VM incidence: 2:1000 births

VM is 2/2 abnormal CSF volume 2/2...

• ...overproduction or abnormal resorption

• Circulation

• Abnormal parenchymal development

• Parenchymal destruction.

Approach is anatomically based via assessment of posterior fossa, midline structures, and cerebral mantle

Ventricular system is composed of

• Paired supratentorial lateral ventricles (LVs)

  • Composed of the body and atria centrally

  • frontal, occipital, and temporal horns extending into the respective cerebral lobes

• Single central 3rd ventricle (3V)

• Infratentorial 4th ventricle (4V)

CSF is produced by specialized ependymal cells of the choroid plexus located in the body of the LVs, 3V, and 4V.

CSF circulates in a predictable rostrocaudal fashion through the LVs into the 3V via foramen of Monro, through cerebral aqueduct into 4V, and then either into central canal of SC or out into extraaxial space via foramina of Luschka and Magendie. CSF is then reabsorbed by arachnoid granulations  and returned to the ventricular system.

AS is MCC of prenatal obstructive VM w/ a 2:1 male:female ratio. Most cases sporadic. Acquiductal gliosis or web ofrmation can sometimes form 2/2 infection or hemorrhage.

Pattern: 

• Mod- severe enlargement of LVs and 3V w/ nml 4V.

• Midline structures intact

  • HPE have absent midline structures

• Cerebellum often looks nml

  • Rhombencephalosynapsis (vermian hypoplasia and fused cerebellar hemispheres) can be associated.

  • 10% incidence of RCS in cases of AS; majority of VM in setting of RCS is 2/2 AS. 

Fetal MRI findings: enlarged 3V inferior recess, LV diverticulum, callosal thinning (signs of severe obstructive physiology)

Serial US imaging useful as progressive VM in AS may reuslt in macrocephaly w/ head circumference exceeding other biometric measures by multiple SDs. 

Head size may determine the timing and route of delivery.

X-linked hydrocephalus is an important syndromic cause of AS. Associated abnormalities can include:

• Adduction of thumbs

• Dsplasia or hypoplasia of the brainstem, pons, cerebellar vermis w/ pyramidal tract agenesis or hypoplasia and variable callosal anomalies.

A multicenter study refined the imaging criteria by defining the phenotypic features of inferior predominant vermian hypoplasia:

• Enlarged tegmentovermian angle

• Inferolateral displacement of the 4V choroid plexus and associated taenia-tela choroidea complex, an obtuse fastigial recess, and an unpaired caudal lobule.

Though experts recommend elimination of posterior fossa size and torcular location from criteria, these signs are useful in triggering further postnatal assessment of DWM in fetus w/ VM.

Characterized by herniation of cerebellar tonsils through the foramen magnum and obliteration of the cisterna magna CSF space.

Fetal MRI quantifies the degree of hindbrain herniation and helps detect callosal agenesis/dysgenesis and cortical heterotopia.

CIIM is characterized by:

• Herniation of cerebellar tonsils and vermis w/ descent of brainstem and 4V below the plane of foramen magnum.

• Sono: obliteration of the posterior fossa CSF space, a "banana" configuration of the cerebellum as it wraps around the brainstem and molds to the concave posterior occiput, and a lemon-shaped head w/ loss of the nml biconvex calvarial shape. 

• Other: posterior fossa funneling, small transcerebellar diameter, and beaked tectum.

Associated w/ open neural tube defects in the lumbar (73%), sacral (17%), thoracic (9%), and rarely cervical (1%)

In utero repair of open neural tube defect prior to 26 weeks gestation led to preservation of peripheral neurologic function with improved ambulation at 30 months and likelihood of resolution of cerebellar ectopia.

Histologic types MC include teratomas (50%), meningiomas, and ependymomas. 

Can be benign or malignant though even benign lesions can be locally aggressive. 

70% of fetal brain tumros are supratentorial.

Identification of a suspected intracranial mass at US should prompt assessment w fetal MRI for an evaluation of both anatomic location and tumoral imaging features such as...

• Cellularity and necrosis on DWI

• +/- hemorrhage on GRE images

An important ddx for an intracranial mass includes intracranial hemorrhage which can appear heterogeneous ad masslike often w associated parenchymal destruction and VM from abnormal CSF resoption.

The CC is a primary white matter commissure b/n the R and L cerebral hemispheres; its absence manifests w/ VM.

Imaging features:

• Absence of crossing callosal fibers

• Parallel LVs demonstrating colpocephaly (teardrop shaped configuration of the ventricles w/ asym enlargement of the occipital horn)

• Loss of the nml branching pattern of the ACA seen at color Doppler US

• Absent cingulate gyrus (normally parallels the course of CC)

• Radial or spoke-wheel configuration of medial gyri.

Asym VM (>50% difference in LV diameters), interhemispheric cyst, and dysgenesis/agenesis of the CC is termined the AVID complex.

HPE spectrum is the result of incomplete cleavage of the cerebral hemispheres in early (5-4 wk) embryogenesis. Severity ranges from alobar (most severe), to semilobar, to lobar (least severe).

Alobar = single forebrain cortical mantle w a primitive monoventricle, often accompanied by a dorsal cyst.

• The CSP, CC, interhemispheric fissure, and falx cerebri are absent.

• Deep brain nuclei (BG, thalami, hypothalami) are fused resulting in malformation of the 3V -> CSF cannot properly circulate -> development of a thinwalled dorsal cyst. 

