Malformations of the posterior fossa: current perspectives

Fetal and neonatal outcomes of posterior fossa anomalies: a retrospective cohort study

The primary aim of this study was to estimate the incidence of posterior fossa anomalies (PFA) and assess the associated outcomes in King Abdulaziz Medical City (KAMC), Riyadh. All fetuses diagnosed by prenatal ultrasound with PFA from 2017 to 2021 in KAMC were analyzed retrospectively. PFA included Dandy-Walker malformation (DWM), mega cisterna magna (MCM), Blake's pouch cyst (BPC), and isolated vermian hypoplasia (VH). The 65 cases of PFA were 41.5% DWM, 46.2% MCM, 10.8% VH, and 1.5% BPC. The annual incidence rates were 2.48, 2.64, 4.41, 8.75, and 1.71 per 1000 anatomy scans for 2017, 2018, 2019, 2020, and 2021, respectively. Infants with DWM appeared to have a higher proportion of associated central nervous system (CNS) abnormalities (70.4% vs. 39.5%; p-value = 0.014) and seizures than others (45% vs. 17.9%; p-value = 0.041). Ten patients with abnormal genetic testing showed a single gene mutation causing CNS abnormalities, including a pathogenic variant in MPL, C5orf42, ISPD, PDHA1, PNPLA8, JAM3, COL18A1, and a variant of uncertain significance in the PNPLA8 gene. Our result showed that the most common PFA is DWM and MCM. The autosomal recessive pathogenic mutation is the major cause of genetic disease in Saudi patients diagnosed with PFA.

Chiari 1 malformation and exome sequencing in 51 trios: the emerging role of rare missense variants in chromatin-remodeling genes

Type 1 Chiari malformation (C1M) is characterized by cerebellar tonsillar herniation of 3–5 mm or more, the frequency of which is presumably much higher than one in 1000 births, as previously believed. Its etiology remains undefined, although a genetic basis is strongly supported by C1M presence in numerous genetic syndromes associated with different genes. Whole-exome sequencing (WES) in 51 between isolated and syndromic pediatric cases and their relatives was performed after confirmation of the defect by brain magnetic resonance image (MRI). Moreover, in all the cases showing an inherited candidate variant, brain MRI was performed in both parents and not only in the carrier one to investigate whether the defect segregated with the variant. More than half of the variants were Missense and belonged to the same chromatin-remodeling genes whose protein truncation variants are associated with severe neurodevelopmental syndromes. In the remaining cases, variants have been detected in genes with a role in cranial bone sutures, microcephaly, neural tube defects, and RASopathy. This study shows that the frequency of C1M is widely underestimated, in fact many of the variants, in particular those in the chromatin-remodeling genes, were inherited from a parent with C1M, either asymptomatic or with mild symptoms. In addition, C1M is a Mendelian trait, in most cases inherited as dominant. Finally, we demonstrate that modifications of the genes that regulate chromatin architecture can cause localized anatomical alterations, with symptoms of varying degrees.

The developing brain by trimester

Transient anatomical entities play a role in the maturation of brain regions and early functional fetal networks. At the postmenstrual age of 7 weeks, major subdivisions of the brain are visible. At the end of the embryonic period, the cortical plate covers the neopallium. The choroid plexus develops in concert with it, and the dorsal thalamus covers about half the diencephalic third ventricle surface. In addition to the fourth ventricle neuroepithelium the rhombic lips are an active neuroepithelial production site. Early reciprocal connections between the thalamus and cortex are present. The corticospinal tract has reached the pyramidal decussation, and the arteries forming the mature circle of Willis are seen. Moreover, the superior sagittal sinus has formed, and at the rostral neuropore the massa commissuralis is growing. At the viable preterm age of around 24 weeks PMA, white matter tracts are in full development. Asymmetric progenitor division permits production of neurons, subventricular zone precursors, and glial cells. Myelin is present in the ventral spinal quadrant, cuneate fascicle, and spinal motor fibers. The neopallial mantle has been separated into transient layers (stratified transitional fields) between the neuroepithelium and the cortical plate. The subplate plays an important role in organizing the structuring of the cortical plate. Commissural tracts have shaped the corpus callosum, early primary gyri are present, and opercularization has started caudally, forming the lateral fissure. Thalamic and striatal nuclei have formed, although GABAergic neurons continue to migrate into the thalamus from the corpus gangliothalamicum. Near-term PMA cerebral sublobulation is active. Between 24 and 32 weeks, primary sulci develop. Myelin is present in the superior cerebellar peduncle, rubrospinal tract, and inferior olive. Germinal matrix disappears from the telencephalon, except for the GABAergic frontal cortical subventricular neuroepithelium.

