MRI in cerebellar hypoplasia

Fetal and perinatal outcome associated with small cerebellar diameter based on second- or third-trimester ultrasonography

OBJECTIVE

To examine the outcome of pregnancy with fetal transverse cerebellar diameter (TCD) below the fifth percentile based on routine second- or third-trimester ultrasonography.

METHODS

We retrospectively analyzed the outcomes of 12 344 women according to TCD Z scores based on systematic second- or third-trimester ultrasound examination between 2007 and 2015. Information on major malformations, chromosomal anomalies, intrauterine or neonatal demise, and other abnormal findings were collected.

RESULTS

In total, 408 fetuses with small prenatal TCD underwent clinical investigation; 160 major malformations were noted, consisting mainly of neurological or cardiac anomalies (39,2%%). Chromosomal anomalies were reported in 39 (9.5%) and intrauterine or neonatal demise in 41 cases (10%). Major malformations and chromosomal anomalies were found in 46.4% and 10% of fetuses with extremely small TCD (Z score < -2.5), respectively, 31.3% and 12.7% of fetuses with small TCD (Z score between -2.0 and -1.645), and 39.6 % and 7.7% of fetuses with subnormal TCD (Z score between -2.0 and -1.645). Intrauterine or neonatal demise was noted in 22%, 8.8%, and 4.8% of fetuses with extremely small, small, and subnormal TCD, respectively (P < .05). Among intrauterine growth-restricted fetuses, fetal demise or neonatal adverse outcome was reported in 75%, 81.8%, and 18.5%, respectively. Of all the fetuses, 2.2% were lost to follow-up.

CONCLUSION

A small cerebellar diameter below the fifth percentile is a relevant marker to detect associated anomalies during routine ultrasound examination in the second or third trimester. This is related to a high rate of fetal malformations, chromosomal anomalies, and genetic disorders, regardless of the severity of the cerebellar small size. Small TCD seems to be a prognostic factor for fetal growth restriction. Therefore, when facing a TCD below the fifth percentile, patients should be referred for further sonography and fetal karyotyping.

Dandy-Walker syndrome and chromosomal abnormalities

Dandy-Walker syndrome (DWS) is a brain malformation of unknown etiology, but several reports have been published indicating that there is a causal relationship to various types of chromosomal abnormalities and malformation syndromes. In the present article, we present a bibliographical survey of several previously issued reports on chromosomal abnormalities associated with DWS, including our case of DWS found in trisomy 18. There are various types of chromosomal abnormalities associated with DWS; most of them are reported in chromosome 3, 9, 13 and 18. We also summarize some other chromosomal abnormalities and various congenital malformation syndromes.

MRI of the fetal posterior fossa

MRI is a useful tool to complement US for imaging of the fetal posterior fossa (PF). In France, the discovery of a PF malformation in the fetus frequently leads to termination of pregnancy (80% in a personal series). However, despite improved accuracy in the diagnosis of PF abnormalities, prognosis remains uncertain. The first objective of this review is to document the normal MRI landmarks of the developing fetal PF. Because of their thinness, the visibility of the cerebellar fissures is dramatically delayed on MRI compared to macroscopic data. An important landmark is identification of the primary fissure of the vermis, normally seen at around 25-26 weeks' gestation (WG) on the sagittal slice, separating the larger posterior lobe from the anterior lobe (volume ratio around 2:1). The prepyramidal and secondary fissures are usually only identifiable after 32 WG and the hemispheric fissures are difficult to see until the end of pregnancy. Considering the signal changes, high signal on T2-weighted (T2-W) sequences is seen from 25 WG in the posterior part of the brain stem (tegmentum and ascending sensory tracts) related to myelination. The low signal intensities seen within the cerebellum on T2-W images correspond to high cellularity of grey matter (deep nuclei), as there is no myelination within the white matter before 38 WG. The second objective is to highlight the signs highly predictive of a poor neurological prognosis. Lack of pontine curvature or vermian agenesis without a PF cyst (small volume of PF) is greatly associated with poor neurological status. The third objective is to propose a diagnostic strategy in difficult cases where prognosis is important, e.g. the Dandy Walker continuum. Analysis of the cerebellum is often impossible if a PF cyst is present (whatever its nature) as the mass effect usually blurs the foliation and even impairs evaluation of the normal ratio between the posterior and anterior lobes of the vermis. Isolated cerebellar hypoplasias raise the question of prognosis and genetic counselling. Such uncertainties require an amniocentesis and a careful search for other anomalies (cerebral and extracerebral). Unilateral abnormalities of a cerebellar hemisphere can be associated with good neurological status if they are isolated. The final objective is to discuss other rare PF fetal abnormalities, such as vascular malformations and tumours.

