 of 18 patients with deep
lobe tumors, and 4 of 28 patients with lower pole tumors developed
paralysis, and the incidence of paralysis was significantly higher among
those with deep lobe tumors than other types (p = 0.04). Among the 20
patients who developed paralysis, 16 had paralysis of only one branch, 3
had paralysis of three branches, and 1 had paralysis of all branches. In
sum, postoperative facial nerve palsy occurred in 29 of 80 branches.
Thirteen patients had paralysis of MB, 7 had paralysis of ZB, 5 had
paralysis of BB, and 4 had paralysis of TB. Among the 113 patients, 372
branches of 93 patients did not develop facial nerve palsy and were
classified as group A. Among 20 patients who developed paralysis, 51
branches without paralysis were classified as group B, and 29 branches
with paralysis were classified as group C.