| Background and purposes of the researchRegardless of functional diseases in the cerebellopontine angle zone such as trigeminal neuralgia (TN),hemifacial spasm (HFS),and glossopharyngeal neuralgia (GPN),or parenchymal diseases such as vestibular schwannoma,the cerebellopontine angle zone meningeal layers will result in anatomical variations respective with the nerves and vessels of posterior fossa. However, the key steps for different modes of MVD techniques are to identify arteries or veins compressing a cranial nerve where the nerve sheath are thinner(especially the root entry/exit zone,REZ),neurovascular conflict,the doubted compression places which should be appropriately separated away. Even though compressive factors involved the distal segment of the nerve part from the REZ. Adhesions betweed the arachnoid membrane,nerve sheath,fraenulum and the involved nerve shoul be separated away. Some nerve tracts and fraenulum had to be interrupted when it was in need in order to push the offending vessels which penetrated intrafascicular away.Vestibular schwannomas originate outside brain. There are pia mater,capsule wall of tumor consisted of vestibular nerve sheath, arachnoid membrane layers covered posterior nerves and brain surface,neurilemmal sheath between the brain stem,cerebellum and the tumor. The cerebellopontine angle zone meningeal layers offer anatomy foundations to remove vestibular schwannomas following with neuroma/ nerve interface. However,the anatomical variations of meningeal layers with vasculonervous structures and neuromas are still unclear or inaccurate with unresolved controversies.More and more neurosurgeons widely offer MVD to treat trigeminal neuralgia (TN),hemifacial spasm (HFS),and glossopharyngeal neuralgia (GPN) patients for the first choice,many MVD modes have some low or high probability of remaining symptomatic relief that late recurrences have also been reported. However,in line with the variety of different theories regarding the site or sites of vascular compression that cause syndrome,there is wide variation in the types of decompression techniques that neurosurgeons recommend. Different MVD modes have more controversies surrounding the issues of the standard mode,the pathogenesis of microvascular decompression (MVD) without insufficient systematic anatomic and histology study to be based on. Although different MVD modes with different techniques resulted in curative effect in variation,MVD is an efficacious method to treat TN,HFS,GPN with good outcomes. Such a high degree of surgical success makes a statistically significant analysis difficult of the very small subset of recurrent,inefficient patients .The exact reasons for surgical failure or recurrence remain unclear. It is debatable for the postoperative evaluation and management of failure or recurrence and present a challenging decision regarding whether a second surgical procedure should be performed. There is no consensus among neurosurgeons regarding which site or sites of compression actually cause this condition,and,thus,which should be operated on to provide relief. Many etiological theories have been proposed,but none have yet been definitively proven.There is no consensus among neurosurgeons regarding which site or sites of compression actually cause trigeminal neuralgia,facial spasm,which should be operated on to provide relief. What different variations based on surgical anatomic and histological features are about decompression site of the nerve. What the theoretical and anatomic evidences are about MVD,et al. All these are waiting for being resolved. So far there have been rare system reports on anatomy and histological evidence of MVD and decompression site.It has been debatable for more than one hundred years to accept that the origin of an vestibular schwannomas is located outside or inside the subarachnoid space. The origin concept of acoustic neuroma which should be paid particular attention to the arachnoidal membrane during surgical procedures is important to remove tumor with low or high probability of preservation of facial and vestibulocochlear nerves.However, in the late two decades, anatomical and functional preservation rate of facial and hearing nerves is still kept in low scope. The study of cerebellopontine angle zone meningeal layers aims to improve anatomical and functional preservation of hearing and facial nerves related to vestibular schwannoma surgery via the retrosigmoid approach. It will bring immportant theoretical and clinical values for the study. So far there have been rare system reports on the study of intraoperative cerebellopontine angle zone meningeal layers related to vestibular schwannoma surgery via the retrosigmoid approach. The purposes of the research are①to study microanatomy and histology of trigeminal,facial and vestibulocochlear nerves,cerebellopontine angle zone meningeal layers and explore the microanatomy and histology of the central nervous system segment (CNS),peripheral nervous system(PNS) segment and transition zone(TZ) in trigeminal and vestibulocochlear nerves,acousticofacial cistern from cadavers so as to offer anatomic references for microvascular decompression and origin of an vestibular schwannomas.