Effect of clival bone growth on the localization of the abducens nerve at the petroclival region: a postmortem anatomical study.

PURPOSE: To investigate the effect of the clival bone pattern on the abducens nerve (AN) localization in the petroclival region between the Pediatric and Adult Groups. METHODS: This study used 12 pediatric and 17 adult heads obtained from the autopsy. The length and width of the clivus and the length of the petrosphenoidal ligaments (PSLs) were measured. The ratio of the length and width of the clivus was accepted as the clival index (CI). The localization of the AN at the petroclival region below the PSL, classified as lateral and medial, were recorded. RESULTS: The average length of the clivus was 26.92 ± 2.88 mm in the Pediatric Group, and 40.66 ± 4.17 mm in the Adult Group (p < 0.001). The average width of the clivus was 22.35 ± 2.88 mm in the Pediatric Group, and 29.96 ± 3.86 mm in the Adult Group (p < 0.001). The average value of the CI was 1.20 in the Pediatric Group and 1.36 in the Adult Group (p = 0.003). The length of the PSL was 7.0 ± 1.47 mm in the Pediatric Group and 11.05 ± 2.95 mm in the Adult Group (p < 0.001). The nerve was located below the medial side of the PSL in the Pediatric Group and below the lateral side in the Adult Group (p = 0.002). CONCLUSIONS: The petrous apex localization of the AN in adults compared with pediatric subjects could be related to the increased growth in the length of the clivus than its width.

Posterior Cranial Fossa Maldevelopment in Infants with Repaired Open Myelomeningoceles: Double Trouble or a Dynamic Process of Posterior Cranial Fossa Abnormalities?

OBJECTIVE: Two degrees of posterior cranial fossa (PCF) maldevelopment can be hypothesized in children with myelomeningocele (MMC). This paper investigates the PCF deformation by quantitative magnetic resonance imaging analysis in MMC subjects with and without Chiari 2 malformation (CM2). METHODS: PCF bone volume (PCFV), PCF brain volume (PCFBV), lengths of PCF, ventriculomegaly, level, and extension of the dysraphism were analyzed by magnetic resonance image scanning of 51 newborns with MMC surgically repaired at birth (and 41 controls). The possible correlation among PCF hypoplasia, level/extension of the spinal dysraphism, and ventriculomegaly was assessed. RESULTS: In MMC and CM2, the dysraphism level was above L4 in 30 and below L4 in 10 subjects. PCFV/PCFBV ratio and supraocciput and exocciput lengths were significantly reduced; foramen magnum diameters, mammillo-pontine distance, and pons length were significantly increased (P < 0.05). In isolated MMC, the dysraphism level was below L4 in all cases. PCFV/PCFBV ratio and supraocciput length were significantly reduced; pons length was significantly increased (P < 0.05). The lower the MMC level, the lower the incidence of CM2. A positive correlation was found between PCF hypoplasia and MMC level above L4 (P < 0.001), while a negative correlation was found among PCF hypoplasia and MMC extension (P = 0.006), PCF hypoplasia, and ventriculomegaly (P = 0.02). CONCLUSIONS: PCF hypoplasia has to be considered a dynamic maldevelopment process in the 2 cohorts rather than 2 separated entities. The level of MMC is the main but not the unique cause influencing the severity of PCF maldevelopment.

The growth of the posterior cranial fossa in FGFR2-induced faciocraniosynostosis: A review.

BACKGROUND: The growth of the posterior fossa in syndromic craniostenosis was studied in many papers. However, few studies described the pathophysiological growth mechanisms in non-operated infants with fibroblast growth factor receptor (FGFR) type 2 mutation (Crouzon, Apert or Pfeiffer syndrome), although these are essential to understanding cranial vault expansion and hydrocephalus treatment in these syndromes. OBJECTIVE: A review of the medical literature was performed, to understand the physiological and pathological growth mechanisms of the posterior fossa in normal infants and infants with craniostenosis related to FGFR2 mutation. DISCUSSION: Of the various techniques for measuring posterior fossa volume, direct slice-by-slice contouring is the most precise and sensitive. Posterior fossa growth follows a bi-phasic pattern due to opening of the petro-occipital, occipitomastoidal and spheno-occipital sutures. Some studies reported smaller posterior fossae in syndromic craniostenosis, whereas direct contouring studies reported no difference between normal and craniostenotic patients. In Crouzon syndrome, synchondrosis fusion occurs earlier than in normal subjects, and follows a precise pattern. This premature fusion in Crouzon syndrome leads to a stenotic foramen magnum and facial retrusion.

