Neural System - Abnormalities

Introduction

Neural groove closing to neural tube, early week 4
(Stage 10)

There are many different congenital and maternally derived abnormalities associated with the nervous system. There are potentially 1000's of neurological abnormalities that could be listed on this page, therefore this current page is only a very brief introduction to some neural abnormalities.

Some Recent Findings

Neural Tube Closure

Dysraphism is the term often used to describe the defective fusion of the neural folds. The position and degree of failure of fusion will result in either embryonic death or a range of different neural defects. The way (mode) in which the human neural tube fuses has been a source of contention. In humans, fusion appears to initiate at multiple sites along the neural groove, but this mode may differ from that found in many animal species used in developmental studies.

Human Embryonic Death:

• 5 weeks with total failure of fusion.
• 6.5 weeks with opening over the rhombencephalon.
• survive beyond 7 weeks with a defect at the frontal and parietal regions.

(Data: Nakatsu etal, 2000

Spina Bifida (Meningomyelocele)

Spina Bifida Rates (USA Data)

There are potentially many different causes of spina bifida and neural tube defects. The basis of the abnormality is a failure of the neural tube to close caudally. At least one known cause is a low maternal diet of folic acid (folate) containing foods. (see Neural Tube Defects and Low Folic Acid or Folic Acid below)

Neural tube defects from failure to close can be screened by amniocentesis or ultrasound.

Alpha-fetoproetin normally present in the CSF, leaks and can be detected in amniotic fluid.

Cephalic Disorders

Cephalic (Greek, kephale = head) are a large group of abnormalities that relate to both skeletal (skull) and neural (brain) associated defects including: anencephaly, hydrocephalus, encephalocele, colpocephaly (occipital horn enlargement), lissencephaly (smooth brain), porencephaly (cyst or cavity in cerebral hemisphere), acephaly (absence of head), exencephaly (brain outside skull), macrocephaly (large head), micrencephaly (small brain), otocephaly (absence of lower jaw), brachycephaly (premature fusion of coronal suture), oxycephaly (premature fusion of coronal suture + other), plagiocephaly (premature unilateral fusion of coronal or lambdoid sutures), scaphocephaly (premature fusion of sagittal suture), trigonocephaly.

Anencephaly

Anencephaly Rates (USA Data)

This is a neural tube defect, the basis of the abnormality is a failure of the neural tube to close cranially. Also called exencephaly or craniorachischisis.

Hydrocephalus

Hydrocephalus (historic image from Hess, 1922)

This is a defect of cerebrospinal fliud (CSF) flow, leading to enlarged ventricles and head, separated skull cranial sutures and fontanelles. Obstruction of CSF flow can occur at any time (prenatally or postnatally) and leads to accumulation of within the ventricles. The time of onset will have different effects and should be compared to the equilivant neurological events that are occuring.

Ventricular obstruction usually occurs at the level of the cerebral aqueduct (narrowest site), but can occur elsewhere, and can be caused by viral infection.

Encephalocele

This defect is generally mesodermal in origin, leading to protrusion of brain and meninges outside the crainal cavity. The severity of the disorder can vary dependent upon the degree of mesodermal abnormality.

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Cite this page: Hill, M.A. (2024, April 27) Embryology Neural System - Abnormalities. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Neural_System_-_Abnormalities

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G