The velum interpositum space represents a potential space between the columns of the fornices and the choroid forming the roof of the third ventricle. If this potential space is dilated, then it is known as the cavum velum interpositum (CVI). This is a median structure located inferior to the hippocampus and anteroinferior to the splenium of the corpus callosum. The columns of the fornices form the superolateral walls of the cavum velum interpositum and the thalami form the inferolateral wall. It is bordered anteriorly by the foramen of Monro and posteriorly by the pineal body [1-3].

• in the axial plane: a well-defined uniloculated anechoic mass located between the cerebral hemispheres and posterior to the thalami (Fig. 1).

3D-imaging shows:

• in the midsagittal view: a mass below the intact splenium of the corpus callosum, above the tentorium cerebelli superior and posterior to the thalami; the columns of the fornix form the superolateral walls of the cavum velum interpositum cyst and separate it from the cavum vergae and the cavum septi pellucidi; a characteristic inverted helmet shape of a mass with the convex side down and the flat side up (Fig. 2). It may extend anteriorly in the roof of the third ventricle as far as the interventricular foramina [1, 3, 6, 8, 10].

cavum velum interpositum cysts have a round shape in the midsaggital plane (Fig. 3) and situated 2,5–4 mm away from the quadrigeminal plate [6, 10].

Figure 3. The midsagittal plane (3D-imaging). Short arrow – quadrigeminal cistern, long arrow – round-shaped cavum velum interpositum cyst.

3D along with Color Doppler provides spatial relationships and the course of the cerebral vessels inferiorly and laterally to the cavum velum interpositum [1-3, 6, 11] (Fig. 4).

Figure 4. The midsagittal plane (3D-imaging+Color Doppler). The internal cerebral veins are visualized as parts of the inferolateral boundaries of the cavum velum interpositum , but are not anatomically within it.

Tomographic imaging helps to establish the relationship between the cyst, the adjacent parenchyma and the ventricular structure (Fig. 5).

Figure 5. Tomographic imaging shows intact adjacent structures.

MRI

The MRI-imaging corresponds to the sonographic appearance of cavum velum interpositum . A typical axial MRI or CT scan displays a triangular-shaped space between the lateral ventricles. On sagittal images, a cavum velum interpositum appears like a helmet-shape slit with collections of cerebro-spinal fluid behind the foramen of Monro, below the fornices, and above the telachoroidea of the third ventricle [2, 4, 6] (Fig. 6).

Figure 6. MRI demonstrates cavum velum interpositum cyst.

Complications

Because of location, a cavum velum interpositum is not usually related to complications, but mass effect was reported with ventriculomegaly, hydrocephaly and macrocephaly [3, 6, 8, 9, 10].

Progression

The gestational age at the time of diagnosis varies between 19 and 36 weeks. The reported diameter is from 9 to 30 mm. Among cases detected prenatally, some decreased in size, some lesions were stable and some enlarged slightly during pregnancy. The cavum velum interpositum may undergo spontaneous resolution after delivery [1-3, 5, 6, 10, 12, 13].

Prognosis

The prognosis in prenatally diagnosed cavum velum interpositum is ususally good [3, 4, 12]. The prognosis associated with cavum velum interpositum cysts is uncertain and may be related to the size of the cyst. Psychotic disorders, low-pressure hydrocephalus, neurodevelopmental delays, epilepsy and multiple cranial neuropathies have been reported in patients with large cavum velum interpositum cysts [1, 3, 14].

Clinical importance

It is suggested that the cavum velum interpositum is a normal developmental phase of the cerebral midline and usually disappears with brain maturation by the end of the first year of life and, hence, has no pathological meaning. But it is not certain whether cavum velum interpositum cysts represent a simple variant of the norm or are developmental malformations since some neurodevelopmental complications are  also reported. An arachnoid cyst of the velum interpositum can cause symptoms similar to those seen with a third ventricular mass [6, 9, 11].

Management

Careful fetal neurosonography is mandatory to rule out associated anomalies which can influence postnatal outcome. When the diagnosis of isolated cavum velum interpositum  cyst is certain, no additional prenatal investigations are necessary and an expectant approach is recommended with monthly sonographic examinations until delivery. Due to the reported association of large cavum velum interpositum cysts and adverse pediatric outcome, postnatal sonographic follow-up with serial examinations at delivery and at 6 and 12 months is recommended [3, 5].

Differential diagnosis

The differential diagnosis of midline intracranial cystic lesions is a diagnostic challenge. The localization of internal cerebral veins is of great importance in the differential diagnosis that include:- an arachnoid cyst (most of them are supratentorial, and an arachnoid cyst at this location is extremely rare: only one third of cases are located in the posterior fossa. An arachnoid cyst is located below the internal cerebral veins. Median diameter of an arachnoid cyst is 40 mm (range, 10–80 mm). Associated findings are rather common);- dilated cavum septi pellucidi  (located anterior to the foramen of Monro, between the frontal horns; also triangular, but with the apex pointing posteriorly);- cavum vergae (is superior and anterior to the columns of the fornix in the midsagittal view, rectangular in cross-section (parallel wall), located superior to the columns of the fornices, which are displaced inferiorly. Internal cerebral veins are the most important landmark that helps in differentiating. Whereas CV lies above these veins, cavum velum interpositum encloses them at its lower and lateral borders);- an arachnoid cysts of the quadrigeminal cistern (located below the internal cerebral veins; may compress the tectum of the midbrain; usually eccentric, and not triangular in cross section);- a vein of Galen aneurysm (color Doppler demonstrates arterio-venous malformation);- a pineal cyst (ovoid or spherical, located below the internal cerebral veins. abutting the colliculi [1- 5, 8-10].

Treatment

Fetuses with progressive hydrocephalus may require ventriculoperitoneal shunting. Surgical fenestration of the cyst is sometimes needed in case of hydrocephalus result from an arachnoid cyst of the velum interpositum [3, 6, 13].

References:

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