Clival canal and clival foramen development in the fetal and infant basioccipital.

PURPOSE: There is a paucity of research regarding both the development and the prevalence of the clival canal or clival foramen in fetuses. Reports that have examined child and adult populations have posited ideas for the development of the canal and foramen; however, they have done so in the absence of anatomical data from the fetal population. Therefore, the present study was performed to elucidate the development of the clival canal and foramen through the assessment of perinatal basioccipitals. METHODS: This study analyzed 104 basioccipital bones, 60 from fetuses and 44 from newborns and infants. Dorsal surfaces of basioccipitals were assessed for the presence of anatomical variation with particular attention to the presence of clival canals and foramina. Among cases in which the presence of a clival canal or clival foramen was suspected, cannulation was performed for verification. RESULTS: Of the 104 basioccipitals analyzed, 1 (0.96%) had a clival foramen. Clival canals were identified in seven basioccipitals (7:104; 6.73%), four of which were from fetuses. Trends in anatomical variations among basioccipitals were also identified and categorized. These categories were then evaluated relative to age in order to elucidate ontogeny. A model is presented to explain the development of the clival foramen, the clival canal, and the basioccipital, in general. CONCLUSIONS: The presence of a clival canal or foramen should be considered even among individuals of fetal age. The findings of this osteological study suggest that the clival canal and foramen develop around vascular structures and, therefore, signify vascular connections among nearby venous plexuses.

Human Foramen Magnum Area and Posterior Cranial Fossa Volume Growth in Relation to Cranial Base Synchondrosis Closure in the Course of Child Development.

BACKGROUND: To date, no study has compared the evolution of the foramen magnum area (FMA) and the posterior cranial fossa volume (PCFV) with the degree of cranial base synchondrosis ossification. OBJECTIVE: To illustrate these features in healthy children. METHODS: The FMA, the PCFV, and the ossification of 12 synchondroses according to the Madeline and Elster scale were retrospectively analyzed in 235 healthy children using millimeter slices on a computed tomography scan. RESULTS: The mean FMA of 6.49 cm in girls was significantly inferior to the FMA of 7.67 cm in boys (P < .001). In both sexes, the growth evolved in a 2-phase process, with a phase of rapid growth from birth to 3.75 years old (yo) followed by a phase of stabilization. In girls, the first phase was shorter (ending at 2.6 yo) than in boys (ending at 4.33 yo) and proceeded at a higher rate. PCFV was smaller in girls (P < .001) and displayed a biphasic pattern in the whole population, with a phase of rapid growth from birth to 3.58 yo followed by a phase of slow growth until 16 yo. In girls, the first phase was more active and shorter (ending at 2.67 yo) than in boys (ending at 4.5 yo). The posterior interoccipital synchondroses close first, followed by the anterior interoccipital and occipitomastoidal synchondroses, the lambdoid sutures simultaneously, then the petro-occipital and spheno-occipital synchondroses simultaneously. CONCLUSION: The data provide a chronology of synchondrosis closure. We showed that FMA and PCFV are constitutionally smaller in girls at birth (P ≤ .02) and suggest that a sex-related difference in the FMA is related to earlier closure of anterior interoccipital synchondroses in girls (P = .01). ABBREVIATIONS: AIOS, anterior interoccipital synchondrosesFMA, foramen magnum areaLS, lambdoid suturesOMS, occipitomastoidal synchondrosesPCFV, posterior cranial fossa volumePIOS, posterior interoccipital synchondrosesPOS, petro-occipital synchondrosesSOS, spheno-occipital synchondrosisyo, years old.

Variation of the slope of the tentorium during childhood.