②To the different MVD modes for idiopathic trigeminal neuralgia,facial spasm and explore the efficacy differences and surgical techniques,findings and outcomes avoiding of recurrence to find clinical evidences for modified MVD.③To study the origin of vestibular schwannomas and effect of cerebellopontine angle(CPA) zone meningeal layers anatomy variation on the surgical technique for removal of vestibular schwannoma with improved preservation of hearing and facial nerve function.Materials and MethodsExperiment data1. Histological and micranatomy study on the membranes structure of the central nervous system segment,peripheral nervous system segment and transition zone in the trigeminal,facial and vestibulocochlear nervesThe cisternal portions of the trigeminal nerve and the acousticofacial nerves were removed enbloc for observation and measurement with micro-operative techniques in 20 sides from 10 cadaveric heads. Each tissue was fixed in formalin and then embedded in paraffin and sectioned horizontally with the aid of scarlet nitroxanthic acid staining method. Specimens were light microscopically measured the length of CNS,PNS,TZ segments and analyzed for structural differences between the CNS and PNS segments. The statistical analysis was performed with SPSS11.5 software package and the data were presented with(Mean±Standard Deviation). The differences between the length of CNS among trigeminal,hemifacial,and vestibulocochlear nerves were analyzed with One-way ANOVA,being significant when P<0.05.If vascular contact with the CNS segment of a cranial nerve is necessary to cause symptoms,we analyzed the correlation of the known incidences of trigeminal neuralgia,hemifacial spasm,and glossopharyngeal neuralgia in the west are related to the length of their respective CNS segments through double-variance correlationship analysis.2.Microsurgical anatomy of cisternal segments of facial and vestibulocochlear nervesThe anatomic relationships and datas of facial and vestibulocochlear nerves were observed and measured using microscope in 5 to 20xmagnification with microsurgical techniques through retrosigmoid approach in 20 sides from 10 formalin-fixed and perfused cadaveric heads with color silicon.Clinical data3. Microvascular decompression with whole range encircling mode versus conventional microvascular decompression in treatment of hemifacial spasmWe retrospectively analyzed the collected datas of all patients with the diagnosis of hemifacial spasm from January 1998 to January 2006. Intraoperative findings,surgical technique,and outcomes were recorded. The patients were divided into Group a (30 cases) and Group b (35 cases) according to different operative modes, Group c (below 40yr) and Group d (above 40yr) according to the onset age of symptom. The statistical analysis was performed with SPSS11.5 software package. Overall effective rate of group a and b were analyzed with 2 independent sample nonparametric test,being significant when P<0.05. The differences between four facial nerve compression categories, total complicated and recurrent incidences in two groups were analyzed with Pearson Chi-square,being significant when P<0.05.4. Modified microvascular decompression for idiopathic trigeminal neuralgiaWe retrospectively assessed surgical technique,findings and outcomes in 87 patients who underwent modified MVD. The outcomes from 41 consecutive patients with a mean age of 64 years(range,60～75yr) were compared with those from 46 younger group(mean age,47yr;range,20～59yr) operated on during the same period. The statistical analysis was performed with SPSS11.5 software package. The differences between the complication incidence,the mean length of hospital stay and pain relief in group a and group b were analyzed with Pearson Chi-square,being significant when P<0.05.5. Surgical treatment for idiopathic trigeminal neuralgia caused by venous compressionSurgical management technique and outcomes in 33 patients were retrospectively analyzed. Tiny veins were coagulated and cut,the offending vein was coagulated and cut when the vein was one of several superior petrosal veins complex(SPVC).However,the vein was preserved,divided away from the nerve and not cut in cases in which it was a large main drainer. After exposure of the entire cisternal trigeminal nerve,teflon graft was properly interpositioned to encircle the whole range of it.6. Study of cerebellopontine angle zone meningeal layers and preservation of hearing and facial nerve function related to vestibular schwannoma surgery via the retrosigmoid approach63 patients with vestibular schwannoma in our department during the last 3 years were divided into three groups on the basis of radiological features and operative record. The clinical features, microsurgical technique and results were analyzed retrospectively. The micranatomical position relationship between the cerebellopontine angle zone meningeal layers and tumor,hearing and facial nerve function were analyzed retrospectively for 63 patients with vestibular schwannoma via the retrosigmoid approach in our department during the last 3 years. 