Clinical analysis of surgical outcome of 89 patients having large cerebral arteriovenous malformations.

OBJECTIVE: The article analyzes the clinical features, morphological characteristics, surgical subtleties and long-term outcome of surgery in 89 cases of 'large' sized AVMs. MATERIALS AND METHODS: During the period 2004 to 2022, 89 cases of 'large' arteriovenous malformations were operated in the neurosurgery departments of the authors. Large AVMs were defined as those that were more than 4 cm on either lateral or antero-posterior view of digital subtraction angiogram. The factors that determined the extent of surgical difficulties included site and eloquence of the area, number of feeding vascular territories and draining veins, degree and rate of flow, presence of flow-related aneurysms, and the physical nature of the arteriovenous malformation. RESULTS: There were 59 males and 30 females and the average age was 32 years. Headache, giddiness and convulsions were the common presenting complaints. Six patients were unconscious after surgery. Of these, five patients died in the immediate post-operative period and one patient gradually recovered. Additionally, seven patients developed unilateral limb weakness that included hemiplegia (4 patients) and hemiparesis (3 patients) following surgery. Clinical follow-up ranged from 6 months to 18 years (average 43 months). All surviving patients are leading normal and essentially symptom free life and have recovered from their symptoms of headache, convulsions and giddiness. CONCLUSIONS: Large AVMs are amenable to 'curative' surgery with 'acceptable' results. The surgery can be challenging and appropriate case selection that is based on the surgeons experience is vital and decisive.

Evolving Concepts of Craniovertebral and Spinal Instability.

Weakness of the muscles of the nape of the neck and back of the spine and its related instability is the nodal point of pathogenesis of a number of clinical and pathological events at the craniovertebral junction and the spine. Whilst acute instability results in sudden and relatively severe symptoms, chronic or long-standing instability is associated with a range of musculoskeletal and structural spinal alterations. Telescoping of the spinal segments results in "vertical" spinal instability in the subaxial spine and central or axial atlantoaxial instability (CAAD) at the craniovertebral junction. Instability in such cases might not be observed on dynamic radiological imaging. Chiari formation, basilar invagination, syringomyelia, and Klippel-Feil alteration are some of the secondary alterations as a result of chronic atlantoaxial instability. Radiculopathy/myelopathy related to spinal degeneration or ossification of posterior longitudinal ligament appears to have their origin from vertical spinal instability. All the secondary alterations in the craniovertebral junction and subaxial spine that are traditionally considered pathological and to have compressive and deforming role are essentially protective in nature, are indicative of instability, and are potentially reversible following atlantoaxial stabilization. Stabilization of unstable spinal segments is the basis of surgical treatment.

Central Atlantoaxial Dislocation: Presenting Symptoms, Diagnostic Parameters, and Surgical Treatment from Reports on 15 Surgically Treated Patients.

AIM: This chapter reviews the clinical entity of central or axial atlantoaxial instability (CAAD). MATERIAL AND METHODS: From January 2018 to November 2020, 15 patients were identified as having CAAD, wherein there was no atlantoaxial instability when analyzed by conventional radiological parameters and wherein there was no evidence of neural or dural compression due to the odontoid process. The patients were identified as having atlantoaxial instability on the basis of the alignment of facets on lateral profile imaging and a range of telltale clinical and radiological indicators. The clinical statuses of the patients were recorded both before and after surgical treatment by using the specially designed Goel symptom severity index and visual analog scale (VAS) scores. All patients were treated via atlantoaxial fixation. RESULTS: There were six men and nine women ranging in age from 18 to 45 years (average: 37 years). The presenting clinical symptoms were relatively subtle and long-standing. Apart from symptoms that are generally related to neural compromise at the craniovertebral junction, a range of nonspecific cranial and spinal symptoms were prominent. The follow-up time after surgery ranged from 6 to 34 months. All patients showed early postoperative and sustained clinical recovery. CONCLUSIONS: The correct diagnosis and appropriate surgical treatment of CAAD can provide an opportunity for quick and lasting clinical recovery.

Central or axial atlantoaxial dislocation and craniovertebral junction alterations: a review of 393 patients treated over 12 years.