BACKGROUND AND PURPOSE: Neural structures in the posterior fossa grow at different rates during development. While there are computationally intensive approaches to analyze growth of the cerebellum and brainstem, there is a paucity of information about summary measures of normal posterior fossa development suitable for real-time clinical use. The present study investigates changes in the trajectory of the tentorium as measured by the occipital and tentorial angles at different stages of development. METHODS: A retrospective study was conducted drawing from a Boston Children's Hospital database of over 1500 magnetic resonance imaging (MRI) studies. The imaging study population included fetuses older than 20 gestational weeks and children between the ages of 0 and 10 years. Two parameters were measured for all subjects: (1) the tentorial angle (the angle between the tentorium and a line from the internal occipital protuberance to the tuberculum sellae) and (2) the occipital angle (the angle between the tentorium and a line from the internal occipital protuberance to the opisthion). Descriptive statistics were used to analyze the study cohort. RESULTS: We reviewed 1510 brain MRI studies, and 367 studies met the inclusion criteria (125 fetal and 242 postnatal studies). During fetal development, the inclination of the tentorium showed an ascending course, while it plateaus after birth. CONCLUSIONS: During the second and third trimesters, the tentorial and occipital angles steadily increase reflecting the dynamic growth of the posterior fossa structures. Postnatally, the tentorial angle decreases and the tentorium slopes downward and plateaus, possibly due to stabilization of posterior fossa development and ongoing growth of the cerebrum. Together, these findings suggest that the tentorial angle can serve as an imaging biomarker of posterior fossa development during the second half of fetal life.

Posterior fossa tumors in children: developmental anatomy and diagnostic imaging.

INTRODUCTION: Modern understanding of the relation between the mutated cancer stem cell and its site of origin and of its interaction with the tissue environment is enhancing the importance of developmental anatomy in the diagnostic assessment of posterior fossa tumors in children. The aim of this review is to show how MR imaging can improve on the exact identification of the tumors in the brainstem and in the vicinity of the fourth ventricle in children, using both structural imaging data and a precise topographical assessment guided by the developmental anatomy. RESULTS: The development of the hindbrain results from complex processes of brainstem segmentation, ventro-dorsal patterning, multiple germinative zones, and diverse migration pathways of the neural progenitors. Depending on their origin in the brainstem, gliomas may be infiltrative or not, as well as overwhelmingly malignant (pons), or mostly benign (cervicomedullary, medullo-pontine tegmental, gliomas of the cerebellar peduncles). In the vicinity of the fourth ventricles, the prognosis of the medulloblastomas (MB) correlates the molecular subtyping as well as the site of origin: WNT MB develop from the Wnt-expressing lower rhombic lip and have a good prognosis; SHH MB develop from the Shh-modulated cerebellar cortex with an intermediate prognosis (dependent on age); recurrences are local mostly. The poor prognosis group 3 MB is radiologically heterogeneous: some tumors present classic features but are juxtaventricular (rather than intraventricular); others have highly malignant features with a small principal tumor and an early dissemination. Group 4 MB has classic features, but characteristically usually does not enhance; dissemination is common. Although there is as yet no clear molecular subgrouping of the ependymomas, their sites of origin and their development can be clearly categorized, as most develop in an exophytic way from the ventricular surface of the medulla in clearly specific locations: the obex region with expansion in the cistern magna, or the lateral recess region with expansion in the CPA and prepontine cisterns (cerebellar ependymomas, and still more intra-brainstem ependymomas are rare). Finally, almost all cerebellar gliomas are pilocytic astrocytomas. CONCLUSIONS: A developmental and anatomic approach to the posterior fossa tumors in children (together with diffusion imaging data) provides a reliable pre-surgical identification of the tumor and of its aggressiveness.

Role of MYC in Medulloblastoma.

Since its discovery as an oncogene carried by the avian acute leukemia virus MC29 in myelocytomatosis (Roussel et al. 1979) and its cloning (Vennstrom et al. 1982), c-MYC (MYC), as well as its paralogs MYCN and MYCL1, has been shown to play essential roles in cycling progenitor cells born from proliferating zones during embryonic development, and in all proliferating cells after birth. MYC deletion induces cell-cycle exit or cell death, depending on the cell type and milieu, whereas MYC and MYCN amplification or overexpression promotes cell proliferation and occurs in many cancers. Here, we review the relationship of MYC family proteins to the four molecularly distinct medulloblastoma subgroups, discuss the possible roles MYC plays in each of these subgroups and in the developing cells of the posterior fossa, and speculate on possible therapeutic strategies targeting MYC.

