MRI of the foetal brain

Transitory and Vestigial Structures of the Developing Human Nervous System

As with many body organs, the human central nervous system contains many structures and cavities that may have had functions in embryonic and fetal life but are vestigial or atrophic at maturity. Examples are the septum pellucidum, remnants of the lamina terminalis, Cajal-Retzius neurons, induseum griseum, habenula, and accessory olfactory bulb. Other structures are transitory in fetal or early postnatal life, disappearing from the mature brain. Examples are the neural crest, subpial granular glial layer of Brun over cerebral cortex, radial glial cells, and subplate zone of cerebral cortex. At times persistent fetal structures that do not regress may cause neurological problems or indicate a pathologic condition, such as Blake pouch cyst. Transitory structures thus can become vestigial. Examples are an excessively wide cavum septi pellucidi, suprapineal recess of the third ventricle, trigeminal artery of the posterior fossa circulation, and hyaloid ocular artery. Arrested maturation might be considered another aspect of vestigial structure. An example is the persistent microcolumnar cortical architecture in focal cortical dysplasia type Ia, in cortical zones of chronic fetal ischemia, and in some metabolic/genetic congenital encephalopathies. Some transitory structures in human brain are normal adult structures in lower vertebrates. Recognition of transitory and vestigial structures by fetal or postnatal neuroimaging and neuropathologically enables better understanding of cerebral ontogenesis and avoids misinterpretations.

Excitatory/Inhibitory Synaptic Ratios in Polymicrogyria and Down Syndrome Help Explain Epileptogenesis in Malformations

BACKGROUND

The ratio between excitatory (glutamatergic) and inhibitory (GABAergic) inputs into maturing individual cortical neurons influences their epileptic potential. Structural factors during development that alter synaptic inputs can be demonstrated neuropathologically. Increased mitochondrial activity identifies neurons with excessive discharge rates.

METHODS

This study focuses on the neuropathological examinaion of surgical resections for epilepsy and at autopsy, in fetuses, infants, and children, using immunocytochemical markers, and electron microscopy in selected cases. Polymicrogyria and Down syndrome are highlighted.

RESULTS

Factors influencing afferent synaptic ratios include the following: (1) synaptic short-circuitry in fused molecular zones of adjacent gyri (polymicrogyria); (2) impaired development of dendritic spines decreasing excitation (Down syndrome); (3) extracellular keratan sulfate proteoglycan binding to somatic membranes but not dendritic spines may be focally diminished (cerebral atrophy, schizencephaly, lissencephaly, polymicrogyria) or augmented, ensheathing individual axons (holoprosencephaly), or acting as a barrier to axonal passage in the U-fiber layer. If keratan is diminished, glutamate receptors on the neuronal soma enable ectopic axosomatic excitatory synapses to form; (4) dysplastic, megalocytic neurons and balloon cells in mammalian target of rapamycin disorders; (5) satellitosis of glial cells displacing axosomatic synapses; (6) peri-neuronal inflammation (tuberous sclerosis) and heat-shock proteins.

CONCLUSIONS

Synaptic ratio of excitatory/inhibitory afferents is a major fundamental basis of epileptogenesis at the neuronal level. Neuropathology can demonstrate subcellular changes that help explain either epilepsy or lack of seizures in immature brains. Synaptic ratios in malformations influence postnatal epileptogenesis. Single neurons can be hypermetabolic and potentially epileptogenic.

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.

The assessment of fetal brain growth in diabetic pregnancy using in utero magnetic resonance imaging

AIM

To assess fetal brain growth over the third trimester in pregnant women with diabetes using in utero magnetic resonance imaging (iuMRI) to determine if greater brain growth occurs in type 1 (T1DM) when compared to gestational (GDM) diabetes mellitus.

MATERIALS AND METHODS

Each consented participant was scanned at three fixed times during the third trimester using iuMRI. One hundred and fifty-seven patients were approached, 48 participants were recruited, and 36 complete data sets were analysed. Three-dimensional (3D) iuMRI volume data sets were manually segmented using software to construct models of the fetal brain from which brain volumes could be calculated. Inter-rater analysis was performed, and volume differences and growth rates were compared between T1DM and GDM.

RESULTS

Recruitment proved difficult with low uptake and high attrition rates (77.1%). Inter-rater analysis revealed excellent correlation (intraclass correlation coefficient=0.93, p<0.001) and agreement with no significant difference between operators (p=0.194). There was no evidence of increased brain volume in the T1DM group. Growth rates between visit 1 and 3 for T1DM and GDM were not significantly different (p=0.095).

