Hydrocephalus and neural tube defects

• Hydrocephalus is the symptomatic accumulation of cerebrospinal fluid (CSF) inside the cerebral ventricles, from obstruction of flow, failure of absorption, or excess production. ^[1]
• Congenital hydrocephalus is often present at birth, commonly as part of a genetic syndrome or a spinal dysraphism. ^[1]
• A neural tube defect is any failure of the neural tube to close in early fetal life. Spina bifida ranges from the hidden occulta to open defects in which the spinal cord and meninges are exposed. ^[2][3]
• Myelomeningocele, the most serious common open defect, brings paralysis, neurogenic bladder and Chiari II malformation with hydrocephalus. ^[3]
• Periconceptional folic acid cuts the risk of neural tube defects. ^[7]
• After birth, exposed neural tissue is closed and any hydrocephalus is relieved with a ventricular shunt or, in selected cases, an endoscopic procedure. ^[1][3]

By the end of this page you should be able to:

1. Explain the three mechanisms that produce hydrocephalus and how the type changes the clinical picture. ^[1]
2. Classify spina bifida and describe what tissue is exposed in an open defect. ^[2]
3. Name the modifiable and non-modifiable risk factors for myelomeningocele. ^[3]
4. Recognise the Chiari malformations and state the imaging measure used for Chiari I. ^[4][5]
5. List the maternal and fetal clues that suggest an open neural tube defect before birth. ^[3]
6. Plan initial management after birth for an infant with myelomeningocele and hydrocephalus. ^[1][3]
7. State the folic acid doses recommended before and in early pregnancy and who needs the higher dose. ^[7]
8. Explain how a ventriculoperitoneal shunt works and what makes it fail. ^[6]

Hydrocephalus and the neural tube defects are problems of the developing nervous system that a paediatrician meets in utero, in the delivery room and for a lifetime after. Hydrocephalus is not one disease but a final common pathway: cerebrospinal fluid (CSF) collects in the ventricles because something blocks its passage, something impairs its reabsorption, or, rarely, something makes too much of it. ^[1] The neural tube defects are earlier and more structural. The neural tube must close within the first month of gestation, and if it does not, the spine or skull may remain open, with the spinal cord or brain exposed and under threat. ^[3]

Because one condition often leads to the other, they are best learned together. An open spinal dysraphism leaks CSF, and that loss pulls the cerebellum and brainstem down through the foramen magnum, producing the Chiari II malformation; that distortion obstructs CSF circulation, and hydrocephalus follows. ^[3][4] Treatment therefore has two aims: close the defect and protect the brain from the fluid.

Hydrocephalus is the symptomatic accumulation of CSF inside the cerebral ventricles. ^[1] Dandy's old division into communicating and non-communicating (obstructive) forms still organises teaching, although many further classifications have been proposed since. ^[1] An adult pattern recognises four types: obstructive, communicating, hypersecretory, and normal pressure hydrocephalus. ^[1] In children, congenital or developmental hydrocephalus is often present at birth and is frequently part of a genetic syndrome or a spinal dysraphism. ^[1]

For practical purposes the useful distinction is which way the CSF is blocked. Obstructive (non-communicating) hydrocephalus has a block within the ventricular system, for example at the aqueduct of Sylvius or the outlets of the fourth ventricle. Communicating (absorptive) hydrocephalus has free passage of CSF over the brain surface, but reabsorption at the arachnoid granulations is impaired, often after bleeding or infection. Hypersecretory hydrocephalus is rare and follows excessive production, classically from a choroid plexus papilloma.

The term spina bifida is nonspecific and refers to any degree of neural tube closure defect. ^[2] It divides into two broad categories. Spina bifida occulta, also known as closed spinal dysraphism, is the mildest form: a concealed vertebral defect with minimal neural involvement, sometimes marked only by a patch of hair, a dimple, or a fatty swelling over the lower back. ^[2] Spina bifida aperta, also known as open spinal dysraphism, is a defect in which neural tissue is exposed and communicates with the outside world, as in a meningocele, a myelomeningocele, or a myeloschisis. ^[2]

Atypical variants exist and include segmental spinal dysgenesis, lipomyelomeningocele, the human tail, membranous meningocele, and myeloschisis. ^[2]

Myelomeningocele is a severe neural tube defect caused by incomplete closure of the spinal cord during early fetal development, typically within the first month of gestation. ^[3] It is a form of open spinal dysraphism, with the spinal cord and its meninges protruding from the vertebral column. ^[3] Closed spinal dysraphism, or spina bifida occulta, consists instead of skin-covered lesions that may not be obvious except for cutaneous clues such as haemangiomas, hypertrichosis, sacral dimples, or subcutaneous lipomas. ^[3]

