The impact of arachnoid structures on skull-base meningioma surgical management: a radiological analysis and narrative review.

The presence of intact arachnoid membranes between skull base meningiomas and critical neurovascular structures is crucial for predicting surgical outcomes, understanding tumor development and growth, and planning the feasibility of tumor resection or the need for adjuvant treatments. While neurosurgeons often utilize the subarachnoid cisterns to enhance access to these tumors and facilitate their removal, a comprehensive review aimed at health professionals involved in the diagnosis and treatment of this complex pathology, including radiologists, neurologists, oncologists, ophthalmologists, and neurosurgeons is still lacking. This study aims to summarize the interaction between skull base meningiomas, subarachnoid cisterns, and arachnoid membranes, emphasizing their significance in both the diagnosis and treatment of this pathology. By conducting a thorough radiological assessment of skull base meningiomas, correlating these findings with intraoperative observations, and reviewing relevant literature, we summarize the critical relationship between skull base meningiomas and the surrounding subarachnoid spaces. We concisely describe how arachnoid structures influence tumor growth and interaction with neurovascular elements. We advocate for the inclusion of tumor-arachnoid relationships in the medical literature concerning the treatment of these tumors. A better understanding and description of the interaction between tumors and neurovascular structures will aid in planning and attempting safer treatments, minimizing surgical risks, predicting potential tumor progression, and the need for adjuvant treatments.

Predicting the presence of 4th ventricular outlet obstruction in Chiari I Malformation.

INTRODUCTION: A subset of children with Chiari 1 malformation (CM-1) have a 4th ventricle arachnoid veil-a thin membrane covering the outlet of the 4th ventricle. Studies suggest that failure to disrupt this veil during posterior fossa decompression can reduce the likelihood of syringomyelia resolution. However, there is no reliable method for predicting the presence of the veil without direct surgical exploration. This study aims to evaluate the association between pre-operative symptoms, radiographic measurements, and the arachnoid veil. METHODS: A retrospective review of an institutional database of children evaluated for CM-I was conducted. For patients treated with surgery, operative notes were reviewed to determine if an arachnoid veil was present. Logistic regression was used to test for relationship of clinical variables and radiographic measurements with the presence of an arachnoid veil. RESULTS: Out of 997 children with CM-1, 226 surgical patients were included in the analysis after excluding those with inadequate documentation. An arachnoid veil was found in 23 patients (10.2%). Larger syrinx, spinal canal, and thecal sac diameters were significantly associated with the presence of a veil, with odds ratios of 1.23 (95% CI 1.2-1.48; p = 0.03), 1.27 (95% CI 1.02-1.59; p = 0.03), and 1.35 (95% CI 1.03-1.77; p = 0.03), respectively. No significant associations were found with any signs or symptoms. CONCLUSIONS: Arachnoid veil was present in 10% of cases. Radiographic measurements indicating larger syrinx size were the only variables found to be significantly associated with an arachnoid veil. Exploration of the 4th ventricular outlet is recommended for CM-I decompression in the setting of expansile syringomyelia.

Arachnoid granulations in idiopathic intracranial hypertension: Do they have an influence?

OBJECTIVE: The aim of this study was to investigate whether the relative narrowing of the dural venous sinuses by arachnoid granulations (AGs) is more pronounced in patients with idiopathic intracranial hypertension (IIH) compared to healthy controls. BACKGROUND: IIH is characterized by increased intracranial pressure, which is associated with symptoms such as headache and visual disturbances. The role of cerebral venous drainage obstruction in IIH is the subject of ongoing research. MATERIALS AND METHODS: In this retrospective case-control study, 3D contrast-enhanced magnetic resonance images of a cohort of 43 patients with IIH were evaluated for (1) the number of AGs per venous sinus and (2) the diameters of the dural venous sinuses at the site of an AG and at standardized measurement points. In addition, the minimum width of the transverse/sigmoid sinus was measured. All data were compared to the same data from a cohort of 43 control participants. RESULTS: Patients with IIH showed less relative sinus narrowing by AG compared to controls (median: 7%, interquartile range [IQR] 10% vs. 11%, IQR 9% in controls; p = 0.009). In patients with IIH, sinus diameter was larger at the site of an AG (70 ± 25 mm2) compared to its diameter at the standardized measurement point (48 ± 23 mm2; p = 0.010). In the superior sagittal sinus (SSS), patients with IIH had smaller AGs (median: 3 mm2, IQR 2 mm2 vs. 5 mm2, IQR 3 mm2 in controls; p = 0.023) while the respective sinus segment was larger (median: 69 mm2; IQR 21 mm2 vs. 52 mm2, IQR 26 mm2 in controls; p = 0.002). The right transverse sinus was narrower in patients with IIH (41 ± 21 mm vs. 57 ± 20 mm in controls; p < 0.001). CONCLUSIONS: In contrast to our hypothesis, patients with IIH showed less pronounced relative sinus narrowing by AG compared to controls, especially within the SSS, where AGs were smaller and the corresponding sinus segment wider. Smaller AGs could result in lower cerebrospinal fluid resorption, favoring the development of IIH. Conversely, the smaller AGs could also be a consequence of IIH due to backpressure in the SSS because of the narrower transverse/sigmoid sinus, which widens the SSS and compresses the AG.