[Dandy-Walker malformation]

The Dandy-Walker malformation is the most frequent cerebral malformation. It is defined by hypoplasia and upward rotation of the vermis cerebelli, a cystic enlargement of the fourth ventricle and in total an enlarged posterior fossa with cranially shifted position of the lateral sinus, tentorium and torcula herophili. This malformation was first described by Dandy and Blackfan in 1914 then supplemented again by Taggart and Walker in 1942. The current description as Dandy-Walker malformation was introduced in 1954 by Bender. In addition to these classical findings, the Dandy-Walker malformation is characterized by other abnormalities and malformations of the central nervous system (CNS) including agenesis of the corpus callosum, heterotopia, occipital meningocele, visual deficits and epilepsy. Neurogenetic and imaging examinations have led to a better understanding of this malformation.

Post-mortem magnetic resonance (PMMR) imaging of the brain in fetuses and children with histopathological correlation

Post-mortem magnetic resonance (PMMR) imaging is rapidly emerging as an alternative, "less invasive", and more widely accepted investigative approach for perinatal deaths in the UK. PMMR has a high diagnostic accuracy for congenital and acquired fetal neuropathological anomalies compared to conventional autopsy, and is particularly useful when autopsy is non-diagnostic. The main objectives of this review are to describe and illustrate the range of common normal and abnormal central nervous system (CNS) findings encountered during PMMR investigation. This article covers the standard PMMR sequences used at our institution, normal physiological post-mortem findings, and a range of abnormal developmental and acquired conditions. The abnormal findings include diseases ranging from neural tube defects, posterior fossa malformations, those of forebrain and commissural development as well as neoplastic, haemorrhagic, and infectious aetiologies. Neuropathological findings at conventional autopsy accompany many of the conditions we describe, allowing readers to better understand the underlying disease processes and imaging appearances.

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.

Inferior cerebellar hypoplasia resembling a Dandy-Walker-like malformation in purebred Eurasier dogs with familial non-progressive ataxia: a retrospective and prospective clinical cohort study

Cerebellar malformations can be inherited or caused by insults during cerebellar development. To date, only sporadic cases of cerebellar malformations have been reported in dogs, and the genetic background has remained obscure. Therefore, this study`s objective was to describe the clinical characteristics, imaging features and pedigree data of a familial cerebellar hypoplasia in purebred Eurasier dogs. A uniform cerebellar malformation characterized by consistent absence of the caudal portions of the cerebellar vermis and, to a lesser degree, the caudal portions of the cerebellar hemispheres in association with large retrocerebellar fluid accumulations was recognized in 14 closely related Eurasier dogs. Hydrocephalus was an additional feature in some dogs. All dogs displayed non-progressive ataxia, which had already been noted when the dogs were 5 – 6 weeks old. The severity of the ataxia varied between dogs, from mild truncal sway, subtle dysmetric gait, dysequilibrium and pelvic limb ataxia to severe cerebellar ataxia in puppies and episodic falling or rolling. Follow-up examinations in adult dogs showed improvement of the cerebellar ataxia and a still absent menace response. Epileptic seizures occurred in some dogs. The association of partial vermis agenesis with an enlarged fourth ventricle and an enlarged caudal (posterior) fossa resembled a Dandy-Walker-like malformation in some dogs. Pedigree analyses were consistent with autosomal recessive inheritance.