Chiari malformations are a spectrum of hindbrain abnormalities involving the cerebellum, brainstem, skull base, and cervical cord. ^[4][5] They are among the most common craniocervical junction malformations seen in adults. ^[4] Classification rests on morphology and severity, usually assessed on imaging. ^[5]

Chiari I is the least severe and is often found incidentally. It is defined by one or both pointed (not rounded) cerebellar tonsils projecting 5 mm below the foramen magnum, measured by a line drawn from the basion to the opisthion, the McRae line. ^[5] Chiari II, the Arnold-Chiari malformation, is usually associated with spina bifida. It is marked by caudal displacement of the cerebellar vermis, the brainstem, and the fourth ventricle, arising from an open distal spinal dysraphism or myelomeningocele. ^[4][5] It is a distinct disease process from Chiari I, despite the shared radiographic findings. ^[4] Chiari II can be accompanied by hydrocephalus and syringomyelia. ^[4] Chiari III involves herniation of the hindbrain into a low-occipital or high-cervical meningoencephalocele. ^[5] Chiari IV, severe cerebellar hypoplasia, is now considered obsolete. ^[5] Several further designations, including Chiari 0, Chiari 1.5, and Chiari V, are reported but remain controversial and are not universally accepted. ^[5]

Hydrocephalus in children carries a mortality of 0 to 3%, depending on the length of follow up. ^[1] Untreated congenital hydrocephalus in a child produces irritabilities, chronic headaches, learning difficulties, visual disturbances, and, in advanced cases, severe mental retardation. ^[6] Myelomeningocele is compatible with survival into adulthood with specialist care, but its associated conditions, Chiari II malformation and hydrocephalus, worsen both morbidity and mortality. ^[3]

Myelomeningocele results from incomplete closure of the spinal cord in the first month of gestation. ^[3] Its risk factors include environmental exposures, maternal health problems, inadequate pre-pregnancy folic acid intake, and genetic predispositions, including chromosomal abnormalities such as trisomy 18 and trisomy 13. ^[3] Inadequate periconceptional folic acid is the principal modifiable factor and is the reason folic acid is recommended before pregnancy. ^[3][7]

Hydrocephalus has a different causal spectrum. Congenital forms may be part of a genetic syndrome or a spinal dysraphism. ^[1] Hydrocephalus is also a complication of an open neural tube defect rather than an independent disease in that setting. ^[3]

CSF is produced, circulates, and is reabsorbed. ^[1] Hydrocephalus appears when that balance fails in one of three ways: obstruction of the normal flow of CSF, failure of absorption into the venous system by the arachnoid granulations, or excessive production of CSF. ^[1] In an infant whose cranial sutures have not fused, the ventricles enlarge by pushing the skull outward, so head circumference grows and the fontanelle may bulge. ^[1] Once the sutures fuse, the same volume within a fixed box raises intracranial pressure and threatens herniation. ^[1]

The link between spina bifida and hydrocephalus is mechanical. In myelomeningocele the CSF leak through the open defect changes the dynamics of the developing hindbrain, pulling the cerebellar vermis, brainstem, and fourth ventricle downwards into the spinal canal. ^[3][4] That Chiari II malformation itself distorts the pathways by which CSF leaves the fourth ventricle and is absorbed, so hydrocephalus develops in most infants with myelomeningocele. ^[3][4]

In infants, the cardinal sign of hydrocephalus is a head growing faster than expected, often noticed first by parents or on routine measurement. ^[1] There may be irritability, vomiting, poor feeding, a high-pitched cry, and delayed milestones. ^[6] In older children and adults the presentation is headache, nausea, visual disturbance, gait difficulty, or cognitive change; normal pressure hydrocephalus in adults presents with gait disturbance, cognitive impairment, and urinary incontinence. ^[1]

In myelomeningocele the findings depend on the level of the lesion. ^[3] Higher lesions produce paraplegia; lower ones produce weakness, sensory loss, and bladder and bowel dysfunction. ^[3] Neurogenic bladder is common and can damage the kidneys if untreated. ^[3] Chiari II may add brainstem and lower cranial nerve signs: stridor, apnoeic spells, poor swallow, and arm weakness. ^[4]

The fontanelle may be full and tense, the scalp veins dilated, and the sutures separated. ^[1] Measurement of head circumference with its normal centiles is the simplest screening test, and plotting it over time matters more than a single value. ^[1] Transillumination and setting-sun sign of the eyes are described in infants. In spina bifida, the open sac on the back is usually obvious at birth, and a careful neurological examination maps motor and sensory levels. ^[3]