OBJECTIVE: The authors reviewed their scientific publications and updated their clinical material obtained over the last 12 years for cases of central or axial atlantoaxial dislocation (CAAD) identified in the presence of craniovertebral musculoskeletal and/or neural alteration(s). The management implications of diagnosing and treating CAAD are highlighted. METHODS: During a 12-year period, CAAD was diagnosed in 393 patients with craniovertebral junction-related musculoskeletal and neural alterations who underwent atlantoaxial fixation. No bone decompression was done. All CAAD-related craniovertebral junction structural changes were identified to have a naturally protective role. Hence, in this paper the term "craniovertebral alterations" is used for "craniovertebral junction anomalies" and the term "Chiari formation" is used instead of the commonly used term "Chiari malformation." RESULTS: The major radiological diagnosis was determined either singly or in cohort with one or more of other so-called pathological entities that included Chiari formation (367 cases), syringomyelia with Chiari (306 cases), idiopathic syringomyelia (12 cases), type B basilar invagination (147 cases), bifid arch of the atlas (9 cases), assimilation of the atlas (119 cases), C2-3 fusion (65 cases), Klippel-Feil alteration (4 cases), and dorsal kyphoscoliosis (15 cases). The follow-up period ranged from 6 to 155 months. Clinical improvement was observed in all patients. CONCLUSIONS: Understanding and treating CAAD may have significant implications in the surgical treatment of a number of clinical entities. The gratifying clinical outcomes obtained in patients after atlantoaxial fixation, without any type of decompression involving bone or soft-tissue resection, consolidate the concept that atlantoaxial instability has a defining role in the pathogenesis.

Lumbar canal stenosis: analyzing the role of stabilization and the futility of decompression as treatment.

OBJECTIVEThe authors report their current experience with their previously published novel form of treatment in 70 cases of lumbar canal stenosis. The treatment consisted of only fixation of the spinal segments by the transarticular screw fixation technique. No bone, ligament, osteophyte, or disc resection was done for spinal canal and neural foraminal decompression. The proposed treatment is based on the concept that vertical instability that results in telescoping of the facets on physical activity forms the nodal point of pathogenesis of lumbar canal stenosis.METHODSDuring the period June 2014 to May 2018, 70 patients presenting with the classically described symptoms of lumbar canal stenosis were treated surgically by only fixation of involved spinal segments. Apart from clinical and radiological guides, instability was diagnosed on the basis of physical observation of the status of articulation by direct manipulation of bones of the region. The operation involved transarticular insertion of 2 or 3 screws for each articulation. The Oswestry Disability Index and visual analog scale were used to assess the patients before and after surgery and at follow-up. Additionally, a personalized patient satisfaction score was used to assess the outcome of surgery.RESULTSClinical symptomatic recovery was observed in all patients in the immediate postoperative period. During the average follow-up period, 100% of patients had varying degrees of symptomatic relief. The patient satisfaction score suggested that all patients were very satisfied with the surgical procedure. The transarticular fixation technique provided strong spinal segment fixation and a reliable ground for bone arthrodesis. No patient needed any additional modality of treatment or reoperation for recurrence of symptoms.CONCLUSIONSSpinal instability is the nodal point of pathogenesis of spinal degeneration-related lumbar canal stenosis. Only fixation of the involved spinal segments is necessary-decompression by bone or soft-tissue resection is not necessary.

Anatomical analysis of white fiber tracts in SMA and its implications related to en-masse tumor resection technique.

OBJECTIVE: Anatomy and connections of the supplementary motor area (SMA) are studied essentially to analyze the SMA syndrome. Experience with surgical treatment of 19 tumors located in SMA is analyzed. MATERIAL AND METHODS: The cortical anatomy and subcortical connectivity of the SMA was studied on ten previously frozen and formalin fixed human cadaveric brain specimens. The white fiber dissection was performed using Klingler's method. Nineteen patients with low grade gliomas in the region of the SMA treated surgically were clinically analyzed. RESULTS: The white fiber connections of the SMA include short arcuate connections with the pre-central, middle and inferior frontal gyri, the medial part of the SLF, the cingulum, the frontal aslant tract (FAT), the claustro-cortical fibers, the fronto-striatal tract and the crossed frontal aslant tract. All tumors were operated using en-masse surgical technique described by us and its subsequent modifications that focused on attempts towards preservation of related critical fiber tracts namely FAT, cingulum and corpus callosum presumed to be responsible for postoperative SMA syndrome. Eight patients developed an SMA syndrome in the immediate post-operative period. Eleven patients did not develop any post-operative neurological deficits. In all these 11 patients it was apparent that the cingulum, FAT and the corpus callosal fibers were preserved during surgery by modifying the tumor resection technique. CONCLUSIONS: SMA syndrome is a frequent occurrence following surgery in patients with tumors in the region of the SMA complex. Surgical strategy that preserves the cingulum and the FAT can prevent the occurrence of the SMA syndrome.

