Daily Archives: 18/12/2021

Skull Defects

Cranial defects vary from a small meningocele, which is very difficult to visualize, to complete absence of the cranium such as with anencephaly or exencephaly. In practice, there is no problem in the diagnosis of complete absence of the skull or a large defect, however, severe bone demineralization, such as osteogenesis imperfecta, may give the appearance of a large skull defect. Partial defects, or small cephaloceles are occasionally difficult to visualize.

Fig 1, Fig 2, Fig 3

Differential diagnosis

The differential diagnoses and some sonographic features of skull defects are as follows:

Anencephaly:

• absent bony calvarium above the orbits
• orbits well visualized
• absence of supratentorial brain
• residual brain
• possibly spina bifida
• polyhydramnios

Excencephaly

• calvarium absent
• disorganized supratentorial brain tissue

Amniotic band syndrome (ABS)

• asymmetric or bizarre cephalocele
• other deformities, including limb amputation

Limb-body wall complex

• asymmetric or bizarre encephaloceles
• similar to ABS except fetal body adherent to placenta
• usually more severe than ABS
• associated bizarre defects of fetal body

Cephalocele

• midline defect
• extracranial cyst or brain tissue
• possibly ventriculomegaly
• lemon sign may be present

Open neural tube defects

• lemon sign
• banana sign
•  mild ventriculomegaly
• spinal defect

Fetal demise

• overlapping sutures (Spalding’s sign)
• poor visualization of intracranial structures
• associated findings of fetal demise

Microcephaly

• calvarium present
• decreased brain tissue
• head circumference 2-3 standard deviations below that expected for the menstrual age

Craniosynostosis

• complete or partial
• deformed skull
• possibly microcephaly
• abnormal cephalic index.

In addition, considerable overlap may be found in these features among abnormalities. For instance, the lemon sign was originally described with opened NTD, however, it may not be present in an opened NTD identified in the third trimester of pregnancy, and a mild lemon sign may be identified in normal fetuses.

Cystic Brain Lesions

Cystic lesions of the brain have a number of causes but ventriculomegaly and choroid plexus cyst are the most common. Usually, the location and associated abnormalities are very helpful in the differential diagnosis. For example, interhemispheric cyst is commonly related to agenesis of the corpus callosum. Likewise, ventriculomegaly and dorsal sac in the case of holoprosencephaly are often associated with other abnormalities, whereas others are mostly isolated findings. The choroid plexus cyst is always a lateral cyst located in the choroid plexus and is often isolated and not difficult to diagnose. Cystic lesions of Dandy-Walker malformations are always located in the posterior fossa.

Fig 1, Fig 2, Fig 3, Fig 4

Differential diagnosis

The differential diagnoses of intracranial cystic lesion are as follows:

Midline cyst

• prominent cavum septum pellucidi (very common)
• dorsal sac of holoprosencephaly (common)
• interhemispheric cyst of agenesis of corpus callosum (uncommon)
• vein of Galen aneurysm (rare)
• arachnoid cyst (rare)
• enlarged cavum vergae interhemispheric cystic lesions

Lateral cyst

• choroid plexus cyst (very common)
• unilateral hydrocephalus (uncommon)
• porencephaly/schizencephaly (uncommon)
• arachnoid cyst (rare)
• neoplastic cyst (very rare)
• periventricular cyst formation secondary to CMV infection

Posterior fossa cyst

• enlarged cisterna magna (>10 mm (very common)
• Dandy-Walker malformation (common)
• arachnoid cyst (very rare)
• median retrocerebellar fluid collections remain the most difficult to prognosticate, retrocerebellar cysts often being difficult to differentiate from enlarged cisterna magna and Dandy-Walker complex.

Massive Intracranial Fluid Collections

The three common causes of massive intracranial fluid collections with an intact cranial vault are hydrocephalus, which is usually secondary to aqueductal obstruction, holoprosencephaly, and hydranencephaly. The unique features of each condition can distinguish these three entities from each other.

Fig 1, Fig 2

Differential diagnosis

• Hydrocephalus secondary to aqueductal stenosis (about 1 in 2000 births):
  • thin cerebral mantle with falx cerebri
  • separated thalami and dilated third ventricle
  • obvious choroid dangling sign
  • usually no midline face defects
• Hydranencephaly (uncommon; 1.0-2.5 in 10,000 births):
  •  no cerebral mantle with falx cerebri
  • normal thalami (not separated or fused)
  • minimal choroid dangling sign
  • no midline face defects
• Holoprosencephaly (rare; 1 in 16,000 neonates):
  • cerebral mantle with or without dorsal sac, no falx cerebri
  • fused thalami
  • minimal choroid dangling sign
  • associated midline face defects (very common).

