Neural Tube Defects: Sonographic Findings

Fetal Central Nervous System

Embryology

• Neurulation initiates with the formation of the neural plate.
• The neural tube fuses and closes after the neural folds form.
• The tube closes from the center but leaves both ends temporarily open.
• By 6 weeks per the LMP, the neural tube should be completely closed.

Normal Anatomy

• Spine examination should be done in two planes.
• The transverse plane is the best plane to detect spina bifida.
• Evaluation in the transverse plane should include location and configuration of ossification centers/vertebrae, integrity of musculature of the back, and skin line.
• Assess cervical and lumbosacral curvature, sacral caudal tapering, and vertebral ossification centers in the sagittal plane.
• Normal spine should demonstrate 3 ossification centers.
• There should be two posterior lamina and one anterior vertebrae that form a triangle or circle.
• Kidneys are the landmark for the lumbar spine.
• The segment below the Thoracic spine (T1-5) is the Lumbar Spine.

Brain

• Must document cavum septum pellucidum, thalami, lateral ventricles, and choroid plexus.
• Measure BPD and HC.
• Measure the atrium of lateral ventricle; normal is less than 10 mm/1 cm.
• Oblique axial sections are obtained through the posterior fossa.
• Document cerebellum and brain stem.
• The normal measurement of the cisterna magna is 3-10 mm.

Neural Tube Defects

• NTDs occur when the neural tube fails to completely close by six weeks.
• These can occur anywhere along the cranium or spine.
• Cerebrospinal fluid can escape into the amniotic fluid, causing elevated fluid levels and elevated MSAFP.
• Elevated MSAFP is grounds for additional screening.
• If an ultrasound does not identify a problem, then amniocentesis is suggested.
• Amniocentesis can detect acetylcholinesterase, which is elevated in the amniotic fluid when a neural tube defect is present.
• Folic acid supplements can help decrease the risk of a NTD.

Anencephaly

• This is the most common NTD.
• There is congenital absence of the cerebral hemispheres and cranial vault.
• Occurs in 1 out of every 1000 births, but varies geographically.
• More common in females and multiple gestations.
• Occurs when the neural tube fails to close by 24 days gestation.
• Characterized by an open defect covered by angiomatous stroma rather than skin or bone.
• Brain stem and bony base of the skull that form other structures are present.
• Associated with polyhydramnios due to ineffective fetal swallowing.
• May be seen with spina bifida.

Anencephaly Sonographic findings

• Fetal cranium should be identifiable by 12 weeks (15 at the latest).
• There is absence of cranial vault and cerebral hemispheres.
• Face and orbits are present, but are frog-like.
• Polyhydramnios occurs in 40-50% of cases.

Acrania/Exencephaly

• There is a developmental abnormality in which the cranium is partially or completely absent with the development of abnormal brain tissue.
• This condition is lethal.
• Increased levels of MSAFP.

Acrania/Exencephaly Sonographic findings:

• There is a lack of echogenic cranium with presence of a large amount of brain tissue.
• Can be demonstrated as early as 12 weeks with TV exam.
• Reliably demonstrated at 16 weeks as a lack of an ossified cranium in the presence of normal long bones
• Abnormal long bones can indicate skeletal dysplasia.

Encephalocele

• Herniation of the brain and meninges or meninges and CSF (menigocele) through a cranial defect.
• Prognosis depends on the amount of brain involved and other findings.
• Associated with Meckel-Gruber syndrome.
• Encephaloceles are usually midline, most commonly occipital and may be frontal or lateral.
• An amniotic band syndrome should be considered if asymmetric or atypical location.

Encephalocele Sonographic findings:

• Purely cystic extracranial mass (meningocele)
• Solid mass contiguous with the cranium (cephalocele)
• Often associated with hydrocephalus and polyhydramnios

Spina Bifida

• General term for the lack of closure of the vertebral column.
• Prognosis depends on the level and severity of the lesion.
• Prognosis is poorest in infants who have total paralysis below the lesion, kyphosis, hydrocephalus and other associated congenital defects.
• It usually occurs in the lumbosacral region.

Types of Spina Bifida - Occulta

• When defect is covered with skin or hair, referred to as spina bifida occulta.
• Associated with normal spinal cord and nerves and normal neurologic development.
• Spina bifida occulta is extremely difficult to detect prenatally, because the defect of the spine is covered by skin.
• Maternal serum AFP level will be normal

Types of Spina Bifida

• Meningocele is when the defect involves only protrusion of meninges
• Meningomyelocele is more common when both meninges and neural elements protrude through defect.
• Rachischisis is when the defect is very large and severe.

Spina Bifida Sonographic Findings

• Best to evaluate for spina bifida in the transverse plane.
• Splaying of posterior elements into a U or V configuration.
• When sac is intact, cystic structure may be seen extending from the back.
• You may see a small cystic structure, a cyst with septations or a solid mass.
• Splaying of parallel ossification centers.
• There is a soft tissue defect or dicontinuity of skin and muscle of the back in the sagittal plane.
• Be aware of NORMAL widening of the spin in the cervical and lumbar regions.

