Unilateral alacrimia as a presenting symptom of Meckel's cave tumour.

Meckel's cave tumour, a rare benign tumour originating from the Schwann cells surrounding the trigeminal nerve within the Meckel's cave region, can present with a variety of clinical manifestations. We report a case of a 44-year-old male patient who presented with symptoms of tear deficiency, including dryness, ocular discomfort, and blurred vision. Diagnostic evaluation revealed the presence of a Meckel's cave tumour harming the trigeminal nerve, leading to alacrimia. This case highlights the association between Meckel's cave tumour and tear deficiency disorders.

Pseudomeningocele Following Posterior Cranial Fossa Surgery Significantly Increases the Risk of Intracranial Infection: A 10-Year Retrospective Analysis.

Background: Posterior fossa craniotomy is commonly performed for various pathologies. However, intra-cranial infection following craniotomy causes morbidity. Pseudomeningocele is one of the main complications following posterior fossa operation. This study aimed to test the hypothesis that the risk of intra-cranial infection is increased in patients who undergo posterior fossa craniotomy with pseudomeningocele compared with those without pseudomeningocele. Methods: We retrospectively analyzed the data of patients undergoing posterior fossa craniotomy for the management of neurological pathologies at our institute from 2011 to 2020. A total of 308 craniotomies were included, and the primary outcome of interest was the occurrence of intra-cranial infection. Standard statistical methods were used to explore associations between several parameters, including pseudomeningocele, intra-cranial infection, and wound leak. Results: Of the 308 craniotomies, 41 (13.3%) developed intra-cranial infection and 59 (19.2%) involved pseudomeningocele. Of cases involving pseudomeningocele, 27 (45.8%) developed an intra-cranial infection compared with only 14 of 249 without pseudomeningocele (5.6%; p < 0.001). In the multi-variable analysis, pseudomeningocele was associated with intra-cranial infection (odds ratio [OR] 8.56; 95% confidence interval [CI] 3.145-23.299; p < 0.001) and wound leak (OR 91.339; 95% CI 10.437-799.364; p < 0.001). Conclusion: The findings indicate that patients undergoing posterior fossa craniotomy are at a greater risk of intra-cranial infection if there is pseudomeningocele after the operation.

Magnetic Resonance Imaging-Related Anatomic and Functional Parameters for the Diagnosis and Prognosis of Chiari Malformation Type I: A Systematic Review and Meta-analysis.

OBJECTIVE: Imaging parameters of Chiari malformation type I (CMI) development are not well established. This study aimed to collect evidence of general or specific imaging measurements in patients with CMI, analyze indicators that may assist in determining the severity of CMI, and guide its diagnosis and treatment. METHODS: A comprehensive search was conducted across various databases including the Cochrane Library, PubMed, MEDLINE, Scopus, and Embase, covering the period from January 2002 to October 2023, following predefined inclusion criteria. Meta-analyses were performed using RevMan (ver. 5.4). We performed a quantitative summary and systematic analysis of the included studies. This study was registered in the PROSPERO (International Prospective Register of Systematic Reviews) prior to initiation (CRD42023415454). RESULTS: Thirty-three studies met our inclusion criteria. The findings indicated that out of the 14 parameters examined, 6 (clivus length, basal angle, Boogard's angle, supraocciput lengths, posterior cranial fossa [PCF] height, and volume) exhibited significant differences between the CMI group and the control group. Furthermore, apart from certain anatomical parameters that hold prognostic value for CMI, functional parameters like tonsillar movement, obex displacement, and cerebrospinal fluid dynamics serve as valuable indicators for guiding the clinical management of the disease. CONCLUSION: We collated and established a set of linear, angular, and area measurements deemed essential for diagnosing CMI. However, more indicators can only be analyzed descriptively for various reasons, particularly in prognostic prediction. We posit that the systematic assessment of patients' PCF morphology, volume, and other parameters at a 3-dimensional level holds promising clinical application prospects.

Foramina and canals of the skull base in Holstein cow: a computed tomography study.

