Severe and progressive neuronal loss in myelomeningocele begins before 16 weeks of pregnancy.

BACKGROUND: Despite undisputable benefits, midtrimester prenatal surgery is not a cure for myelomeningocele (MMC): residual intracranial and motor deficits leading to lifelong handicap question the timing of prenatal surgery. Indeed, the timing and intensity of intrauterine spinal cord injury remains ill defined. OBJECTIVE: We aimed to describe the natural history of neuronal loss in MMC in utero based on postmortem pathology. STUDY DESIGN: Pathology findings were analyzed in 186 cases of myelomeningocele with lesion level between S1 and T1. Using a case-control, cross-sectional design, we investigated the timewise progression and topographic extension of neuronal loss between 13 and 39 weeks. Motor neurons were counted on histology at several spinal levels in 54 isolated MMC meeting quality criteria for cell counting. These were expressed as observed-to-expected ratios, after matching for gestational age and spinal level with 41 controls. RESULTS: Chiari II malformation increased from 30.7% to 91.6% after 16 weeks. The exposed spinal cord displayed early, severe, and progressive neuronal loss: the observed-to-expected count dropped from 17% to ≤2% after 16 weeks. Neuronal loss extended beyond the lesion to the upper levels: in cases <16 weeks, the observed-to-expected motor neuron count was 60% in the adjacent spinal cord, decreasing at a rate of 16% per week. Progressive loss was also found in the upper thoracic cord, but in much smaller proportions. The observed-over-expected ratio of motor neurons was not correlated with the level of myelomeningocele. CONCLUSIONS: Significant neuronal loss is present ≤16 weeks in the exposed cord and progressively extends cranially. Earlier prenatal repair (<16 weeks) could prevent Chiari II malformation in 69.3% of cases, rescue the 17% remaining motor neurons in the exposed cord, and prevent the extension to the upper spinal cord.

Spinal Dysraphia, Chiari 2 Malformation, Unified Theory, and Advances in Fetoscopic Repair.

Fetal spina bifida, the most common nonlethal birth defect of the central nervous system, results in substantial neurologic morbidity. The unified theory describes the complex relationship between local spinal lesions and development of Chiari 2 malformation, contributing to hydrocephalus. Prenatal ultrasonography reliably allows diagnosis, but fetal MR imaging is an important complement to identify additional brain abnormalities. Fetal surgery improves neurologic and motor outcomes, but various approaches, either open hysterotomy or minimally invasive to the uterus, carry substantial obstetric risks. Optimization of the fetoscopic approach aims to minimize maternal and obstetric risks, but data regarding longer-term outcomes are awaited.

Embryology of the craniocervical junction and posterior cranial fossa, part I: Development of the upper vertebrae and skull.

Although the embryology of the posterior cranial fossa can have life altering effects on a patient, a comprehensive review on this topic is difficult to find in the peer-reviewed medical literature. Therefore, this review article, using standard search engines, seemed timely. The embryology of the posterior cranial fossa is complex and relies on a unique timing of various neurovascular and bony elements. Derailment of these developmental processes can lead to a wide range of malformations such as the Chiari malformations. Therefore, a good working knowledge of this embryology as outlined in this review of its bony architecture is important for those treating patients with involvement of this region of the cranium. Clin. Anat. 31:466-487, 2018. © 2018 Wiley Periodicals, Inc.

Embryology and pathophysiology of the Chiari I and II malformations: A comprehensive review.

Although the Chiari malformations are well-studied and described developmental anomalies, there remains some incongruity in regards to their underlying etiologies. A number of theories have been proposed with the purpose of accounting for the embryology and pathogenesis of the Chiari I and II malformations and their associated complications and clinical syndromes. The present review aims to review the pertinent literature for all of the main theories that have been proposed, and outline their validity and relevance to our contemporary understanding of these anomalies. Clin. Anat. 31:202-215, 2018. © 2017 Wiley Periodicals, Inc.

Evolution of posterior fossa and brain morphology after in utero repair of open neural tube defects assessed by MRI.

