Coblation is a relatively novel technique applied in neuropathic pain. Some previous studies found19 that the coblation group had lower postoperative NRS scores than RFT group and the risk of postoperative numbness in patients with V1, V2, and V3 ITN was reduced. However, the previous studies were limited to small sample sizes and short-term follow up. Besides, because of the specificity of V1, some studies indicated that the appropriate temperature of RFT for the treatment of V1 ITN was lower than V2/V3 ITN22, 23, which was to avoid complications such as diplopia and keratitis. To prevent the influences of different temperatures on the evaluation of surgical outcomes, we only included V2/V3 ITN in the present study with a 12-months follow-up.
In terms of the efficacy of coblation, the postoperative NRS scores at discharge were significantly lower than before surgery and it continuously remained at a low level during our 12-months follow up, which was identical with RFT group in respect of postoperative analgesic effect. Compared with the initial remission rate of 96.9% in RFT group, it was 94% in coblation group, which was consistent with the result of a previous study ranging from 85.3–97.7%19. At the endpoint of follow up, the remission rate was 75.3% and 78.4%, respectively in coblation group and RFT group. Despite the slightly lower remission rate in coblation group, there was no statistically significant difference. Some literature review reported that the remission rate after RFT22, 24 ranged from 50.4–80.7% in a follow-up of 5 years. However, as a relatively novel technique for the treatment of ITN, it was difficult to perform the long-term results according to the available and limited clinical data. Therefore, a big-sample of long-term follow-up study is still required to illustrate the efficacy of coblation.
Concerning postoperative complications, postoperative facial numbness had been the most disturbing issue among patients who accepted minimally invasive treatment of ITN. In contrast to the remission rate, the coblation group had some advantages in the degree of postoperative facial numbness compared with the RFT group. Our study results indicated that the degree of postoperative facial numbness tended to increase in RFT group, mainly due to the different mechanisms of the two techniques. Coblation14, 15 is a unique modality that uses bipolar radiofrequency energy to ablate and coagulate soft tissue at low temperatures (40℃-70℃) with minimal thermal damage to surrounding tissues. During coblation, conductive saline solution is converted in the gap between the needle tip and the tissue into an ionized plasma layer. Once the plasma layer meets the tissue, intercellular bonds would be destroyed by the ions. However, the mechanism of RFT is thermal damage as Aδ and C-type nerve fibers for pain transmission would be coagulated and denatured by temperatures of ≤ 80℃, but Aα and Aβ nerve fibers25, 26 would not be affected. Yao22, 23 and Tang9 had investigated the eligible temperature of RFT for V2/V3 ITN and the recommended temperature was 68–75℃, which could reduce the incidence of postoperative facial numbness, but the temperature was still relatively high compared with coblation. Neither of the two procedures could selectively destroy C-type nerve fibers. But thermal damage to the nerves is minimized by coblation, which attributes to 40 °C-70 °C operating temperatures. This may be explanations of the fact that lower degree of postoperative facial numbness occurred in coblation group rather than RFT group. Other complications such as oral ulcer, masticatory weakness, corneal hypoesthesia, hypoesthesia of temperature and tinnitus were no significant differences between the 2 groups.
In addition, complications are also important aspects in evaluating the effectiveness of treatment and improving the satisfaction of patients. However, due to the mechanism of treatment, postoperative facial numbness was almost inevitable among patients who accepted minimally invasive treatment. To explore the risk factors of the degree of postoperative facial numbness in ITN patients, we conducted a multivariate analysis and the results showed that the pain characteristics, previous treatment at the affected side, and procedure were risk factors significantly associated with the degree of postoperative facial numbness. The continuous pain27, 28 was described as dull, aching, burning and other different from the sharp-like. Some studies reported27 that the prognosis of continuous ITN patients was less effective than paroxysmal ITN patients, with 80% initial remission rate and 54% 5-years remission rate. However, we also observed more severe postoperative facial numbness in patients with continuous ITN in our study, which might be explained by the central sensitization hypothesis29. The hypotheses rated that the intense, repetitive and sustained stimulus could increase the membrane excitability and synaptic efficacy, and decrease the thresholds of activation. The chronic and continuous stimulus to our trigeminal nerve would lead to pain hypersensitivity, that is, innocuous stimulation could be considered as pain-causing noxious stimulation. We speculated that the pain hypersensitivity caused by pathophysiological changes in pain pathway would also affect the superficial sensation, which would be the possible cause of the severe postoperative facial numbness. In addition, some scholars also rated28 that the continuous pain was transformed by paroxysmal pain if left treated, and the changes involved the development of sensory impairment. But it is just supposition; evidence is needed to support the speculation.
Meanwhile, we also found that patients with recurrent ITN, who had accepted RFT, suffered more severe facial numbness compared with primary ITN patients. The destruction of the Gasserian ganglion by neurosurgeries would result in denaturation and necrosis of partial trigeminal nerve, and the damaged nerve fibers would be gradually repaired physiologically, which may lead to the recurrence of ITN30. And we supposed that the second damage to the repaired nerves would result in severe facial numbness, compared with those who accepted primary destructive neurosurgeries. But more researches are needed to support our supposition.
There are still some limitations about our study. First, patients with V1 ITN were excluded because of the appropriate ablation temperature differed from V2/V3 ITN. Second, it was a retrospective study and selection bias could not be avoided. Large-scale, randomized, double-blind studies will be needed to validate our findings in the future.