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Birth Defects: Neural tube defects
 

Birth Defects: Neural tube defects

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Birth Defects was written for healthcare workers who look after individuals with birth defects, their families, and women who are at increased risk of giving birth to an infant with a birth defect. ...

Birth Defects was written for healthcare workers who look after individuals with birth defects, their families, and women who are at increased risk of giving birth to an infant with a birth defect. This book is being used in the Genetics Education Programme which trains healthcare workers in genetic counselling in South Africa. It covers: modes of inheritance, medical genetic counselling, birth defects due to chromosomal abnormalities, single gene defects, teratogens, multifactorial inheritance

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    Birth Defects: Neural tube defects Birth Defects: Neural tube defects Document Transcript

    • 6 Neural tube defectsBefore you begin this unit, please take the INTRODUCTION TOcorresponding test at the end of the book toassess your knowledge of the subject matter. You NEURAL TUBE DEFECTSshould redo the test after you’ve worked throughthe unit, to evaluate what you have learned. 6-1 What are neural tube defects? Neural tube defects (NTDs) are congenital Objectives malformations of the neural tube, caused by failure of the neural tube to close at the end of the fourth week after conception. Neural tube When you have completed this unit you defects include the following three conditions: should be able to: • Define a neural tube defect. 1. Anencephaly. 2. Encephalocoele. • List the three types of neural tube defect. 3. Spina bifida. • Give the birth prevalence of neural tube defects. Neural tube defects are typical examples of a multifactorial congenital malformation. • Explain the causes of neural tube defects. These birth defects result from an interaction • Describe the clinical features of neural between genetic factors (usually a number of tube defects. inherited genes) and an environmental factor • Understand how to care for children with (probably viral, dietary, toxic or radiation). neural tube defects. Multifactorial defects, such as neural tube • Understand the prevention of neural defects, occur in both males and females. tube defects. 6-2 What is the neural tube? At 22 days after conception, the embryo is a flat, pear-shaped plate made up of three layers of cells. By a process of folding in the midline, the top layer of cells forms a tube within the middle layer of the plate.
    • NEURAL TUBE DEFECTS 101 Anencephaly is called an open neural tube Outer layer (ectoderm) Middle layer (mesoderm) defect because brain (neural tissue) is exposed. Inner layer (endoderm) 6-5 What is an encephalocoele? An encephalocoele is a failure of closure in the midline of the skull anywhere from a position between the eyes (frontal area) to the back of the skull (occipital area). With an encepaholcoele the brain coverings (the meninges), with or without brain tissue, protrude through the skull defect into a membranous sac which is covered by skin. The most common site of an encephalocoele is in the occipital area. Frontal encephalocoeles are also seen. Encephalocoeles are called closed neural tube defects because neural tissue is not exposed as the defect is covered by skin.Figure 6-1: The three layers of the embryonic plateshowing the folding to form the neural tube 6-6 What is spina bifida? Spina bifida (split spine) is an opening in the spinal column due to failure of closure of theThis tube, the neural tube, runs from top bony vertebral arches. Spina bifida may occurto bottom of the developing embryo and is anywhere down the spinal column. There areformed by 28 days post conception. three forms of spina bifida:6-3 What develops from the neural tube? 1. Meningomyelocoele. This is the most severe form of spina bifida.The neural tube is the structure from which 2. Meningocoele. This is a less severe thanthe skull, brain, spinal cord and nerves will meningomyelocoele.develop, as well as the spinal column (made up 3. Spina bifida occulta. This is the least severeof vertebrae). If the neural tube fails to close at form as it only involves the bony spine.the head end, the defect results in anencephalyor an encephalocoele. If it fails to close lower Spina bifida may be either open or closed,down along the spine, the result is spina bifida. depending on the type. NOTE Meningomyelocoele (or Anencephaly, encephalocoele and spina bifida myelomeningocoele) is also referred to as are the three types of neural tube defect. spina bifida cystica while meningocoele is also called spina bifida aperta. Cystica is Latin for cyst and aperta means an opening.6-4 What is anencephaly?Anencephaly (no brain) is the most serious 6-7 What is a meningomyelocoele?of all neural tube defects and always results A meningomyelocoele is an opening anywherein stillbirth or early neonatal death. The top along the spinal column, due to failure of one(vault) of the skull is absent, exposing the or more vertebral arches to close. Neural tissuebrain, which is malformed. The cerebral (spinal cord and nerves) and the coverings ofhemispheres do not develop with anencephaly. the spinal cord (the meninges) bulge through