Post-Traumatic Central or Axial Atlantoaxial Dislocation Presenting with "Atypical" Symptoms-Analyzing the Role of Dynamic Imaging on the Basis of Experience with 14 Patients Treated by Atlantoaxial Fixation surgery.

BACKGROUND: This is a report of a series of 14 patients who presented with a range of "atypical" cranial, spinal, and systemic symptoms that started after they suffered a relatively severe injury to the head and/or neck several months or years before surgical treatment. The implications of diagnosing and treating central or axial atlantoaxial dislocation (CAAD) is discussed. Also, the role of dynamic rotatory and lateral head tilt imaging in the diagnosis and treatment is analyzed. METHODS: Of the 14 patients, 7 were men and 7 were women, with an age range of 21-64 years (average, 42 years). Due to the severity of the presenting neurological and non-neurological symptoms, all the patients had lost their occupation and were heavily dependent on painkillers and/or antidepressant drugs. In addition to other characteristic clinical and radiological evidence, CAAD was diagnosed made based on the facet alignments on lateral profile imaging in the neutral head position. Dynamic head flexion-extension, lateral head tilt, and neck rotation imaging findings confirmed and subclassified CAAD. All 14 patients underwent atlantoaxial fixation surgery. RESULT: A personalized self-assessment clinical scoring parameter and the World Health Organization Disability Assessment Schedule 2.0 was used to evaluate the outcome. One patient did not follow-up after surgery. At a minimum follow-up of 6 months after atlantoaxial fixation surgery, the remaining 13 patients experienced relief from all major symptoms. CONCLUSIONS: Diagnosing and treating CAAD can have major therapeutic implications for patients presenting with progressively worsening disabling clinical symptoms following relatively severe head and/or neck trauma.

Lumbar parafacetal cyst-spinal segmental instability is the cause and stabilization is the treatment: A clinical report of eight surgically-treated patients.

Objective: The authors report the results of "only-fixation" of the affected spinal segment without any decompression of the bones or soft tissue or manipulation of the cyst wall or contents in eight cases having lumbar parafacetal cyst (LPFC). This surgical strategy was based on the concept that LPFCs are secondary to spinal instability, has a protective or adaptive role, and is reversible following stabilization. Materials and Methods: During the period from January 2018 to January 2023, eight consecutive patients having LPFC were surgically treated. There were 5 males and 3 females, and their ages ranged from 48 to 72 years (average 63 years). Seven patients had a single cyst and one patient had multiple cysts. The patients presented with symptoms classically attributed to lumbar canal stenosis. Apart from the cyst-affected spinal segment, degenerative alterations were observed in adjoining spinal segments in six out of seven patients having a single cyst. All patients underwent "only fixation" of the unstable spinal segments without any kind of bone or soft-tissue resection and without any manipulation or handling of the cyst wall or contents. Results: During the follow-up period that ranged from 12 to 57 months (average 29 months), all patients improved from their symptoms. The recovery was observed in the immediate postoperative period and was lasting. Conclusions: LPFCs are one of the several secondary alterations observed in spinal degeneration. Identification of unstable spinal segments and their fixation constitutes rational treatment of lumbar parafacetal cysts. Direct handling and resection of cysts are unnecessary.

Defining role of atlantoaxial and subaxial spinal instability in the pathogenesis of cervical spinal degeneration: Experience with "only-fixation" without any decompression as treatment in 374 cases over 10 years.