Midbrain-hindbrain malformations in patients with malformations of cortical development and epilepsy: a series of 220 patients

Midbrain-hindbrain malformations (MHM) may coexist with malformations of cortical development (MCD). This study represents a first attempt to investigate the spectrum of MHM in a large series of patients with MCD and epilepsy. We aimed to explore specific associations between MCD and MHM and to compare two groups of patients: MCD with MHM (wMHM) and MCD without MHM (w/oMHM) with regard to clinical and imaging features. Two hundred and twenty patients (116 women/104 men, median age 28 years, interquartile range 20-44 years at the time of assessment) with MCD and epilepsy were identified at the Departments of Neurology and Pediatrics, Innsbruck Medical University, Austria. All underwent high-resolution MRIs (1.5-T) between 01.01.2002 and 31.12.2011. Midbrain-hindbrain structures were visually assessed by three independent raters. MHM were seen in 17% (38/220) of patients. The rate of patients wMHM and w/oMHM differed significantly (p=0.004) in three categories of MCD (category I - to abnormal neuronal proliferation; category II - to abnormal neuronal migration; and category III - due to abnormal neuronal late migration/organization): MCD due to abnormal neuronal migration (31%) and organization (23%) were more commonly associated with MHM compared to those with MCD due to abnormal neuronal proliferation (9%). Extensive bilateral MCD were seen more often in patients wMHM compared to those w/oMHM (63% vs. 36%; p=0.004). In wMHM group compared to w/oMHM group there were higher rates of callosal dysgenesis (26% vs. 4%; p<0.001) and hippocampal abnormalities (52% vs. 27%; p<0.001). Patients wMHM were younger (median 25 years vs. 30 years; p=0.010) at the time of assessment and had seizure onset at an earlier age (median 5 years vs. 12 years; p=0.043) compared to those w/oMHM. Patients wMHM had higher rates of learning disability (71% vs. 38%; p<0.001), delayed developmental milestones (68% vs. 35%; p<0.001) and neurological deficits (66% vs. 47%; p=0.049) compared to those w/oMHM. The groups (wMHM and w/oMHM) did not differ in their response to antiepileptic treatment, seizure outcome, seizure types, EEG abnormalities and rate of status epilepticus. Presence of MHM in patients with MCD and epilepsy is associated with severe morphological and clinical phenotypes.

Neurodevelopmental abnormalities in children with PHACE syndrome

Prior case reports have identified neurodevelopmental abnormalities in children with PHACE syndrome, a neurocutaneous disorder first characterized in 1996. In this multicenter, retrospective study of a previously identified cohort of 93 children diagnosed with PHACE syndrome from 1999 to 2010, 29 children had neurologic evaluations at ≥ 1 year of age (median age: 4 years, 2 months). In all, 44% had language delay, 36% gross motor delay, and 8% fine motor delay; 52% had an abnormal neurological exam, with speech abnormalities as the most common finding. Overall, 20 of 29 (69%) had neurodevelopmental abnormalities. Cerebral, but not posterior fossa, structural abnormalities were identified more often in children with abnormal versus normal neurodevelopmental outcomes (35% vs. 0%, P = .04). Neurodevelopmental abnormalities in young children with PHACE syndrome referred to neurologists include language and gross motor delay, while fine motor delay is less frequent. Prospective studies are needed to understand long-term neurodevelopmental outcomes.

Dominantly inherited nonprogressive cerebellar hypoplasia identified in utero

Cerebellar hypoplasia is the hallmark of a heterogeneous group of disorders that are caused by genetic and metabolic disorders. Prenatal identification of cerebellar hypoplasia and accurate prediction of outcome are challenging. Autosomal dominant nonprogressive cerebellar ataxia is a rare disorder that typically presents with early hypotonia and delayed motor milestones followed by the onset of mild ataxia and occasionally cognitive impairment. We present a case of a mother and her female fetus. Fetal sonography and magnetic resonance imaging (MRI) showed generalized cerebellar hypoplasia. The mother had mild learning difficulties and clinically showed minor features of cerebellar ataxia. Her MRI also demonstrated extreme cerebellar hypoplasia. The diagnosis of autosomal dominant nonprogressive cerebellar ataxia was suggested. This is the first report of prenatal diagnosis of autosomal dominant nonprogressive cerebellar ataxia. We recommend obtaining a family history, examining the parents, and when appropriate obtaining an MRI before counseling parents of a fetus with a brain malformation.

Cochlear implantation in patient with Dandy-walker syndrome

INTRODUCTION

 Dandy Walker Syndrome is a congenital abnormality in the central nervous system, characterized by a deficiency in the development of middle cerebelar structures, cystic dilatation of the posterior pit communicating with the fourth ventricle and upward shift of the transverse sinuses, tentorium and dyes. Among the clinical signs are occipital protuberances, a progressive increase of the skull, bowing before the fontanels, papilledema, ataxia, gait disturbances, nystagmus, and intellectual impairment.

