Fetal Anatomy Survey: CNS and Face

Quick Answer: Transtentorial for ventricles/CSP; transcerebellar for posterior fossa. Atria <10 mm. Cisterna magna 2–10 mm. Face: profile + lips. Know banana cerebellum, lemon skull, holoprosencephaly.

CNS Survey: Plane-First Thinking

The fetal brain anatomy survey is the most image-heavy portion of second-trimester OB on the RDMS exam. Questions test standard plane recognition before pathology labeling. Learn what must be visible in each plane; if a structure is absent or malformed, document it and correlate with spine and genetic counseling pathways.

Standard CNS Planes

Transtentorial (Axial) Plane

• Thalami symmetric and central
• Cavum septi pellucidi (CSP) between frontal horns
• Falx midline
• Lateral ventricles with choroid plexus
• Atria measured if dilatation suspected

Absent CSP in second trimester triggers corpus callosum agenesis workup—do not dismiss as normal variant without detailed neurosonography or MRI referral.

Transcerebellar Plane

• Cerebellar hemispheres and vermis
• Cisterna magna (CM) depth 2–10 mm
• Banana-shaped cerebellum + lemon-shaped skull → classic association with open spina bifida (Chiari II)

Midsagittal and Coronal Brain

Midsagittal assesses corpus callosum arch when protocol requires. Coronal views evaluate orbits, parietal symmetry, and upper lip.

Major CNS Anomalies (Registry Recognition)

Worked Scenario: Enlarged Cisterna Magna

CM measures 12 mm. Before calling Dandy-Walker, assess vermis continuity on midsagittal imaging. Isolated mega cisterna magna with normal vermis may be benign; vermian hypoplasia requires MFM/genetics referral.

Face and Profile Assessment

Cleft palate is poorly seen on 2D—document "upper lip intact; palate not assessed" when 2D incomplete. 3D or targeted views may help in specialized centers.

Worked Scenario: Cleft Lip vs Palate

Coronal lip view normal; clinical suspicion for palate cleft. Correct documentation: cleft lip not identified; fetal palate not adequately visualized on 2D—not "normal face."

Soft Markers vs Major Anomalies

Choroid plexus cysts (CPC): common, often bilateral, usually resolve. Isolated CPC has low aneuploidy risk but registry stems may pair with trisomy 18 context when other findings present.

Echogenic intracardiac focus (EIF): crosses cardiac domain—know as soft marker.

Soft markers do not equal anomalies but prompt correlation with maternal age, screening, and complete anatomic survey.

Spine Linkage (Mandatory Correlation)

Open neural tube defect affects posterior fossa morphology. Always complete longitudinal and transverse spine sweeps assessing skin line continuity and vertebral alignment. Open defect: splayed arches, cystic meningocele possible. Closed defect: subtle—may need high-frequency targeted scan.

Ventriculomegaly Follow-Up

Mild ventriculomegaly near 10 mm may resolve on follow-up. Persistent or progressive dilatation warrants MRI and genetics consultation. Measure atria consistently on transtentorial plane at the level of the choroid plexus glomus.

Microcephaly and Macrocephaly

Diagnose microcephaly by serial HC percentiles trending below 3rd percentile—not a single borderline measurement. Macrocephaly similarly requires trend and ventricular assessment to exclude hydrocephalus.

Documentation Checklist

List systematically: CSP present/absent, ventricular size, cerebellum, CM measurement, facial profile, upper lip, spine reference, intracranial symmetry.

Exam Traps

• Measuring ventricles on transcerebellar plane
• Calling Dandy-Walker without vermian assessment
• Missing absent CSP on otherwise "normal" brain
• Labeling isolated CPC as lethal anomaly
• Ignoring lemon/banana signs without spine sweep

CNS items reward plane recognition—name the plane, list required structures, then decide if the image shows normal, variant, or malformation.

Corpus Callosum and Midline Integrity

When CSP is absent or abnormal, perform targeted midsagittal imaging for corpus callosum continuity. Agenesis may show teardrop-shaped lateral ventricles (colpocephaly) and parallel ventricular walls on axial imaging. Interhemispheric cyst may accompany callosal anomalies. Registry items test whether absent CSP alone mandates referral—answer is yes for detailed neurosonography or fetal MRI when feasible.

Posterior Fossa Differential Diagnosis

Always measure cisterna magna depth on transcerebellar plane and document vermian continuity on midsagittal sweep before finalizing posterior fossa impression.

Nasal Bone and Profile Markers

Absent or hypoplastic nasal bone in second trimester is a soft marker for aneuploidy when combined with other findings—not a standalone diagnosis. Profile view assesses premaxillary bone, forehead slope, and chin. Retrognathia may suggest Pierre Robin sequence—correlate with palate when possible.

Ventriculomegaly Measurement Technique

Measure atrial width perpendicular to ventricular axis at level of choroid plexus glomus on transtentorial plane. Bilateral symmetric mild dilatation near 10 mm may resolve; asymmetric or progressive dilatation warrants MRI referral. Document whether cerebellar vermis and spine were assessed when ventriculomegaly is present.

Neural Tube Defect Screening Integration

CNS survey is incomplete without spine evaluation. Open NTD produces Chiari II intracranial signs before skin defect is obvious. Closed NTD may show only subtle vertebral defects—high-frequency targeted sweep improves detection. Lemon skull and banana cerebellum should trigger immediate longitudinal and transverse spine documentation in the report.