Anomalies of the Fetal Head and BrainChapter 1

In the late first trimester ultrasound the cranial vault, brain tissue and lateral ventricles occupied by the choroid plexuses can be visualized.

In the anomaly scan done between 18–22 weeks of gestation the ventricular system, subarachnoid space, the diencephalic and rhombencephalic structures can be evaluated in detail.

Visualization of the ventricular atrium, the cisterna magna and the cavum septum pellucidum on the standard imaging planes excludes most of the cranial anomalies.

Meticulous cranial biometry consisting of the biparietal diameter, head perimeter, occipitofrontal diameter, ventricular width, interhemispheric distance, cerebellar transverse diameter and depth of cisterna magna is essential as this can help in diagnosing many anomalies.

Section for cranial biometry consists of the thalamus, the third ventricle and the cavum septum pellucidum.

Biparietal diameter: Side to side measurement from the outer table of the proximal skull to the inner table of the distal skull.

Head perimeter: The total cranial circumference, which includes the maximum anteroposterior diameter.

Occipitofrontal diameter: Front to back measurement from the outer table on both sides.

The choroid plexus should occupy the whole of the body of the lateral ventricle (Figs 1.2 and 1.3)

Upper limit of normal being 10 mm.3

The width of the body, anterior horn and posterior horn of the lateral ventricle are taken. (Normal value < 08 mm, borderline 08–10 mm and > 10 mm is abnormal).

The lateral ventricle/interhemispheric width ratio is not very sensitive for early dilatation so is not used very often for defining hydrocephalus (Fig. 1.4).

When the choroid plexus does not occupy the whole of the body of the lateral ventricle see for the measurement of the medial separation of the choroid plexus from the wall of the lateral ventricle. (Normal value < 02 mm, borderline 02–03 mm and > 03 mm is abnormal) (Fig. 1.5).

The cerebellum is seen as a ‘W’ turned 90 degrees (Fig. 1.6).

Measure cerebellar transverse diameter.5

The cerebellar transverse diameter (CTD) is measured from the edges of both cerebellar hemispheres. The CTD in mm from 14–22 weeks is equal to the gestational age of the fetus in weeks (Fig. 1.7).

Look for any hypoplasia of the superior and inferior cerebellar vermis.

Whether there is any communication between fourth ventricle and cisterna magna.

The cisterna magna is seen posterior to the cerebellar vermis and anterior to the occipital bone (Fig. 1.8). (Normal value < 08 mm, borderline 08–10 mm and > 10 mm is abnormal).

Should not be more than 10 mm.

Few strands seen traversing the cisterna magna are normal.

Carefully check for any communication between the fourth ventricle and the cisterna magna with an abnormal cerebellar vermis.7

If there is any communication at gestational age less than 16 weeks revaluate the fetus after 2 weeks.

Check for any posterior fossa cyst.

Partial or complete absence of cranial vault (Fig. 1.9).

Brain tissue (disorganized) always present (Figs 1.10 and 1.11).

Can be diagnosed on transvaginal scan at 10 ½ weeks (Fig. 1.12).

The cranial vault and telencephalon are absent and the brainstem and parts of the mesencephalon are present which are covered by a vascular membrane (Fig. 1.13).

Failure to identify normal bony structure and brain tissue cephalad to the bony orbits is the most reliable feature of this anomaly (Fig. 1.14).

Diagnosis can be made as early as 10–11 weeks of gestation by a transvaginal scan.

Mostly occurs in the midline in the occipital area.

Can occasionally occur in the parietal and frontal bones.

The defect in the bony skull can be demonstrated on ultrasound (Figs 1.15 and 1.16).

Can be a meningocele, meningomyelocele, encephalomeningocele (Fig. 1.17) or encephalocystomeningocele depending on the contents within the herniation.12

There is a bony defect in the occipital region of the skull with a partial or total absence of the cervical and thoracic vertebra with spina bifida and a fixed retroflexion of the head (Fig. 1.18)

It arises from a failure of diverticulation of the embryonic forebrain and the degree of disordered prosencephalic development decides the anatomical and clinical classification.

Interhemispheric fissure and the falx cerebri are totally absent.14

There is a single primitive ventricle (holoventricle).

Dorsal sac between skull and cerebral convexity can be seen.

Thalami are fused in the midline.

Third ventricle, neurohypophysis, olfactory bulbs and tracts are absent.

The midbrain, brainstem and cerebellum are structurally normal.

Invariably poor prognosis.

Monoventricular cavity with rudimentary occipital horns is seen.15

Falx and interhemispheric fissure form caudally with partial separation of occipital lobes.