    • 102 BIR TH DEFECTS Normal spinal cord above meningomyelocoele Vertebral body Thin membrane (meninges) but no skin Cerebrospinal fluid Damaged neural tissue (spinal cord) in the meningomyelocoeleFigure 6-2: A cross-section of the spinal column showing a meningomyelocoele with both the spinal cord andmeninges protruding through a defect in the vertebral arches Normal spinal cord Vertebral body Meningocoele covered with skin Meningeal sac filled with cerebrospinal fluid onlyFigure 6-3: A cross-section of the spinal column showing a meningocoele with only the meninges protrudingthrough a defect in the vertebral archesthe opening. The skin over this defect does not Meningomyelocoeles are open neural tubeclose, but the defect may be covered by a thin defects because neural tissue (spinal cord andmembrane which tears easily. The neural tissue nerves) is exposed and not covered by skin.that bulges through the bony defect is usuallydamaged, resulting in nerve abnormalities 6-8 What is a meningocoele?below the level of the defect. A meningocoele is an opening anywhere along the spinal column, due to failure of closure
    • NEURAL TUBE DEFECTS 103 Normal spinal cord Vertebral body Patch of hair in the midline Absent vertebral archFigure 6-4: A cross-section of the spinal column showing spina bifida occulta with neither the spinal cord northe meninges protruding through a defect in the vertebral archesof one or more vertebral arches. Only the problems. The diagnosis of spina bifida occultacoverings of the spinal cord (the meninges) can be confirmed on X-ray which shows theprotrude through the defect, forming a sac defect in the spinal column.which is filled with cerebrospinal fluid (CSF). Spina bifida occulta is a closed neural tubeThe spinal cord and nerves are normal and defect.do not bulge through the opening. There isno associated spinal cord or nerve damage. NOTEOcculta is a Latin word thatThe meningocoele usually is covered on the means hidden or secret.outside by skin.A meningocoele is a closed neural tube defect 6-10 How common are neural tube defects?when it is covered by skin. Neural tube defects occur throughout the world. Their birth prevalence (number of6-9 What is spina bifida occulta? infants with neural tube defects per 1000Spina bifida occulta is a defect of the spinal live births) varies according to the areacolumn, due to failure of one or more (geographic location), the ethnicity and thevertebral arches to close. This usually occurs socio-economic status of the population. Inin the lumbar and sacral regions of the spine industrialised countries the birth prevalence(lower back). Unlike a meningomyelocoele or of neural tube defects has decreasedmeningocoele, the spinal cord and meninges significantly over the last 40 to 50 years, andare normal and do not protrude through is now about 1/1000 live births. Of thesethe defect. The defect may be covered by an infants about 50% spina bifida, 40% haveoverlying abnormality such as a midline patch anencephaly and 10% encephalocoeles.of hair, a lipoma or a dimple. Neurological In urban areas of South Africa (Cape Town,abnormality is usually not associated with Johannesburg and Pretoria), the birthspina bifida occulta although spina bifida prevalence in the Black population is aboutocculta may present later in life with back 1/1000 live births. In contrast, the birth
    • 104 BIR TH DEFECTSprevalence has been recorded as 6.1/1000 in Folic acid is an important fetal environmentalrural areas of the Eastern Cape Province and factor in the cause of neural tube defects.3.6/1000 in rural Limpopo Province. The reasonfor the difference in birth prevalence betweenurban and rural populations is not known. 6-12 What are the other causes of neural tube defects?The prevalence (number of infants withneural tube defects per 1000 in a population) Neural tube defects can also be caused byof neural tube defects in South Africa chromosomal abnormalities, single gene defectsis small as most infants born with these and teratogens including alcohol and sodiumdefects die young. In rural Limpopo more valproate (Epilim or Convulex). Sodiumthan 90% of infants born with anencephaly, valproate is used to treat epilepsy. Some ofencephalocoele or meningomyelocoele die these drugs may cause neural tube defects bybefore the age of two years. working against the effect of folic acid. In black populations in South Africa, the birth 6-13 What are the clinical features of neural tube defects? prevalence of neural tube defects in urban areas is about 1/000 live births while in rural areas it is The clinical presentation of neural tube defects about 4/1000 live births. depends on the type of defect, whether it is open or closed, its position and size. The NOTE The birth prevalence of neural tube different presentations vary greatly, from no defects in industrialised countries has obvious clinical features in spina bifida occulta decreased greatly over the last 40 years as the to a gross abnormality in anencephaly. socio-economic situation has improved and prevention strategies have been put in place. 