Development of the human fetal pons: in utero ultrasonographic study

OBJECTIVES

To examine the ultrasonographic feasibility of imaging the fetal pons and to construct a reference chart for its normal development during gestation.

METHODS

A cross-sectional, prospective study on 293 healthy fetuses of low-risk pregnancies between 19 and 34 weeks was performed. The transfontanel approach, via the abdominal or vaginal routes, was used to evaluate the fetal metencephalon (pons and cerebellum). The anteroposterior diameter of the fetal pons was measured in a mid-sagittal plane. The longitudinal diameter of the cerebellar vermis was measured at the same plane and the vermis-pons ratio (VPR) was established.

RESULTS

One hundred and forty-four fetuses were in vertex position. In 140 (97.2%) satisfactory visualization and measurements of the pons and cerebellar vermis were obtained. One hundred and forty-nine fetuses were breech presentations and measurements were successfully performed in 147 (98.6%). The pons anteroposterior and vermis longitudinal diameters showed a linear correlation with gestational age (GA) (r = 0.95 for both measurements; P < 0.001). The mean VPR was 1.5 (+/-0.1 SD) and did not change in the gestational interval that was considered.

CONCLUSION

By using the transfontanel approach, evaluation of the fetal pons is feasible via the mid-sagittal plane. The nomograms developed and the ratio to fetal vermis provides reference data that may be helpful when evaluating anomalies of the brainstem.

Posterior fossa abnormalities seen on magnetic resonance brain imaging in a cohort of newborn infants

OBJECTIVE

To describe the nature and frequency of posterior fossa (PF) lesions in infants who underwent magnetic resonance (MR) brain imaging in the neonatal period and to correlate with cranial ultrasound (CUS) findings and clinical outcome.

STUDY DESIGN

A retrospective review of all neonatal MR brain imaging from 1996 to 2001 (n=558). MR images, CUS and case notes were reviewed in infants with PF abnormality.

RESULTS

A total of 20 infants had abnormalities in the PF, which represents 4.7% of abnormalities seen on MR. Out of 10, six term infants had PF extra-axial hemorrhage, three had cerebellar hypoplasia, while one had cerebellar hemorrhage. In the preterm, 8/10 lesions were unilateral; focal cerebellar hemorrhage was seen in 5/10 and extensive hemorrhage with secondary atrophy in 3/10. Out of 20, 17 infants also had supratentorial lesions. Out of 20, 19 had CUS performed, of which 7/19 showed PF abnormality.

CONCLUSION

Intracerebellar hemorrhage was more common in preterm infants than in term infants. These hemorrhages tended to be focal, unilateral and were associated with atrophy.

