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.