OBJECTIVES

 To describe a case of female patient, 13 years old with a diagnosis of this syndrome and bilateral hearing loss underwent cochlear implant surgery under local anesthesia and sedation.

CASE REPORT

 CGS, 13 years old female was referred to the Otolaryngological Department of Otolaryngology Institute of Parana with a diagnosis of "Dandy-Walker syndrome" for Otolaryngological evaluation for bilateral hearing loss with no response to the use of hearing aids. Final Comments: The field of cochlear implants is growing rapidly. We believe that the presence of Dandy-Walker syndrome cannot be considered a contraindication to the performance of cochlear implant surgery, and there were no surgical complications due to neurological disorders with very favorable results for the patient who exhibits excellent discrimination. It has less need for lip reading with improvement in speech quality.

Neuroimaging of Dandy-Walker malformation: new concepts

Dandy-Walker malformation (DWM) is the most common human cerebellar malformation, characterized by hypoplasia of the cerebellar vermis, cystic dilation of the fourth ventricle, and an enlarged posterior fossa with upward displacement of the lateral sinuses, tentorium, and torcular. Although its pathogenesis is not completely understood, there are several genetic loci related to DWM as well as syndromic malformations and congenital infections. Dandy-Walker malformation is associated with other central nervous system abnormalities, including dysgenesis of corpus callosum, ectopic brain tissue, holoprosencephaly, and neural tube defects. Hydrocephalus plays an important role in the development of symptoms and neurological outcome in patients with DWM, and the aim of surgical treatment is usually the control of hydrocephalus and the posterior fossa cyst. Imaging modalities, especially magnetic resonance imaging, are crucial for the diagnosis of DWM and distinguishing this disorder from other cystic posterior fossa lesions. Persistent Blake's cyst is seen as a retrocerebellar fluid collection with cerebrospinal fluid signal intensity and a median line communication with the fourth ventricle, commonly associated with hydrocephalus. Mega cisterna magna presents as an extraaxial fluid collection posteroinferior to an intact cerebellum. Retrocerebellar arachnoid cysts frequently compress the cerebellar hemispheres and the fourth ventricle. Patients with DWM show an enlarged posterior fossa filled with a cystic structure that communicates freely with the fourth ventricle and hypoplastic vermis. Comprehension of hindbrain embryology is of utmost importance for understanding the cerebellar malformations, including DWM, and other related entities.

Species differences in the expression of Ahi1, a protein implicated in the neurodevelopmental disorder Joubert syndrome, with preferential accumulation to stigmoid bodies

Joubert syndrome (JBTS) is an autosomal recessive disorder characterized by cerebellum and brainstem malformations. Individuals with JBTS have abnormal breathing and eye movements, ataxia, hypotonia, and cognitive difficulty, and they display mirror movements. Mutations in the Abelson-helper integration site-1 gene (AHI1) cause JBTS in humans, suggesting that AHI1 is required for hindbrain development; however AHI1 may also be required for neuronal function. Support for this idea comes from studies demonstrating that the AHI1 locus is associated with schizophrenia. To gain further insight into the function of AHI1 in both the developing and mature central nervous system, we determined the spatial and temporal expression patterns of the gene products of AHI1 orthologs throughout development, in human, mouse, and zebrafish. Murine Ahi1 was distributed throughout the cytoplasm, dendrites, and axons of neurons, but was absent in glial cells. Ahi1 expression in the mouse brain was observed as early as embryonic day 10.5 and persisted into adulthood, with peak expression during the first postnatal week. Murine Ahi1 was observed in neurons of the hindbrain, midbrain, and ventral forebrain. Generally, the AHI1/Ahi1/ahi1 orthologs had a conserved distribution pattern in human, mouse, and zebrafish, but mouse Ahi1 was not present in the developing and mature cerebellum. Ahi1 was also observed consistently in the stigmoid body, a poorly characterized cytoplasmic organelle found in neurons. Overall, these results suggest roles for AHI1 in neurodevelopmental processes that underlie most of the neuroanatomical defects in JBTS, and perhaps in neuronal functions that contribute to schizophrenia.