54 large tumors in a series of 63 patients were divided into three groups(Ventral brainstem group, Brainstem-cerebellum group and Incuneation group)on the basis of tumor exceeding 3.5cm in size. Surgical technique and operative duration on these three groups were analyzed retrospectively. The statistical analysis was performed with SPSS11.5 software package and the data were presented with(Mean±Standard Deviation). Statistical significance was set at P<0.05.The differences between the large tumor size and operative duration in three groups were analyzed with One-way ANOVA. Comparisons of postoperative facial nerve functions between the 3 groups were performed using Kruskal Wallis Test.Results1. Microscopic measurement demonstrate that the cisternal portions of trigeminal, hemifacial,and vestibulocochlear nerves respective were 13.08±2.12(9.26～15.7)mm,23.23±1.29(20.72～25.30)mm,28.02±3.33(21.79～31.82)mm.The histological examination revealed that the CNS segment of trigeminal,hemifacial, and vestibulocochlear nerves respective were 3.42±0.22(3.14～3.89)mm,2.39± 0.197(2.05～2.6) mm,9.59±0.84 (8.56～11.17) mm.The cone-like organizationobserved at the top of the CNS segment was characteristic. It pushed into the PNSregion,which surrounded the CNS segment. The PNS region revealed a parallel-aligned organization of the axons,which was loosely arranged to be rich inconnective tissue.The CNS segment was surrounded by thinner connective tissue,but to a far lesser extent than in the PNS segment. The length of the CNS segmentof vestibulocochlear nerve is longer than the CNS segment of the trigeminal nerve,which is longer than the CNS segment of the facial nerve(P=0.000).We found aclear epidemiological correlation between the length of the CNS segment, whichdiffered among cranial nerves, and the incidence of the microvascularcompression syndrome in the west(P=0.000).2.The cisternal segments of facial and vestibulocochlear nerves were divided into three segments,namely,REZ,cerebellopontine angle, internal acoustic meatus(IAM) segment. There was an important anatomical triangle in the area of REZ.The facial nerve motor root of cerebellopontine angle segment exposed a regular relation with the vestibulocochlear nerve in their pathways. The motor root had its location in the anterior-superior vestibulocochlear nerve and converged into one with the intermediate nerve in the middle of IAM. The arachnoid membrane of cerebellopontine cistern invaginated into the fundus of IAM as a muff.3. Results of MVD for facial spasmWe ascertained the facial nerve intraoperative compressed by degree was classified into 4 categories: Single contact,Contact and indentation, Adhesion and encasement,No identified offending vessels type.The offending vessels included arteries,veins,vascular loops. In the most patients of Group c,arachnoid membrane around the facial nerve was thick and encased the offending vessel;In Group d,the characteristic changes of the vasculature were the offending artery,which compressed the facial nerve was elongated,redundant,and focal arteriosclerosis. In Group a,the overall effective rate was 80%, the recurrent rate was 20.8%.In Group b,the overall effective rate was 97.1%,the recurrent rate was 3.2%.The chief permanent complications were hearing impairment found in 10.0% and ataxia in 6.7% of patients in Group a. In Group b,the rate of hearing impairment was 2.9 %,ataxia was 2.9%.There was no significant difference in terms of four facial nerve compression categories(P=0.856).Overall effective rate in group b was significantly higher than that of group a(P=0.019),however,the complicated incidence(P=0.020),recurrent incidence(P=0.038) in Group a were significantly higher than that of Group b.4. Results of modified MVD for idiopathic trigeminal neuralgiaThe trigeminal nerve compressed by mode was classified into 6 types:Single contact,Contact and indentation,Adhesion and encasement,No identified offending vessels,Transfixation and Complicated type. The offending vessels include arteries,veins,vascular loops. Location of the neurovascular conflict was often in the midthird and distal segment of the trigeminal nerve in Group a and at the trigeminal root entry/exit zone in Group b.Pain relief achieved 100% in 87 cases. In our series of modified MVD procedures,there was no significant difference in terms of postoperative complications between two groups (P=0.981).There were no operative mortalities or life-threatening morbidities. After average follow-ups of 2.5yr,no case recurred. The complications improved after discharge through treatments. 