Chiari malformation type I: a case-control association study of 58 developmental genes.

Chiari malformation type I (CMI) is a disorder characterized by hindbrain overcrowding into an underdeveloped posterior cranial fossa (PCF), often causing progressive neurological symptoms. The etiology of CMI remains unclear and is most likely multifactorial. A putative genetic contribution to CMI is suggested by familial aggregation and twin studies. Experimental models and human morphometric studies have suggested an underlying paraxial mesoderm insufficiency. We performed a case-control association study of 303 tag single nucleotide polymorphisms (SNP) across 58 candidate genes involved in early paraxial mesoderm development in a sample of 415 CMI patients and 524 sex-matched controls. A subgroup of patients diagnosed with classical, small-PCF CMI by means of MRI-based PCF morphometry (n = 186), underwent additional analysis. The genes selected are involved in signalling gradients occurring during segmental patterning of the occipital somites (FGF8, Wnt, and retinoic acid pathways and from bone morphogenetic proteins or BMP, Notch, Cdx and Hox pathways) or in placental angiogenesis, sclerotome development or CMI-associated syndromes. Single-marker analysis identified nominal associations with 18 SNPs in 14 genes (CDX1, FLT1, RARG, NKD2, MSGN1, RBPJ1, FGFR1, RDH10, NOG, RARA, LFNG, KDR, ALDH1A2, BMPR1A) considering the whole CMI sample. None of these overcame corrections for multiple comparisons, in contrast with four SNPs in CDX1, FLT1 and ALDH1A2 in the classical CMI group. Multiple marker analysis identified a risk haplotype for classical CMI in ALDH1A2 and CDX1. Furthermore, we analyzed the possible contributions of the most significantly associated SNPs to different PCF morphometric traits. These findings suggest that common variants in genes involved in somitogenesis and fetal vascular development may confer susceptibility to CMI.

Hyperpneumatization of the skull base: case report.

BACKGROUND AND IMPORTANCE: Temporal bone and skull base pneumatization is a naturally occurring process that begins before birth and continues into early adulthood. Occasionally this process surpasses normal limits, resulting in hyperpneumatization, which is usually obvious, but on rare occasions may mimic more aggressive skull base disorders. An awareness of this rare anatomical variant may help clinicians avoid more extensive investigations. CLINICAL PRESENTATION: We present the case of a 37-year-old man with severe headache and multiple, partially opacified lytic lesions in the skull base noted after minor head trauma. At presentation, a computed tomographic (CT) head scan revealed multiple lucent areas in the skull base after which magnetic resonance imaging (MRI) further suggested the diagnosis of an extensive lytic skull base process associated with a small clival fracture. Needle biopsy revealed nonspecific inflammation. An earlier head CT, not available at the time of admission, demonstrated extensive pneumatized air cells in the basiocciput. During the course of the 2-year follow-up, the originally pneumatized skull base was noted to become permanently opacified with areas of new bone growth. CONCLUSION: We concluded that the skull base abnormality was an anatomical variant associated with a clival fracture and hemorrhage, which led to opacification of the pneumatized air cells. No specific treatment was offered and symptoms resolved completely. Long-term follow-up CT demonstrated opacification of the skull base. This is one of very few cases in the literature reporting the clinical course of a patient with a hyperpneumatized skull base and the subsequent evolution of the disorder after minor head trauma.

Remodeling patterns of occipital growth: a preliminary report.

Occipital growth depends on coordinated deposition and resorption on the external and internal surface and includes interrelated processes of movement: cortical drift, displacement, and relocation. The current work aspires to map patterns of remodeling activity on the endocranial surface of the occipital bone from childhood to adulthood using a larger study sample compared with previous studies. The study sample consists of 5 adult and 10 immature (2(1/4) to 8 years old) occipital bones from skeletal remains from the eighteenth and nineteenth century. Preparation of the samples includes the elaboration of negative impressions, positive replicas coated with gold, and observed with the reflected light microscope. Cerebellar fossae are typically resorptive in both immature and adult specimens. Cerebral fossae, on the other hand, exhibit a resorptive surface in early childhood and turn into depository around the age of 7 years, which places this transition within the age interval of the completion of cerebral development. Depository fields are also observed in adult cerebral fossae. The remodeling map presented here is consistent with the results of Mowbray (Anat Rec B New Anat 2005;283B:14-22) and differs from cellular patterns described by Enlow. Future research implicating more elements of the neurocapsule can shed light on the factors affecting and driving occipital growth.