Rhombencephalosynapsis: association with single umbilical artery

A 6-year-old girl who presented with developmental delay and non-progressive ataxia is described. MRI of brain showed agenesis of cerebellar vermis with fusion of cerebellar hemispheres and dentate nuclei. MRI findings were characteristic of rhombencephalosynapsis. Partial agenesis of corpus callosum and absent septum pellucidum were also seen. The child had also been noted to have a single umbilical artery at birth: a hitherto undescribed association.

Nomogram of the cerebellar vermis height at birth in small-for-gestational-age neonates

AIM

To establish a nomogram of the height of the cerebellar vermis at birth in small-for-gestational-age neonates.

METHODS

Eighty-five consecutively born healthy singletons who were small for gestational age were enrolled in a cross-sectional observational study. The height of the cerebellar vermis was measured by ultrasonography in the midsagittal plane. Reference intervals for the vermian height at 5th, 50th and 95th centiles were estimated at any week of gestational age using a mean and standard deviation model based on least squares polynomial regression.

RESULTS

All attempts at obtaining neonatal vermian height were successful. Mean (standard deviation) maternal age was 29.6 (4.7) years; median parity was 1 (range, 1-3). No difference was found between males and females for the vermian height (mean difference [95% confidence interval], -0.03 [-0.16; 0.10] cm). Mean vermian height (in cm) against gestational age (in weeks) was suitably modelled by a quadratic-cubic polynomial (r2= 0.790, p < 0.0001) as 0.712 + 0.00129 x gestational age2- 0.0000053 x gestational age3 (SD =-1.009 + 0.0504 x gestational age -0.000014 x gestational age3).

CONCLUSION

A nomogram of the height of the cerebellar vermis at birth against gestational age was established for use in small-for-gestational-age neonates.

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.

The Fetal Posterior Fossa

OBJECTIVES

Recognize posterior fossa anomalies on ultrasound and magnetic resonance imaging and appreciate imaging pitfalls that may lead to misdiagnosis.

MATERIALS AND METHODS

Cases are presented to illustrate normal development and various anomalies. Postnatal studies and autopsy are used for correlation with prenatal imaging.

RESULTS

Normal anatomy and anomalies are demonstrated. Pitfalls such as cystic hygroma, pseudomasses, and use of nonstandard scan planes are illustrated.

CONCLUSIONS

Recognizing normal developing structures is an important component of performing fetal ultrasound. Documentation of the cerebellum, vermis, and cisterna magna are required for posterior fossa evaluation in any American Institute of Ultrasound in Medicine-certified practice. Normal variations are common, and understanding the anatomy is vital to avoid misdiagnosis and to accurately characterize abnormalities.

Nomograms of the axial fetal cerebellar hemisphere circumference and area throughout gestation

OBJECTIVE

The widely applied transcerebellar diameter (TCD) obtained at axial cranial imaging, measures the distance between the lateral aspects of the cerebellum and incorporates the width of the cerebellar vermis. Our objective was to create reference ranges of axial fetal cerebellar hemisphere circumference (CHC) and area (CHA), independent of the cerebellar vermis, throughout gestation.

METHODS

This cross-sectional study involved pregnant patients between 14 and 41 weeks of gestation. Inclusion criteria consisted of well-established dates (confirmed by early ultrasound), non-anomalous singleton fetuses and intact amniotic membranes. Sonographic measurements included biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL), humerus length (HL), TCD, and estimated fetal weight (EFW). Values of axial fetal CHC and CHA were each calculated as the mean of three separate measurements. The 5th, 50th and 95th centiles were estimated at each week of gestational age (GA) by least-squares regression for the mean and standard deviation (SD) of the CHC and CHA as functions of GA. r2 and associated P-values for the relationships of CHC and CHA with other sonographic biometric measurements were calculated.

RESULTS

The study included 651 consecutive patients. All attempts at obtaining axial fetal CHC and CHA were successful. Mean maternal age was 27.3+/-6.7 years, median gravidity was 1 (range 1-16), and median parity was 1 (range 0-6). Mean CHC (cm) throughout gestation was modeled as -2.091+0.2563xGA (weeks) (SD=-0.075+0.0164xGA), and mean CHA (cm2) was modeled as 0.245-0.0765xGA+0.00506xGA2 (SD=1.167-0.1565xGA+0.006785xGA(2)-0.00008028xGA3). Fetal axial CHC and CHA correlated significantly and strongly with BPD, HC, AC, HL, FL, TCD and EFW (all R2 values were >or=0.95, and all P-values were <0.001).