6-14 What are the clinical As an example, in people of Celtic origin living features of anencephaly? in Ireland, the birth prevalence of neural tube defects used to be above 6/1000 live births Infants with anencephaly are often born but is now less than 1/1000 live births. preterm and may be stillborn. If they are live born they seldom live longer than 24 hours.6-11 What causes neural tube defects? The infants are born with the top of their skull missing and brain exposed. The eyesThe development of the neural tube, including appear to bulge. General examination of theits closure, is under the control of several rest of the infant is usually normal but maygenes working together with environmental reveal other abnormalities.factors. Most neural tube defects are, therefore,caused by multifactorial inheritance, i.e. they NOTEAt postmortem examination 40% ofresult from an interaction between genetic and anencephalics have abnormal internal organs.environmental factors. 6-15 What are the clinical features Most neural tube defects are due to of an encephalocoele? multifactorial inheritance. An encephalocoele develops because of failure of complete closure of the skull. TheFolic acid is one of the important fetal infant presents at birth with a midline massenvironmental factors involved in closure of anywhere from between the eyes to the backthe neural tube. of the skull (occiput). The most common site for an encephalocoele is over the occiput. The clinical presentation will depend on the size and site and whether the encephalocoele
    • NEURAL TUBE DEFECTS 105contains neural tissue (brain matter) or not. 5. Incontinence of bowel.Encephalocoeles that only contain meninges 6. Clubfeet.and no neural tissue usually have problems 7. Hydrocephalus (80% of cases).only related to the defect in the skull. However, Hydrocephalus (excessive cerebrospinalif the encephalocoele contains brain tissue, this fluid in the ventricles of the brain) oftencan be damaged or be associated with severe presents in utero but may only presentbrain abnormalities. The resulting neurological in the first weeks after delivery. Earlyabnormalities will depend on the size and site diagnosis of hydrocephalus is important toof the encephalocoele. In severe cases, most of obtain the best results from surgery.the brain may be in the encephalocoele. NOTE Hydrocephalus is caused by a malformationAssociated neurological abnormalities include: at the base of the brain, called an Arnold–Chiari malformation, where the cerebellum bulges1. Intellectual disability. through the opening at the base of the skull. This2. Microcephaly. blocks the normal flow of cerebrospinal fluid.3. Cerebral palsy.4. Visual disability (blindness). 6-17 What are the complications5. Epilepsy. of a meningomyelocoele?Depending on the size and site of the Complications of meningomyelocoele mayencephalocoele, early death, even with present early or repeatedly. These include:treatment, is a common outcome in many ofthese infants. 1. Meningitis. Infection can easily and rapidly enter the nervous system through the6-16 What are the clinical features open meningomyelocoele. Therefore, theof a meningomyelocoele? meningomyelocoele should be carefully and aseptically covered (e.g. sterileA meningomyelocoele presents at birth with gauze) at birth and then closed, as soona mass anywhere along the spine, but usually as possible, by surgery. Sometime thein the thoracic, lumbar or sacral regions. The covering membrane may leak or rupturemass may or may not be covered by a thin (tear) during delivery.membrane, but neural tissue (spinal cord 2. Raised intracranial pressure due toand nerves) is usually visible. The associated hydrocephalus. Early clinical signs ofclinical features depend on the site and size of increased intracranial pressure are athe defect. bulging anterior fontanelle, irritability,As a meningomyelocoele contains neural vomiting, and a high-pitched cry.tissue which is usually damaged, the body and 3. Intellectual disability. The hydrocephalus,limbs of the affected infant are paralysed below which may develop before or after delivery,the level of the defect. The effect is similar can result in intellectual disability. Epilepsyto traumatic cutting of the spinal cord. The from the hydrocephalus may also result.associated clinical features include: 4. Urinary tract infections. The bladder incontinence can result in urinary tract1. Flaccid paralysis (floppiness, weakness and infections in both males and females. absent reflexes) of the legs. 5. Bed sores. These result from the lack of2. No feeling (touch or pain) below the level feeling (sensation) and movement in the of the defect. body below the meningomyelocoele.3. Kyphosis. A forward bend in the spine. With large meningomyelocoele this may In low resource countries, infant and early also be associated with a lateral bend childhood death is a common outcome of (scoliosis). meningomyelocoeles.4. Incontinence of bladder.