Description and clinical relevance of the variable conformation of canine spinal arachnoid diverticula.

The conformation of spinal arachnoid diverticula (SAD) and their clinical implications are poorly characterized in dogs. This retrospective cross-sectional study describes different SAD conformations in dogs and aims to identify if there is an association between SAD conformation and clinical features, localization, syringomyelia (SM) presence, concurrent vertebral condition, treatment option, and short as well as long-term outcome. Sixty-two dogs were included (12 cervical and 50 thoracolumbar SAD). All dogs with a cervical SAD had a cranial tethered conformation and were not included in the statistical analysis. Half of the dogs with a thoracolumbar SAD were cranial tethered, and the other half were caudal tethered. SM associated with SAD had a moderate prevalence in the cervical region (58.3%) and a high prevalence in the thoracolumbar region (82%). All dogs with the presence of SM and caudal tethered SAD had a cranial positioned SM, and all dogs with SM and a cranial tethered SAD had a caudal positioned SM. The SM absolute length and SM length/L2 ratio were significantly higher (P = .018, respectively) in the caudal tethered SAD compared with the cranial tethered SAD. The short-term outcome was statistically different (P = .045) between caudal and cranial tethered thoracolumbar SAD, but not the long-term outcome (P = .062). Multivariable logistic regression identified thoracolumbar caudal tethered SAD conformation had a better short-term outcome (P = 0.017, OR: 0.043, CI: 0.003-0.563), independently of SM length measurements. SAD conformation in dogs can influence SM formation. A possible link between short-term outcome and SAD conformation was found, but further research is warranted.

Intraoperative Ultrasound: Real-Time Surgical Adjunct for Complete Resection of Spinal Arachnoid Webs.

Spinal arachnoid webs are abnormal formations of arachnoid membranes that reside in the arachnoid space. Clinically, they may present as an incidental finding or in patients with progressively worsening myelopathy. Early detection and surgical intervention are recommended in patients with progressive symptoms. Several methods have been described for the surgical treatment of these web formations.1-4 The success of surgery and the ability to prevent recurrence is dependent on complete surgical resection of these lesions, which in some cases can appear complex and intricate in nature. A few reports have highlighted the use of intraoperative ultrasound to localize the lesion; however, none have highlighted its value in establishing successful web resection and restoration of normal cerebrospinal fluid flow.3,4 Herein, we demonstrate the use of intraoperative ultrasound as an effective adjunct to assessing and establishing complete resection of arachnoid webs. We illustrate how intraoperative ultrasound allows for real-time, direct visualization of arachnoid lysis with restoration of normal cerebrospinal fluid flow (Video 1). Our patient was symptomatic for 12 months with rapid progression of myelopathic symptoms in the 3 months before presentation. Following surgery, she remained asymptomatic at 4-year follow-up with no reoccurrence at 24-month magnetic resonance imaging. Intraoperative ultrasound is a useful adjunct to successfully performing dorsal arachnoid web surgery and ensuring improved surgical outcomes through complete web resection and decompression of the spinal cord.

Importance of Arachnoid Dissection in Arteriovenous Malformation Microsurgery: A Technical Note.

