Central nervous system. Fetal CNS: Brain and Spine Martha L. Decker, MD. Central nervous system. Central nervous system

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1 Central nervous system Fetal CNS: Brain and Spine Martha L. Decker, MD Malformations some of the most common of all congenial abnormalities Neural tube defects the most frequent at 1-2 cases per 1000 births Long term studies suggest intracranial abnormalities may be as high as 1 in 100 births Central nervous system Sonography is THE tool to visualize anatomy, pathology and developmental changes Most common approach is transabdominal usually can obtain axial planes or views Highly skilled sonographers use transvaginal scanning to obtain coronal and sagittal views 3D sonography allows one to view all 3 orthogonal planes Central nervous system Appearance of brain and spine changes dramatically throughout gestation want to be familiar with anatomy in the first, second and third trimesters Get best images earlier in gestation due to less ossification For basic exam use 3-5 MHz abdominal transducer with harmonics on International Society of Ultrasound in Obstetrics and Gynecology guidelines 2007 basic examination targeted t exam or neurosonogram basic examination Three intracranial views and three spine views 1

2 basic examination Intracranial 3 views and 2 components Views: transventricular, transthalamic and transcerebellar Components: biometry and evaluation of brain anatomy Transventricular Transthalamic Transcerebellar basic examination Transventricular plane Components: anterior and posterior portions of the lateral ventricle, cavum septum pellucidum, atrium and choroid plexus CSP seen from weeks Atrium of the lateral ventricle filled with choroid plexus Choroid plexus is very echogenic and produces the spinocerebral fluid basic examination Medial and lateral walls of the lateral ventricle parallel to midline and easily measured At any gestational age, should measure 10 mm or less Transventricular Transthalamic Transcerebellar 2

3 butterfly sign basic examination Transthalamic plane Inferior and parallel to transventricular plane Contains: frontal horns, CSP, thalami and hippocampal gyri Plane used to perform all measurements: BPD, HC, and OFD Transventricular Transthalamic Transcerebellar basic examination Transcerebellar plane Inferior to the transthalamic plane and tilted posteriorly to image the posterior fossa Structures: frontal horns, CSP, thalami, cerebellum and cisterna magna Cerebellum composed of 2 rounded cerebellar hemispheres and joined by vermis; measured from the outer-to-outer edge; 18 th and 24 th the TCD equals # of weeks 3

4 basic examination Cisterna magna: fluid filled space; measured from posterior aspect of cerebellum and inner edge of occipital bone; > 10 mm considered abnormal Thin septa commonly seen within the CM Caution: inferior vermis is not completely formed until 20 weeks Transventricular Transthalamic Transcerebellar basic examination Cavum septum pellucidum Fluid filled cavity in between the frontal horns Visible between 18 and 37 weeks If not seen, abnormalities such as agenesis of the corpus callosum and septo-optic dysplasia could be present Fetal spine Axial, sagittal and coronal views Fetal spine Visualized starting at 14 weeks 3 planes should be obtained: sagittal, coronal and axial or transverse planes Very dependent on fetal position usually can obtain at least two views 4

5 Fetal spine Axial or transverse plane Sweep up and down evaluating the cervical, thoracic, lumbar and sacral segments of the spine 3 ossification centers - body and 2 posterior arches - seen in a triangular shape Cervical Thoracic Lumbar Sacral Fetal spine Sagittal plane Usually see 2 parallel lines: the ossification center of the vertebral body (one line) and one of the posterior arches (second line) Lines flare in upper cervical region and converge towards the sacrum Intact skin should be seen above 2 lines 5

6 Fetal spine Coronal plane Can see one, two or three parallel lines depending on fetal position Choroid plexus cyst Choroid plexus cyst Cysts within the CP > 2 mm Resolve in third trimester Seen in 1% of second trimester scans 50% of T18 fetuses have CP cysts Refer 6

7 Spina bifida Myelomeningocele, meningocele or myeloschisis Spina bifida spinal findings Splayed posterior arch ossification centers Myelomeningocele sac 80% 73% lumbar Transverse view best to see bony defect Almost 100% are detectable Spinal bifida cranial and intracranial findings Easier to see than spinal defect Frontal bone scalloping or lemon sign 99% have Chiari II malformation Cerebellum curved around midbrain or banana sign Obliteration of cisterna magna Ventriculomegaly - > 10 mm 7

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10 Ventriculomegaly Ventriculomegaly Measured at atrium of lateral ventricle mm: mild ventriculomegaly > 15 mm: moderate to severe Normal posterior fossa (CB and CM) Dangling choroid Head size may be large Thinning of cortical mantle 10

11 Anencephaly Anencephaly No calvarium Varying amounts of neural tissue CRL less than expected Protuberant eyes Polyhydramnios common Can be diagnosed between 10 and 14 weeks 100% detected in the second trimester 11

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13 Encephalocele Encephalocele Herniation of intracranial structures through a skull defect Paracranial mass with boney defect Occipital, parietal, vertex and frontal locations 80% occipital Diverse appearance of herniated tissue mixed cystic/solid mass 70 % ventriculomegaly; 25% microcephaly 13

14 Arachnoid cyst Arachnoid cyst Cerebrospinal fluid collection enclosed within layers of arachnoid Extra-axial cyst Most common over cerebral convexities One third of cases occur in the posterior fossa Remaining brain usually appears normal 14

15 Holoprosencephaly Holoprosencephaly Single ventricle Absent midline structures Fused thalami Facial anomalies Alobar, semilobar and lobar Dandy Walker 15

16 Absence of CSP 16

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19 FIGURE Fetal CNS Scanning Less of a Headache Than You Think. MONTEAGUDO, ANA; TIMOR TRITSCH, ILAN Clinical Obstetrics & Gynecology. 55(1): , March DOI: /GRF.0b013e d65 FIGURE 1. Axial scan at the transthalamic plane. A, Demonstrates the biometric measurements of the biparietal diameter (BPD), head circumference (HC), and occipitofrontal diameter (OFD), which are routinely performed during the screening or basic scan. B, The anatomic landmarks of the transthalamic plane are demonstrated. (AH indicates anterior horns; CP, choroid plexus; CSP, cavum septi pellucidi; Th, thalamus) Lippincott Williams & Wilkins, Inc. Published by Lippincott Williams & Wilkins, Inc. 2 19

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