CONCLUSION

Nomograms of axial fetal cerebellar hemisphere circumference and area throughout gestation, independent of the cerebellar vermis, have been provided.

Disorders of cerebellar growth and development

PURPOSE OF REVIEW

This review summarizes for the pediatrician the current understanding of normal cerebellar and brainstem development, and then discusses selected malformations to highlight advances in the area. The impact of prematurity on cerebellar growth and development is then examined. The important insights provided by recent neuroimaging and genetic advances are reviewed.

RECENT FINDINGS

Previous areas of dispute are being addressed by advances in two major areas. Advanced neuroimaging studies during fetal and postnatal life are now providing important insights into the nature of normal and abnormal development of the brainstem and cerebellum. These powerful new techniques for defining morphology in vivo, together with major advances in genetics, are accelerating our understanding of genotype-phenotype relationships. Conversely, the ability to link early brain injury to subsequent cerebellar development has challenged previous understanding of the distinction between acquired and primary dysgenesis, presumed to be genetic in origin.

SUMMARY

The synthesis of a rational and clinically useful classification of posterior fossa malformations has been elusive. Recent developments promise to resolve ongoing disputes that have delayed progress. However, these insights into disturbed structural development demand rigorous examination of their long-term functional significance and caution before their prognostic significance is applied clinically.

An unusual combination: Aortic arch coarctation associated with Dandy–Walker variant

We report on a 29-year-old woman with aortic arch coarctation with cerebral anomalies including posterior fossa cyst, hydrocephalus, cerebellar vermis hypoplasia and multiple congenital anomalies including hirsutism, hipotelorism, shortened philtrum, unregulated teeth and short alveolar crest, rotated auricles, short and webbed neck, hypopigmentation on the scalp, bilateral clinodactyly, bilateral hallux valgus, brachydactyly on the left foot 3rd finger, hemangioma on the sacrum. An extra-anatomical bypass was made by grafting from the ascending to the distal descending aorta. Some of these features are consistent with the diagnosis of the other clinical syndromes except genetical expression, no chromosomal deletions in our patient with normal familial pedigree, however, cerebral anomalies are consistent with the Dandy-Walker variant. To the best of our knowledge, literature contains no other report of the association of aortic coarctation, Dandy-Walker variant with these clinical features. These previously undescribed combinations, however, raise the possibility of a newly recognized disorder.

La malformation de Dandy-Walker

The aim of this review is to summarize our knowledge about Dandy-Walker malformation (DWM) and introduce recent notions about its prognosis. DWM is a malformation associating hypoplasia of the vermis, pseudocystic fourth ventricle, upward displacement of the tentorium, torcular and lateral sinuses and anterio-posterior enlargement of the posterior fossa. It is frequently associated with genetic anomalies, brain malformations (anomalies of gyration, grey matter heterotopias, meningoceles, corpus callosum agenesis...) or systemic malformations (heart, orthopedic, intestinal, urogenital and facial anomalies). It is also part of many syndromes. Its rarity, the difficulty of its diagnosis, the fact that the malformation is mainly known through hospitalized patients, mainly from neurosurgical departments, have made its definition variable and prevents us from having an accurate knowledge of its natural history and prognosis. Hydrocephalus, so frequent in neurosurgical series that some have included it in the definition, is actually rare as has been seen in prenatal series. Treatment, when necessary, is still controversial, the main options remaining cyst fenestration, ventriculo- and/or cystoperitoneal shunts and more recently endoscopic third ventriculostomy. The prognosis, commonly said to be unpredictable, is actually foreseeable after scrutinous observation of the brain anatomy, and mainly of the vermis. A cerebellar vermis with three groups of lobes and two main fissures, as identified on MRI T2 sagittal views, not only has the greatest chance to not be associated with other malformation but also to have a favorable neurocognitive outcome. On the contrary, a deeply dysgenetic vermis with only two or one recognizable lobes is not only constantly associated with other brain malformation but also with poor prognosis. This is a recent advance that may be important for those involved in prenatal counselling.

