The effect of partial superficial parotidectomy on amplitude, latency and threshold of facial nerve stimulation.

The aim of this study is to assess the effect of partial superficial parotidectomy and facial nerve dissection to electrophysiologic parameters of intraoperative facial nerve monitoring such as nerve stimulation threshold, stimulus amplitude and latency. Twenty-five patients who underwent partial superficial parotidectomy for benign parotid gland mass were included in the study. After the identification of the facial nerve main trunk, minimum stimulation threshold, latencies and amplitudes of the orbicularis oculi (electrode 1) and orbicularis oris (electrode 2) electrodes at 0.50 milliamperes (mA) were recorded. All of the recordings were repeated after the completion of parotidectomy. Median nerve dissection duration was calculated and size of the tumors was measured during macroscopic pathology. The median minimum nerve stimulation threshold was 0.15 mA [interquartile range (IQR) = 0.05] before and 0.15 mA (IQR = 0.08) after the parotidectomy (p = 0.02). Median nerve dissection duration was 49 min (IQR = 38). Median amplitude and latency in electrode 1 before and after the facial nerve dissection were 322 millivolts (mV) (IQR = 330), 370 mV (IQR = 370) (p = 0.02), 3 milliseconds (ms) (IQR = 1) and 4 ms (IQR = 2) (p = 0.05), respectively. Median amplitude and latency in electrode 2 before and after the facial nerve dissection were 396 mV (IQR = 275), 365 mV (IQR = 836) (p = 0.86), 3 ms (IQR = 1.5) and 4 ms (IQR = 1.5) (p = 0.17), respectively. Minimal nerve stimulation threshold and amplitude of electrode 1 were affected by facial nerve dissection among the electrophysiologic parameters (p = 0.02 and p = 0.02). Of the electrophysiological parameters only the latency of electrode 2 was significantly correlated with tumor size (p = 0.03). Besides, none of the parameters were predictive for a possible postoperative facial nerve dysfunction regarding superficial partial parotidectomy.