BACKGROUND: The aim of the study was to describe the comprehensive morphological and morphometric features of the foramina and canals at the base of the cranial cavity in Holstein cow using CT images. MATERIALS AND METHODS: The study was performed on fourteen adult Holstein cow head cadavers. Images taken with MSCT were transferred to the DICOM Viewer program. The MPR and 3D reconstructive tools of the program were used to analyse the foramina and canals. RESULTS: Although they varied in shape and size, foramina and canals were found bilaterally in all animals. It was observed that the orbitorotund foramen, jugular foramen and oval foramen had a canalicular structure, with the distance between the extra-intra cranial openings measured as 15.0 mm, 5.9 mm and 6.2 mm, respectively. The hypoglossal canal, which was found to be single in 43%, double in 50% and triple in 7% in each body half, was the canal with the most variation in number and shape. The orbitorotund foramen, a canal with an area of 180.6 mm² and a diameter of 18.1 × 12.4 mm is the widest at the skull base, while the optic canal is the narrowest and longest opening with an area of 33.4 mm², a diameter of 8.4 × 5.5 and a length of 17.5 mm. CONCLUSIONS: This study shows that our knowledge of skull base morphometry in animals is extremely limited. Although the study was conducted on a limited number of materials, it may benefit both regional anatomy knowledge in terms of the data presented and veterinary anatomists, radiologists and clinicians in terms of methodology.

Feasibility, safety, and efficacy of endovascular treatment of anterior cranial fossa dural arteriovenous fistulas: a systematic review and meta-analysis with a subanalysis for Onyx.

Dural Arteriovenous Fistulas (dAVFs) of the anterior cranial fossa (ACF) are uncommon but carry a high risk of hemorrhage and pose substantial treatment challenges. Recent advancements in endovascular treatment (EVT), including the introduction of novel liquid embolic agents, have markedly bolstered EVT's role in managing ACF-dAVFs, with notable series published in the last five years. We aimed to assess the feasibility, safety, and efficacy of EVT for ACF-dAVFs. We searched Medline, Scopus, Web of Science, and Cochrane Library databases following PRISMA guidelines. Eligible studies included those with ≥ 5 patients undergoing embolization of ACF-dAVFs, detailing both angiographic and clinical outcomes. We used single proportion analysis with 95% confidence intervals under a random-effects model, I2 to assess heterogeneity, and Baujat and sensitivity analysis to address high heterogeneity. Publication bias was assessed by funnel-plot analysis and Egger's test. Outcomes included complete occlusion following embolization, unsuccessful endovascular embolization attempts, incomplete occlusion following embolization, symptom resolution or clinical improvement following embolization, recurrence; procedure-related complications, morbidity, and mortality. Additionally, a subanalysis for studies exclusively utilizing Onyx™ embolic system was done. Eighteen studies comprising 231 ACF-dAVF were included. Unsuccessful endovascular embolization attempts rate was 2%. Complete occlusion rate was 85%, with 4% of complications. Incomplete occlusion rate was 10%. Successfully embolized patients experienced either symptom resolution or clinical improvement in 94% of cases. Morbidity and mortality rates were 1% and 0%, respectively. Onyx subanalyses showed an overall rate of 0% for unsuccessful attempts, 95% for complete occlusion, and 5% for incomplete occlusion. Symptom resolution or clinical improvement was 98% and recurrence rate was 0%. EVT for ACF-dAVF is highly feasible, effective, and safe, with a low rate of complications, morbidity, and mortality. The subanalyses focusing on Onyx embolizations revealed superior efficacy and safety outcomes compared to the findings of the primary analyses involving all included studies.

Posterior Cranial Fossa Surgery with a 3 Dimensional Exoscope: A Single-Center Survey-Based Analysis and a Literature Review.

OBJECTIVE: The potential advantages of exoscopy have been discussed theoretically for a long time. Such a concept holds significance, especially in the specific setting of the posterior cranial fossa (PCF), characterized by complex anatomy and long and narrow surgical corridors with relatively extreme working angles. We aimed to: 1) analyze the institutional preliminary case-based data on the use of the Robotic 3D Exoscope AEOS Aesculap in three different PCF approaches: retrosigmoid (RSA), midline suboccipital (MSA), and supracerebellar infratentorial via feedbacks was collected using a 20-point questionnaire, and 2) perform a comprehensive review of the literature concerning the use of EX in PCF surgery. RESULTS: A total of 38 patients with neurosurgical pathologies underwent a neurosurgical procedure using the EX (Robotic 3D exoscope AEOS Aesculap) at our institution between January and March 2022. 21 surgeons were involved in the abovementioned PCF surgeries and answered the questionnaire. The main perceived advantages were in terms of ergonomics (67%), magnification (52%), and visualization of extreme angles. The main reported disadvantage was color vision (16, 76%), followed by manual mobility (24%). Concerning the review, the search of the literature yielded a total of 177 results. Upon full-text review, 17 articles were included, including 153 patients. CONCLUSIONS: In conclusion, our study provides a comprehensive evaluation of the advantages and challenges associated with using the exoscope in posterior fossa surgery, setting a precedent as the first to report on a questionnaire-based analysis of exoscope utilization in this specific domain.