    • 106 BIR TH DEFECTS6-18 What are the clinical available health facilities. In all patients thefeatures of a meningocoele? best possible care available must be given. This will include:A meningocoele presents at birth with a skin-covered mass in the midline anywhere along 1. Diagnosisthe spine. As the spinal cord and nerves are Because of the obvious physical featuresnot involved there usually are no neurological of most forms of neural tube defect theabnormalities in the trunk, limbs, bladder diagnosis is made at birth or shortlyand bowels. However, hydrocephalus is thereafter. The exception is spina bifidapresent as a complication in 20% of infants occulta which is usually not clinicallywith meningocoeles. obvious at birth. 2. Treatment Infants with anencephaly do not survive Hydrocephalus develops in 80% of infants with and are given palliative (hospice) care with meningomyelocoele and in 20% of infants with warmth, feeds if hungry and support for meningocoele. the parents. a) Medical treatment:6-19 What are the clinical features Medical treatment may be needed forof spina bifida occulta? the complications of encephalocoele, and meningomyelocoele. These include:Most people with a spina bifida occulta do not • Antibiotics for meningitis if it developsknow that they have a neural tube defect, i.e. it in infants with a meningomyelocoele.often remains hidden for life, therefore, the use • Antibiotics for urinary tract infectionsof the word ‘occulta’ (hidden). They usually in meningomyelocoele. .have no signs or symptoms. Occasionally, • Recurrent catheterisation for urinaryspina bifida occulta is diagnosed on an X-ray incontinence in meningomyelocoele.which is taken for some other reason. In some • Anticonvulsants for epilepsy ininfants the presence of the bony defect in the encephalocoele.vertebral arches is suggested by an overlying b) Surgical treatment:midline abnormality, usually a hairy patch. • Infants with encephalocoele, meningomyelocoele and meningocoele NOTE Very occasionally a child may develop should be referred to a neurosurgical neurological complications of spina bifida occulta, especially during periods unit for assessment. Meningocoeles of rapid growth, presenting with urinary should be simply covered with a piece incontinence, neurological signs of nerve of sterile gauze after delivery to reduce damage in a limb or clawing of the toes the risk of infection before surgery. due to weakness of muscles in the feet. • In encephalocoeles, meningo- myelocoeles and meningocoeles, surgery is used to close the defect and removeCARING FOR INFANTS the mass caused by the protruding brain, spinal cord or meninges. Care isAND CHILDREN WITH taken not to damage the spinal cord andNEURAL TUBE DEFECTS nerves in the repair of meningocoeles nor cause further damage to the brain in encephalocoeles that contain neural6-20 What care is available for tissue. These infants with neural tubeinfants with neural tube defects? defects need to be referred to a tertiary care hospital for surgical assessment asThe care required will depend on the type of soon as possible after birth.neural tube defect, its site and size, and the
    • NEURAL TUBE DEFECTS 107 • In meningomyelocoeles and people with neural tube defects and their meningocoeles, surgery to insert a families. ventriculo-peritoneal (VP) shunt to treat hydrocephalus may be required. A shunt is a tube that drains the fluid GENETIC COUNSELLING from the ventricles of the brain into FOR NEURAL TUBE the abdomen. If the affected infant does not have hydrocephalus at birth, DEFECTS it may develop after the spinal defect is repaired. Therefore, regular careful 6-21 What genetic counselling is head circumference measurements needed by parents who have a must be taken and plotted on a centile child with a neural tube defect? chart to help make the diagnosis of hydrocephalus as early as possible Genetic counselling is a very important part (head circumference measurements of the care of people with neural tube defects weekly for the first six weeks and and their family, especially the parents and then monthly to one year of age). If siblings. The parents need to be educated and ultrasound facilities are available, serial informed about: head ultrasound examination can be 1. The diagnosis. used to assist in the diagnosis. 