Intracranial arteriovenous malformations (AVMs) are congenital anomalies where arteries and veins connect without a capillary bed. AVMs are the leading cause of nontraumatic intracerebral hemorrhages in people younger than 35 years old.1 The leptomeninges (arachnoid and pia) form from the meninx primitiva.2,3 Endothelial channels produce a vascular plexus in the meninx connected by primitive arachnoid. Remodeling of the plexus in response to changing metabolic demands results in a recognizable pattern of arteries and veins.2,3 Defects at the level of capillaries during arteriovenous specification are most likely responsible for arteriovenous fistula formation.4-6 Interplay between the congenital dysfunction and flow-related maturation in adulthood, when vasculogenesis has stopped, produces the AVM.6,7 The relationship between the primitive arachnoid and aberrant AVM vessels is preserved and forms the basis of microsurgical disconnection discussed in Video 1. Several authors have described dissecting these natural planes to delineate the abnormal AVM vessels, relax the brain, and avoid morbidity during AVM surgery.8-10 We recommend sharp arachnoid dissection with a scalpel or microscissors, occasionally helped by blunt dissection with patties or bipolar forceps. We present a 2-dimensional video of the microsurgical resection of a right parietal AVM. The patient, a healthy 30-year-old female, presented with intermittent headaches and mild impairment of arithmetic and visuospatial ability. Magnetic resonance imaging and digital subtraction angiography showed a compact 3.5-cm supramarginal gyrus AVM supplied by the middle cerebral artery, with superficial drainage. Complete microsurgical resection was performed without morbidity. We demonstrate the principles of arachnoid dissection requisite to disentanglement of the nidus and safe resection of the AVM.

Microstructure and Reasonable Management of the Arachnoid Associated with the Infrafloccular Approach for Microvascular Decompression of the Facial Nerve.

AIM: To understand the arachnoid microstructure during infrafloccular approach for facial nerve microvascular decompression (MVD). MATERIAL AND METHODS: This study recruited 55 patients with hemifacial spasm who underwent MVD. Retrospective analyses of the MVD surgical videos were performed to reveal the arachnoid microstructure during the procedures. Cadaveric head specimens (n=8, on 16 sides) were dissected for observation of the microstructure of the arachnoid in the cerebellopontine angle. RESULTS: The arachnoid membrane surrounding the facio-cochleovestibular and lower cranial nerves forms two arachnoid sheaths. Both arachnoid sheaths contain two parts: the outer membranous and inner trabecular part. The membranous part is an intact and translucent membrane that wraps around nerves. The inner trabecular part is located beneath the membranous part and forms a trabecular network that connects the membranous arachnoid, nerves, and blood vessels to form a physical structure. CONCLUSION: The arachnoid connects the facio-cochleovestibular and lower cranial nerves, blood vessels, and cerebellum as a complex physical entity. Therefore, during MVD surgery, sharply dissecting the arachnoid before retracting the flocculus and relocating the offending vessels helps reduce nerve injury.

Evaluation of anterior and middle cranial fossa intraosseous arachnoid granulations with 3D T2-SPACE sequence.

OBJECTIVES: Arachnoid granulations (AG) can be located anywhere outside the dural sinuses. Their presence is thought to be associated with idiopathic intracranial hypertension (IIH) and cerebrospinal fluid (CSF) leaks. It was aimed to evaluate the intraosseous AGs located in the middle and anterior cranial fosses in detail with three-dimensional T2-SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolution-Siemens) imaging and to investigate their clinical significance. MATERIALS AND METHODS: Sixty-five intraosseous AG of 46 patients were included in this retrospective study. The highest diameter, bone indentation degree (in the inner tabula, diploe distance, reaching and exceeding the outer tabula), content (CSF/+parenchyma) of each AG were evaluated by 2 experienced radiologists. In addition, the presence of other MRI findings supporting IIH was examined. RESULTS: Additional signs of IIH were detected in 25 patients, and they were statistically significantly more common in the middle cranial fossa. Parenchymal herniation (in four patients) was more common in the young population. CONCLUSIONS: Intraosseous AGs can be evaluated in detail with T2-SPACE imaging. Determining intraosseous AG is very important both as an indicator of IIH and in terms of its content. T2-SPACE imaging is superior to CT and conventional sequences in this regard.