OBJECTIVES: To describe characteristics of foetuses undergoing in utero repair of open neural tube defects (ONTD) and assess postoperative evolution of posterior fossa and brain morphology. METHODS: Analysis of pre- and postoperative foetal as well as neonatal MRI of 27 foetuses who underwent in utero repair of ONTD. Type and level of ONTD, hindbrain configuration, posterior fossa and liquor space dimensions, and detection of associated findings were compared between MRI studies and to age-matched controls. RESULTS: Level of bony spinal defect was defined with exactness of ± one vertebral body. Of surgically confirmed 18 myelomeningoceles (MMC) and 9 myeloschisis (MS), 3 MMC were misdiagnosed as MS due to non-visualisation of a flat membrane on MRI. Hindbrain herniation was more severe in MS than MMC (p < 0.001). After repair, hindbrain herniation resolved in 25/27 cases at 4 weeks and liquor spaces increased. While posterior fossa remained small (p < 0.001), its configuration normalised. Lateral ventricle diameter indexed to cerebral width decreased in 48% and increased in 12% of cases, implying a low rate of progressive obstructive hydrocephalus. Neonatally evident subependymal heterotopias were detected in 33% at preoperative and 50% at postoperative foetal MRI. CONCLUSION: MRI demonstrates change of Chiari malformation type II (CM-II) features. KEY POINTS: • Hindbrain herniation is significantly more pronounced in myeloschisis than in myelomeningocele • Resolution of hindbrain herniation 4 weeks after in utero closure of ONTD • MRI is valuable for preoperative assessment and postoperative evaluation following in utero repair.

Cryopreserved human umbilical cord patch for in-utero spina bifida repair.

OBJECTIVE: To identify a patch system to repair surgically created spina bifida in a sheep model for its efficacy in healing the skin defect, protecting the underlying spinal cord and reducing the Chiari II malformation. METHODS: Spina bifida was created surgically in 16 fetuses from eight timed-pregnant sheep at gestational age of 75 days. Two fetuses did not survive the procedure. Repeat hysterotomy was performed at 95 days' gestation to cover the defect with either biocellulose film with underwater adhesive (BCF-adhesive) (n = 7) or human umbilical cord with suture (HUC-suture) (n = 7). Three fetuses without formation of the defect served as reference controls. The skin healing was examined by direct visualization after a planned Cesarean section at term, followed by histological analysis using hematoxylin and eosin and Masson's trichrome stains. Mid-sagittal sections of the fetal cranium and upper cervical spine were analyzed by a pediatric neuroradiologist who was blinded to the type of patch received. RESULTS: Three fetuses that received the BCF-adhesive and six fetuses that received the HUC-suture survived to term for final analysis. As a result of dislodgment of the BCF-adhesive, all spina bifida defects repaired using BCF-adhesive were not healed and showed exposed spinal cord with leakage of cerebrospinal fluid. In contrast, all spinal defects repaired by HUC-suture were healed with complete regrowth of epidermal, dermal and subdermal tissue components, with no exposed spinal cord. The maximal skin wound width was 21 ± 3.6 mm in the BCF-adhesive group but 3 ± 0.8 mm in the HUC-suture group (P < 0.001). The spinal cord area (P = 0.001) and the number of anterior horn cells (P = 0.03) was preserved to a greater degree in the HUC-suture group than in the BCF-adhesive group, whilst psammoma bodies, signifying neuronal degeneration, were only observed in the BCF-adhesive group. Anatomic changes, indicative of Chiari II malformation, were seen in all three fetuses of the BCF-adhesive group but in none of the HUC-suture group (P < 0.01). CONCLUSION: Cryopreserved umbilical cord graft is a promising regenerative patch for intrauterine repair of spina bifida.

Fetal diffusion tensor quantification of brainstem pathology in Chiari II malformation.

