Although a sizable body of studies demonstrated the importance of objective clinical predictors of curative effect, the selection of TN patients for PBC was still subjective[24, 21, 18]. Until now, there was no objective scale established for the prediction of TN patients’ after PBC. Some may doubt whether such a scale would be complicated or offer any additional value. While, in our study, we developed a nomogram, including response to carbamazepine, compression severity score and TN type, to predict the probability of effective outcome for TN patients following PBC. Moreover, the ROC analysis and decision curves demonstrated relatively good performance for the model in terms of clinical application. In more practical terms, our results revealed that applying the nomogram added significant clinical value above the current paradigm of subjective consideration of clinical factors.
Even though the TN was not life-threatening, it influenced the life and work of patients seriously. PBC of trigeminal ganglion (TG) is an acknowledged minimally invasive treatment for TN, which was firstly reported by Mullan and Lichtor[20, 19]. PBC can block the abnormally discharging “short circuit” by compressing the demyelinating Aβ fiber selectively. Subsequently, the pain relieved by generating new myelin sheath[17]. Previous studies showed that the rates of pain relief ranged from 80–100% and the rate of complications was about 20%[18, 9, 6]. The complications of PBC included facial numbness, weak chewing, keratitis, act[28, 25]. In our study, we found the pain relief rate of PBC was 81.2% at 3 months follow-up which was parallel with previous studies. Previous studies showed the preoperative factors, such as TN type, response to medication, radiographic data and symptom duration, and intra-operative factors such as compression time, balloon shape and volume were associated with the TN patients’ outcome after PBC[24, 16]. While, in our study, we focused on the preoperative factors which would influence prognosis and drawn different conclusions.
For the intra-operative factors, such as compression time, balloon shape and volume, were reported influenced the outcome after PBC. Meckel cave is a cleft-like dural pocket extending from the posterior fossa into the middle cranial fossa through the porus trigeminus which is a natural connection between the two fossae. The anatomical features were basics of the pear-shaped balloon during PBC. A pear-shaped balloon appears when the balloon extends into the pontine cistern after filling the Meckel cave through the porus trigeminus[3, 2]. Numerous clinical studies have classified the shape of the balloon into pear-shaped, oval, round, irregular and inverted pear-shaped. Round shape indicates that the balloon has not entered or did not completely enter the Meckel cave and is inflated in the epidural cavity. A balloon with this shape may not produce effective treatment or may be associated with early recurrence. According to Asplund et al. a pear-shaped balloon results in longer pain relief and lower risk for pain recurrence than non-pear-shaped balloons[4]. Several previous studies showed the parallel conclusion. Consequently, in our study, we exclude the cases with non-pear balloon shape to reduce errors of our score system. Mullan and Lichtor concluded that 1 minute of compression was enough to relieve the pain after analyzed 100 cases of TN following PBC[19]. Brown and Gouda reported a series of TN cases treated by PBC[7]. During the operation, the volume of balloon was between 0.75 to 1 mL and compression duration was 1 to 1.5 minutes. While, in the study, they found that there was no significant difference between different balloon volume or compression duration. They concluded that 0.75 mL of contrast medium and 60-second compression was enough for getting satisfactory results which was also demonstrated by other studies. Consequently, in our study, we used the 0.75 mL of balloon volume and 60-second compression as the standard treatment during the PBC operation.
Several previous studies showed clinical factors such as symptom duration, preoperative NRS, previous surgery, TN type and compression severity score of TN nerve may influence the curative effect. Previous studies showed that atypical pain was associated with a poorer prognosis not only in ablative procedures such as GR and SRS, but also in non-ablative procedures such as MVD[22, 12]. Tyler-Kabara et al. showed that 74% of patients with typical pain had excellent outcomes following MVD, compared with only 35% of patients with atypical pain[26]. In our study, we found similar results that the atypical TN patients achieved a lower rate of excellent pain relief. Previous studies showed that vascular contact with the trigeminal nerve is universal (occurring in 82 to 92%)[15]. Nevertheless, several papers showed that neurovascular compression on MRI remains important for predicting postoperative outcome following MVD[22, 29]. A meta-analysis showed that artery compression to be an independent predictor of pain relief following MVD (OR 3.35, 95% CI 1.91–5.88)[14]. In our study, we found parallel results that the grade II or III compression severity score had a 14.2 times risk against the no vessel or grade I compression severity score of pain relief at 3 months follow-up. Until now, the carbamazepine was still the preferred drug for TN patients, which provides pain relief in 70% of patients. Previous study showed that response to carbamazepine positively remains significant for prognosis of TN following MVD[22]. In our study, we found parallel results that response to carbamazepine positively significantly predicted satisfying outcome following PBC. In my opinion, response to carbamazepine negatively means the nerve was compressed for a long time and injured seriously. The curative effect of other therapy, including MVD, PBC and radiofrequency rhizotomy (RFT), would be limited because of the serious lesion of nerve.
The nomogram system provides an objective means to select TN patients for PBC. While, the simplicity is critical for the nomogram system readily adopted. The complex disease factors which was validated in previous studies were simplified in our nomogram system. The TN type was simplified to classical vs nonclassical. Responsiveness to medication was divided into response to carbamazepine positively or positively. The degree of vascular compression was divided into two levels: grade I vs grade II or grade III. The ROC analysis and decision curves indicated that the nomogram system was reliable for clinical application. The TN nomogram system realize the goal of being simple to be used without sacrificing predictive ability.
Limitations
The main limitation of this study was that the sample size was small. Consequently, the predictive effect was susceptible to bias. Moreover, this is a retrospective study and as such provides only retrospective validity, so a prospective validation of the prognostication tool is still needed. The retrospective nature of the study creates the inherent bias. Depending on the best current understanding of the disease process, we selected three variables into the nomogram system. We excluded the cases caused by space-occupying lesion or had undergone MVD, Percutaneous glycerol rhizotomy (PGR) and glycerol rhizotomy (GR). Other factors not included or assessed in the nomogram system may also influence the prognosis of TN following the PBC. In addition, the long-term effects of PBC on TN need a longer patient follow-up period. At this stage, more research with validation in multiple, external independent patient populations is needed