Lateral Suboccipital Infrafloccular Approach with Extensive Arachnoid Dissection for Vertebral Artery-Associated Hemifacial Spasm: Two-Dimensional Operative Video.

Hemifacial spasm (HFS) is generally caused by compression of the root exit zone (REZ) of the facial nerve by the anterior and posterior inferior cerebellar arteries and occasionally the vertebral artery (VA). Owing to its large caliber and high stiffness, microvascular decompression (MVD) for VA-associated HFS is considered more difficult, and the result is worse than for HFS not associated with the VA.1,2 Therefore, a safer, more reliable MVD is required for VA-associated HFS. In Video 1, we demonstrate our MVD technique in a 57-year-old woman who presented with left HFS owing to facial nerve compression by a dolichoectatic VA. A lateral suboccipital infrafloccular approach with extensive arachnoid dissection was performed. Arachnoid dissection was started from the cisterna magna and continued from the caudal to the rostral direction. This extensive arachnoid dissection provided access to the facial nerve REZ through the infrafloccular route with gentle retraction of the flocculus in the caudorostral direction, while avoiding strong retraction of cranial nerve VIII and the cerebellum. In addition, we were able avoid damaging the neurovascular structures in the operative field. This is mandatory to make the operative field bloodless and facilitate identifying the relationship between the facial nerve REZ and the offending vessels. MVD of the facial nerve REZ was achieved. The patient's postoperative course was uneventful, and her HFS resolved postoperatively. Patient consent was obtained to perform the surgery and to publish the surgical video.

Brain herniation (encephalocele) into arachnoid granulations: prevalence and association with pulsatile tinnitus and idiopathic intracranial hypertension.

PURPOSE: Brain herniation into arachnoid granulations (BHAG) of the dural venous sinuses is a recently described finding of uncertain etiology. The purpose of this study was to investigate the prevalence of BHAG in a cohort of patients with pulsatile tinnitus (PT) and to clarify the physiologic and clinical implications of these lesions. METHODS: The imaging and charts of consecutive PT patients were retrospectively reviewed. All patients were examined with MRI including pre- and post-contrast T1- and T2-weighted sequences. Images were reviewed separately by three blinded neuroradiologists to identify the presence of BHAG. Their location, signal intensity, size, presence of arachnoid granulation, and associated dural venous sinus stenosis were documented. Clinical records were further reviewed for idiopathic intracranial hypertension, history of prior lumbar puncture, and opening pressure. RESULTS: Two hundred sixty-two consecutive PT patients over a 4-year period met inclusion criteria. PT patients with BHAG were significantly more likely to have idiopathic intracranial hypertension than PT patients without BHAG (OR 4.2, CI 1.5-12, p = 0.006). Sixteen out of 262 (6%) patients were found to have 18 BHAG. Eleven out of 16 (69%) patients had unilateral temporal or occipital lobe herniations located in the transverse sinus or the transverse-sigmoid junction. Three out of 16 (19%) patients had unilateral cerebellar herniations and 2/16 (13%) patients had bilateral BHAG. CONCLUSION: In patients with PT, BHAG is a prevalent MRI finding that is strongly associated with the clinical diagnosis of IIH. The pathogenesis of BHAG remains uncertain, but recognition should prompt comprehensive evaluation for IIH.

External Neurolysis in Microvascular Decompression for Magnetic Resonance Imaging-Negative Idiopathic Trigeminal Neuralgia.

OBJECTIVE: Internal neurolysis has been proposed as an alternative to microvascular decompression in patients with idiopathic trigeminal neuralgia (TN) in whom neurovascular compression is not confirmed by magnetic resonance imaging (MRI). External neurolysis, which straightens and realigns the trigeminal nerve root axis by dissecting the arachnoid membranes around the nerve, was reported 20 years ago in the context of so-called negative exploration when MRI did not confirm the absence of the offending vessel, but is not currently used. METHODS: External neurolysis was performed in 4 patients with idiopathic TN with typical evoked neuralgic pain despite the absence of suspected offending vessels on MRI. The surgical findings that caused TN were summarized and the outcomes were evaluated using the Barrow Neurological Institute Pain Intensity Scale (BNI-PS). RESULTS: Tethering and distortion of the nerve root by surrounding arachnoid membranes were commonly found. All 4 patients showed complete pain relief immediately after surgery. During the follow-up period of 26.5 ± 16.92 months (±standard deviation), 3 of 4 patients had no pain (score I, BNI-PS). One patient received a score of IIIa on the BNI-PS assessment. There was no instance of recurrence or side effects associated with the surgery. CONCLUSIONS: Idiopathic TN can be induced by individual variation of the surrounding inner arachnoid membranes supporting the trigeminal nerve root, and the condition cannot be identified by MRI. Intradural external neurolysis may be considered an effective treatment for MRI-negative idiopathic TN.

