Pial Fistula Research Articles

Imaging Assessment of Fetal Cerebrovascular Anomalies.

Four distinct vascular anomalies can be seen to affect the brain on fetal imaging: vein of Galen malformations, non-galenic arteriovenous pial fistulas, dural sinus malformations, and intracranial venous malformations. These congenital disorders affect the arteries and veins of the developing brain and are rarely seen beyond the neonatal stage. The four fetal cerebrovascular anomalies are associated with quite disparate natural histories and prognoses. MRI plays a pivotal role in the evaluation of fetuses with these conditions due to its ability to definitively establish the diagnosis, to detect subtle parenchymal injuries, to delineate the course of abnormal vessels in detail and to some extent the nature of vascular flow, and to identify ischemic, thrombotic, and hemorrhagic complications. Recently, an investigational transurterine embolization procedure targeted at treating fetuses with vein of Galen malformations who are at high risk for neonatal decompensation has emerged as a promising alternative to expectant management and post-natal embolization, with imaging being used to identify suitable patients for the intervention and in pre-procedural planning. This manuscript reviews the essential imaging and clinical features of these four fetal neurovascular anomalies and underscores the practical aspects related to counseling, prognosis, and the multidisciplinary management of these entities.ABBREVIATIONS: ACVRL1= activin A receptor like type 1; b-SSFP=Balanced Steady State Free precession; DSM= Dural Sinus Malformation; Ephrin B4= Ephrin type-B receptor 4; icVM= Intracranial Venous Malformation; ITGB1= Integrin Subunit Beta 1; NOTCH1= Neurogenic locus notch homolog protein 1; PTPN11= Protein Tyrosine Phosphatase Non-Receptor Type 11; RASA1= RAS P21 Protein Activator 1; SSFSE= Single-shot fast spin echo; VOGM=Vein of Galen Malformation.

Spontaneous thrombosis of high flow pediatric arteriovenous fistulae: Case series of two patients and a comprehensive literature review.

Pediatric pial arteriovenous shunts in the brain and spine are challenging to understand because of low incidence, variable presentation, and associations with genetic syndromes. What is known about their natural history comes from reviews of small series. To better understand the natural history and role for intervention, two cases are presented followed by a review of the literature. In the first case, an infant with a prior history of intracranial hemorrhage from a ruptured pial fistula returns for elective embolization for a second pial fistula which was found to be spontaneously thrombosed 2weeks later. In the second case, a 5-year-old with a vertebro-vertebral fistula, identified on work up for a heart murmur and documented with diagnostic angiography, is brought for elective embolization 6weeks later where spontaneous thrombosis is identified. In reviewing the literature on pediatric single-hole fistulae of the brain and spine, the authors offer some morphologic considerations for identifying which high-flow fistulae may undergo spontaneous thrombosis to decrease the potentially unnecessary risk associated with interventions in small children.

Pediatric Interventional Neuroradiology.

Pediatric neurovascular disorders can lead to substantial mortality and morbidity if not diagnosed early and adequately managed. Children with neurovascular diseases cannot be treated as small adults as the vascular and central nervous system anatomy, physiology, and pathologies in children differ greatly from those of adults. In addition, some neurovascular pathologies are seen exclusively in children such as aneurysmal malformation of the vein of Galen, pial fistulas, and dural fistulas in the context of dural sinus disease. In this review, we aim to present an overview of the common pediatric neurovascular diseases along with their endovascular management.

Abstract Number ‐ 280: A Unique Case of Pial AVF Presenting as Hemispheric Cerebral Edema