Posterior fossa morphometry in symptomatic pediatric and adult Chiari I malformation.

The linear dimensions and volume of the posterior fossa, and the length of the supra-occiput and the clivus in children with Chiari I malformations (CMI) were studied. A statistical relationship between patient demographics, radiological features, posterior fossa and occipital bone morphometry in the study and control groups was investigated. The results of 21 pediatric patients was compared to those of a matched control group. The posterior fossa volume (PFV) of an adult CMI group was also studied. Linear measurements were used to calculate the length of the occipital bone, spherical PFV and intracranial volume (ICV) using pre-operative MRI and CT scans. A PFV to ICV ratio was obtained to standardize the comparison. The volumetric measurement in the pediatric study group was also compared to the adult CMI group. The antero-posterior dimension, width, and volume of the posterior fossa and the PFV to ICV ratio in pediatric CMI patients was significantly lower than in the control group (p<0.05). There was no statistical difference in the length of the clivus and the supra-occiput between the two pediatric groups. The difference in the PFV to ICV ratio between pediatric and adult CMI patients was not statistically significant. Thus, the authors provide a simple, yet accurate, and reproducible method of comparison of posterior fossa volume in patients with CMI in different age groups, based on linear measurements. There is no significant difference between the length of the occipital bone at the base of the skull in pediatric CMI patients and the normal pediatric population. Development of the clivus due to late fusion of the sphenoid and occipital synchondroses in the second decade could result in manifestation of the disease in adulthood in CMI patients who were asymptomatic as children.

Spontaneous resolution of a 13-mm Chiari malformation Type I in relation to differential growth of the posterior fossa volume.

The case of a 3-year-old patient with tuberous sclerosis and a 13-mm Chiari malformation Type I that spontaneously disappeared over the course of 4 years is presented. Using morphometric measurements of the posterior fossa and cerebellum in this patient, the authors show that the volume of the posterior fossa at the time of initial evaluation was consistent with that reported as normal in the literature (180.24 cm3; normal volume 132-198 cm3). Moreover, the patient showed a normal rate of growth of his posterior fossa over the period of observation (201.05 cm3; normal range 153-230 cm3). Cerebellar volumes were found to increase only minimally during this time period, which is compatible with observations in healthy controls. The posterior fossa volume, on the other hand, was shown to increase significantly more than that of the cerebellum (p=0.0185). This differential growth may permit the tonsils to ascend back up into the posterior fossa. Therefore, pediatric patients with normal posterior fossa volumes and normal development may have a spontaneous resolution of their asymptomatic Chiari malformation Type I.

A quantitative and descriptive approach to morphological variation of the endocranial base in modern humans.

The cranial base is one of the major foci of interest in functional craniology. The evolution and morphogenesis of this structure are still poorly known and rather controversial because of multifactorial influences and polyphasic stages. Endocranial dynamics are associated anteriorly with the upper facial structures, laterally with the mandibular system and midsagittally with brain development. In the present study, we investigated the endocranial morphology of modern humans using 3D landmark-based approaches, i.e. geometric morphometrics and Euclidean distance matrix analysis. The structure of endocranial variation is poorly integrated, with only weak reciprocal influences among the three fossae. Some major variations are associated with changes in the posterior fossa, with possible consequences on the anterior areas. These main patterns of integration are hypothesized to be influenced by the connective tensors of the dura layers. Static allometry and sex differences are largely related to the ontogenetic sequences, characterized by early maturation of the anterior fossa with respect to the middle and posterior regions (i.e., relatively shorter posterior part of the planum sphenoideum and vertical lengthening of the clivus in males). The relative independence between the endocranial fossae, as well as their structural connection through the meningeal tensors, must be carefully considered in studies on the evolutionary dynamics, since they lead to mosaic changes through phylogeny.

Gender, head size and disease: a hypothesis related to posterior fossa growth.