OBJECTIVES: This prenatal MRI study evaluated the potential of diffusion tensor imaging (DTI) metrics to identify changes in the midbrain of fetuses with Chiari II malformations compared to fetuses with mild ventriculomegaly, hydrocephalus and normal CNS development. METHODS: Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were calculated from a region of interest (ROI) in the midbrain of 46 fetuses with normal CNS, 15 with Chiari II malformations, eight with hydrocephalus and 12 with mild ventriculomegaly. Fetuses with different diagnoses were compared group-wise after age-matching. Axial T2W-FSE sequences and single-shot echo planar DTI sequences (16 non-collinear diffusion gradient-encoding directions, b-values of 0 and 700 s/mm(2), 1.5 Tesla) were evaluated retrospectively. RESULTS: In Chiari II malformations, FA was significantly higher than in age-matched fetuses with a normal CNS (p = .003), while ADC was not significantly different. No differences in DTI metrics between normal controls and fetuses with hydrocephalus or vetriculomegaly were detected. CONCLUSIONS: DTI can detect and quantify parenchymal alterations of the fetal midbrain in Chiari II malformations. Therefore, in cases of enlarged fetal ventricles, FA of the fetal midbrain may contribute to the differentiation between Chiari II malformation and other entities. KEY POINTS: • FA in the fetal midbrain is elevated in Chiari II malformations. • FA is not elevated in hydrocephalus and mild ventriculomegaly without Chiari II. • Measuring FA may help distinguish different causes for enlarged ventricles prenatally. • Elevated FA may aid in the diagnosis of open neural tube defects. • Elevated FA might contribute to stratification for prenatal surgery in Chiari II.

Congenital basis of posterior fossa anomalies.

The classification of posterior fossa congenital anomalies has been a controversial topic. Advances in genetics and imaging have allowed a better understanding of the embryologic development of these abnormalities. A new classification schema correlates the embryologic, morphologic, and genetic bases of these anomalies in order to better distinguish and describe them. Although they provide a better understanding of the clinical aspects and genetics of these disorders, it is crucial for the radiologist to be able to diagnose the congenital posterior fossa anomalies based on their morphology, since neuroimaging is usually the initial step when these disorders are suspected. We divide the most common posterior fossa congenital anomalies into two groups: 1) hindbrain malformations, including diseases with cerebellar or vermian agenesis, aplasia or hypoplasia and cystic posterior fossa anomalies; and 2) cranial vault malformations. In addition, we will review the embryologic development of the posterior fossa and, from the perspective of embryonic development, will describe the imaging appearance of congenital posterior fossa anomalies. Knowledge of the developmental bases of these malformations facilitates detection of the morphological changes identified on imaging, allowing accurate differentiation and diagnosis of congenital posterior fossa anomalies.

Trans-amniotic stem cell therapy (TRASCET) minimizes Chiari-II malformation in experimental spina bifida.

PURPOSE: We sought to study the impact of trans-amniotic stem cell therapy (TRASCET) in the Chiari-II malformation in experimental spina bifida. METHODS: Sprague-Dawley fetuses (n=62) exposed to retinoic acid were divided into three groups at term (21-22 days gestation): untreated isolated spina bifida (n=21), isolated spina bifida treated with intra-amniotic injection of concentrated, syngeneic, labeled amniotic fluid mesenchymal stem cells (afMSCs) on gestational day 17 (n=28), and normal controls (n=13). Analyses included measurements of brainstem and cerebellar placement on high resolution MRI and histology. Statistical comparisons included ANOVA. RESULTS: In parallel to the expected induced coverage of the spina bifida in the afMSC-treated group (P<0.001), there were statistically significant differences in brainstem displacement across the groups (P<0.001), with the highest caudal displacement in the untreated group. Significant differences in cerebellar displacement were also noted, albeit less pronounced. Pairwise comparisons were statistically significant, with P=0.014 between treated and normal controls in caudal brainstem displacement and P<0.001 for all other comparisons. Labeled afMSCs were identified in 71% of treated fetuses. CONCLUSIONS: Induced coverage of spina bifida by TRASCET minimizes the Chiari-II malformation in the retinoic acid rodent model, further suggesting it as a practical alternative for the prenatal management of spina bifida.

Fetal syringomyelia.

We explored the prevalence of syringomyelia in a series of 113 cases of fetal dysraphism and hindbrain crowding, of gestational age ranging from 17.5 to 34 weeks with the vast majority less than 26 weeks gestational age. We found syringomyelia in 13 cases of Chiari II malformations, 5 cases of Omphalocele/Exostrophy/Imperforate anus/Spinal abnormality (OEIS), 2 cases of Meckel Gruber syndrome and in a single pair of pyopagus conjoined twins. Secondary injury was not uncommon, with vernicomyelia in Chiari malformations, infarct like histology, or old hemorrhage in 8 cases of syringomyelia. Vernicomyelia did not occur in the absence of syrinx formation. The syringes extended from the sites of dysraphism, in ascending or descending patterns. The syringes were usually in a major proportion anatomically distinct from a dilated or denuded central canal and tended to be dorsal and paramedian or median. We suggest that fetal syringomyelia in Chiari II malformation and other dysraphic states is often established prior to midgestation, has contributions from the primary malformation as well as from secondary in utero injury and is anatomically and pathophysiologically distinct from post natal syringomyelia secondary to hindbrain crowding.