A newborn has a lumbar midline sac containing meninges and spinal cord. Movements in both legs are weak, the anus is lax, and the head circumference is enlarged. The mother took no folic acid before pregnancy. The likely diagnosis is myelomeningocele with hydrocephalus; an urgent neurosurgical assessment is needed to close the defect and relieve the fluid. ^[3][4]

Elevated maternal serum alpha-fetoprotein and prenatal ultrasonography detect open neural tube defects; the ultrasound shows characteristic cranial and spinal signs. ^[3] Amniotic fluid acetylcholinesterase has been used in specialist settings. Where myelomeningocele is found, detailed imaging of the fetal head assesses ventriculomegaly and hindbrain herniation.

For hydrocephalus, cranial ultrasonography through the open fontanelle is the first test in a newborn; computed tomography or magnetic resonance imaging of the head defines the ventricular system and the cause in older infants. ^[1] Magnetic resonance imaging of the whole neuraxis and craniovertebral junction establishes the presence and type of Chiari malformation. ^[4][5] Spinal imaging in spina bifida defines the level and any associated cord tethering.

In countries with fortification programmes and routine antenatal care, the combination of maternal serum screening and the detailed anomaly scan has reduced the number of babies born with open defects. ^[3][7]

There is no single numeric diagnostic threshold for hydrocephalus that stands alone, because ventricular size must be interpreted against age, skull maturity, and symptoms. ^[1] The diagnosis is clinical and radiological together: enlargement of the ventricles with evidence of raised pressure or a progressive increase in size. ^[1] Chiari I has an established imaging criterion, with one or both pointed cerebellar tonsils projecting at least 5 mm below the foramen magnum on the McRae line. ^[5] Chiari II is diagnosed in the setting of an open distal spinal dysraphism with caudal displacement of the vermis, brainstem, and fourth ventricle. ^[4][5]

Associated conditions dominate management. Myelomeningocele commonly brings neurogenic bladder, renal problems, Chiari II malformation, and hydrocephalus, each of which worsens morbidity and mortality. ^[3]

Acute hydrocephalus raises intracranial pressure and can cause brain herniation and death if not treated promptly. ^[1] The first step is to relieve pressure urgently, usually by drainage, while the cause is established. ^[1] Definitive treatment depends on type. Surgical placement of a shunt is the first treatment option for hydrocephalus. ^[1] Endoscopic third ventriculostomy and choroid plexus cauterisation are possible in suitable forms, in which a natural or created opening bypasses the obstruction. ^[1]

Long-term care has two parallel tasks: keep the ventricles at a safe size, and support the developing child. Children with shunts need regular review of head circumference, development, and imaging, and any deterioration should be treated as possible shunt failure until proved otherwise. ^[1][6]

Myelomeningocele is managed by early postnatal surgical closure or by prenatal repair, both aimed at protecting the spinal cord and reducing complications such as hindbrain herniation. ^[3] NICE guidance on open prenatal repair is specific: efficacy evidence is adequate, but safety concerns for the mother and fetus mean the procedure should only be done with special arrangements for clinical governance, consent, and audit or research, in specialised centres with teams specifically trained, and with patient selection made by a multidisciplinary team that includes fetal medicine, obstetric surgery, paediatric neurosurgery, fetal imaging, and anaesthesia. ^[8] After closure, associated hydrocephalus and Chiari II malformations are treated with shunt placement or endoscopic surgery. ^[3]

Treatment does not stop at the spine. Neurogenic bladder needs surveillance and management to protect kidney function, and orthopaedic, mobility, and developmental needs should be addressed early. ^[3]

There is no drug that reverses hydrocephalus or an open neural tube defect. Folic acid is prevention, not treatment: the doses are given below. ^[7]

The complications of hydrocephalus and neural tube defects dominate later life.

• Hydrocephalus that is acute and untreated can cause brain herniation and death. ^[1]
• After shunt placement, malfunction allows CSF to accumulate again, raising intracranial pressure and potentially causing cerebral oedema and herniation. ^[6]
• A myelomeningocele commonly brings neurogenic bladder, renal problems, Chiari II malformation, and hydrocephalus. ^[3]
• Neural tube defects are often accompanied by other developmental abnormalities, so an interprofessional approach is needed to optimise survival and outcome. ^[2]

Prognosis in myelomeningocele follows the level of the lesion: the higher the lesion, the greater the motor deficit. ^[3] Late diagnosis or untreated disease has a worse prognosis. ^[3] Surgical repair improves outcome but does not eliminate disability, and long-term follow up into adulthood is the norm. ^[3] In children with hydrocephalus, modern treatment has reduced mortality, although the range of 0 to 3% still depends on the duration of follow up. ^[1]