5.Results of surgical treatment for idiopathic trigeminal neuralgia caused by venous compressionLocation of the neuroveins conflict can be at all along the cisternal trigeminal nerve. The venous conflict was associated with one (or several) offending artery(ies) in 22 cases. The venous compression was pure in 11(12.6%), among 3 was unnamed vein while,8 was SPVC near Meckel's cave. The patterns of drainage of the SPVC were classified into three groups. Complete pain relief in all patients after microvascular decompression. After mean follow-ups of 2.5yr,no case recurred. Four patients developed trigeminal nerve impairment and 2 cases suffered from disordered cerebellum function, which improved through treatment.6.Results of Study of cerebellopontine angle zone meningeal layers and preservation of hearing and facial nerve function related to vestibular schwannoma surgery via the retrosigmoid approach①Cerebrospinal fluid emerged between the distal portion of vestibular schwannoma and the fundus of auditory meatus. DSA showed the some vestibular schwannomas with abundant blood supply on MRI examination was fed by the branch(es) of vertebral or basilar artery.②the vestibular schwannoma base can wholly locate in CPA cistern. The capsule wall of tumor consisted of vestibular nerve sheath. Arachnoid membrane layers covered posterior and ventral surface, superior and inferior poles of tumor,no arachnoid membrane interface was present between tumor and facial,vestibulocochlear nerves,also at side opposite to brainstem.③)Total tumor removal was accomplished in 96.8% of the patients. Anatomical preservation of the facial nerve and the cochlear nerve were achieved in 85.5% and 39.7%,respectively. There was no death in this group. The resection of groupⅢtumor with the longest time-consuming was the most difficult (P=0.000).However,it is not significant different from three groups for preservation of facial nerve function(P=0.751).Conclusions1.①Histological differences between the PNS and CNS segments suggest that the PNS segment is more resistant to compression. The evidence we present supports that vascular compression syndromes prone to arise from vascular contact along the CNS segment of the cranial nerves.②The data definitively prove that the root entry zone (REZ, nerve-pons junction) and TZ of the nerves are distinct sites and that these terms should never be used interchangeably.③The peripheral nervous system segment in the vestibulocochlear nerves could extend from cerebellopontine angle cistern to the fundus of internal acoustic meatus.2.①The origin of acoustic neuroma was inside the subarachnoid space,the cousticofacial cistern was a lateral extension of the cerebellopontine cistern.②The initial segments of facial and vestibulocochlear nerves can be located according to REZ anatomical triangle.The acoustic neuroma can be classified 3 types according to different tumor growth directions and anatomic relationships with REZ anatomical triangle. The functions preservation of facial and vestibulocochlear nerves can benefit from the categories.③To avoid entering the labyrinth,the bone exposure of IAM does not extend to the lateral end about 1cm.3. The report demonstrated that microvascular decompression with whole range encircling mode was more safety associated with higher cure rate. The key steps were to expose the entire facial nerve from the pons to internal auditory canal to identify neurovascular conflict,especially the REZ,the doubted compression places which should be appropriately separated away. Teflon graft was properly interpositioned to encircle the cisternal portion of facial nerve. Fewer disturbance to cranial nerves avoiding injury to the penetrating vessels from REZ and the pons were essential to improve the efficacy,reduce recurrence and complications.4.The report demonstrated modified MVD was safe and higher effective without recurrence. It was also suitable for the elderly. The key steps were to expose the entire trigeminal nerve from Meckel's cave to pons to identify neurovascular conflict,especially the REZ,the doubted compression places which should be appropriately separated away. Teflon graft was properly interpositioned to encircle the whole range of trigeminal nerve. All these done are essential to improve the efficacy,reduce recurrence and complications.5.SPVC is most frequently the offending vein. Whether the venous compression was main compressing factor or not,it was essential to deal with them and adjust surgical strategy. The trigeminal nerve was completely decompressed from veins followed with or without arterial compression in all patients. MVD may identify the venous compression accurately and reduce the recurrence. However,it could result in higher postoperative complications.6.①Vestibular schwannoma locates inside of arachnoid membrane layers. Arachnoid membrane layers covering tumor is intrinsic extension of cerebellopontine cistern. Arachnoid membrane layers covered posterior and ventral surface,superior and inferior poles of tumor,no arachnoid membrane interface was present between tumor and facial,vestibulocochlear nerves,also at side opposite to brainstem. Intraoperative difficulties in total tumor removal and preservation of hearing and facial nerve function can be avoided in atraumatic dissection and separation between the duplication of arachnoidal layers or cyst wall of tumor.?Surgical results of large vestibular schwannoma can be improved by familiarity with the anatomy of cerebellopontine angle zone meningeal layers and adoption of corresponding surgical strategy according to its anatomical relationship to brain stem and cerebellum.
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