Intracranial Findings associated with Spina Bifida

• Associated findings are secondary to the Arnold Chiari II Malformation.
• Banana sign (A) – obliteration of the cisterna magna by abnormal configuration of the cerebellum.
• Lemon sign (B, C) – flattening of the temporal/frontal bones due to decreased intracranial pressure.
• Hydrocephalus (D) (lateral ventricle greater than 10 mm)
• Iniencephaly is a rare malformation in which the occiput is fused to the cervical region and cervical spina bifida and occipital encephalocele are present.
• Marked neck and head hyperextension

Spinal Anomalies

Sacrococcygeal Teratoma

• A rare tumor arising from the embryonic cells of the sacrum and coccyx.
• There are three types: benign (mature), immature, and malignant.
• The tumors are frequently hypervascular and consist of solid and cystic components.
• They may be external, intrapelvic, or intra-abdominal.
• Sonographic Findings include:
  • Complex large mass inside or outside of the body
• Polyhydramnios
• Associated with increased MSAFP
• May have hydrops

Caudal Regression Syndrome

• Associated with diabetes in 16% of cases.
• Includes a spectrum of skeletal anomalies of the lower spine and lower limbs.
• Sacral agenesis
• Lumbar or thoracic agenesis
• Associated anomalies of the GI and GU tracts, CNS and heart anomalies.

Scoliosis and Kyphosis

• Abnormal curvature of the spine that may involve any segment.
• Most frequently it is the thoracolumbar region.
• Associated with other structural defects like CNS and VATER association.
• Severe curvature is associated with lethal anomalies such as anencephaly, amniotic band syndrome, and Limb-body wall complex.

Intracranial Abnormalities

The Ventricles and CSF

• CSF is produced by the choroid plexus.
• It flows from the lateral ventricles to the interventricular foramen/foramen of Monro.
• From there it goes to the third ventricle and then passes through the cerebral aqueduct/aqueduct of Sylvius, then it moves to the fourth ventricle.

Ventriculomegaly/Hydrocephalus

• This is a dilatation of the ventricular system secondary to an increase in volume of CSF.
• Effects the flattening of the parenchyma and spread of CSF which causes demyelnation and brain damage.

Classifications of Hydrocephalus

• Obstructive/non-communicating hydrocephalus is caused by obstruction of CSF due to aqueductal stenosis (narrowing of aqueduct of Sylvius).
• Narrowing is due to inflammation or a developmental process, CNS anomaly such as spina bifida, Dandy Walker malformation or a tumor.
• Communicating hydrocephalus is a dilation of all ventricles and subarachnoid space caused by an obstruction to CSF flow OUTSIDE of the ventricular system.
• It is Caused by faulty absorption of CSF or increased CSF production.

Hydrocephalus Sonographic Findings

• Normal ventricular configuration, but dilated
• Presence of excess fluid in lateral ventricles (measurement greater than 10 mm).
• “dangling” choroid plexus
• Possible dilation of third and fourth ventricles
• Fetal head enlargement when biparietal and head circumference measurements exceed those for established gestational age.
• Associated findings:
  • Polyhydramnios
  • Abnormal fetal lie
  • Hepatomegaly
  • Fetal ascites with infection
  • Meningomyelocele, Dandy-Walker malformation and Encephalocele
  • Intracranial tumor

Holoprosencephaly

• Spectrum of disorders resulting from absent or incomplete diverticulation (division) of the forebrain into cerebral hemispheres and lateral ventricles.
• Can be sporadic, due to chromosomal anomalies (Especially Trisomy 13) or maternal infection.
• Associated with facial anomalies due to the common embryonic origin, the face predicts the brain: Cyclopia, proboscis, hypotelorism, facial clefts.
• Holoprosencephaly Sonographic Findings depend on types and have a normal cerebellum and posterior fossa.

Types of Holoprosencephaly

• Alobar is the most severe form.
• Includes monoventricle, fused thalami, and absence of falx.
• Semi-lobar includes partial separation of the ventricles and hemispheres, occipital lobe present, incompletely fused thalami.
• Lobar is the least severe form
• Can include normal separation of thalami, hemispheres and ventricles.
• Absent Cavum Septum Pellucidum and olfactory tracts

Hydranencephaly

• Destructive disorder due to bilateral internal carotid artery occlusion/malformation.
• Characterized by a near total lack of hemispheres with an intact and normally developing meninges and skull.
• The newborn with hydranencephaly may have normal neurologic functions but does not develop, and computed tomographic scans indicate an absence of cerebral tissue.
• Conditions that can occur with hydranencephaly include deafness, blindness, paralysis, and cognitive impairments.
• Treatment is supportive and the condition should not be confused with hydrocephalus, which is an accumulation of cerebrospinal fluid in the ventricles.
• No associated abnormalities

Hydranencephaly Sonographic Findings:

• Large fluid-filled cranium
• Absent cerebral tissue
• Falx is present
• Normal midbrain and basal ganglia (thalami)
• Polyhydramnios

Dandy-Walker Malformation

• Complete or partial absence of the cerebellar vermis and posterior fossa cystic dilatation communicating with the 4th ventricle.
• 80% also have hydrocephalus
• Some autosomal recessive syndromes, maternal infection, diabetes mellitus, and exposure to ETOH and Coumadin are all associated.