2. The cause of neural tube defects. Theyc) Neurodevelopmental therapy need to know that neural tube defects • Neurodevelopmental therapy includes are usually the results of multifactorial physiotherapy, hearing assessment inheritance, but occasionally have other (audiology) and speech therapy, and causes. It is important to rule out these occupational therapy. One or more other causes if possible. of these therapies may be needed. All 3. The clinical features, complications and these forms of neurodevelopmental prognosis of the particular neural tube therapy are available in major centres. defect their child has, and what treatment However, in both rural and urban is available. areas with fewer resources, hospitals 4. The increased risk for parents of a child may only have a physiotherapist or with a neural tube defect of having another an occupational therapist. Some child with a neural tube defect in future may also have a community-based pregnancies. They need to know their rehabilitation programme. options for reducing their risk of having • In South Africa, rehabilitation for another affected child through primary infants and children with disabilities prevention, genetic screening, prenatal can be assisted by the use of a locally diagnosis and genetic counselling. produced stimulation programme called START (Strive Towards The parents, family and child with a neural Achieving Results Together). This tube defect need to be offered on-going was designed to use affordable, locally psychosocial support, as do all individuals available, materials. Information on who have a congenital disability. They have START can be obtained from: Sunshine problems that require lifelong care. The burden Centre, P O Box 41167, Craighall, 2024. of the disorder and the care is experienced not Telephone 011 642 2005. Internet: only by the affected person, but also the family, www.sunshine.org.za especially parents, brothers and sisters.3. Genetic counselling and psychosocial support This is an important part of the care of
    • 108 BIR TH DEFECTS6-22 Where can parents who have a child 80 percent. In the southern, wealthier andwith a neural tube defect get support? more urban part of China, where the birth prevalence of neural tube defects is aboutSupport, help and reassurance may be 1/1000, live births periconceptional folic acidobtained from: reduced the birth prevalence by 40 percent.1. Doctors, nurses (especially nursing staff 2. Secondary prevention: This is based on with genetic training), genetic counsellors genetic screening, prenatal diagnosis of and neurodevelopmental therapists. neural tube defects, and genetic counselling.2. Social workers.3. The Southern African Inherited Disorders Association (SAIDA). Many neural tube defects can be prevented4. Parent Support Group. by periconceptional folic acid, and by genetic screening, prenatal diagnosis and geneticThe Southern African Inherited Disorders counselling.Association (SAIDA) can be contacted atthe Division of Human Genetics, NationalHealth Laboratory Service, P O Box 1038, 6-24 What is periconceptionalJohannesburg, 2000, South Africa. Telephone folic acid supplementation?and fax 011 4899213. Folic acid is a group B vitamin. It is very cheap and safe to give as it has few and only minor side effects even in large doses.PREVENTION OF NEURAL If folic acid is given as a medicine in theTUBE DEFECTS form of a pill, capsule or tablet, this is called supplementation.6-23 Can neural tube defects be prevented? With periconceptional supplementation folic acid is given around the period of conception,Yes. There are two approaches for the i.e. for three months before and three monthsprevention of neural tube defects. These are: after conception. This is the recommendation.1. Primary prevention: This aims to ensure However, even if the folic acid is given for only the conception of infants without neural a month, there is some benefit. tube defects. The pre-conception approach The recommended dose of folic acid to is the preferred method of prevention. prevent the occurrence of neural tube It is based on the knowledge, confirmed defects is a minimum of 0.4 mg daily. This in Europe in the early 1990s, that if a can be taken alone or in combination with woman takes periconceptional folic acid other vitamins in a multivitamin tablet. supplements, she can reduce her risk of One periconceptional multivitamin tablet having an infant with a neural tube defect containing folic acid a day is recommended. by 50 percent. It was also confirmed, that if a woman had previously had an infant with Vitamins A and D, if given to a pregnant a neural tube defect, her increased risk of mother in high doses, are teratogenic and can having another child with a neural tube damage the fetus. Therefore, more than one defect could be decreased by 70 percent. multivitamin tablet a day can be dangerous for the fetus and the mother. NOTE Research done recently in China showed that periconceptional folic acid supplementation taken by women in the poorer, more rural, northern region where the birth prevalence of neural tube defects is greater than 6/1000 live births, reduced the birth prevalence by