Superior semicircular canal dehiscence postoperative outcomes: a case series of 350 repairs.

BACKGROUND: Superior Semicircular Canal Dehiscence (SSCD) is a dehiscence of the otic capsule which normally lies over the superior semicircular canal. This database constitutes the largest series of SSCD patients to date. OBJECTIVE: To determine what preoperative factors, if any, contribute to postoperative outcomes and evaluate symptom resolution in a large SSCD patient cohort. METHODS: A single-institution, retrospective chart review collected patient demographics, intraoperative findings, and pre-and postoperative symptoms. Fisher's exact t-test was performed for unpaired categorical variables, with a significance level of p < 0.05. RESULTS: 350 SSCD repairs were performed. The median age was 52 years (range: 17-86 years, ± 6.4 years), and the median follow-up duration was 4.6 months (range: 0.03-59.5 months, ± 6.8 months). Preoperative hearing loss was significantly associated with female sex (p = 0.0028). The most reported preoperative symptoms were tinnitus (77.4%), dizziness (74.0%), autophony (66.3%), amplification (63.7%), and disequilibrium (62.6%). Between patients who received unilateral versus bilateral SSCD repair, the greatest postoperative symptomatic resolution was seen in autophony (74.9%, p < 0.001), amplification (77.3%, p = 0.00027), hyperacusis (77.4%, p = 0.023), hearing (62.9%, p = 0.0063), and dizziness (54.6%, p < 0.001) for patients with unilateral SSCD repair. CONCLUSION: Surgical repair via the middle cranial fossa approach can significantly resolve auditory, vestibular, and neurological symptoms of patients with SSCD. Although this is one of the largest single-institution SSCD studies to date, future multi-institutional, prospective studies would be beneficial to validate these results.

Imaging of cerebrovascular complications from blunt skull base trauma.

Cerebrovascular complications from blunt trauma to the skull base, though rare, can lead to potentially devastating outcomes, emphasizing the importance of timely diagnosis and management. Due to the insidious clinical presentation, subtle nature of imaging findings, and complex anatomy of the skull base, diagnosing cerebrovascular injuries and their complications poses considerable challenges. This article offers a comprehensive review of skull base anatomy and pathophysiology pertinent to recognizing cerebrovascular injuries and their complications, up-to-date screening criteria and imaging techniques for assessing these injuries, and a case-based review of the spectrum of cerebrovascular complications arising from skull base trauma. This review will enhance understanding of cerebrovascular injuries and their complications from blunt skull base trauma to facilitate diagnosis and timely treatment.

Comparing Approaches for Repair of Superior Semicircular Canal Dehiscence.

OBJECTIVE: Compare outcomes for subjects who underwent middle cranial fossa (MCF) or transmastoid (TM) repair of superior semicircular canal dehiscence (SSCD). STUDY DESIGN: Retrospective cohort study. SETTING: Quaternary-care, academic neurotology practice. METHODS: Subjects who underwent MCF or TM repair of SSCD between December 1999 and April 2023 were identified. Main outcome measures included demographic data, length of surgery and hospital stay, clinical presentation, and audiometric testing. RESULTS: Ninety-three subjects (97 ears) who underwent surgery for SSCD met inclusion criteria: 58.8% (57) via MCF, 39.2% (38) via TM, and 2.0% (2) via TM + MCF. Median operative time was shorter for the TM (35) compared to the MCF (29) approach (118 vs 151 minutes, P < .001). Additionally, median hospital stays were shorter for TM (36) compared to the MCF (56) approach (15.3 vs 67.7 hours, P < .001). Overall, 92% (49/53) of MCF and 92% (33/36) of TM surgeries resulted in an improvement or resolution of one or more symptoms (P = .84). There was no significant preoperative to postoperative change in the median air conduction pure-tone average (PTA), air-bone gap, or word recognition score in both the MCF and TM groups (P > .05). Improvements of >10 dB in the pre- to postoperative absolute change in bone conduction PTA were noted in 3 subjects in the MCF group and 4 subjects in the TM group (P = .49). CONCLUSION: The TM approach for SSCD demonstrates shorter operative times and length of hospital stay. The TM and MCF approaches have comparable audiometric and clinical outcomes.