Utility of True Fast Imaging with Steady-State Precession in Detecting Arachnoid Veils of the Posterior Fossa.

INTRODUCTION: Arachnoid membranes are well recognized as a cause of cerebrospinal fluid (CSF) flow impairment in disorders such as obstructive hydrocephalus and syringohydromyelia, but can be difficult to detect with standard noninvasive imaging techniques. True fast imaging with steady-state precession (TrueFISP) can exhibit brain pulsations and CSF dynamics with high spatiotemporal resolution. Here, we demonstrate the utility of this technique in the diagnosis and management of arachnoid membranes in the posterior fossa. CASE PRESENTATIONS: Three symptomatic children underwent cine TrueFISP imaging for suspicion of CSF membranous obstruction. Whereas standard imaging failed to or did not clearly visualize the site of an obstructive lesion, preoperative TrueFISP identified a membrane in all 3 cases. The membranes were confirmed intraoperatively, and postoperative TrueFISP helped verify adequate marsupialization and recommunication of CSF flow. Two out of the 3 cases showed a decrease in cerebellar tonsillar pulsatility following surgery. All children showed symptomatic improvement. CONCLUSION: TrueFISP is able to detect pulsatile arachnoid membranes responsible for CSF outflow obstruction that are otherwise difficult to visualize using standard imaging techniques. We advocate use of this technology in pre- and postsurgical decision-making as it provides a more representative image of posterior fossa pathology and contributes to our understanding of CSF flow dynamics. There is potential to use this technology to establish prognostic biomarkers for disorders of CSF hydrodynamics.

Extradural Remodeling in Empty Sella Syndrome: 3-Dimensional Operative Video.

Sellar arachnoidocele is a term used to define the herniation of the subarachnoid space to the sella.1 This is a rare radiologic finding that, in most cases, does not require treatment.2-5 When symptoms appear, the term empty sella syndrome is used. Two varieties exist: primary and secondary empty sella syndrome.2 The aim of this 3-dimensional operative video (Video 1) is to demonstrate the extradural microsurgical remodeling of the sellar fossa with autologous bone in 2 cases of primary empty sella syndrome. Both patients signed an informed consent for the procedures and agree with the use of their images for research purposes. In both cases, magnetic resonance imaging scans showed herniation of the subarachnoid space into the pituitary fossa and an anchor-like silhouette on coronal view. Patients evolved favorably, improving their visual deficit after the surgery, as can be observed in the postoperative visual field study. If surgery is indicated due to visual loss, the procedure is known as chiasmapexy. Recently, Guinto et al3 described a technique for chiasmapexy. Our team considers this procedure to be useful, technically simple, and low cost. Being autologous, rejection possibilities are almost null. This 3D video serves as a complement to illustrate the technique.

The Role of Arachnoid Granulations and the Glymphatic System in the Pathophysiology of Idiopathic Intracranial Hypertension.

PURPOSE OF REVIEW: Idiopathic intracranial hypertension (IIH) is a disorder characterized by long-standing elevated intracranial pressure (ICP). As the name applies, no uniform cause has been identified. IIH is typically characterized by headaches, pulsatile tinnitus, and visual deterioration. RECENT FINDINGS: Anomalies in cerebrospinal fluid (CSF) absorption are implicated in the pathophysiology of IIH. Non-invasive imaging of the brain parenchyma and the cerebral venous sinus has improved, and research has gained a better understanding of the role of cerebral venous sinus stenosis. Both have led to a better delineation of the role of arachnoid granulations (AG) and the glymphatic system in the development of IIH. IIH may occur as a result of restrictions of CSF absorption from the venous system, and or the congestion and overflow of the glymphatic system. Elucidating these mechanisms will lead to greater understanding of its underlying pathophysiologic mechanisms.