Assessment of fetal midbrain and hindbrain in mid-sagittal cranial plane by three-dimensional multiplanar sonography. Part 2: application of nomograms to fetuses with posterior fossa malformations.

OBJECTIVES: To apply fetal midbrain (MB) and hindbrain (HB) nomograms, developed using three-dimensional multiplanar sonographic reconstruction (3D-MPR) in the mid-sagittal cranial plane, to fetuses with known posterior fossa malformations. METHODS: In this retrospective study we examined sonographic volumes obtained by sagittal acquisition in 43 fetuses diagnosed with posterior fossa abnormalities and evaluated in the mid-sagittal cranial plane, using 3D-MPR, the following: MB parameters tectal length (TL) and anteroposterior midbrain diameter (APMD), and HB parameters anteroposterior pons diameter (APPD), superoinferior vermian diameter (SIVD) and anteroposterior vermian diameter (APVD). Fetuses were grouped, according to malformation, into eight categories: cobblestone malformation complex (CMC, n = 3), Chiari-II malformation (C-II, n = 7), pontocerebellar hypoplasia (PCH, n = 2), rhombencephalosynapsis (RES, n = 4), Dandy-Walker malformation (n = 8), vermian dysgenesis (VD, n = 7), persistent Blake's pouch cyst (n = 6) and megacisterna magna (n = 6). In each case and for each subgroup, the MB-HB biometric parameters and their z-scores were evaluated with reference to our new nomograms. RESULTS: The new MB-HB nomograms were able to identify the brainstem and vermian anomalies and differentiate fetuses with MB-HB malformations from those with isolated enlarged posterior fossa cerebrospinal fluid spaces. Use of the nomograms enabled detection of an elongated tectum in fetuses with CMC, C-II and RES, and a flattened pontine belly in cases of CMC, PCH and VD. In the fetuses with VD, the nomograms enabled division into three distinctive groups: (1) those with small SIVD and APVD, (2) those with normal SIVD but small APVD, and (3) those with small SIVD but normal APVD. CONCLUSIONS: Application of our new reference data, that for the first time include the MB, enables accurate diagnosis of brain malformations affecting the MB and HB and makes possible novel characterization of previously described features of posterior fossa anomalies.

Diffusion-weighted imaging of the cerebellum in the fetus with Chiari II malformation.

BACKGROUND AND PURPOSE: Diffusion-weighted imaging can be used to characterize brain maturation. MR imaging of the fetus is used in cases of suspected Chiari II malformation when further evaluation of the posterior fossa is required. We sought to investigate whether there were any quantitative ADC abnormalities of the cerebellum in fetuses with this malformation. MATERIALS AND METHODS: Measurements from ROIs acquired in each cerebellar hemisphere and the pons were obtained from calculated ADC maps performed on our Avanto 1.5T imaging system. Values in groups of patients with Chiari II malformations were compared with those from fetuses with structurally normal brains, allowing for the dependent variable of GA by using linear regression analysis. RESULTS: There were 8 fetuses with Chiari II malformations and 23 healthy fetuses, ranging from 20 to 31 GW. There was a significant linear decline in the cerebellar ADC values with advancing gestation in our healthy fetus group, as expected. The ADC values of the cerebellum of fetuses with Chiari II malformation were higher [1820 (±100) × 10⁻6 mm²/s] than ADC values in the healthy fetuses (1370 ± 70) × 10⁻6 mm²/s. This was statistically significant, even when allowing for the dependent variable of GA (P = .0126). There was no significant difference between the pons ADC values in these groups (P = .645). CONCLUSIONS: While abnormal white matter organization or early cerebellar degeneration could potentially contribute to our findings, the most plausible explanation pertains to abnormalities of CSF drainage in the posterior fossa, with increased extracellular water possibly accounting for this phenomenon.

Single-Access Fetal Endoscopy (SAFE) for myelomeningocele in sheep model I: amniotic carbon dioxide gas approach.

