City clinical hospital #70, perinatal center, Moscow, Russia.

Case report

The cavum veli interpositi (CVI) is a space within the double-layered tela choroidea of the third ventricle. Occasionally, this space is fluid-filled and sonographically visible as an interhemispheric anechoic cyst.

CVI¹ is a normal variation where there is a dilated CSF space involving the velum interpositum.

The velum interpositum is a small membrane containing a potential space just above and anterior to the pineal gland, which can become enlarged to form a cavum velum interpositum. When CVI is larger than 1cm in axial transverse measurement, with outwardly bowed margins and positive mass effect, the term of cavum veli interpositi cyst should be used.³

A CVI is median structure inferior to the hippocampus, anteroinferior  to the splenium of the corpus callosum and superior to the tela choroidea of the third ventricle.² The CVI extends below the splenium of the corpus callosum and the column of the fornix and above of the internal cerebral veins. It is triangular in shape, the apex pointing anteriorly reaching as far forward as the foramen of Monro. The  CVI originates from the fold of the pia mater protrusion, which forms the final tela choroidea of the third ventricle.²

The main differential diagnosis is  an arachnoid cyst. Most of them are supratentorial, and one third of cases are located in the posterior fossa, they are usually eccentric and not triangular in cross section.² According to Chen et al. the difference between arachnoid cyst and CVI is that the former is topographically below the internal cerebral veins  abutting the colliculus whereas the latter is 2,5 to 4mm away from the quadrigeminal plate above the internal cerebral veins.

The  differential diagnosis includes also a pineal cyst , which is ovoid or spherical, located below the internal cerebral veins  abutting the colliculi of the quadrigeminal plate.⁽³⁾

The difference between the  cavum Vergae and CVI is that the former is rectangular in shape and located superior to  the columns of the fornices, which are displaced inferiorly.³

CVI has not been linked to neuropsychiatric disorders or disfunction of the limbic system. ² In cases where cystic dilation is present with positive mass effect and obstruction to normal CSF flow, then surgical fenestration may be curative.⁽³⁾

Case 1

A 27-year old woman (G1P0) was referred to our institution at 20 weeks for routine second trimester scan.

Following images revealed a cavum veli interpositi.

Images 1, 2 and 3

Case 2

A 39-year old woman (G2P1) was  referred to our institution at 18 weeks 6 days for routine second trimester scan. Also, there was a choroid plexus cysts. No other anomalies were detected.

Images 4-11; Videos 1 and 2: 2D and doppler color sagittal, axial and coronal images of the fetal head showing the the cavum veli interpositi. Noted the characteristic triangular shape in the axial plane.

We rescaned the patient at 20 weeks 4 days. The CVI was slightly decreased in size.

Image 12; video 3:   2D and doppler images show the internal cerebral vein located beneath the CVI.

References

1. D’Addario V, Pintucci A, Rossi AC, Pinto V, Di Cagno L. Prenatal diagnosis and outcome of cavum veli interpositi cysts [abstract]. Ultrasound Obstet Gynecol 2008;
2. Immacolata Blasi, MD, Wolfgang Henrich, MD, Cindy Argento, MD, Rabih Chaoui, MD Prenatal diagnosis of a cavum veli interpositi2008
3. Radiopedia.org Cavum veli interpositi
4. Chen CY, Chen FH, Lee CC, Lee KW, Hsiao HS. Sonographic characteristics of the cavum velum interpositum. AJRN Am J Neuroradiol 1998; 19:1631–1635. 6.