Developmental instability of the cerebellum and its relevance to Down syndrome

It has been recognized for many years that cerebellar abnormalities are frequently observed in association with Down syndrome (DS). An important question to be asked about these and other findings in DS is whether their occurrence (i) is attributable to specific loci on the triplicated chromosome or chromosomal segment or (ii) derives from exaggerated responses secondary to the genetic imbalance resulting from trisomy (Ts). Recently, similar cerebellar alterations were observed in subjects with DS and in Ts65Dn mice (Baxter et al., 2000), mice segmentally trisomic for a portion of chromosome 16, which is homologous for loci on the long arm of human chromosome 21. It was concluded by these authors that the occurrence of similar cerebellar changes in DS and in the DS mouse model resulted from triplication of these homologous loci in the two trisomic organisms, i.e. cerebellar development is affected similarly by homologous loci in each species. They wrote that their study of Ts65Dn mice "correctly predicts an analagous pathology in humans". . . and that. . . "The candidate region of genes on chromosome 21 affecting cerebellar development in DS is therefore delimited to the subset of genes whose orthologs are at dosage imbalance in Ts65Dn mice, providing the first localization of genes affecting a neuroanatomical phenotype in DS." Findings described in this review suggest otherwise--that cerebellar findings in DS and in the Ts65Dn mouse are a result of exaggerated vulnerability in general of the cerebellum to disturbing events and that liability to expression of response(s) is exacerbated by trisomy. This conclusion is based on the following: (i) the cerebellum has an extended postnatal development; (ii) numerous genetic, environmental, epigenetic and metabolic conditions express cerebellar changes similar to those observed in Down syndrome; (iii) most if not all chromosomal imbalance syndromes express similar cerebellar abnormalities; (iv) the cerebellum is particularly sensitive to diverse toxic agents which may act prenatally, postnatally and/or in the mature organism; and (v) cerebellar abnormalities similar to those found in Ts65Dn mice have been described in Ts19 mice which have no segments homologous to any segment of human chromosome 21. An unavoidable conclusion from the review is that triplication of specific loci on 21q is an unlikely explanation for the cerebellar findings in DS. A simple positive control, in which the effect of triplication of loci other than those in question on a specific phenotype, should be used in experiments comparing human and experimental trisomies. As pointed out many years ago by Lorke and his coworkers (Lorke et al., 1989; Lorke, 1994; Lorke and Albrecht, 1994) similar phenotypic findings in the presence of different trisomies in the same species would suggest that the trisomic state itself rather than the gene content of a particular trisomy is responsible for the genesis of traits at issue.

Unappreciated agenesis of cerebellum in an adult: case report of a 38-year-old man

An unexpected finding at autopsy of almost complete agenesis of the cerebellum in an apparently functional, mentally subnormal 38-year-old man who died as the result of an accidental electrocution is reported. The posterior fossa was normal in appearance despite nearly complete absence of the cerebellum. A number of syndromes of cerebellar atrophy or dysgenesis have been reported, but congenital agenesis is considered a very rare condition. It does not resemble most common cerebellar malformations or acquired conditions, especially in an adult, who apparently had reasonable motor and coordinative function. The relevant literature is reviewed.

Diffuse pachygyria with cerebellar hypoplasia: a milder form of microlissencephaly or a new genetic syndrome?

We report on 2 families with diffuse pachygyria and cerebellar hypoplasia, who presented hypotonia, ataxia, seizures, and developmental delay since infancy. Computed tomography (CT) and magnetic resonance imaging (MRI) revealed decreased gyral formation in the cerebral cortex and marked hypoplasia in the cerebellum. Cerebellar hypoplasia is often associated with type 2 lissencephaly; however, our cases showed no polymicrogyria, and their clinical findings were quite mild compared with those of microlissencephaly. Their characteristic phenotype suggested a new genetic syndrome, which was possibly inherited as an autosomal recessive trait.

Joubert's syndrome and prenatal hydrocephalus

Joubert's syndrome is an autosomal-recessive condition characterized by dysgenesis of the cerebellar vermis, hypotonia, developmental delay, a respiratory pattern of alternating tachypnea and apnea, and abnormal eye movements. Radiologic findings include a midline cerebellar cleft in place of the vermis and a characteristic shape of the fourth ventricle. Prenatal hydrocephalus has been proposed as a possible etiology for the cerebellar abnormalities but has not previously been described in association with this syndrome. The authors report a patient with clinical and radiographic features consistent with Joubert's syndrome who presented with congenital hydrocephalus.