The endaural subtemporal keyhole: a novel minimally invasive approach to the middle cranial fossa.

OBJECTIVE: The goal of this study was to evaluate the feasibility of a minimally invasive approach to the middle cranial fossa using a novel endaural keyhole. METHODS: The charts of all patients who underwent this novel minimally invasive approach to the middle cranial fossa were retrospectively reviewed. In addition, cadaveric dissection was performed to demonstrate the feasibility of the endaural keyhole to the middle cranial fossa. RESULTS: Six patients (5 female and 1 male; age range 47-77 years) who underwent craniotomy for CSF leak (n = 3), intracerebral hematoma evacuation (n = 2), and tumor resection (n = 1) via the endaural subtemporal approach were identified. There were no approach-related complications noted. Representative imaging from cadaveric dissection is provided with a stepwise discussion of the procedure. CONCLUSIONS: The endaural subtemporal keyhole craniotomy provides a novel approach to middle fossa skull base pathology, as well as a minimally invasive approach to intra-axial pathology of the temporal lobe and basal ganglia. Further research is needed to establish the limitations and potential complications of this novel approach.

Treatment of Clival Giant Cell Tumor: A Case Report and Literature Review.

Giant cell tumors (GCTs) are locally aggressive primary bone tumors of osteoclast-like cells. Most GCTs occur within the long bones, and primary GCTs involving the clivus are extremely rare. We present the case of an 18-year-old boy with binocular horizontal diplopia with an insidious onset who was found to have a hypointense enhancing mass involving the clivus and left side dorsum sellae on magnetic resonance images. The tumor was completely resected via an endoscopic endonasal transclival approach, and histopathologic examination via immunohistochemistry indicated a GCT. The patient's left abducens nerve palsy improved slightly after surgery. Because of the rarity of GCTs, there is no consensus about the definitive treatment protocol. However, we suggest that gross total resection is the treatment of choice, and denosumab plays a critical role in patients with subtotal resection.

Captive Bolt Gun-Related Vascular Injury: A Single Center Experience.

This article investigates the clinical and radiological characteristics of captive bolt gun head injuries, a rare form of low-velocity penetrating brain injury. Eleven consecutive patients were included in the study. Vascular injuries and the rate of infection were systematically analyzed. Radiological findings reveal common bolt trajectories in the anterior cranial fossa, with identified risk factors for a poor outcome including trajectory crossing midline, hematocephalus, and paranasal sinus involvement. Only one patient had a good outcome. Despite meticulous microsurgical techniques, this study highlights often unfavorable clinical outcomes in captive bolt gun injuries, with vascular injury identified as a potential contributing risk factor for a poor outcome. Knowledge of variant vascular tree anatomy and corresponding vascular territory is important. To avoid potential vascular injuries, a complete removal of bone fragments was not always performed and it did not increase the rate of infection, challenging the conventional wisdom advocating for the complete removal of bone fragments. These findings contribute novel insights into captive bolt gun-related injuries, paving the way for further research.

Liponeurocytoma: Rare Neoplasm of the Central Nervous System.

In this case report, we characterize an instance of diagnosis, treatment, characteristics, and outcomes of a patient with a liponeurocytoma, a rare WHO grade II brain tumor first described in 1978. This tumor has been described with a wide array of radiographic, microscopic, and histologic features, and there remains no consensus regarding the role of radiation therapy. Most patients have favorable outcomes after surgical resection. Here we present the case of a 46-year-old female who underwent suboccipital craniectomy for resection of a cerebellar mass, which was diagnosed as liponeurocytoma on final pathology. The patient experienced resolution of symptoms and is neurologically intact two years after resection of the tumor.

Multiple Rare Anatomic Variations in Anterior and Posterior Cerebral Circulation: A Case Report.

Anatomic variations of intracranial arteries are of paramount importance in neurosurgery and interventional radiology. Three extremely rare arterial variants were found by observing the intracranial vascular anatomy on the magnetic resonance angiography files of a 56-year-old female patient. Firstly, on the left side of the vertebrobasilar axis, a persistent primitive lateral basilovertebral anastomosis was found uniting the left anterior inferior and posterior inferior cerebellar arteries; further, the left anterior inferior cerebellar artery looped above the nerves of the internal auditory canal. Secondly, the right posterior inferior cerebellar artery was shown to be leaving the vertebral artery and had a distal fenestration of the telovelotonsillar segment. Such cases of distal fenestrated posterior inferior cerebellar artery are rare. Thirdly, a partly duplicated anterior communicating artery was also found in the anterior circulation. In conclusion, magnetic resonance angiography helps distinguish and detail discrete and delicate rare arterial variants.