    • NEURAL TUBE DEFECTS 1096-25 Does a mother who previously had 6-27 What are the risks for a motheran infant with a neural tube defect need who has an infant with a neural tubemore periconceptional folic acid? defect having a further affected child?Yes. If a mother had a previous child with a Women who have previously had an infantneural tube defect she is at greater risk for with a neural tube defect of multifactorialhaving another infant with a neural tube origin are at greater risk of having futuredefect in future pregnancies. To reduce this children affected with the same type of neuralincreased risk for an infant with a neural tube defect. This is also true for the childrentube defect it is recommended that she take of a parent who has a neural tube defect. The1 mg of folic acid daily for three months risks involved in these situations are:before conception and for three months afterconception in all future pregnancies. Family relationship Approximate risk One affected sibling 5% (1 in 20)6-26 Is supplementation the only (brother or sister)way that folic acid can be given? Two affected siblings 10% (1 in 10)No. As many pregnancies are not planned, Three affected siblings 20% (1 in 5)it is important to put folic acid into a staple One affected parent 5% (1 in 20)food to reduce the risk of neural tube defects.When an essential nutritional factor, such as One affected second 2% (1 in 50)folic acid, is added to the diet of the general degree relative (unclepopulation in this manner, this is called or aunt)food fortification. Research of folic acidfortification of flour and other wheat products NOTE With three or more affected siblings,(from Canada, the USA, Chile and South consideration must be given to theAfrica) has shown that this reduces the birth possibility of autosomal recessive inheritance,prevalence of neural tube defects. and if the affected siblings are all male, to X-linked recessive inheritance.In South Africa maize meal and wheat flourare now fortified by law with folic acid. Since Although most neural tube defects are causedthe start of fortification in 2004 there has by multifactorial inheritance, care must alwaysbeen a more than 30% decrease in the birth be taken to exclude other causes of neural tubeprevalence of neural tube defects. defects before genetic counselling, including risk assessment, is given.Although food fortification with folic acidis being done, it is recommended thatwomen still take periconceptional folic acid SCREENING FOR NEURALsupplementation as some people may not getsufficient folic acid from fortification. TUBE DEFECTS Periconceptional supplementation and food 6-28 How can neural tube defects be fortification with folic acid reduces the birth screened for during pregnancy? prevalences of neural tube defects. There are a number of methods which can be used to screen the fetus for neural tube defects. In countries that have well organised screening programmes, all fetuses with anencephaly and 70–80% of fetuses with spina bifida can be detected. The screening includes:
    • 110 BIR TH DEFECTS1. Maternal serum alpha-fetoprotein (AFP) Whenever a maternal serum alpha-fetoprotein aternal screening. (AFP) screening test is abnormal, a fetal2. Fetal ultrasound screening. ultrasound examination must be done to decide whether a neural tube defect or other6-29 What is maternal serum birth defect is present or not.alpha-fetoprotein screening? NOTE Occasionally, the level of maternal serumMaternal serum alpha-fetoprotein (AFP) alpha-fetoprotein is raised but a neural tube defectlevels are significantly raised in ‘open’, but cannot be seen on fetal ultrasound examination.not in ‘closed’, neural tube defects. This is This may be because the neural tube defect iswhy different types of neural tube defects small and cannot be detected by ultrasoundare classified into ‘open’ or ‘closed’ defects. scan, or the raised alpha-fetoprotein is due to some other cause. In this situation the mother,Maternal serum alpha-fetoprotein screening preferably with her partner, should receive geneticwill not detect ‘closed’ defects. counselling and be offered an amniocentesis. IfMaternal serum alpha-fetoprotein screening is the amniocentesis is accepted and performed,best performed at