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Fetal CNS: Head and Spine

Frequency of congenital neuro anomalies is 1:500. Anencephaly and spina bifida 1:1000, cephalocele 1:4000


Normal anatomy

  • Midline hypoechoic paired thalami
  • Cavum septi pellucidi (cystic structure immediately anterior to the 3rd ventricle), Cavum Vergae, Cavum Velum Interpositi
  • Slit like 3rd ventricle < 2 mm
  • Lateral ventricles measured on axial views w/ choroid plexus at the level of the atrium < 10mm, minimal ventriculomegaly 10 - 12 mm, choroid plexus should fill 60-90% of the width of the atria, if ventriculomegaly - choroid plexus is dangling ==> dangling choroid plexus sign

Ventriculomegaly

Ex vacuo - fill in the space previously occupied by brain parenchyma


Anencephaly

At 12-14 weeks, absence of normal brain tissue and calvarium above the level of orbits produce “frog-like” appearance. Residuum of dysmorphic brain - angiomatous stroma or area cerebrovasculosa. Polyhydramnios is secondary finding.


Hydrocephalus

Enlarged ventricles due to increased intraventricular pressure. 3rd ventricle rounded or triangular shape > 3 mm. Posterior fossa abnormalities: Mega Cisterna Magna, Dandy-Walker malformation, Small or obliterated cisterna magna and abnl cerebellar shape due to Arnold-Chiari type II malformation. Congenital Aqueductal Stenosis.

There is always residual rim or mantle of brain parenchyma


Hydranencephaly

“Water head”, destruction of the brain parenchyma in the frontal, temporal and parietal regions due to thrombosis of b/l MCA or in utero infection. Ex vacuo fill in with the lateral ventricles. Posterior cerebral circulation Normal → normal development of occipital lobes, posterior fossa and brain stem. No mantle is seen. Lethal condition. Intact choroid plexus w/ abnl resorption → increased pressure later.


Microcephaly

TORCH (Toxo, Other, Rubella, CMV, HSV), trisomies; BPD or HC > 3SD below the norm.


Ventriculomegaly w/ small head and periventricular and parenchymal calcifications ⇒ CMV.

Periventricular and parenchymal calcifications in normal head ⇒ Toxoplasmosis

Calcifications do not have to produce posterior shadowing. In appropriate clinical setting these areas may represent the tubers of TS


Holoprosencephaly

Midline developmental anomalies due to failure of cleavage of brain parenchyma during forebrain development.


Alobar Holoprosencephaly

Single monoventricle MV, fused thalami, absence of midline structures (falx cerebri, cavum septi pellucidi, facial abnl [cyclopia, proboscis, hypotelorism, cleft lip/palate]). MV may communicate w/ dorsal cyst. Brain tissue is of boomerang shape.


Semilobar Holoprosencephaly

Partial development and separation of the lateral ventricles w/ a MV anteriorly communicating w/ partially formed lateral ventricles posteriorly. Thalami and BG are fused.


Lobar Holoprosencephaly

Absence of the frontal horns of the lateral ventricles and cavum septi pellucidi, may resemble agenesis of corpus callosum.


Cephalocele

Cranial meningoceles, encephaloceles. 80% are posterior cephaloceles extending through a defect in the occipital bone. Small anterior cephaloceles within the ethmoidal sinus region.


Posterior fossa


Obliterated cisterna magna

Is caused by an Arnold-Chiari type II owing to downward displacement of the medulla and cerebellar structures through the foramen magnum (banana sign is proof of a neural tube defect).


Dandy-Walker malformation

Enlarged cisterna magna w/ splayed cerebellar hemispheres and agenesis of cerebellar vermis (vermian defect), posterior fossa cyst communicate w/ 4th ventricle --> keyhole. Frequently associated w/ agenesis of corpus callosum, cephalocele, meningocele.


Dandy-Walker variant

Less severe findings, posterior fossa cyst is smaller, vermian hypoplasia is milder


Mega cisterna magna

nl variant if not associated w/ above abnl


Arachnoid cyst

Asymmetric location of the fluid, rounded cyst margins, nl cerebellum


Choroid plexus cyst

Cyst in 2nd trimester may be assoc. w/ Trisomy 18 if other abnormalities found --> heart defect, clubfeet, rocker-bottom foot, clenched hands w/ overlapping index or 3rd and 4th digits, facial clefts, neural tube defects, radial ray abnls, two-vessel cord.

DDx: Connatal cyst: coarctation of ventricular horns, subependymal cyst, choroid plexus papilloma or carcinoma, PVL, porencephaly


Agenesis of the Corpus Callosum

Isolated or in assoc. w/ other abnormalities.

  • Partial: posterior portion of the CC is missing
  • Complete: Ventriculomegaly, lateral ventricles located more laterally and more parallel to each other, 3rd ventricle extends cephalad and can herniate between hemispheres → midline interhemispheric cyst.
Colpocephaly: disproportionate enlargement of the posterior portion of the lateral ventricles --> teardrop config of lateral ventricles. Absent cavum septum pellucidum.


The vein of Galen aneurysm

Rounded or ovoid fluid collection in the midline, posterosuperior to the 3rd ventricle and thalamus. Drain into sagittal sinus. May cause high-output failure. Freq. seen w/ cardiomegaly and fetal hydrops, may cause obstruction of the aqueduct of Sylvius.


Porencephalic cyst

Parenchymal loss in vascular distribution, cystic area of destroyed brain communicates with or enlarges ventricle (ex vacuo)


Meningocele and Meningomyelocele

Splaying of posterior vertebral ossification centers which normally converges, cystic or complex mass --> meningocele sac. Lumbosacral region. The cord ends at L3 after birth. Lipoma or bony spur (diastematomyelia) may be assoc w/ myelomeningocele. Whenever a neural abnl is identified look for Arnold-Chiari II.


Arnold-Chiari II

Obliteration of the cisterna magna, distorted shape of posterior fossa structures, altered head shape (Lemon), hydrocephalus.

Lemon sign: flattening of the bilateral frontal bones, present before 24 weeks in 98% fetuses w/ spinal defect.

Strawberry sign: Lemon sign + fattening of the occiput ==> Trisomy 18.

Banana sign: abnl curved cerebellum, cerebellum and medulla are pulled downward by the tethered cord (from myelomeningocele) resulting in obliteration of the cisterna magna and loss of nl shape of cerebellum --> C shape, banana - more specific than the lemon sign


Lymphatic malformation - Cystic hygroma

Spoked wheel shaped structure in posterior and lateral neck - lymphocele in this location is called cystic hygroma seen w/ Turner sy (XO), trisomy 21 and 18


DDx of mass along the back of fetal neck

  • Cephalocele (calvarial defect)
  • Cystic Hygroma (septations)
  • Cervical Teratoma (solid)
  • Cervical Meningocele (defect in spine)


Sacrococcygeal Teratoma

combined cystic-solid to mostly solid mass


CHESS

Chiari, Holoprosencephalies, Enecphalocele, SeptoOptic dysplasia, Schizencephaly and other abnl