This hypothesis proposes that the difference in head size between the genders has a bearing on disease incidence. Smaller head size predisposes to spina bifida and larger head size predisposes to autism. In the fetus, the posterior fossa has a regulatory influence on maximum intracerebral cerebrospinal fluid (CSF) pressure. CSF pressure pulsations are generated by movements in the chest and abdomen and transmitted to the intracranial CSF spaces via the posterior fossa. The efficiency of transmission of pressure to the head is influenced by the size of posterior fossa CSF spaces. The larger posterior fossa in the male fetus allows higher peaks of pressure in the lateral ventricles than the female, resulting in larger head size. Restriction of posterior fossa growth has a greater influence on females than males. This leads to a greater incidence of spina bifida in females, where reduced head size is characteristic in the fetal stages. Fetal surgery for spina bifida expands posterior fossa CSF spaces and improves transmission of pressure waves generated in the fetal abdomen and thorax into the head. This results in an increase in head circumference following surgery. Males are susceptible to autism where head growth is accelerated in early childhood. This is because they have a larger average head size.

Posterior fossa dimension and volume estimates in pediatric patients with Chiari I malformations.

INTRODUCTION: Chiari I malformations (CMI) involve pathological hindbrain abnormalities reported to be correlated with a hypoplastic posterior fossa. CMI was traditionally characterized by the downward herniation of the cerebellar tonsils with a descent of 5 mm or more below the foramen magnum. The fullness of the cisterna magna and CSF flow at the level of the cervicomedullary junction have been shown to be more useful in selecting symptomatic patients for surgical decompression. The present study calculates posterior fossa dimension and volume estimates in pediatric patients using magnetic resonance imaging. The combination of neuroradiological and clinical findings is used to re-examine the criteria used for diagnosis and treatment of pediatric CMI patients. MATERIALS AND METHODS: A retrospective chart review was conducted on patients who were admitted to the Division of Neurosurgery of the Children's hospital of Eastern Ontario between 1990 and 2007. Clinical and radiological assessments were performed on all patients. Posterior fossa volumes (PFV) and intracranial volumes (ICV) were measured from sagittal head magnetic resonance imaging scans using the Cavalieri method. RESULTS: Sixty-one CMI patients were identified. There were 32 male and 29 female patients with a mean age of 10 years (range: 8 weeks-18 years). Thirty-four (55%) of these patients were symptomatic with scoliosis (38%), suboccipital headaches (29%), and motor/sensory deficits (26%) being the most prominent symptoms. The mean PFV/ICV ratio for all the CMI patients (0.110) was found to be statistically smaller than that of the control patients (0.127, p=0.022). Mean PFV/ICV ratios for asymptomatic and symptomatic CMI patients were found to be similar for children aged 0-9 years (p=0.783) but different for children aged 10-18 years (p=0.018). DISCUSSION: Mean PFV values were found to be smaller in pediatric CMI patients than control patients; this complements earlier studies in adults and supports the present theory concerning the pathophysiological mechanism of CMI. Subtle morphometric differences among asymptomatic and symptomatic patients aged 0-9 years stress the importance of monitoring asymptomatic patients for the onset of symptoms in their adult years. Symptom development in CMI is likely multifactorial and is much more extensive than the degree of cerebellar tonsillar herniation.

A pathological approach to anomalies of the posterior fossa.

BACKGROUND: Both clinical and postmortem diagnoses of posterior fossa anomalies remain difficult to make and to corroborate. This is particularly true for Dandy-Walker malformation and variant. Difficulties arise for a variety of reasons, including technical and methodological ones, but also because the conditions may overlap anatomically with others, most notably mega cisterna magna, Blake's pouch cyst, and posterior fossa (arachnoid) cysts. Family counseling is difficult and complicated not only by diagnostic uncertainties but by the highly variable prognosis. METHODS: In this study, a systematic pathologic approach to study of the posterior fossa is put forth. The benefits of postmortem imaging and in situ and ex situ examination of posterior fossa contents are demonstrated. RESULTS: Normative data for cerebellar width (transverse cerebellar diameter) and height and vermian width and height are derived for the fetal period. These data will help workers recognize changes in these structures, particularly hypoplasia. CONCLUSIONS: Basic morphologic definitions of posterior fossa anomalies are advanced, in the hopes that better agreement can be reached between clinical and pathologic diagnoses and that better patient and family care will result.