Chiari 1 Malformation Surgery: Comparing Non-violation of the Arachnoid versus Arachnoid Opening and Thermocoagulation of the Tonsils.

OBJECTIVE: Evidence is lacking concerning the myriad surgical techniques for type 1 Chiari malformation. This study evaluated the impact of arachnoid violation with tonsil thermocoagulation during surgical craniovertebral junction decompression. METHODS: The evaluation included aspects of the neurologic examination and parameters of cerebrospinal fluid flow on magnetic resonance imaging during preoperative and postoperative periods. All patients underwent craniovertebral junction decompression and opening of the dura mater. Patients were divided into 2 study groups. Patients in group 1 did not undergo arachnoid violation. Patients in group 2 underwent tonsil manipulation and systematic opening of the fourth ventricle outlet. RESULTS: There were 16 patients enrolled in each group (total of 32 patients). Regarding clinical improvement, there were no significant differences between groups in the postoperative period. Group 2 had more adverse events (relative risk 2.45, 95% confidence interval 1.55-3.86). In terms of cerebrospinal fluid flow parameter analyses, patients in group 1 achieved better results (P < 0.05). CONCLUSIONS: For treatment of symptomatic type 1 Chiari malformation, craniovertebral junction decompression with arachnoid preservation (i.e., without tonsillar manipulation) seems more suitable than the addition of arachnoid opening and thermocoagulation of the tonsils.

How Should Syringomyelia be Defined and Diagnosed?

OBJECTIVE: Syringomyelia is considered as a fluid-filled cavitation inside the spinal cord. However, there is no agreement whether a dilated central canal should be included under this heading or how glioependymal cysts, myelomalacias, or cystic tumors should be distinguished from syringomyelia. This article provides a definition of syringomyelia and guidelines for its diagnosis. METHODS: Between 1991 and 2015, of 3206 patients with spinal cord pathologies 2276 demonstrated cystic features. All patients underwent magnetic resonance imaging. Syringomyelia was differentiated from cystic intramedullary tumors, glioependymal cysts, myelomalacias, and dilatations of the central canal by clinical and radiologic criteria. RESULTS: A total of 1535 patients were diagnosed with syringomyelia, 635 with dilatations of the central canal, 52 with glioependymal cysts, 52 with mylomalacias, and 2 with cystic intramedullary spinal cord tumors. Additional neuroradiologic studies revealed the causes of syringomyelia. As a result 604 patients showed pathologies at the craniocervical junction leading to disturbances of cerebrospinal fluid (CSF) flow. The commonest was a Chiari I malformation in 543 patients. Nine hundred thirty-one patients presented with pathologies in the spinal canal. The commonest causes were spinal arachnopathies, leading to CSF flow obstructions in 533 patients, intramedullary tumors in 152 patients, and tethered cord syndromes in 69 patients. CONCLUSIONS: The diagnosis of syringomyelia should be reserved for patients with a fluid-filled cavity in the spinal cord related to either a disturbance of CSF flow, spinal cord tethering, or an intramedullary tumor. For patients in whom such a relation cannot be established, the diagnosis of syringomyelia should be withheld.

Minimally Invasive Excision of Thoracic Arachnoid Web.

OBJECTIVE: Arachnoid webs are rare intradural lesions that can cause direct spinal cord compression or alteration of the cerebrospinal fluid flow with syringomyelia. Surgery has been historically performed through wide-open laminectomies. The aim of this study is to prove the feasibility of minimally invasive techniques for the excision of arachnoid webs. METHODS: A retrospective review of two cases of minimally invasive excision of thoracic arachnoid webs was performed. Surgery was undertaken through expandable tubular retractors. RESULTS: Complete excision was achieved through the described approach, with minimal bony removal and soft tissue disruption. There were no intraoperative or perioperative complications. Both patients were mobilized early and discharged home within 24 hours after surgery. Postoperative imaging showed good re-expansion of the spinal cord, with no evidence of residual compression or tethering. CONCLUSIONS: For symptomatic arachnoid webs, surgery remains the only definitive treatment. In expert hands, the excision of arachnoid webs can be achieved with tubular retractors and minimally invasive techniques.