BACKGROUND: This study aimed to assess the feasibility of single-access fetal endoscopy (SAFE) for the management of myelomeningocele (MMC) using intrauterine carbon dioxide as a distension medium in a sheep model. METHODS: This prospective experimental case-control study investigated 12 lamb fetuses that had a myelomeningocele-like defect surgically created on the 75th day of gestation. Four fetuses remained untreated (control group), and eight fetuses had MMC repair using two fetoscopic approaches with carbon dioxide used to distend the amniotic cavity. A collagen patch was placed over the defect and secured with surgical sealant. Four animals had a two-port fetoscopic procedure, and four animals had SAFE. Clinical and pathologic studies were performed after delivery. RESULTS: This study confirmed the validity of the animal MMC model. None of the control animals was able to stand or walk, and all had a significant defect in the lumbar area with continuous leakage of cerebrospinal fluid, ventriculomegaly, and a Chiari-II malformation. All the treated animals, independently of the number of ports used in the repair, were able to walk and had a closed defect with resolution of the Chiari malformation. CONCLUSIONS: The SAFE patch and glue coverage of surgically created fetal MMC is feasible and effective in restoring gross neurologic function in the fetal lamb model.

Cerebral ventricular system in fetuses with open spina bifida at 11-13 weeks' gestation.

OBJECTIVE: To determine if in fetuses with open spina bifida at 11-13 weeks' gestation there are alterations in the cerebral ventricular system. METHODS: In this study we selected 10 cases of open spina bifida and 410 normal singleton pregnancies which subsequently resulted in the delivery of phenotypically normal neonates. In all cases transvaginal sonography was carried out at 11-13 weeks' gestation and three-dimensional (3D) brain volumes were acquired. The fetal head was systematically assessed in a series of transverse views and measurements were obtained of the area of the lateral ventricles, the diameter of the roof of the third ventricle, the diameter of the aqueduct of Sylvius and the diameter of the fourth ventricle. The measurements obtained on the normal and affected fetuses were compared. RESULTS: In normal fetuses the area of the lateral ventricles and the diameter of the roof of the third ventricle increased, the diameter of the aqueduct of Sylvius decreased and the diameter of the fourth ventricle did not change significantly with biparietal diameter (BPD). In fetuses with open spina bifida, compared with normal fetuses, the measurements of the lateral ventricle area, the diameter of the roof of the third ventricle, the diameter of the aqueduct of Sylvius and the diameter of the fourth ventricle were significantly decreased (P < 0.01). CONCLUSION: In fetuses with open spina bifida at 11-13 weeks' gestation the intracranial collection of cerebrospinal fluid is substantially reduced.

Microscopía electrónica de barrido de la vellocidad placentaria en la malformación de Chiari tipo II / Scanning electron microscopy of the placental villi in Chiari type II malformation

Nuestro proposito es el evaluar la morfología de la vellocidad placentaria en un caso de malformación de Chiari tipo II, diagnosticada ultrasonográficamente antes del naciemiento, para determinar variaciones de forma o tipo de ramificación, como las encontradas cuando existe un cariotipo anormal, según datos de la literatura, utilizando la microscopía electrónica de barrido y análisis previo con microscop¡a de luz. La malformación de Chiari es una hidrocefalia congénita comunicante, que coexiste con trisomías del par 13 o 18 y meningomielocele. Las alteraciones cromosómicas no solamente implican anomalías morfológicas características en embriones o fetos sino tambien en las vellosidades placentarias. Los resultados obtenidos de una placenta de la paciente de 24 semanas de gestación complicada con muerte neonatal confirman vellosidades troncales, intermedias maduras e inmaduras y vellosidades terminales originándose de intermedias maduras que fueron rastreadas sin observarse modificaciones en el patrón morfológico normal o su ramificación, cuando se comparan con vellosidades normales de la misma edad gestacional. La superficie de las vellosidades se mostraron lisas, con escasas microvellosidades. No se encontraron variaciones con respecto al diámetro de los diferentes tipos de vellosidades. La deposición de fibrinoide es similar a la normal. No se observó hiperplasia de trofoblasto ni cambios en los elementos estromales. Los resultados reportan la inmadurez vellosa confirmada previamente con microscopía de luz, con retraso en la maduración de árbol vellosos. Estas imágenes muestran una visión tridimensional de la vellosidad con inmadurez persistente, ligeramente asincrónica, sin la expresión de alguna anomalía morfológica específica indicativa de algún cariotipo anormal, proporcionando una mayor confianza al histopatólogo en la evaluación completa de la vellosidad en esta malformación

Chiari Type II malformation: past, present, and future.