Aim: The authors analyze their published work and update their experience with 374 cases of cervical radiculopathy and/or myelopathy related to spinal degeneration that includes ossification of the posterior longitudinal ligament (OPLL). The role of atlantoaxial and subaxial spinal instability as the nodal point of pathogenesis and focused target of surgical treatment is analyzed. Materials and Methods: During the period from June 2012 to November 2022, 374 patients presented with acute or chronic symptoms related to radiculopathy and/or myelopathy that were attributed to degenerative cervical spondylotic changes or due to OPLL. There were 339 males and 35 females, and their ages ranged from 39 to 77 years (average 62 years). All patients were treated for subaxial spinal stabilization by Camille's transarticular technique with the aim of arthrodesis of the treated segments. Atlantoaxial stabilization was done in 128 cases by adopting direct atlantoaxial fixation in 55 cases or a modified technique of indirect atlantoaxial fixation in 73 patients. Decompression by laminectomy, laminoplasty, corpectomy, discoidectomy, osteophyte resection, or manipulation of OPLL was not done in any case. Standard monitoring parameters, video recordings, and patient self-assessment scores formed the basis of clinical evaluation. Results: During the follow-up period that ranged from 3 to 125 months (average: 59 months), all patients had clinical improvement. Of 130 patients who had clinical evidences of severe myelopathy and were either wheelchair or bed bound, 116 patients walked aided (23 patients), or unaided (93 patients) at the last follow-up. One patient in the series was operated on 24 months after the first surgery by anterior cervical route for "adjacent segment" disc herniation. No other patient in the entire series needed any kind of repeat or additional surgery for persistent, recurrent, increased, or additional related symptoms. None of the screws at any level backed out or broke. There were no implant-related infections. Spontaneous regression of the size of osteophytes was observed in 259 patients where a postoperative imaging was possible after at least 12 months of surgery. Conclusions: Our successful experience with only spinal fixation without any kind of "decompression" identifies the defining role of "instability" in the pathogenesis of spinal degeneration and its related symptoms. OPLL appears to be a secondary manifestation of chronic or longstanding spinal instability.

Basilar invagination associated with brainstem tumor opposite the odontoid process.

An 8-year-old girl presented with the symptom of nasal regurgitation and hoarseness of voice. Investigations revealed severe basilar invagination and a large intra-axial pons-medulla brainstem tumor adjoining the tip of the odontoid process. The child underwent lateral mass plate and screw atlantoaxial fixation and attempted craniovertebral junction realignment. Biopsy of the brainstem tumor was done during the same surgical procedure. Histology of the brainstem tumor revealed it to be a pilocytic astrocytoma. Simultaneous presence of brainstem tumor and basilar invagination is a rare clinical association and a complex therapeutic challenge. The possible cause of genesis of brainstem tumor in an unusual location is speculated.

Unilateral facet fixation as a salvage procedure: Report of two cases with cervical primary bone tumors with extensive bone destruction.

We describe cases of two patients with primary cervical bone tumors that resulted in extensive destruction of bones of the region. In the first patient, the tumor and its growth resulted in the destruction of C3-C5 vertebral bodies and the unilateral destruction of facets and pedicles. In the second case, there was focal destruction of the body and odontoid process and unilateral pedicle and facet of C2. Tumor resection and spinal stabilization were the aims of the surgery. In both cases, unilateral fixation of facets was done. In the first patient, multi-segmental transarticular fixation by Camille's technique was done, and in the second patient, unilateral lateral mass plate and screw fixation was done by the Goel technique. Solid bone fusion of spinal segments was observed at more than a 2-year follow-up in both cases. Based on this experience, it appears that unilateral facetal fixation can provide firm stability and can be used when other more frequently and popularly used techniques are either not available or possible.

"Only fixation" in cases with failed decompression for lumbar canal stenosis - Analysis of outcome in 14 cases.

Aim: The rationale of "only fixation" of affected spinal segments without any form of bone or soft-tissue decompression in cases with failed decompressive laminectomy for lumbar canal stenosis is discussed on the basis of an experience with 14 cases. Materials and Methods: During the period between 2010 and 2022, 14 patients who symptomatically worsened or did not improve following a long-segment "wide" decompressive laminectomy for multisegmental lumbar canal stenosis were identified. All patients were treated by segmental spinal stabilization aimed at arthrodesis by facetal distraction by Goel's facetal spacers (6 cases) or Camille's transarticular facetal fixation (8 cases). No bone, soft tissue, or disc resection was done for spinal or neural canal "decompression." Oswestry Disability Index and Visual Analog Scale were used to clinically assess the patients before and after the surgery and at follow-up. In addition, video recordings of patient's self-assessment of clinical outcome were used to monitor the outcome. Results: During the average period of follow-up of 71 months (range 6 months to 16 years), all patients recovered in majority of their major symptoms, the recovery was observed in the immediate postoperative period. During the period of follow-up, none of the patients complained of recurrent symptoms or needed any additional surgery. There was firm stabilization and evidences of bone fusion of the treated spinal segments in all patients. There were no infections or implant failure. No patient worsened after treatment. Conclusions: Instability of the spinal segments is the primary issue in cases with lumbar canal stenosis and stabilization in the treatment.