• Abnormal sulcation in alobar HPE manifests as absence of the sylvian fissure and oligogyria.

Lobar HPE

• Some fusion of anterior frontal lobes w separation of the posterior frontal lobes and more posterior structures usually including the thalami and BG.

• While the splenium of the CC is present in lobar HPE, the rostrum and genu are generally absent w/ varying amounts of the callosal body present.

• The falx cerebri is hypoplastic anteriorly but can be normally formed posteriorly.

Semilobar HPE

• Cerebral nonseparation pattern is like that of lobar HPE w the distinction that less than half of the frontal lobes are separated.

• There is fusion of the frontal horns of the LVs w absent CSP but w/ separation of the occipital horns of the LVs and varying degrees of thalamic and deep brain nuclei separation.

• The splenium and some of the posterior body of the CC can be present but the anterior body, genu, and rostrum are absent. As in lobar HPE, the falx and interhemispheric fissure can be present posteriorly but are absent anteriorly.

Implies an abnormally smooth brain.

Cortical mantle is smooth and featureless w/ a thickened layered appearance w/ loss of the nml stepwise formation of the parietooccipital, calcarine, and cingulate sulci medially and the central, postcentral, and superior temporal sulci laterally.

Lissencephaly should NOT be suggested at less than 20 weeks. B/n 20 and 24 weeks, FP can occur.

An increased angle of the sylvian fissure is a key imaging features of abnormal cortical development.

There are 3 types:

• Type 1 (classic): complete agyria or regional pachygyria (no gyri or few broad gyri, respectively) with subcortical band heterotopia (a layer of heterotopic gray matter ectopically located in subcortical WM).

  • Can be isolated but is associated w/ Miller-Dieker and Normal-Roberts syndromes.

• Type 2 (cobblestone): diffuse fine cortical nodules w/ VM and posterior fossa abnormalities such as DW or AS.

• Type 3 is an overlap w/ features of both type 1 and 2.

GMH is an intrinsic error in neuronal migration resulting in abnormal distribution of neurons b/n the cortex and LVs manifesting as either periventricular nodular heterotopia or subcortical band heterotopia.

Evaluation of a fetus w/ VM should include careful assessment of the ventricular lining to detect ependymal nodularity.

US: subependymal nodules can be seen lining the walls of the ventricles in a dot-dash pattern replacing the nml smooth ependymal surface.

MRI: nodules follow cortical GM SI w/ all sequences.

DDX is ependymal nodularity 2/2 adherent clot in the setting of intracranial hemorrhage and TS w/ subependymal tubers.

Near-complete destruction of the cerebral hemispheres w/ replacement of the brain by a membranous CSF-filled sac.

Result of a catastrophic vascular injury, such as may occur w/ placental abruption, monochorionic twin demise, or as a complication of fetal intervention.

Brain liquefaction and resoprtion is in an anterior circulation distribution.

Posterior fossa structures and those supratentorial structures supplied by the PCA (temporal/occipital lobes) are usually preserved.

Pathogenesis is thought to be 2/2 either early b/l occlusion or obliteration of the supraclinoid ICA or a catastrophic insult causing global hypoperfusion of nml vessels.

Midline structures (Falx and meninges) are intact bc it is an otherwise nml developed brain. This is a useful feature to distinguish hydranencephaly from alobar holoprosocencephaly. 

No cortical mantle supports the diagnosis of hydranencephaly instead of massive hydrocephalus or severe open lip schizencephaly.

Few live past infancy.

Refers to incomplete parenchymal destruction from an insult or injury.

Causes include vascular compromise, hypoxic-ischemic injury, infectious, inflammatory, metabolic, and toxic processes.

Distribution is often periventricular due to the relatively immature vascular supply to cerebral WM and vulnerability of the oligodendroglial precursor cells in this locaiton.

Impaired perfusion leads to neuronal injury, cell death, and resorption.

Imaging patterns: 

• Increased echogenicity of ventricular lining.

• Small cystic spaces w/n periventricular WM

• Larger cystic spaces w/n the brain parenchyma connected to the ventricular system w/o mass effect (porenchaplic cysts)

• VM; 2/2 combination of diminished volume of brain tissue and impaired CSF resportion 2/2 intraventricular debris and blood products.

Use Color doppler US in evaluation of apparent intracranial cysts bc porenchepalic cysts are abascular while high flow vascular lesions such as vein of Galen malformations will be immediately apparent at color Doppler US and can be fully characterized w spectral Doppler US.

Acute vascular compromise can result from placental abruption, monochorionic twin demise, or complications from intrauterine intervention.

Early sonographic findings are often nml w progression of findings over time.

Repeat imaging performed 0-14 days after an acute event is important to assess for developing encephalomalacia indicated by VM and irregular ependymal lining.

MRI is useful to assess ischemic encephalomalacia. 

• TwI best demonstrate VM and areas of parenchymal destruction and the flow voids created by fast-flowing blood can be used to map feeding and draining vessels in intracranial vascular malformations.

• T1WI show blood products w high SI, DWI show increased diffusion SI w/ decreased SI on apparent diffusion coefficient maps along the margin of ischemic areas and GRE sequences highlight magnetic susceptilibity suggestive of blood products.

Congenital infections (TORCH), parvovirus, varicella, zika virus, malaria, listeriosis, syphilis and TB can result in brain injury. VM and growth restriction are common.