Molar tooth sign in fetal brain magnetic resonance imaging leading to the prenatal diagnosis of Joubert syndrome and related disorders

Joubert syndrome is a rare autosomal recessive disorder characterized by ataxia, developmental delay, and oculomotor and respiratory abnormalities in relation to cerebellar vermian and midbrain dysgenesis. The midbrain dysgenesis is responsible for the molar tooth sign on axial magnetic resonance imaging (MRI). This classic hallmark of Joubert syndrome has been identified in other disorders sharing overlapping clinical and radiologic features with Joubert syndrome. Recent identification of two different genes points to genetic heterogeneity in this group of disorders, now entitled Joubert syndrome and related disorders, making a genetic prenatal diagnosis not readily available. In addition, fetal ultrasonography lacks sensitivity in regard to posterior fossa malformation. Fetal MRI is now acknowledged as the method of choice to delineate posterior fossa malformation in a fetus. The identification of a molar tooth sign has, however, rarely been documented by a fetal brain MRI. We report a case of Joubert syndrome diagnosed prenatally using fetal MRI. We also discuss the etiology of Joubert syndrome in view of the recent genetic advances and murine models of cerebellar dysgenesis.

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.

Diagnostic approach to cerebellar disease in children

We reviewed the clinical records of 51 extensively investigated pediatric patients with structural abnormalities of the cerebellum as revealed by magnetic resonance imaging (MRI). Ten had hypoplasia of the vermis, 21 had hypoplasia of the vermis and cerebellar hemispheres, 2 had pontocerebellar hypoplasia, and 18 had progressive cerebellar atrophy. A clear diagnosis was reached in 37 (72.5%). Initial characterization of the cerebellar alterations by MRI separated hypoplastic from atrophic cases and confirmed MRI as an essential preliminary means for distinguishing malformations from metabolic-degenerative conditions. However, the diagnostic possibilities are so numerous that it is not feasible to propose a standardized diagnostic protocol for pediatric patients with an altered cerebellum. Subsequent investigations should be suggested by the neuroradiologic and clinical peculiarities of each case.

Prenatal diagnosis of herniated Dandy-Walker cysts

OBJECTIVE

The purpose of this series is to describe the prenatal diagnosis and pregnancy outcome of fetuses affected with Dandy-Walker malformation in which a posterior cyst herniated through a bony defect of the occipital skull, foramen magnum, or both.

METHODS

Two- and 3-dimensional sonography were used to examine 2 fetuses with poorly delineated cerebellar structures and a large posterior cystic neck mass. Fetal magnetic resonance imaging (MRI) was added to this evaluation as a complementary diagnostic modality.

RESULTS

Three-dimensional sonography helped characterize the precise site of cyst herniation through the occipital skull or foramen magnum. Fetal MRI confirmed the sonographic findings. Neonatal MRI studies identified heterotopic gray matter as evidence of a neuronal migration disorder in both fetuses. The second fetus also had agenesis of the corpus callosum. Retrospective review of the fetal MRI (25.9 weeks' menstrual age) and 3-dimensional sonographic (18.7 weeks' menstrual age) studies confirmed ventricular wall nodularity involving the occipital horns of the second fetus.

CONCLUSIONS

The antenatal detection of a large posterior cystic neck mass and a poorly defined or nonvisualized cerebellar vermis suggest Dandy-Walker malformation with a herniated cyst. Three-dimensional sonography and fetal MRI are important adjunctive methods that can be used to evaluate the herniation site and a possible neuronal migrational disorder.

Abnormal cerebellar development and axonal decussation due to mutations in AHI1 in Joubert syndrome

Joubert syndrome is a congenital brain malformation of the cerebellar vermis and brainstem with abnormalities of axonal decussation (crossing in the brain) affecting the corticospinal tract and superior cerebellar peduncles. Individuals with Joubert syndrome have motor and behavioral abnormalities, including an inability to walk due to severe clumsiness and 'mirror' movements, and cognitive and behavioral disturbances. Here we identified a locus associated with Joubert syndrome, JBTS3, on chromosome 6q23.2-q23.3 and found three deleterious mutations in AHI1, the first gene to be associated with Joubert syndrome. AHI1 is most highly expressed in brain, particularly in neurons that give rise to the crossing axons of the corticospinal tract and superior cerebellar peduncles. Comparative genetic analysis of AHI1 indicates that it has undergone positive evolutionary selection along the human lineage. Therefore, changes in AHI1 may have been important in the evolution of human-specific motor behaviors.