Periconceptional folic acid is the single most effective prevention against neural tube defects. ^[3][7] NICE advises that a woman who may become pregnant, is planning pregnancy, or is in the first 12 weeks of pregnancy should take 400 micrograms of folic acid a day, starting ideally 3 months before trying for a baby, or as early as possible after a positive pregnancy test, and continuing for at least the first 12 weeks. ^[7] Healthy Start vitamins provide a daily 400 microgram dose. ^[7] A higher dose of 5 mg a day is recommended for women with an increased risk of having a baby with a neural tube defect, for example a personal or family history of a neural tube defect, a previous affected pregnancy, type 1 or type 2 diabetes, sickle cell anaemia or thalassaemia, or use of medicines that alter folic acid absorption or metabolism such as anti-epileptic drugs or medicines for HIV. ^[7] A body mass index of 25 kg/m2 or more does not by itself justify more than 400 micrograms a day unless one of those risk factors is present. ^[7] Even where flour is fortified, NICE recommends taking the supplement as well. ^[7]

Folic acid should be taken in addition to a healthy diet, not as a substitute for it, and the reasons to take it should be discussed at every opportunity. ^[7][9]

Open neural tube defects are detectable before birth by elevated maternal serum alpha-fetoprotein and ultrasonography showing characteristic cranial and spinal signs. ^[3] Globally, WHO provides evidence-informed recommendations on routine antenatal care, including the nutritional and preventive measures that underpin detection. ^[9] WHO technical guidance also describes assessment of serum and red blood cell folate concentrations as useful for monitoring folate status trends and evaluating the impact of public health interventions. ^[10]

Prenatal repair remains restricted to specialised centres under the governance conditions described by NICE. ^[8]

Because closure happens within the first month, folic acid started after a first missed period is often too late to prevent the defect. ^[3][7] Any clinician who meets a woman who could become pregnant should opportunistically advise on periconceptional folic acid. ^[7]

Infants with hydrocephalus and a shunt need coordinated neurosurgical, paediatric, and developmental follow up. ^[1][6] Parents should be taught the signs of shunt failure: increasing head size, irritability, vomiting, or a change in behaviour or tone. ^[6]

Normal pressure hydrocephalus in older adults is a different entity from the congenital forms, with a triad of gait disturbance, cognitive impairment, and urinary incontinence, and it is treated differently. ^[1]

• Where routine antenatal ultrasound and maternal serum screening are unavailable, open defects are detected at birth. Community and maternity staff should recognise the open sac and refer promptly. ^[3]
• Folic acid tablets are cheap and widely available, so primary prevention remains the highest-value intervention even where surgery is scarce. ^[7]
• VP shunt surgery requires resources and long-term follow up. Where shunts are not available, endoscopic procedures may be an option in selected centres. ^[1]
• A shunt malfunction can kill quickly. Training parents and primary workers to recognise the early signs is a life-saving measure wherever follow up is intermittent. ^[6]

• Hydrocephalus is the symptomatic build-up of CSF in the ventricles from obstruction, failed absorption, or excess production. ^[1]
• In infants the sutures are open, so pressure first makes the head enlarge; after fusion, it threatens herniation. ^[1]
• Congenital hydrocephalus is often part of a genetic syndrome or a spinal dysraphism. ^[1]
• Spina bifida is an umbrella term for any failed neural tube closure; occulta is closed, aperta is open and exposes neural tissue. ^[2]
• Myelomeningocele is the severe open form, with level-dependent paralysis, neurogenic bladder, and associated Chiari II and hydrocephalus. ^[3]
• Chiari I is tonsillar herniation of at least 5 mm below the foramen magnum; Chiari II is the spina bifida-associated descent of vermis, brainstem, and fourth ventricle. ^[4][5]
• A shunt is first-line surgery for hydrocephalus, with endoscopic options in selected cases. ^[1]
• Shunt malfunction raises intracranial pressure and can lead to cerebral oedema and herniation. ^[6]
• Folic acid 400 micrograms a day, ideally from 3 months before conception through the first 12 weeks, is the key preventive measure, with 5 mg a day for women at increased risk. ^[7]
• Open neural tube defects can be screened for before birth with maternal serum alpha-fetoprotein and ultrasonography. ^[3]
• Prenatal repair is restricted to specialised centres under strict governance conditions. ^[8]

• Infantile hydrocephalus and its long-term follow up
• Chiari malformation
• Spina bifida and myelomeningocele
• Periconceptional folic acid and preconception care
• Prenatal screening for structural anomalies
• Paediatric neurosurgery and rehabilitation