Dandy-Walker Malformation Sonographic Findings:

• This includes complete or partial agenesis of the cerebellar vermis with flattened cerebellar hemispheres
• There will be a large midline cystic structure in the posterior fossa associated with ventriculomegaly and polyhydramnios

Agenesis of the Corpus Callosum

• Anatomy review: The corpus callosum is a broad band of nerve fibers joining the two hemispheres of the brain.
• Development of the CC should be complete by 20 weeks
• Agenesis occurs in 1-3 per 1000 births.
• Due to chromosomal abnormality or chromosomal translocation
• May be complete or partial and is frequently associated with other anomalies or syndromes.
• Prognosis depends on high incidence of associated anomalies; many carry poor prognosis
• May be asymptomatic or associated with mental retardation and/or seizures

Agenesis of the Corpus Callosum Sonographic Findings

• Absence of Cavum Septum Pellucidum
• Elevated, dilated third ventricle
• Widely separated frontal horns of the lateral ventricles with enlarged occipital horn
• Teardrop shaped ventricles displaced outward and upward

Vein of Galen Aneurysm

• Rare arteriovenous malformation
• Can causes increased flow through the vein of Galen
• Well defined midline vascular structure superior and posterior to the thalamus
• Turbulent and arterial flow may be seen

Choroid Plexus Cysts

• Small cysts within the choroid plexus are common
• Significant because rarely they can be associated with trisomy 18.
• They can be unilateral or bilateral, multiples, single, large or small.
• In the absence of other abnormalities, it is considered a normal variant.
• When cysts are found in a patient who has not had a genetic work-up it is important to follow up.

Intracranial Tumors

• Congenital brain tumors are rare and the prognosis is not good.
• The most common is Teratomas but other types include: Epidermoids, Germinomas, Glioblastomas, and craniopharyngomas.

Intracranial Tumors Sonographic Findings

• This includes loss of normal intracranial architecture
• presence of a space occupying lesion with changes in normal anatomic structures and relationships
• tumor type can't be determined by US

Lissencephaly

• Caused by an abnormal migration
• The brain lacks sulci and gyri and appears smooth
• The diagnosis is made in the third trimester
• Associated with mild ventriculomegaly and possibly an abnormal corpus callosum

Schizencephaly

• Rare Clefts in the cerebral hemispheres
• Only in the region of the primary fissures
• Usually bilateral and symmetric, but not always
• The Brain appears “split” into anterior and posterior portions.

Porencephaly

• Presence of cystic areas within the cerebral parenchyma
• Cysts are varied in size and location and may communicate with the ventricular system.
• Thought to be caused by intracranial hemorrhage or encephalomalacia, infarction, delivery trauma, or inflammatory changes in nervous system

Porencephaly caused by

• Encephalomalacia, or softening and loss of brain tissue after cerebral infarction, cerebral ischemia, infection, or craniocerebral trauma
• Affected brain parenchyma undergoes necrosis, brain tissue is resorbed, and a cystic lesion remains
• Sonographic Findings includes Simple cystic structures in the cerebral parenchyma

Microcephaly

• Defined as decreased head size (more than 3 SD below the mean).
• Symptom of several etiologic disturbances.
• Causes:
  • cranyiosynostosis, where the skull prematurely fuses by turning into bone changing the growth pattern of the skull.
  • chromosomal anomalies
  • exposure to teratogens

Flow Chart for Evaluation of the Fetal CNS

• Is the skull present? If No, anencephaly or acrania
• Is the skull intact? If No - encephalocele, evaluate contents, consider Meckel-Gruber (look at kidneys, hands, feet). If asymmetric encephalocele, consider amniotic band syndrome
• Is the falx present? If No - holoprosencephaly – check the face
• Are the thalami separate? If NO - holoprosencephaly
• Are the lateral ventricles normal? If NO - ventriculomegaly/hydrocephalus (look for choroids), Evaluate spine (look at head shape (lemon and banana signs)), and evaluate cerebellar vermis for Dandy-Walker.
• Is the choroid plexus identifiable and normal? If NO - hydranencephaly or choroid plexus cyst? Then, evaluate for trisomy 18 findings.
• Perform a Color Doppler – check vein of Galen

Explore sonographic findings associated with neural tube defects (NTDs) such as anencephaly, acrania, encephalocele, and spina bifida. Identify key indicators, including elevated amniotic fluid substances, structural absences, and associated conditions.