around 16 weeks (between the amniotic fluid levels of alpha-fetoprotein and acetyl-cholinesterase are measured. If these15 and 18 weeks) gestation on a sample of the are raised, and no other obvious cause for theirmother’s blood. It is important to ensure that being raised can be found, then the diagnosisthe gestational age is correct. Ultrasound dating can be considered to be a neural tube defect.of the fetus confirms the gestation based onthe date of the last menstrual period. A raised 6-31 What is the management if theserum level of alpha-fetoprotein indicates a fetus has a neural tube defect?high risk for an open fetal neural tube defect. If a prenatal diagnosis of a neural tube defect NOTE The maternal serum screening for Down is confirmed, the woman, preferably with syndrome (the Triple Test) measures the her partner, should urgently receive genetic concentration of maternal alpha-fetoprotein counselling regarding the diagnosis and (AFP), unconjugated oestriol (uE3) and human their choices of management. For women at chorionic gonadotrophin (hCG). The level of alpha-fetoprotein in the Triple Test can be increased risk, or with a prenatal diagnosis of a used to screen for neural tube defects. neural tube defect, the choice of which options to take is theirs alone. Many will choose a Other fetal causes of a raised maternal serum termination of pregnancy. Their medical care alpha-fetoprotein include incorrect estimation providers must respect this choice. Parents must of the gestational age, multiple pregnancy, exomphalos, nephrotic syndrome, fetal death, also know that no matter what their choice, this Turner syndrome, ectodermal dysplasia and will not influence their future routine care. Rhesus disease. Maternal causes include diabetes, liver or gut cancer, and hepatitis. Rarely no cause for a raised alpha fetoprotein can be found. With genetic screening and prenatal diagnosis The prognosis of these pregnancies is poor. people are entitled to genetic counselling and always have the right of choice.6-30 What is fetal ultrasound screening? creening?It is recommended that a screening ultrasoundscan for fetal abnormalities is done at 18 weeks CASE STUDY 1gestation (18–23 weeks). During this scansigns of neural tube defects should be detected A female infant is born at term and a severeby an experienced ultrasonographer. Fetal abnormality is noticed by the midwife as soonultrasound scanning can detect both open and as the infant is delivered. A doctor is called toclosed neural tube defects. examine the infant. She notices that the top of the infant’s skull is missing and the brain is
    • NEURAL TUBE DEFECTS 111visible. The parents are told that their infant CASE STUDY 2has a serious birth defect. After delivery, a newborn infant is noticed to1. What is a neural tube defect? have an abnormality over the lower spine andIt is an abnormality of the neural tube which also has club feet. The infant has a big headdoes not close correctly towards the end of the and does not move his legs. Otherwise hefirst month after conception. The neural tube is appears healthy and feeds well at the breast.a structure in the embryo from which the brain,spinal cord, spinal column and nerves develop. 1. What is the diagnosis? The infant has spina bifida. This is a defect2. What are the forms of neural tube in the spinal column due to failure of onedefect that affect the brain? or more vertebral arches to close normally.• Anencephaly. The defect is usually in the lower spine• Encephalocoele. (lumbosacral region).3. Which form of neural tube 2. What are the forms of spina bifida?defect is present in this infant? • Meningomyelocoele.Anencephaly. This results from failure of • Meningocoele.closure of the midline of the skull, exposing the • Spina bifida occulta.brain. The brain is always very abnormal with This infant must have a meningomyelocoelemost of it (the cerebral hemispheres) missing. because the nerves to the legs have been damaged. As a result he has paralysed legs4. Will this infant survive? and club feet.No. Infants with anencephaly are usuallystillborn or die in the first day of life. 3. What are the clinical features of a meningomyelocoele?5. Are all infants with anencephaly females? A midline mass which is covered by a thinNo. All forms of neural tube defect, including membrane. Neural tissue is visible throughanencephaly, may occur equally in both male the membrane. A meningomyelocoele is notand female infants. covered with skin.6. What are the clinical features 4. Why does this infant have a big head?of an encephalocoele? About 