Thalami fused in midline (Fig. 1.19).

Olfactory bulbs and corpus callosum usually absent.

Dorsal sac between skull and cerebral convexity can be seen.

Invariably poor prognosis.

IUD or NND and those who survive have severe mental retardation.

Septum pellucidum is absent.

Interhemispheric fissure is well-developed posteriorly.

The outcome depends on the severity of hydrocephalus affecting the neurologic development.

Patients could vary from a near normal intellectual development to severe mental retardation.16

The largest fiber tract within the CNS develops between 12–18 weeks of gestation. The development begins anteriorly (rostrum and genu) and proceeds posteriorly (splenium).

The ventricles are displaced laterally with indentation of the medial walls.

Ventriculomegaly seen in the atrial and occipital regions (colpocephaly) because of poorly developed white matter surrounding these areas (Tear drop configuration) (Fig. 1.22).

Enlarged elevated third ventricle between the hemispheres is seen as an interhemispheric cyst (Fig. 1.23).

Cavum septum pellucidum is absent (Fig. 1.24).

Usually seen in the posterior region.

Frontal horns are normal.

There is a dilatation of the occipital and temporal horns.

Cavum septum pellucidum is present.

May show the tear drop configuration of the ventricles.

Enlarged posterior fossa/ deep cisterna magna is seen (Fig. 1.25).18

Midline cyst in the posterior cranial fossa communicating with the fourth ventricle.

Cerebellar vermis is either absent or small or abnormally developed.

Hydrocephalus is seen quite commonly.

Can result from a bilateral in utero internal carotid artery occlusion or cytomegalovirus or toxoplasmosis infection.

Complete or almost complete destruction of cerebral cortex and basal ganglia with intact meninges and skull of normal appearance (Fig. 1.26)

They are usually stillborn or die soon after birth.

It should be less than 3 standard deviation below the expected head circumference.

Can be a part of holoprosencephaly, porencephaly or lissencephaly or associated with Trisomies 9,13 and 18. This is what decides the prognosis.

Sloping forehead on a sagittal view usually suggests the diagnosis.

Aneurysmal dilatation of the vein of Galen is seen which lies within the subarachnoid space posterior and superior to the thalami (Fig. 1.27).21

One sees a midline cyst with flow (Fig. 1.28) of an arterio-venous malformation which is diagnostic.

Hydrocephalus can be seen.

Seen as thin-walled clear cysts which can be single or multiple, unilateral or bilateral (Figs 1.29 to 1.31).

Multiple foci of calcification are seen in the brain substance.

Ventriculomegaly and microcephaly can also be associated.

Usually due to intrauterine infection usually cytomegalovirus and toxoplasmosis.23

Ventriculomegaly refers to an enlargement of the lateral ventricles.

The width of the body, anterior horn and posterior horn of the lateral ventricle are taken (Normal value < 08 mm, borderline 08–10 mm and > 10 mm is abnormal) (Fig. 1.32).

An additional sonographic sign of ventriculomegaly is loss of approximation between the choroid plexus and the medial border of the lateral ventricle. In early stages of ventriculomegaly the choroid plexuses are seen detached from the medial wall. When the choroid plexus does not occupy the whole of the body of the lateral ventricle see for the measurement of the medial separation of the choroid plexus from the wall of the lateral ventricle (Normal value < 02 mm, borderline 02–03 mm and > 03 mm is abnormal) (Figs 1.33 and 1.34).

Ventriculomegaly can include hydrocephalus, brain atrophy and abnormal development of the brain.25

Hydrocephalus results from an excessive accumulation of cerebrospinal fluid within the brain due to an obstruction to normal outflow and absorption.

The cerebrospinal fluid flows through foramen of Monro to the third ventricle, and obstruction at this level results in ventriculomegaly with a normal third ventricle.

If obstruction occurs at the level of aqueduct of Sylvius both lateral ventricles and third ventricle are dilated.

A careful evalution of the spine (Fig. 1.35) should always be performed once ventriculomegaly is diagnosed.26

In a communicating hydrocephalus there is overlapping of frontal bones (lemon shape) and downward displacement of cerebellum (banana sign).

Other causes of ventriculomegaly (Fig. 1.36) include: choroid plexus papilloma, Intracranial hemorrhage, Agenesis of corpus callosum, Lissencephaly, Dandy-Walker 28malformation and congenital infections with cytomegalovirus or toxoplasmosis.

The prognosis depends upon the underlying cause of ventriculomegaly and the type and severity of associated malformations.

Very occasionally, ventriculomegaly is transitory and can resolve spontaneously.