Introduction To describe a unique case of pial arteriovenous fistula presenting as left hemispheric cerebral edema with hemianopsia, sensory change and ataxia. Methods Case report Results Our patient is a 38 year old male with a history of diabetes, hyperlipidemia and congenital deafness with a left cochlear implant who presented to an outside hospital for visual complaints. Non‐contrast CT of the head was negative at that time; he could not obtain an MRI due to his cochlear implant. He developed worsening right hemianopia and aphasia, prompting his transfer to our facility. Upon arrival, repeat imaging was done, including a CTA of the head and neck. This revealed a large region of hypodensity involving the left PCA and distal MCA territories. An MRI was attempted but artifact from his implant obscured visualization of the left hemisphere. Post‐contrast CT was negative for evidence of tumor. CT venogram was negative for veno‐occlusive disease. He complained of worsening headache and demonstrated worsening word‐finding ability. He received100g total of mannitol divided across 3 boluses which improved his symptoms mildly. Over the following days he began to become ataxic and clumsy in the right upper extremity and his headache worsened further and he began to develop focal seizures electrographically localized to the left parietal lobe. Repeat CTA demonstrated expanding hypodensity with marked vascular engorgement in the left hemisphere. He underwent 6‐vessel digital subtraction angiography (DSA) of the head which revealed gross left hemispheric venous congestion with early filling of the left hemispheric venous system by a parasagittal pial ateriovenous fistula believed to be responsible for his hemispheric edema. He was discharged on acetazolamide and acetaminophen for headache management and plans for outpatient follow‐up and potential intervention. Follow‐up DSA revealed improvement in his venous congestion and reduction in the degree of his early venous filling with some symptomatic improvement by medical management. Conclusions Pial arteriovenous fistulae (pAVF) are a rare entity and can present in a variety of ways, with symptomatic intracranial hemorrhage and ischemia being two of the most common. This case illustrates an unusual instance of hemispheric edema initially mistaken for venous infarct caused by a pAVF, and prompts the physician to consider angiography and further exploration when initial workup for cerebral edema and/or venous infarct is initially unrevealing.

Endovascular Management of Intracranial Pial Arteriovenous Fistula with Giant Venous Varix: Case Report and Review of Literature.

Abstract: Background: Intracranial pial arteriovenous fistulas (AVFs) are rare intracranial vascular malformation and their treatment is challenging because of high-flow pial supply of the lesion. We report a case of intracranial pial AVF with associated giant venous varices managed successfully by embolization. Case Description: A 25-year-old male presented with sudden onset headache and altered sensorium. Computed tomographic scan was suggestive of frontal lobar hematoma that had dissected into the third ventricle with associated intraventricular hemorrhage. Magnetic resonance imaging revealed presence of large vascular channels with the presence of multiple flow voids in bilateral frontal location. Digital subtraction angiography revealed pial fistula with arterial supply from hypertrophied left anterior cerebral artery. Treatment was done using combined coil and onyx embolization after careful analysis of angioarchitecture of the lesion using three-dimensional rotational angiogram cross-sectional reconstructions. Postprocedure angiogram revealed closure of fistula and giant varices. Conclusion: Pial AVFs with giant venous varix in adults are rare vascular malformation and understanding the angioarchitecture with exact identification of fistulous point is important to plan for surgical/endovascular management strategies. Combined strategy of embolization using coils and onyx is a safe option in such cases with giant varices.

High-flow vascular malformations of the brain in pediatrics: Experience in a tertiary care children's hospital.

High-flow vascular malformations of the brain are uncommon in pediatrics. The objective of this study is to establish the differences among these pathologies and group them by age at onset, clinical manifestations, and angioarchitecture. This was a retrospective and observational study. The medical records, imaging studies, and procedure protocols of patients seen at Hospital J. P. Garrahan diagnosed with vascular malformations of the brain between January 2010 and January 2020 were analyzed. A total of 183 patients met the inclusion criteria. It was possible to identify 131 patients with arteriovenous malformations with a nidus (AVMs) and 52 with direct fistulas (without a nidus), including 19 vein of Galen aneurysmal malformations, 23 pial fistulas, and 10 dural fistulas. The average age of patients was 105 months for AVMs, 1.7 months for vein of Galen aneurysmal malformations, 60.5 months for pial fistulas, and 41 months for dural fistulas. Based on their angioarchitecture, high-flow vascular malformations of the brain presented a nidus (AVMs) or direct fistulas (vein of Galen aneurysmal malformations, pial fistulas, and dural fistulas). AVMs were observed in early childhood, especially due to intracranial hemorrhage. Direct fistulas occurred in the first stage of life, commonly with heart failure.

Intradural spinal cord arteriovenous shunts in a personal series of 210 patients: novel classification with emphasis on anatomical disposition and angioarchitectonic distribution, related to spinal cord histogenetic units.

