This study analyzed IPD from 192 patients with PTC, showing that PTC shared many similarities with TN and HFS in terms of epidemiology, but may differ in conflicting vessel and prognosis predictor.
Epidemiology and conflicting vessel
Our results suggest that PTC is more likely to afflict middle-aged women between 40 to 60. The gender ratio and onset age of PTC is similar to that of TN and HFS [20, 22, 23, 26–28]. Left side symptom is more common than the right side in PTC, the same as HFS but differing from TN, which are more right-side dominant [24, 25]. We found that PTC initiated more as HFS than as TN, a finding also observed by Liu et al.  This can possibly be explained by the compression of the VBA, as the stem of the VBA is more adjacent to the root entry zone (REZ) of the facial nerve than that of the trigeminal nerve. So, when VBA gets enlarged and stiffened due to arteriosclerosis, it is more likely to induce HFS firstly.
Identical to previous reports [2, 52], the most common cause for PTC was vascular compression, observed in 85% of the cases. Interestingly, our results showed that, when analyzed separately, the offending vessel for TN and HFS in PTC is very similar to that for TN and HFS alone. In PTC, the facial nerve was commonly compressed by AICA, PICA, and VA. These 3 vessels are also the most common causes of HFS . Similarly, the trigeminal nerve was most compressed by SCA, AICA, and BA in PTC, identical to the SCA, vein, and AICA compression observed in TN . Given that the cause and conflicting vessel of PTC are so alike to that of TN and HFS alone, we speculated that in most cases, PTC should be regarded as a simple coexistence of the 2 CNs dysfunction, rather than an independent syndrome. This is also supported by that the prevalence of hypertension in patients with PTC (approximately 30%) is similar to that in TN and HFS [23, 29]. After the first CN is compressed, whether the second CN will be affected depends on the degree of dolichoectasia, the position of the nerve, and the volume of the posterior fossa . Importantly, in 46 PTC cases whose symptoms were caused by a single vessel compression, a tortuous dolichoectatic vertebral artery was the major offender, consisting of over 65% of the cases, a finding different to that when TN and HFS exist alone. This is relevant to the anatomy as VBA has the largest vessel diameter among arteries in the posterior fossa . For patients with arteriosclerosis, the ectatic VBA shifted to the affected side could compress multiple CNs along its running area, resulting in combined CN dysfunction.
Surgical outcome and prognosis predictor
In our included patients, the cure-rate after MVD surgery was 84%. It reflects a satisfactory surgical outcome although the rate is slightly lower than that in TN (82% to 91%) and HFS (89% to 91%) [31–34, 51]. Previous literature suggested that the course of PICA is the most variable and complex in all the cerebellar vessels , while we observed that the involvement of AICA/PICA is the only predictor of excellent surgical outcome. The reason could be that the ALCA/PICA are generally slimmer than the VBA. Once the course and neural-vascular conflicting relationship is established, surgeons can decompress the CN though placing Teflon sponge with relative ease. While in VBA compression, Teflon sponge alone may not be sufficient for adequate decompression . Montava et al. have similar findings that PICA compression is predictive of better surgical outcomes. Other correlative factors have also been indicated in the previous report, e.g. single vascular compression and artery compression are predictive of good surgical outcome , while venous compression foresees a suboptimal outcome .
Our results showed that postoperative complication was not rare after MVD surgery, occurred in 26% of patients, with facial weakness being the most common transient complication. As reported, the complication rate of TN is slightly higher than that of HFS, occurring in approximately 30% of patients [38, 39]. Alford  and Liu  suggested that physical conditions such as preoperative anemia, and life habits such as excessive drinking and smoking are predictors for postoperative complication [40, 43]. Amagasaki et al. indicated that PICA offending is the only independent risk factor of postoperative lower CN palsy, possibly because PICA and lower CN belong to the same vascular nerve complex . We did not identify any significant predictor on postoperative complication in the multivariate regression analysis, but observed a weak relationship between postoperative complication and left side surgery in the univariate analysis. We assumed that VBA compression may play a role here, as left-side VBA is often larger and more likely to have sclerosis than right-side VBA [56, 57], causing difficulty for decomposition surgery .
In our study, PTC recurred in 14 patients after MVD surgery, with a recurrence rate of 11%. Among the 14 patients, 11 patients had TN recurred and 3 patients had HFS recurred. Previous findings agreed that TN may have a higher rate of recurrence than HFS [34, 46], possibly because venous compression is more seen as a cause for TN . For TN, previous literature also reported that female gender, long disease duration, and atypical pain are associated with a higher risk of recurrence [26, 47, 48]. Notably, several previous studies indicated that older age is not only a predictive factor for successful decomposition [36, 48], but also a protective factor for postoperative recurrence in patients with TN . The authors hold that older patients are oftentimes concomitant with cerebellar atrophy. The reduced cerebellar volume contributes to better posterior fossa exposure, which not only helps better identify neural-vessel conflict but also shortens operation time. On the contrary, in our study, older age is a risk factor for PTC recurrence. Three possible reasons may explain this. First, for patients with PTC, the conflicting vessel in the REZ is oftentimes already obvious. This makes identification of the offending vessel less difficult for younger patients without significant cerebellar atrophy. Second, PTC is a rare syndrome that neurosurgeons may pay extra attention to. This may offset the bonus brought by cerebellar atrophy because careful exposure and decomposition of the CNs is already needed. Third, older patients are at a higher risk of developing arteriosclerosis and diabetes. These factors could result in the dolichoectatic vertebral artery, increasing the difficulty of decomposition . Besides, recurrence could result from newly developed responsible vessels, which can be more commonly seen when concomitant arteriosclerosis and diabetes exist.
Several limitations of the study should be noted. First, we did not choose the two-step method, which better considers the clustering of patients within studies [10, 16], for main analyses. The reason for doing so is that most of the included studies are case reports, making it impossible to calculate a pooled statistic for each study. Besides, the effect of patient clustering would not significantly bias our result, since most of our outcomes are calculated from count data. But undeniably, the effect of patient clustering could influence regression outcomes. Thus, an additional two-step meta-regression was conducted to validate outcomes obtained in the one-step regression.
Another flaw is that the included studies inconsistently report outcomes. Not each of the 57 studies clearly reported all the demographics and surgical outcomes. And not every study judged surgical outcomes on the same basis. Therefore, we can only estimate surgical outcomes according to descriptions in the report. Future studies should employ standard scales (e.g. barrow neurological institute score) in the evaluation of surgical results.