CONCLUSION

T1DM brain volumes were not significantly larger than GDM volumes and there was no significant divergence of brain growth over the third trimester. Constructing volume models from 3D iuMRI acquisitions is a novel technique that can be used to assess fetal brain growth. No specialist software or knowledge is required. Larger studies attempting to recruit pregnant women in the later stages of pregnancy should employ multicentre recruitment to overcome recruitment difficulties and high attrition rates.

Diagnosis and treatment of vein of Galen aneurysmal malformations

INTRODUCTION

Vein of Galen aneurysmal malformations (VGAM) are rare but clinically significant intracranial arteriovenous shunt lesions that most often present in neonates and infants.

METHODS

Retrospective clinical data were collected for patients evaluated with a diagnosis of VGAM from 1994 to 2007.

RESULTS

Thirteen patients with VGAM were evaluated from 1994 to 2007. Seven patients presented emergently with medically intractable cardiac failure, and six were treated in the first 2 weeks of life. Five children treated after this period (1.5-31 months of age) manifested enlarging head circumference, abnormal development, or subarachnoid hemorrhage. Eleven patients were managed endovascularly. Four disease or procedure-related complications occurred. Two complications were associated with poor outcome, both of which occurred in patients treated at less than 2 weeks of age. Two other patients experienced transient neurological deficits with no evidence of permanent sequelae. Outcome in the six patients treated emergently in the first 2 weeks of life included two patients who developed normally, one with mild to moderate neurological deficits, one with severe neurological deficits, and two deaths. Outcome in the five older patients (treated between 1.5 and 31 months) was considerably better than in the group treated early and included three with normal outcome and two with mild neurological deficits.

CONCLUSIONS

Contemporary endovascular techniques remain the preferred treatment for VGAM in all age groups. Early diagnosis and multimodality treatment are essential for the best management and treatment of the complex constellation of clinical problems often arising from this disorder.

Assessment of sulcation of the fetal brain in cases of isolated agenesis of the corpus callosum using in utero MR imaging

BACKGROUND AND PURPOSE

There is gathering evidence to suggest that agenesis of the corpus callosum is associated with delayed fetal sulcation; it is possible that the corpus callosum facilitates normal gyral development. In this paper we sought to confirm whether delayed sulcation is found in fetuses with isolated agenesis of the corpus callosum as judged by in utero MR imaging.

MATERIALS AND METHODS

Retrospective analysis of 20 fetuses with isolated corpus callosum agenesis investigated by in utero MR imaging and 20 aged-matched normal fetuses was performed in the second or third trimester. All fetuses were singleton pregnancies with known gestational age, imaged on a 1.5T superconducting MR system. Estimation of sulcation maturity was made with reference to a standard atlas and subgroup analysis of earlier gestation (group 1, 21-26 weeks) and later gestation (group 2, 30-34 weeks) fetuses was performed.

RESULTS

Group 1 (n = 12) did not show a statistically significant difference between the 2 subgroups (P = .44) in terms of sulcation. A significant difference was demonstrated in the later gestation, group 2 (n = 8) fetal analyses; mean difference between consensus and actual gestation for normal fetuses was 0.9 weeks (SD of 1.5 weeks) versus -0.5 weeks (SD of 1.1 weeks) for the agenesis of corpus callosum cases (P = .046), suggestive of delayed sulcation in callosal agenesis.

CONCLUSIONS

Delayed sulcation encountered in third trimester fetuses with agenesis of the corpus callosum may be seen and does not in itself imply an additional brain abnormality.

The surgical management of pediatric hydrocephalus

The surgical management of hydrocephalus has undergone incredible changes over the past generation of neurosurgeons, including dramatic improvements in imaging, especially computed tomographic scanning and magnetic resonance imaging, and remarkably innovative advances in cerebrospinal fluid valve technology, complex computer models, and endoscopic equipment and techniques. In terms of overall patient outcomes, however, one could conclude that things are a little better, but "not much." This frustrating yet fascinating dichotomy between technological advancements and clinical outcomes makes hydrocephalus, first described by the ancients, as one of the most understated and complex disorders that neurosurgeons treat. The challenge to the next generation of neurosurgeons is to solve this vexing problem through better understanding of the basic science, improved computer models, additional technological advances, and, most importantly, a broad-based, concerted multidisciplinary attack on this disorder. This review focuses on the evolution of surgery for hydrocephalus over the last 30 years, the current state of the art of hydrocephalus treatment, and what appear to be the most promising future directions.