Few classifications of intradural spinal arteriovenous shunts (ID-SAVSs) have considered their anatomical localization in relation to their phenotype and angioarchitectonics. The authors propose another vision of ID-SAVSs allowing a reappraised classification based on analysis of the anatomical disposition, angioarchitecture, and histogenetic location of these vascular malformations. The radiological and clinical records of 210 patients with ID-SAVSs were retrospectively reviewed, considering their localization, vascular architectonics, and correlation with the 5 histogenetic units of the spinal cord. Among these, 183 files with complete data allowed precise analysis of the ID-SAVSs. Among these 183 files (162 and 21 cases with single and multiple lesions, respectively), different entities were identified: 13 pial macro arteriovenous fistulas (MAVFs), 92 pial micro arteriovenous fistulas (mAVFs), 33 superficial pial niduses, and 69 intramedullary niduses. Thirteen sulcal shunts (either fistulas or niduses) were considered subtypes of pial lesions. Among the 21 multiple cases, 11 were monomyelomeric while 10 were multimyelomeric. Pial lesions, either fistulas or niduses, were dominantly vascularized by pial arteries (anterior or posterior depending on the localization of the shunt) and occasionally (except for MAVFs) by transmedullary arteries. Pial niduses occasionally extended into the funiculus by recruiting intrinsic veins or by extension of the nidus itself inside the white matter. Intramedullary niduses were always vascularized by both centrifugal and centripetal feeders, respectively, from sulcal arteries (SAs) and pial arteries. Sulcal lesions are pial lesions located within the ventral median sulcus and vascularized by SAs and veins. Single or multiple ID-SAVSs can be part of various syndromes such as hereditary hemorrhagic telangiectasia, Parkes-Weber, RASA1, CLOVES, and spinal arteriovenous metameric syndromes. Histogenetic analyses revealed a specific distribution of each ID-SAVS in the 5 histogenetic units of the spinal cord: intramedullary niduses were found almost equally from cervical to thoracic units, while MAVFs and mAVFs were mostly found from thoracic to postcrural ones. Pial niduses showed intermediate features between intramedullary and fistulous lesions and were mostly distributed from brachial to crural segments. Precise analysis of the anatomical disposition of ID-SAVSs in relation to functional histogenetic units allows a better understanding of these lesions and improved therapeutic management.

Four-dimensional digital subtraction angiography for exploration of spinal cord vascular malformations: preliminary experience

BackgroundThe precise understanding of the angioarchitecture of spinal vascular malformations (SVMs) is often difficult to reach with conventional digital subtraction angiography (DSA). The purpose of our study was to evaluate...

Neurovascular Manifestations of Hereditary Hemorrhagic Telangiectasia: A Consecutive Series of 376 Patients during 15 Years.

Hereditary hemorrhagic telangiectasia is associated with a wide range of neurovascular abnormalities. The aim of this study was to characterize the spectrum of cerebrovascular lesions, including brain arteriovenous malformations, in patients with hereditary hemorrhagic telangiectasia and to study associations between brain arteriovenous malformations and demographic variables, genetic mutations, and the presence of AVMs in other organs. Consecutive patients with definite hereditary hemorrhagic telangiectasia who underwent brain MR imaging/MRA, CTA, or DSA at our institution from 2001 to 2015 were included. All studies were re-evaluated by 2 senior neuroradiologists for the presence, characteristics, location, and number of brain arteriovenous malformations, intracranial aneurysms, and nonshunting lesions. Brain arteriovenous malformations were categorized as high-flow pial fistulas, nidus-type brain AVMs, and capillary vascular malformations and were assigned a Spetzler-Martin score. We examined the association between baseline clinical and genetic mutational status and the presence/multiplicity of brain arteriovenous malformations. Three hundred seventy-six patients with definite hereditary hemorrhagic telangiectasia were included. One hundred ten brain arteriovenous malformations were noted in 48 patients (12.8%), with multiple brain arteriovenous malformations in 26 patients. These included 51 nidal brain arteriovenous malformations (46.4%), 58 capillary vascular malformations (52.7%), and 1 pial arteriovenous fistula (0.9%). Five patients (10.4%) with single nidal brain arteriovenous malformation presented with hemorrhage. Of brain arteriovenous malformations, 88.9% (88/99) had a Spetzler-Martin score of ≤2. Patients with brain arteriovenous malformations were more likely to be female (75.0% versus 57.6%, P = .01) and have a family history of hereditary hemorrhagic telangiectasia (95.8% versus 84.8%, P = .04). The prevalence of brain arteriovenous malformation was 19.7% in endoglin (ENG) mutations and 12.5% in activin receptor-like kinase (1ACVRL1) mutations. Our study of 376 patients with hereditary hemorrhagic telangiectasia demonstrated a high prevalence of brain arteriovenous malformations. Nidal brain arteriovenous malformations and capillary vascular malformations occurred in roughly equal numbers.