Ventriculomegaly

Ventriculomegaly is the enlargement of the cerebral ventricles with the cause unknown. The term hydrocephalus is usually reserved for more severe cases of ventriculomegaly, and this usually implies obstruction of flow of the CSF. Obstructive hydrocephalus within the fetus is most commonly non-communicating (blockage within ventricles) rather than communicating (extraventricular blockage). Moreover, underdevelopment of the brain and destructive brain lesions can also cause ventriculomegaly.

Brain destruction is another cause of ventriculomegaly and may include in utero damage due to a CNS infection. CNS infections may be associated with brain atrophy, with resultant microcephaly and ventriculomegaly.

The main causes of fetal ventriculomegaly are aqueductal stenosis, Chiari II malformation, Dandy-Walker complex and agenesis of the corpus callosum.

Fig1, Fig2, Fig3, Fig4

Mild (borderline) ventriculomegaly [Fig5, Fig6] is used when the atrial width is 10-15 mm. Some consider early ventricular dilatation as separation (>3 mm) of the dependent choroid from the medial ventricular wall. This occurs bilaterally in 0.15-0.7% of fetuses and unilaterally in 0.07% of pregnancies. This can be related to an increased risk of CNS and non-CNS anomalies. Isolated borderline ventriculomegaly is normal variant in most cases, however, it may represent the earliest manifestation of cerebral anomalies (4%), chromosomal abnormalities (4%) (mostly trisomy 21) and overall abnormal outcomes (20%).

However, early and unexplained mild ventriculomegaly appears to have a good prognosis but a long postnatal clinical follow-up is required.

Overt (or moderate to severe) ventriculomegaly [Fig7, Fig8, Fig9, Fig10] is usually defined for an atrial width of more than 15 mm in the second and third trimesters(8). This may involve isolated cases (30-60%) or cases associated with other CNS anomalies, most frequently neural tube defects (NTDs) and midline defects. The degree of antenatal ventriculomegaly is related to pediatric neurological morbidity and, when it is >15 mm, it is associated with an increase in abnormal neurological development.

Unilateral ventriculomegaly is a rare finding usually due to focal obstruction of the CSF pathway at the level of Monro’s foramen. Examination of the lateral ventricles using the angled technique helps to distinguish between unilateral and bilateral hydrocephalus. Unilateral ventriculomegaly is usually an isolated finding and when isolated has little measurable effect on developmental outcome.

Sonographic findings:

• Enlargement of the lateral ventricle atrium >10 mm.
• Minimal separation of the choroid plexus in either proximal or distal cerebral ventricle.
• Dangling choroid plexus with an abnormal choroid angle (more than 29 degrees)(14). Dangling can be easily recognized without measuring it in most cases.
• When ventriculomegaly is detected, a careful anatomic survey of the fetus is necessary to determine any associated anomalies. Furthermore, hydrocephalus is associated with a significant risk of chromosomal abnormalities. When objective difficulties preclude detailed sonographic examination, MRI is helpful(15).
• Sonographic differential diagnoses of ventriculomegaly include
  • Arnold-Chiari malformation type II:
    • deformed cranium (lemon sign), which usually disappears in the third trimester
    • obliteration of cisterna magna (banana sign)
    • associated open spinal defect
    • minimal hydrocephalus
    • pointed frontal horns
  • Dandy-Walker malformation:
    • midline posterior fossa cyst
    • cyst communicates with the fourth ventricle
    • vermian agenesis
    • hypoplastic cerebellar hemispheres
    • elevated tentorium
    • mild to moderate hydrocephalus
  • Agenesis of the corpus callosum:
    • third ventricle elevated
    • separation of frontal horns
    • associated colpocephaly
    • mild ventriculomegaly
  • Aqueduct stances:
    • moderate to massive hydrocephalus
    • normal fourth ventricle
    • normal posterior fossa
    • normal cranial configuration
  • Cephalocele:
    • open defect, usually occipital skull base
    • obliteration of the cisterna magna
    • occasional lemon sign
    • mild to moderate hydrocephalus
    • polyhydramnios.