Radiological Evaluation of Anatomical Extensions of Gliomas Based on White Fiber Tracts: Proposal of a Novel Classification.

Background: We speculate on the origin and extensions of gliomas based on our understanding of anatomy of the subcortical network, radiological evaluation and surgical experience. Objective: We recently classified the white fibers of the brain into four horizontal and one vertical group. The role of this classification in understanding the origin and spread of gliomas is analyzed. Materials and Methods: A series of 220 consecutive radiological images of patients with low and high grade gliomas were retrospectively evaluated. The classification discussed was applied to these images to study the location and direction of growth of the gliomas. Results: Our results identify that gliomas arise and spread along a named white fiber tract and displace the adjoining tracts. Gliomas that arose from arcuate fibres either remained limited to a gyrus or extended to the adjacent gyri along the course of the arcuate fibres. Gliomas arising from the LAF spread in the direction of the affected tract. Gliomas arising from the CF tended to involve both the hemispheres and extended across the midline. Conclusion: This radiological evaluation of gliomas correlated well with our proposed anatomical classification of white fibers. Gliomas probably originate in the fiber tract and grow along its extension. The adjoining fiber tracts form a reliable barrier to its spread. The surgical implication of the understanding is outlined in brief here and evaluated in details in part B of this study.

"Flow Reversal" and Cure in a Case of Giant Intracranial Aneurysm: A Case Report.

We describe the case of an 11-year-old girl having a giant anterior circulation aneurysm. The ipsilateral internal carotid artery was entirely blocked and the aneurysm was supplied by posterior circulation. Following a high-flow bypass that connected the external carotid artery to the middle cerebral artery, the giant aneurysm thrombosed spontaneously. We discuss several relatively rare and unique features of the case.

Cavernous Hemangioma of Cavernous Sinus: An Outcome Report of 45 Surgically Treated Patients.

OBJECTIVE: To report an outcome analysis following surgical treatment of 45 patients with cavernous hemangiomas involving cavernous sinus. METHODS: From 1992 to 2020, 45 cases of cavernous hemangiomas involving the cavernous sinus were surgically treated. These patients were retrospectively analyzed. RESULTS: Patients included 12 males and 33 females; the average age was 34 years (age range, 15-61 years). Two patients had only headache as presenting symptom, and 43 patients had headache and diplopia as presenting symptoms. Investigations showed characteristic radiological imaging and encasement of internal carotid artery (35 cases), extension toward the sella, and displacement of cranial nerves III-V. An entirely extradural surgical approach was adopted in 39 cases. Cranial nerves III-V had a discrete dural covering and were always displaced over the dome of the tumor. The sixth cranial nerve was displaced on the dome of the tumor adjacent to the floor of the cavernous sinus. Three patients died in the early postoperative period, all related to excessive bleeding. In 36 patients, ocular movement dysfunction did not recover or worsened. During an average follow-up period of 110 months (range, 6 months to 27 years), 3 patients had tumor recurrence after postoperative imaging had shown complete tumor resection. CONCLUSIONS: Surgery on cavernous hemangiomas of cavernous sinus can be a challenge owing to the vascular profile and complex anatomical location. An extradural approach provides satisfactory exposure for radical tumor resection.

Rotatory atlantoaxial dislocation presenting as spinal kyphoscoliosis.

A 16-year-old male presented with primary complaint of worsening dorsal spinal kyphoscoliosis (SKS) for 3 years. More recently, he developed spasticity in legs, breathlessness on mild exertion, and sleep apneas. Apart from SKS, investigations revealed rotatory atlantoaxial dislocation. Atlantoaxial fixation resulted in rapid recovery from all symptoms including from spinal deformity. Observations in this patient suggest that rotatory dislocation can be a cause of spinal deformity.