Intracranial pial fistulas in pediatric population. Clinical features and treatment modalities

The purpose of the study is to describe the clinical manifestations and treatment modalities of patients having intracranial pial arteriovenous fistulas (PAVFs). We retrospectively analyzed the cases of PAVFs from January 2004 to December 2013. Medical charts, diagnostic images, surgical, and endovascular reports were reviewed retrospectively during each of the procedures and follow-up. We recorded patient demographics, clinical presentation, treatment modalities, and outcome. Ten patients with single PAVFs were identified, one of them with multiple holes. The median age was 7.5 years old (20 days to 14 years). Six patients were male (60% of cases). Four PAVFs were localized in the posterior fossa, and six were supratentorial (60%). Two patients had intracranial bleeding, three presented seizures, one was studied for chronic headaches, three manifested by growth retardation, one had hydrocephalus, and one had a congestive heart failure (CHF) and vein of Galen aneurysmal malformation (VGAM). The latter did not improve after embolization and died few days later. Endovascular therapy was used in eight, whereas two patients were surgically managed. Total occlusion of the fistula was achieved in all cases. PAVF affects pediatric population at different ages with miscellaneous clinical manifestations. Endovascular treatment is safe and effective when the venous side of the fistula can be occluded.

Spinal Arterio- venous lesions: Variable Clinical and Angiographic Feature

Background: Spinal Arterio-Venous Shunts are rare but treatable diseases. AVF has a direct shunt between artery and vein. AVM has a nidus between artery and vein. Differentiation is necessary because, the clinical presentation, angiographic architecture, and treatment options are different.
 Rationale: Presenting clinical features are nonspecific and vary in age and sex. MR images raise the suspicion of diagnosis. For proper understanding of disease and planning of treatment DSA is necessary. Objectives: To evaluate the nature of clinical feature, angiographic findings, and initial outcome after embolization in patients of spinal arterio-venous shunt.
 Materials & Methods: It was prospective study. Patients were referred for spinal DSA. Risk of complication was properly discussed with the patient and attendant and informed written consent was taken.
 Results: Among 9 patients, 5 were female and 4 were male. Male female ratio was 1: 1.25. Three patients were diagnosed as type I, three as type II, and three as type IV. All 3 patients (100%) of dural fistula were male, all 3 patients (100%) of pial fistula were female. Average age at presentation was 36 yrs. And mean age of dural AVF was 60.33 yrs, pial AVF was 18.33 yrs and spinal AVM was 29.33 yrs. Spinal DAVF occurred in patients ranging from 57 to 64 years of age, with an average age of 60.33 years. The average length of time between onset of symptoms and diagnosis was 16.44 months (ranging from 3 to 36 month). All 9 patients (100%) of patients had motor weakness, sensory disturbance was found in 66.66% (six of nine patients) and urinary disturbance was found in 77.77% (seven of nine patients). Progressive clinical course was followed in 100% of patients (all of nine patients). MRI findings revealed flow void in 77.77% of patients (seven of nine patients). Increased T2 signal in the spinal cord was present in 88.88% of patients (eight of nine patients). After DSA total 77.7% (seven of nine patients) feeder was located in low thoracic and lumbar region.
 Conclusion: Spinal AVM & AVF remain undiagnosed for a long period. They should be treated early for prevention of progressive morbidity and disability. MRI features of cord edema, contrast enhancement, and peri-medullary vessels may lead to the diagnosis of these vascular lesions.
 Bangladesh Journal of Neuroscience 2014; Vol. 30 (2): 62-68

Nocturnal Headaches and Pulsatile Cranial Mass: The Tip of an Iceberg

Capillary malformation-arteriovenous malformation (CM-AVM) disorder is a newly defined hereditary disorder of the vasculature with typical defining features that include cutaneous capillary malformations associated with high-flow lesions in various other organ systems. Mutations on the RASA1 gene are reported to be associated with a variety of vascular malformations and present with a widely varying phenotype. A healthy 3 year old presented with acute onset of severe nocturnal headaches, nausea, and vomiting associated with a 2-cm pulsatile mass and prominent superficial veins on her forehead. Neuroimaging demonstrated a complex vascular malformation with multiple arteriovenous fistulae and cavernous angiomas present in multiple locations in the brain, but not in any other organ system. The patient was found to have a mutation of the RASA1 gene, which has not been previously described in the literature. This case describes a new RASA1 mutation with a phenotype that has not been previously described with a combination of pial fistulae and intracranial AV fistula in the absence of arteriovenous malformations.

