Fetopathology and developmental pathology of the embryo and fetus: Disorders of formation of the structures derived from the mediobasal prosencephalon

+ Introduction

+ Support

+ Atlas of fetal pathology

+ Congenital malformations of individual organ systems

Disorders of formation of the structures derived from the mediobasal prosencephalon

Introduction

The prosencephalon is the anterior end of the neural tube. It consists of the telencephalon which gives rise to the cerebral hemispheres and striatum and the diencephalonwhich gives rise to thalamus, hypothalamus globus pallidus and eye. The term “midline field defects” is also used for this group of defects.

Classification

Several malformations belong to this category:

Holoprosencephaly

the most serious defect from this group of anomalies; prosencephalon has not divided into two parts supposed to become the hemispheres
3 forms (according to the degree of division):
- alobar:very small spherical brain undivided into hemispheres or lobes
- semilobar: two hemispheres partially separated by shallow interhemispheric fissure which is best seen occipitally
- lobar: almost complete separation of the hemispheres except for small parts of anterior lobes or gyrus cinguli which is fused
other CNS anomalies are also often present, depending on the severity of the original defect:
- absence of bulbus and tractus olphactorius
- absence of corpus striatum and corpus callosum
- fused thalamus
- anomalies of the cortex
- glioneural heterotopy
- anomalies of the circle of Willis
crainofacial dysmorphology is always found, the rule is the face predicts the brain
- cyclopia: single eye, absent nose, supraorbital proboscis
- ethmocephaly: closely set eyes, absent nose, interorbital proboscis
- cebocephaly: closely set eyes, flattened nose with single nostril
- median cleft lip, closely set eyes, small flattened nose
- hypertelorism, microphtalmy, unilateral or bilateral cleft lip can be seen as well
etiology: often is associated with these chromosomal aberrations: trisomies and abnormalities of chromosomes 13 and 18; it is also associated with chemical teratogens (for example alcohol)
most of the affected fetuses are stillborn or die right after the birth

Corpus callosum agenesis

It is a common malformation. The agenesis can be complete or partial. Agenesis corpus callosum is either an isolated malformation or associated with other CNS defects (holoprosencephaly, Arnold Chiari) and a wide range of variable human malformations. Clinical symptomatology largely depends on associated anomalies. Psychomotoric delay is frequent but isolated cases can be asymptomatic. Occipital horns of the lateral ventricles are dilated. This sign is also usefull in prenatal diagnosis.

Arhinencephaly