Morphometric Analysis of the Retrolabyrinthine Approach to the Posterior Fossa.

INTRODUCTION: The retrolabyrinthine approach provides shorter working distance and less cerebellar retraction compared with the retrosigmoid approach to the internal acoustic canal (IAC) and cerebellopontine angle cistern. However, exposure of the ventral surface of the brainstem and petroclival region may be restricted. Trautmann's triangle (TT), an area intimately related to this region, demonstrates significant anatomical variability, which may adversely affect the ease of the approach. The aim of this study is to evaluate anatomic parameters of the posterior fossa that may anticipate a challenging situation in approaching the IAC and the petroclival region through the retrolabyrinthine approach. METHODS: It was performed a radioanatomic analysis of 75 cerebral angiotomography exams to identify parameters that could potentially reduce areas of surgical exposure. RESULTS: Large variations were observed in the area of exposure of the TT (553%) and the height of the jugular bulb (234%). Shorter distances from the sigmoid sinus to the posterior semicircular canal and high-riding jugular bulb were associated with smaller areas of exposure. Dominant and laterally positioned sigmoid sinuses and less pneumatized mastoids were associated with potentially unfavorable conditions, including a narrower angle of attack to the IAC. Increased petrous slopes and petroclival angles were associated with smaller petroclival areas and shallower clival depths. CONCLUSIONS: This study of the posterior fossa reveals remarkable anatomic variation in the region. These findings should be taken into consideration during the preoperative planning of retrolabyrinthine approaches in order to offer safer and more effective surgical procedures.

Multimodal management of giant solid hemangioblastomas in two patients with preoperative embolization.

Background: Hemangioblastomas are benign vascular neoplasms, World Health Organization grade I, with the most frequent location in the cerebellum. Complete microsurgical resection can be a challenge due to excessive bleeding, which is why preoperative embolization takes importance. Case Description: Two clinical cases are presented, a 25-year-old woman and a 75-year-old man, who presented with intracranial hypertension symptoms due to obstructive hydrocephalus; a ventriculoperitoneal shunt was placed in both cases; in addition, they presented with cerebellar signs. Both underwent embolization with ethylene vinyl alcohol copolymer, with blood flow reduction. After that, they underwent microsurgical resection within the 1st-week post embolization, obtaining, in both cases, gross total resection without hemodynamic complications, with clinical improvement and good surgical outcome. It is worth mentioning that surgical management is the gold standard that allows a suitable surgical approach, like in our patients, for which a lateral suboccipital craniotomy was performed. Conclusion: Solid hemangioblastomas are less frequent than their cystic counterparts. The treatment is the surgical resection, which is a challenge and always has to be considered as an arteriovenous malformation in the surgical planning, including preoperative embolization to reduce perioperative morbidity and mortality and get good outcomes.

The Posterior Auricularis Muscle as Superficial Landmark for the Sigmoid Sinus and Transverse-Sigmoid Sinus Junction: An Anatomical Study.

OBJECTIVE: Neuronavigation systems coupled with previously reported external anatomical landmarks assist neurosurgeons during intracranial procedures. We aimed to verify whether the posterior auricularis muscle (PAM) could be used as an external landmark for identifying the sigmoid sinus (SS) and the transverse-sigmoid sinus junction (TSSJ) during posterior cranial fossa surgery. METHODS: The PAM was dissected in 10 adult cadaveric heads and after drilling the underlying bone, the relationships with the underlying SS and TSSJ were noted. The width and length of the PAM, and the distance between the muscle and reference points (asterion, mastoid tip, and midline), were measured. RESULTS: The PAM was identified in 18 sides (9 left, 9 right). The first 20 mm of the muscle length (mean 28.28 mm) consistently overlay the mastoid process anteriorly and the proximal half of the SS slightly posteriorly on all sides. The superior border was a mean of 2.22 mm inferior to the TSSJ and, especially when the muscle length exceeded 20 mm, this border extended closer to the transverse sinus; it was usually found at a mean of 3.11 mm (range 0.0-13.80 mm) inferior to the distal third of the transverse sinus. CONCLUSIONS: Superficial landmarks give surgeons improved surgical access, avoiding overexposure of deep neurovascular structures and reducing brain retraction. On the basis of our cadaveric study, the PAM is a reliable and accurate direct landmark for identifying the SS and TSSJ. The PAM could potentially be used for guiding the retrosigmoid approach.