Endovascular Treatment of Cerebral Dural and Pial Arteriovenous Fistulas

Dural arteriovenous fistulas (DAVFs) are arteriovenous shunts from a dural arterial supply to a dural venous channel, typically supplied by pachymeningeal arteries and located near a major venous sinus. Pial arteriovenous fistulas (PAVFs) are composed of one or more arterial feeders draining into a single vein in the absence of an intervening nidus. Fistulas manifesting features of high risk for rupture should be treated aggressively, the spectrum of treatment varies from endovascular, surgical resection, and stereotactic radiosurgery. This article describes the natural history, clinical presentation, and treatment of dural and pial fistulas, with emphasis on endovascular treatment.

Cranial Fistulae: Venous Occlusion, the Road to Cure and Misery

Dural and pial fistulae can be associated with complex pathology on the venous side. In the two cases presented by Gonzalez et al. in this issue of WORLD NEUROSURGERY, venous thrombosis after treatment of fistula resulted in significant complications. The questions that arise are why this occurred and whether this complication be avoided. In both cases large venous pouches with significant venous ectasia were found, both were managed via state-of-the-art techniques. Sudden stasis in these large venous pouches with propagation of clot is the likely mechanism involved, but other factors could be at play. Undiagnosed hypercoagulable states could potentially contribute to such complications (2, 3).

Multistage Indocyanine Green Videoangiography for the Convexity Dural Arteriovenous Fistula with Angiographically Occult Pial Fistula

Recently, intraoperative indocyanine green (ICG) videoangiography has become a common technique for treating cerebrovascular diseases. We report a case of dural arteriovenous fistula (AVF) treated with direct surgery using intraoperative ICG videoangiography. A 41-year-old man with right hemiplegia caused by a left subcortical hemorrhage was transferred to our hospital. Digital subtraction angiography (DSA) revealed a left convexity parasagittal dural AVF. Surgical resection of the dural AVF was performed using step-by-step ICG videoangiography 4 times in each dissection procedure, which precisely delineated the structure of the dural AVF. After a circular incision of the dura around the fistular point, repeated ICG videoangiography identified the residual fistula between the pial artery from the middle cerebral artery and the draining vein. Complete disappearance of the AVF was confirmed by ICG videoangiography after this pial fistula was removed. Postoperative DSA revealed no residual AVF. Accurate detection of all fistular points and complete resection, including the dura mater and pial vessels, are necessary to avoid rebleeding caused by the residual dural AVF due to incomplete obliteration of the fistular points. Intraoperative ICG videoangiography could provide information on angiographically occult vascular malformation, such as pial fistulas, that cannot be detected by preoperative DSA. Our findings suggest that multistage intraoperative ICG videoangiography can be quite useful for complete resection of a dural AVF with angiographically occult pial fistula.

Endovascular Treatment of Ruptured Brain AVMs in the Acute Phase of Hemorrhage

Patients with ruptured brain AVMs are at considerable risk of repeat hemorrhage, particularly when associated intranidal or flow-related aneurysms are present. There is controversy about the timing of diagnosis and treatment of patients with hemorrhagic stroke. We present our results of endovascular treatment of ruptured AVMs in the acute phase. Between January 2008 and March 2011, 23 patients (16 men, 7 women; mean age 42 years) with AVM-related hemorrhagic stroke were treated with endovascular techniques within 10 days of the ictus. There were 10 micro-AVMs (< 1 cm) and 1 single-hole pial fistula. In 9 patients, an intranidal or flow-related aneurysm was the likely cause of hemorrhage. Complete obliteration of the AVM with Onyx was achieved in 13 of 23 patients (57%). Eight of the 13 AVMs were micro-AVMs and 3 had an intranidal aneurysm. Partial obliteration of the AVM was achieved in 10 of 23 patients (43%). In 6 of these 10 patients, an intranidal (n = 1) or flow-related aneurysm (n = 5) was obliterated with Onyx or coils. There were no complications of treatment. During a mean follow-up of 21 months in 22 surviving patients, no repeat hemorrhage occurred. Endovascular treatment with Onyx in the acute phase cured most ruptured AVMs. All 9 AVM-associated aneurysms that were considered the source of hemorrhage could be excluded from the circulation. In patients with AVM-related hemorrhagic stroke, prompt angiographic diagnosis and treatment may improve prognosis by reducing repeat hemorrhage rate.