Is "En Masse" Tumor Resection a Safe Surgical Strategy for Low-Grade Gliomas? Feasibility Report on 74 Patients Treated Over Four Years.

BACKGROUND: Gliomas are "confined" tumors arising from a named white fiber tract and displacing adjoining "normal" white fibers. The surgical strategy of "en masse" resection of gliomas based on this concept is evaluated. OBJECTIVE: We evaluate the feasibility of the surgical strategy of "en masse" tumor resection for low-grade gliomas arising from the short arcuate fibers. METHODS: We retrospectively evaluated our series of 74 patients with low-grade gliomas involving the short arcuate fibers who were operated on between the years January 2016 and June 2019. The tumor resection was done on the premise that gliomas arise from and grew along a specific white fiber tract and the expanding tumor displaced but did not transgress the border formed by adjoining tracts. Although modified as per the situation, an en masse tumor resection strategy was the basis of surgical resection. Intraoperative motor cortical and subcortical mapping was performed in 14 cases. Awake surgery was performed on 11 patients. RESULTS: There were 46 males and 28 females. Total/supratotal tumor resection was achieved in 62 (83.8%) patients. Forty-seven patients had an essentially en masse tumor resection. Seventy-one patients improved in their preoperative complaints. The follow-up ranged from 11 to 56 months. Sixty-two patients who underwent a total or supratotal resection were not given any adjuvant treatment. Twelve patients with subtotal resection were subjected to adjuvant radiotherapy with or without additional chemotherapy. CONCLUSIONS: En masse tumor resection of low-grade gliomas is possible and "safe" based on understanding that gliomas are "confined" tumors and have a well-defined plane of surgical dissection.

Lumbar canal stenosis in "young" - How does it differ from that in "old" - An analysis of 116 surgically treated cases.

OBJECTIVE: Patients treated for lumbar canal stenosis (LCS) were retrospectively analyzed to evaluate the differences in clinical management in those below (Group A) and those above (Group B) the age of 50 years. All patients were treated with the premise that instability is the nodal point of the pathogenesis of LCS and "only-stabilization" is the surgical treatment. MATERIALS AND METHODS: During the period June 2014 to June 2020, 116 cases were diagnosed to have LCS and surgically treated by the Goel modification of Camille's transarticular screw fixation technique. RESULTS: Twenty-four patients in Group A and six patients in Group B had a history of "significant" injury to the back at the onset of clinical symptoms. The indices suggested that the intensity of symptoms was relatively more severe in Group A than in Group B. Unilateral leg symptoms were more common in Group A (68%) than in Group B (31.8%). Neurological motor deficits were more common in Group A (28%) than in Group B (12%) patients. Spinal segments surgically treated in Group A ranged from 1 to 4 (average 2 levels) and in Group B it ranged from 2 to 5 (average 3 levels). During the follow-up period that ranged from 6 to 72 months (average 37 months), 100% of patients had varying degrees of relief from symptoms. CONCLUSIONS: LCS is confined to a lesser number of spinal segments in the Group A patients. The symptoms were radicular in nature and relatively severe in Group A than in Group B patients.

"Apoplectic" Recurrence of Massive Trigeminal Neurinomas: Report of 7 Cases.

OBJECTIVE: The pattern of recurrence of large trigeminal neurinomas is analyzed on the basis of experience with 7 cases. METHODS: This is a report of 7 cases of large trigeminal neurinomas that were operated on an average of 11 years earlier. After being relatively asymptomatic over the years, these patients worsened relatively suddenly in their neurologic symptoms and were reoperated. RESULTS: Imaging showed massive recurrence with tumor having cystic and solid components with sizes ranging from 4.5-11 cm. In 4 cases, the cysts at the time of initial presentation and at the time of recurrence showed a well-defined fluid level within the fluid content of the cyst. During operation it was seen that the tumor contained "hemorrhagic" fluid that was under significant pressure. The solid component in the circumferential periphery of the cyst fluid was soft, necrotic, and vascular. The entire recurrence in the middle and posterior cranial fossa and in the extracranial compartment was "interdural" or within the dural confines. Radical tumor resection within the dural confines by deploying relatively small surgical exposure resulted in "unusually" rapid recovery in the symptoms. Histological examination of the tumor did not reveal any malignant transformation. CONCLUSIONS: The cases add further insight to the growth pattern and characteristics of large trigeminal neurinomas.