[Chiari type 1 malformation and magnetic resonance imaging]. / La malformation de Chiari de type 1 à l'ère de l'imagerie par résonance magnétique.

The Chiari type 1 malformation is common. Unlike the Chiari type 2 and 3 malformations, it may remain latent for a long time, becoming symptomatic only in adulthood. The introduction of MRI has resulted in an increased number of diagnoses of this malformation in pediatric patients. It appears to be related to underdevelopment of the posterior cranial fossa. It must be differentiated from acquired tonsillar herniation, particularly when herniation results from intracranial hypotension; these cases are sometimes reported as acquired Chiari I malformation with spontaneous resolution. Tonsillar ectopia may cause symptoms by its direct effect on any or all of the medulla and the cerebellar and upper spinal cord. The most suggestive of the oculomotor disturbances is oscillopsia with downbeat nystagmus. Dysphonia and dysphagia are common. Potentially serious autonomic disturbances are also frequent: sleep apnea, respiratory failure, syncope and even sudden death. Another risk is syrinx formation, resulting from obstruction of CSF circulation in the cisterna magna. Syringomyelia is detected in 32 to 74% of patients with Chiari I malformation. Treatment is surgical. Posterior fossa decompression is achieved by suboccipital craniectomy combined with laminectomy of the upper cervical segments. Surgical intervention is indicated when the malformation is symptomatic and there is no doubt that it is the cause of the symptoms. When a Chiari I malformation is identified fortuitously on MRI, long-term monitoring is essential. The risk of developing symptoms increases over time. Patients should be advised not to participate in contact sports.

Postnatal changes of the clivus.

We studied the size of the clival region (length, widest and narrowest diameter, and distance between two jugular tuberculi) in 77 skull bases or occipital and sphenoid bones from skeletons aged 3-80 years. The comparison of the measurement points among the age groups (3-10, 11-25, 26-60, and 61-80 years) showed that the final length of the clivus was reached by 11 years of life in both men and women, and then remained constant throughout life. All other measurements did not differ among the age groups. Also, only the length of the clivus strongly correlated with age (Pearson's r = 0.336, P = 0.0028). It seems that the postnatal age up to 11 years of life is the crucial time of the development of the clivus, when the final adult width of the clivus is first reached, followed by the finalization of its growth in length.

Differential growth rates of the cerebellum and posterior fossa assessed by post mortem magnetic resonance imaging of the fetus: implications for the pathogenesis of the chiari 2 deformity.

PURPOSE: To evaluate post mortem magnetic resonance imaging of the fetus to provide data on the rate of growth of the cerebellum, bony posterior fossa, supratentorial bony compartment and cerebrum. MATERIAL AND METHODS: Twenty fetuses subsequently shown to have normal brain and spines on autopsy were studied using MRI post mortem. MRI from 20 normal pediatric brain examinations in children aged 6 years or younger were studied for comparison. Post mortem MRI was performed using a high-resolution fast spin echo technique providing T2 weighted images. The area of the cerebellar vermis, posterior fossa, supratentorial skull cavity and cerebral hemispheres was measured in the sagittal plane in all cases. These measurements were compared over the age ranges studied. RESULTS: We have shown that there are differences in the rate of growth and the apparent commencement of growth between the structures under study. The cerebellum appeared to start its significant growth at 16.5 weeks with a rate of 16 mm2/week throughout pregnancy, while the cerebral hemisphere appeared to commence significant growth at 13 weeks at a rate of 184 mm2/week throughout pregnancy. This is in contrast to the bony posterior fossa, whose growth paralleled the supratentorial bony compartment at all ages studied. CONCLUSION: We interpret our findings as showing relatively late commencement of cerebellar growth compared to the cerebral hemispheres, whereas the growth of the bony posterior fossa appears to be in advance and independent of cerebellar growth. Our results support the hypothesis that posterior fossa development depends on raised hydrostatic pressure in the CSF containing structures of the posterior fossa. When this mechanism fails, as in some cases of myelomeningocoele, a small posterior fossa is formed which characterizes the Chiari 2 malformation.