OBJECT: The Chiari Type II malformation (CM II) is a unique hindbrain herniation found only in patients with myelomeningocele and is the leading cause of death in these individuals younger than 2 years of age. Several theories exist as to its embryological evolution and recently new theories are emerging as to its treatment and possible prevention. A thorough understanding of the embryology, anatomy, symptomatology, and surgical treatment is necessary to care optimally for children with myelomeningocele and prevent significant morbidity and mortality. METHODS: A review of the literature was used to summarize the clinically pertinent features of the CM II, with particular attention to pitfalls in diagnosis and surgical treatment. CONCLUSIONS: Any child with CM II can present as a neurosurgical emergency. Expeditious and knowledgeable evaluation and prompt surgical decompression of the hindbrain can prevent serious morbidity and mortality in the patient with myelomeningocele, especially those younger than 2 years old. Symptomatic CM II in the older child often presents with more subtle findings but rarely in acute crisis. Understanding of CM II continues to change as innovative techniques are applied to this challenging patient population.

Chiari I malformation and neurofibromatosis type 1.

Single case reports exist in the medical literature of patients with tonsillar ectopia, i.e., the Chiari I malformation and neurofibromatosis type 1. However, large series of patients with either of these entities have not been examined for the presence of both defects. We have retrospectively examined two large groups of pediatric patients: Group I, with the primary diagnosis of Chiari I malformation, who have undergone posterior fossa decompression for symptomatology; and Group II patients, who have been observed in our hospital's neurofibromatosis clinic for evaluation. Of 130 surgically addressed Chiari I malformations (Group I), we determined that 5.4% of these patients had the additional diagnosis of neurofibromatosis type 1. Of Group II patients (198) who underwent imaging of the brain, 8.6% were found to have a concomitant Chiari I malformation. These data suggest that Chiari I malformation and neurofibromatosis type 1 are not spurious findings but rather true associations. We hypothesize that the same early dysgenesis of mesoderm that is widely accepted as a culprit in the genesis of many Chiari I malformations is the same pathology affecting primitive development of tissues involved in many patients with neurofibromatosis type 1. Perhaps these data will aid in the determination of a genetic locus for the Chiari I malformation.

Spinal dysraphism: MR imaging rationale.

Spinal cord development occurs through the three consecutive periods of gastrulation (weeks 2-3), primary neurulation (weeks 3-4), and secondary neurulation (weeks 5-6). Spinal cord malformations derive from defects in these early embryonic stages, and are collectively called spinal dysraphisms. Spinal dysraphisms may be categorized clinically into open and closed, based on whether the abnormal nervous tissue is exposed to the environment or covered by skin. Open spinal dysraphisms include myelomeningocele and other rare abnormalities such as myelocele, hemimyelomeningocele, and hemimyelocele, and are always associated with a Chiari II malformation. Closed spinal dysraphisms are further divided into two subsets based on whether a subcutaneous mass is present in the low back. Closed spinal dysraphisms with mass comprise lipomyelocele, lipomyelomeningocele, meningocele, and myelocystocele. Closed spinal dysraphisms without mass comprise simple dysraphic states (tight filum terminale, filar and intradural lipomas, persistent terminal ventricle, and dermal sinuses) and complex dysraphic states. The latter category involves abnormal notochordal development, either in the form of failed midline integration (ranging from complete dorsal enteric fistula to neurenteric cysts and diastematomyelia) or of segmental agenesis (caudal agenesis and spinal segmental dysgenesis). Magnetic resonance imaging is the imaging modality of choice for evaluation of this complex group of disorders.

The Chiari II malformation: cause and impact.

INTRODUCTION: It is the Chiari II malformation and its effects that determine the quality of life of the individual born with spina bifida. DISCUSSION: The cause of this malformation has been a source of debate for many years. Understanding the cause enables strategies for the management of problems created by this malformation to be developed. An open neural tube defect allows fluid to escape from the cranial vesicles, altering the intracranial environment and leads to all of the brain changes seen in the Chiari II malformation. Decompression of the intracranial vesicles causes overcrowding, decrease in the size of the third ventricle, and changes in the fetal skull. It also permanently links the intracranial ventricular system to the spinal cord central canal.