80% of infants with a meningo-An encephalocoele is less severe than myelocoele develop a hydrocephalus. Thisanencephaly. In an encephalocoele, only part may be present at birth but can also develop inof the skull does not close completely in the early infancy.midline. As a result, the meninges and oftenpart of the brain push through the hole in the 5. Why is this not a meningocoele?skull. Unlike anencephaly, which is an open A meningocoele is a less severe defect asneural tube defect, encephalocoeles are closed only the meninges bulge through the hole inneural tube defects as they are covered with the vertebral column. As there is no neuralskin. The defect is usually in the occipital tissue in the meningocoele, there usually willregion but may also occur in the frontal region. be no paralysis of the legs. A meningocoele is covered with skin. Therefore, it is called a
    • 112 BIR TH DEFECTS‘closed’ defect and would not be detected with 4. How can the frequency of neural tubea maternal serum alpha-fetoprotein screen. defects be lowered in a community? By fortifying an essential food, such as maize6. What is a spina bifida occulta? meal or wheat flour, with folic acid. FortificationThis is a mild form of spina bifida, which of maize meal in South Africa started in 2004is often not noticed clinically. There is a and has reduced the birth prevalence of neuralsmall defect in the arch of a vertebra but the tube defect by more than 30%.meninges do not prolapse. There may be apatch of hair or abnormal skin over the defect. 5. What is the correct management of a child with a meningomyelocoele? Urgent referral to a neurosurgical unit forCASE STUDY 3 assessment. A meningomyelocoele should be covered with sterile gauze after birth.A young couple, who plan to start a family,visit their general practitioner, as they wantto know about neural tube defects. Their CASE STUDY 4neighbour recently delivered an infant witha meningocoele. The defect was successfully A young mother has a child with a meningo-corrected by surgery. myelocoele. She and her partner want another child but are unsure of the risk of further1. How common are neural tube defects? children also having a neural tube defect. TheyIn industrialised countries the prevalence of attend a genetic clinic for counselling.neural tube defects is about 1/1000 live births.In rural populations in South Africa, the birth 1. What is the risk of this woman havingprevalence is about 4/1000. another child with a neural tube defect? There is an increased risk if there is a family2. What is the cause of neural tube defects? history of neural tube defects. If a previousThe failure of the neural tube to close child has a neural tube defect the risk is 5%normally is usually due to multifactorial (1 in 20).inheritance. The influence of several genes,acting together with environmental factors, 2. How can she lower the risk ofresults in the birth defect. Rarely, the neural having another affected child?tube defect may be due to chromosomal or She should take 1 mg folic acid daily until shesingle gene defects, or teratogens. falls pregnant and then continue to take folic acid until three months after conception.3. What is the most importantenvironmental factor that plays a role 3. How effective is periconceptional folicin causing neural tube defects? acid supplementation in lowering theFolic acid. A relative lack of folic acid in the risk for another neural tube defect?diet may act together with genetic factors to It should significantly lower the risk of neuralresult in neural tube defects. tube defect by up to 70%.
    • NEURAL TUBE DEFECTS 1134. How can a pregnant woman be screened and three months after falling pregnant. Thisfor a fetus with a neural tube defect? is often taken in the form of one multivitamin tablet containing folic acid daily, and couldEither with maternal serum alpha-fetoprotein lower the risk of having an infant with ascreen around 16 weeks of pregnancy or by neural tube defect by up to 50%. However,fetal ultrasound scanning at around 18 weeks most women falling pregnant in South Africaof pregnancy. Ultrasound examination is also are provided with extra folic acid through theused to confirm gestational age needed for fortification of maize meal.maternal serum alpha-fetoprotein screening. 6. What should parents do if the fetus is5. Should all women planning a pregnancy found to have a neural tube defect?take periconceptional supplements? They must be referred for genetic counselling.It is recommended that all women take at least0.4 mg folic acid daily for three months before