Angioarchitecture of Dural Arteriovenous Fistulas: Definition of Fistulous Points.

Because histopathological aspects of dural arteriovenous fistulas (DAVFs) are often lacking, there can be controversies regarding their angioarchitecture. Depending on various statements or DAVF types, the shunts can be situated directly on the sinuses, at the confluence of sinuses and cortical veins, in adjacent vascular structures surrounding the sinus, or, even, in the bone.1-5 Comprehensive knowledge of the intricate arteriovenous shunt anatomy is crucial for the secure and effective management of DAVFs.1 It has been confirmed that the dural arteries communicate with crack-like veins that connect ≥1 dural veins near the affected sinuses.3 In DAVFs, it is certain that ≥1 draining veins exist before draining into the venous sinus, in contrast to the commonly stated direct artery-to-sinus communication. Arteries branch and taper, and veins receive branches and thicken their lumens. We define fistulous points as the locations where the feeding arteries transition from thick to thin and the draining veins transition from thin to thick. We provide an image description based on superselective angiography.

Cerebellar Glioblastoma: A Literature Review and Case Analysis.

Glioblastoma multiforme is one of the most common primary intracranial tumors with a particularly aggressive behavior. It usually develops in the cerebral hemispheres, with infratentorial localization being extremely rare. If located in the posterior cranial fossa, glioblastoma most often presents with symptoms of increased intracranial pressure and impaired cerebellar function. In this article, we present a case of small-cell glioblastoma, which is a rare histological variant of this type of high-grade glioma, situated in the cerebellum. A 31-year-old woman was admitted to the neurosurgery department with severe headache, impaired balance, and weakness in the right arm. Magnetic resonance imaging of the brain showed evidence of a lesion with solid and cystic components in the right cerebellar hemisphere. The latter was surgically removed and the histological examination determined the diagnosis of cerebellar small-cell glioblastoma. The treatment of this patient included a combined approach, i.e., radiotherapy and chemotherapy with temozolomide after surgery. Follow-up for a period of more than two years was done and the patient showed no significant clinical symptoms. There was no evidence of recurrence on follow-up imaging studies.

Evaluation and Treatment of Patients with Small Posterior Cranial Fossa and Chiari Malformation, Types 0 and 1.

The diagnosis of Chiari I malformation is straightforward in patients with typical signs and symptoms of Chiari I malformation and magnetic resonance imaging (MRI) confirming ≥5 mm of cerebellar tonsillar ectopia, with or without a syrinx. However, in many cases, Chiari I malformation is discovered incidentally on MRI to evaluate global headache, cervical radiculopathy, or other conditions. In those cases, the clinician must consider if cerebellar tonsillar ectopia is related to the presenting symptoms. Surgical decompression of the cerebellar tonsils and foramen magnum in patients with symptomatic Chiari I malformation effectively relieves suboccipital headache, reduces syrinx distension, and arrests syringomyelia progression. Neurosurgeons must avoid operative treatments decompressing incidental tonsillar ectopia, not causing symptoms. Such procedures unnecessarily place patients at risk of operative complications and tissue injuries related to surgical exploration. This chapter reviews the typical signs and symptoms of Chiari I malformation and its variant, Chiari 0 malformation, which has <5 mm of cerebellar tonsillar ectopia and is often associated with syringomyelia. Chiari I and Chiari 0 malformations are associated with incomplete occipital bone development, reduced volume and height of the posterior fossa, tonsillar ectopia, and compression of the neural elements and cerebrospinal fluid (CSF) pathways at the foramen magnum. Linear, angular, cross-sectional area, and volume measurements of the posterior fossa, craniocervical junction, and upper cervical spine identify morphometric abnormalities in Chiari I and Chiari 0 malformation patients. Chiari 0 patients respond like Chiari I patients to foramen magnum decompression and should not be excluded from surgical treatment because their tonsillar ectopia is <5 mm. The authors recommend the adoption of diagnostic criteria for Chiari 0 malformation without syringomyelia. This chapter provides updated information and guidance to the physicians managing Chiari I and Chiari 0 malformation patients and neuroscientists interested in Chiari malformations.