Differential Diagnosis of Fetal Intracranial ‘Cyst’
· Large cistern magna- The cisterna magna lies between the cerebellum and the occipital bone, with a normal AP diameter of 4—10 mm. A cisterna magna larger than 10 mm suggests Dandy—Walker malformation. Other associations include communicating hydrocephalus, cerebellar hypoplasia and retrocerebellar arachnoid cyst.
· Non-obstructive venreiculomegaly- Ventriculomegaly means dilatation of the ventricles, which may be obstructive (hydrocephalus) or non-obstructive. These are difficult to differentiate antenatally. The non-obstructive variety may be associated with agenesis of the corpus callosum, which shows mildly dilated lateral ventricles, absent falx and normal thalami. Dandy— Walker malformation, alobar haloprosencephaly and hydranencephaly are other conditions associated with non-obstructive ventriculomegaly.
· Obstructive hydrocephalus- Since it is difficult to determine intraventricular pressure safely in the fetus, obstructive hydrocephalus should not be diagnosed in utero unless an enlarging head and ventricles are documented on serial scans. The majority of cases of obstructive hydrocephalus in the fetus are due to aqueduct stenosis. In this instance there is dilatation of the lateral and third ventricles. A normal BPD or head circumference does not exclude hydrocephalus.
· Choroid plexus cysts- CPCs are common. They usually disappear by 26 weeks. Cysts persisting longer should be considered of different origin. They may be multilocular, varying in size from 3 to 13 mm. They have been reported in association with anomalies, which include trisomy 21, 18, Goldenhar’s syndrome, agenesis of the corpus callosum, frontal lipomas, frontal meningoceles, median cleft nose, hypertelorism, cervical spinal anomalies and cardiac defects.
· 'midiline cyst' in ACC- In agenesis of the corpus callosum the third ventricle may enlarge dramatically and herniated upwards, sandwiched between the two hemispheres. This produces the so called midline or ‘interhehiispheric cysts’.
· Holoprosencephaly- This is related to failure of the brain to divide into two hemispheres and two ventricles. This results in a monoventricle with an absent falx and fused midline thalami. Other associated anomalies include trisomy 13, 18, triploidy. omphalocele and renal dysplasia. Facial abnormalities such as cyclopia, midline clefting and hypotelorism are usually associated.
The alobar type is the most severe brain abnormality. The midbrain, brainstem and cerebellum are normal. The corpus callosum, fornix. falx cerebri, optic tracts and olfactory bulbs are absent. The cerebral cortex is small, lies anteriorly and forms a ‘horse shoe’ shape. The dorsal cyst has no cortical mantle.
· Arachnoid cysts- These are either abnormalities of development of the leptomeninges or secondary to hemorrhage or infection. These cysts may produce a mass effect and secondary hydrocephalus.
· Schizencephaly- This term is applied to bilateral clefts in the cerebral cortex in the region of the middle cerebral arteries. Sonography reveals fluid collections which communicate with the lateral ventricles bilaterally and extend to the calvarium.
· Porencephaly- Porencephaly results from infarction or hemorrhage into the brain, necrosis follows. The necrotic brain parenchyma may evacuate into the ventricular cavity or subarachnoid space. The result is a cyst in free communication with the ventricle or a subarachnoid cistern. The porencephalic cyst does not produce a mass effect.
· Posterior fossa arachnoid cysts- A retrocerebellar arachnoid cyst is a less common and more benign abnormality than the Dandy— Walker malformation. The cyst may produce a mass effect, displacing the hemispheres.
· Cystic periventricular leukomalacia- CPL usually appears as cystic structures adjacent to the angle of the frontal horns. The cysts vary from 2 to 5 mm. Although the diagnosis has been made antenatally, it is often not diagnosed due to the relatively low position of the fetal head in the pelvis making imaging of the cortical region difficult late in pregnancy. It has a significant association with handicap.
· Vein of Galen aneurysm- Vein of Galen aneurysms are basically arteriovenous malformations. The venous limb is represented by cystic dilatation of the vein of Galen. They are rare lesions. The cystic dilatation of the vein of Galen is supratentorial in location, in communication with tubular vessels leading to and from it. Turbulent high frequency Doppler signals are obtained. There may be secondary hydrocephalus or high output failure secondary to a large AV shunt.
· Subdural collections- Rarely seen antenatally, usually occur secondary to a fetal bleeding diathesis.
· Cystic tumors- Fifty percent of all brain tumors in the neonate are teratomas. The tumors may be benign or malignant but even the benign lesions have severe neurological sequelae. The tumors are often complex with cystic areas. There may be a mass effect.
· Unilateral hydrocephalus- Unilateral hydrocephalus is rare, ventriculomegaly is therefore assumed to be bilateral and symmetric even if the near field ventricle is obscured by artifact. The causes include agenesis/stenosis of the foramen of Monro, transient obstruction of the foramen, intraventricular hematoma and underlying brain dysplasia. Prognosis is usually favourable. True unilateral ventricular dilatation may be seen in megaloencephaly — a migration disorder.
· In utero cerebral hemorrhage- Classified into four types: type I confined to the germinal matrix; type II subependymal hemorrhage with rupture into the ventricle:
type Ill same as II but with ventricular dilatation; type IV extension of hemorrhage into the brain parenchyma. All cases diagnosed in utero have been type III or TV. Intraparenchymal hemorrhage may show progressive liquefaction which eventually results in a porencephalic cyst. The associated hydrocephalus secondary to aqueduct stenosis may be progressive.
NB- The transverse diameter of the ventricular atrium measures approximately 7—8 mm with 10 mm being the upper limit of normal. The atrial diameter is used to determine the normality of ventricular size. The choroid usually fills the atrial lumen. If fluid is seen between the choroid and the ventricular wall, measurement becomes necessary.