Cavum veli interpositi

A cavum veli interpositi (CVI), often incorrectly termed a cavum velum interpositum, is an anatomic variation where there is a dilatation of the normal cistern of the velum interpositum. When larger than 1 cm in axial transverse measurement, with outwardly bowed margins and positive mass effect, the term cyst of the velum interpositum or cavum veli interpositi cyst should be used. 

Clinical presentation

This variation is often detected incidentally and individuals are usually asymptomatic. Sometimes this space is not only visible but expanded as a cystic space, in which case patients may have symptoms relating to altered CSF flow (obstructive hydrocephalus leading to a headache). 

Unlike the cavum septi pellucidi and cavum vergae, a cavum veli interpositi has not been associated with neuropsychiatric disorders ¹. 

Gross anatomy

The cavum veli interpositi extends below the splenium of the corpus callosum and the column of the fornix and above the internal cerebral veins. It is triangular in shape, the apex pointing anteriorly reaching as far forward as the foramen of Monro ^1,4-5. 

Epidemiology

A cavum veli interpositi is a relatively frequent finding on imaging of infants and young children, seen in ~25% (range 18-34%) of infants younger than 2 years of age ¹. In most cases, this space closes as the individual ages. 

Radiographic features

Neonatal ultrasound

May be seen as a cystic space in the pineal region and is more common in this age group than either pineal cysts or arachnoid cysts ¹. The internal cerebral veins (best seen on color Doppler) can be seen below the cystic space ¹.  

CT/MRI

Classically seen as an enlarged CSF space situated between the atria/trigones of the lateral ventricles, behind the foramen of Monro, beneath the columns of the fornices and above the tela choroidea of the 3^rd ventricle. 

On axial images may have a triangular configuration pointing anteriorly, its anterior extent never extending further than the foramen of Monro ⁵.

On coronal imaging, the fornices are elevated and splayed ⁵. 

Treatment and prognosis

In the vast majority of cases, patients are asymptomatic. In cases where cystic dilatation is present with positive mass effect and obstruction to normal CSF flow, surgical (endoscopic) fenestration may be curative ¹. 

Differential diagnosis

• cavum septi pellucidi
  • located anterior to the foramen of Monro, between the frontal horns of the lateral ventricles
  • also triangular, but with the apex pointing posteriorly
• cavum vergae
  • rectangular in cross-section (parallel wall)
  • located superior to the columns of the fornices, which are displaced inferiorly
• pineal cyst
  • ovoid or spherical
  • located below the internal cerebral veins
  • abutting the colliculi
• arachnoid cyst of the quadrigeminal cistern 
  • usually eccentric, and not triangular in cross section
  • located below the cerebral veins
  • abutting the colliculi