Congenital malformations of the human brainstem

Congenital disorders of the brainstem often result in multiple severe neurodevelopmental problems. With the advent of magnetic resonance imaging and discovery of genes directing brainstem formation, a more coherent clinical picture of these disorders is emerging. Proper evaluation, management, and counseling for these disorders rests on the clinician having a framework through which to approach them.

Brain expansion and comparative prenatal ontogeny of the non-hominoid primate cranial base

The basicranium is the keystone of the primate skull, and understanding its morphological interdependence on surrounding soft-tissue structures, such as the brain, can reveal important mechanisms of skull development and evolution. In particular, several extensive investigations have shown that, across extant adult primates, the degree of basicranial flexion and petrous orientation are closely linked to increases in brain size relative to cranial base length. The aim of this study was to determine if an equivalent link exists during prenatal life. Specific hypotheses tested included the idea that increases in relative endocranial size (IRE5), relative infratentorial size (RIE), and differential encephalization (IDE) determine the degree of basicranial flexion and coronal petrous reorientation during non-hominoid primate fetal development. Cross-sectional fetal samples of Alouatta caraya (n=17) and Macaca nemestrina (n=24) were imaged using high-resolution magnetic resonance imaging (hrMRI). Cranial base angles (CBA), petrous orientations (IPA), base lengths, and endocranial volumes were measured from the images. Findings for both samples showed retroflexion, or flattening, of the cranial base and coronal petrous reorientation as well as considerable increases in absolute and relative brain sizes. Although significant correlations of both IRE5 and RIE were observed against CBA and IPA, the correlation with CBA was in the opposite direction to that predicted by the hypotheses. Variations of IDE were not significantly correlated with either angle. Correlations of IPA with IRE5 and RIE appeared to support the hypotheses. However, partial coefficients computed for all significant correlations indicated that changes to the fetal non-hominoid primate cranial base were more closely related to increases in body size than the hypothesized influence of relative brain enlargement. These findings were discussed together with those from a previous study of modern human fetuses.

Human malformations of the midbrain and hindbrain: review and proposed classification scheme

Although a great deal of interest in the genetics and etiology of cerebral, particularly forebrain, malformations has been generated in the past decade, relatively little is known about the basis of congenital malformations of the structures of the posterior fossa, namely the midbrain, cerebellum, pons, and medulla. In this review, we present a classification scheme for malformations of the midbrain and hindbrain based on their embryologic derivation, highlight four of the conditions associated with such abnormalities, and describe the genetics, prognosis, and recurrence risks for each. We describe several disorders in addition to Joubert syndrome with the distinctive radiologic sign known as the "molar tooth sign," comprised of midbrain and hindbrain malformations. We discuss Dandy-Walker malformation, its classical definition, and the surprisingly good outcome in the absence of other brain malformations. We consider the heterogeneous entity of cerebellar vermis hypoplasia and describe the recently identified gene associated with an X-linked form of this condition. Finally, the pontocerebellar hypoplasias are discussed in the context of their generally progressive degenerative and severe course, and the differential diagnosis is emphasized. We anticipate that as imaging technologies improve, differentiation of the various disorders should aid in efforts to identify the causative genes.

RECOGNITION OF THE CLINICAL SIGNS AND SYMPTOMS OF JOUBERT SYNDROME

Joubert Syndrome is a rare, autosomal recessive disorder that affects the cerebellum and brain stem. It presents with a distinct respiratory pattern and profound tachypnea in the newborn period. This article provides an overview of the condition and discusses the embryologic origins of this syndrome. A focused history and systematic physical assessment provide a step-by-step guide to enhance the early recognition of clinical signs and symptoms of this disorder. A series of clinical photographs and a brief case report offer insight into the classic presentation of this uncommon disorder. The diagnosis of Joubert syndrome is confirmed with magnetic resonance imaging, which reveals a classic neuroradiologic finding, characterized as the molar tooth sign. A discussion of the range of developmental outcomes and complex multispecialty care and intensive support that these infants and their families require is also provided.