Dural arteriovenous fistulae in pediatric patients: associated conditions and treatment outcomes

ObjectiveDural arteriovenous fistulae (dAVF) are rare vascular lesions of the CNS with potentially significant morbidity and mortality. Our experience in the treatment of children with dAVF is reported.DesignOur neurosurgical and...

Endovascular treatment of intracranial pial arteriovenous fistula

Medical School of Ribeirao Preto, University of Sao Paulo, Ribeirao Preto SP, Brazil: Department of Internal Medicine, Division of Radiology; Department of Surgery, Division of Pediatric Neurosurgery. Intracranial pial arteriovenous fistulas (AVFs) are an extremely rare cerebrovascular disease, accounting for 1.6% of all intracranial vascular malformations. It has been recognized as a pathological entity distinct from the arteriovenous malformations (AVMs), due to the direct connection between the feeder artery and the draining vein, without an intervening true nidus. Pial AVFs differ from dural AVFs in that their arterial supply derive from pial and cortical arteries and they are not involved by the leaflets of the dura-mater. They are in their majority considered to be congenital, but they may also occur sporadically or in association with congenital hereditary vasculopathies. The pial AVFs have a significant morbid-mortality secondary to its rupture when not treated. We report a case of intracranial pial fistula in a 2 years-old patient presenting seizures since the neonatal period, emphasizing the angiographic findings and the results of the endovascular treatment.

Endovascular occlusion of high-flow intracranial arteriovenous shunts: technical note

Endovascular closure of high-flow arteriovenous (AV) shunts in intracranial AV malformations or pial fistulas is technically challenging. In this paper, we illustrate two simple methods to occlude large high-flow AV shunts in a controlled manner.

Benefits and limitations of image guidance in the surgical treatment of intracranial dural arteriovenous fistulas

Despite major advances in endovascular embolization techniques, microsurgical resection remains a reliable and effective treatment modality for dural arteriovenous fistulas (DAVF). However, intraoperative detection of these lesions and identification of feeding arteries and draining veins can be challenging. In a series of 6 patients who were not candidates for definitive treatment by endovascular embolization we evaluated the benefits and limitations of computer-assisted image guidance for surgical ablation of DAVF. Of the 6 patients, 5 presented with haemorrhage and one with seizures. Diagnosis of DAVF was made by conventional angiography and dynamic contrast enhanced MR angiography (CE-MRA). All patients were surgically treated with the assistance of a 3D high resolution T1-weighted MR data set and time-of-flight MR angiography (MRA) obtained for neuronavigation. Registration was based on cranial fiducials and image-guided surgery was performed with the navigation system. Four of the 6 patients suffered from DAVF draining into the superior sagittal sinus, one fistula drained into paracavernous veins adjacent to the superior petrosal sinus and one patient had a pial fistula draining in the straight sinus. DAVF diagnosed with conventional angiography could be located on CE-MRA and MRA prior to surgery. MRI and MRA images were combined on the neuronavigation workstation and DAVF were located intraoperatively by using a tracking device. In 4 out of 6 cases neuronavigation was used for direct intraoperative identification of DAVF. Brain shift prevented direct tracking of pathological vessels in the other 2 cases, where navigation could only be used to assist craniotomy. Microsurgical dissection and coagulation of the fistulas led to complete cure in all patients as confirmed by angiography. Neuronavigation may be used as an additional tool for microsurgical treatment of DAVF. However, in this small series of 6 cases, surgical procedures have not been substantially altered by the use of the neuronavigation system. Image guidance has been beneficial for the location of small, superficially located DAVF, whereas a navigated approach to deep-seated lesions was less accurate due to the familiar problem of brain shift and brain retraction during surgery.