REPLY

We would like to thank Dr. Kier for pointing out issues regarding the evolutionary and embryologic basis for the development and anatomy of cavum basis for the development and anatomy of cavum veli interpositi (CVI) in our recent article. First, we want to emphasize that we did use the term “cavum veli interpositi” in our original submission. During the peer review process, however, a referee opined that “cavum veli interpositi” is not an appropriate term. We agree with Dr. Kier that “cavum veli interpositi” is probably more appropriate than “cavum velum interpositum” (1, 2), which is rarely used in the literature. Relative to the issues of embryogenesis and formation of CVI and its related radiologic anatomy there are, to my knowledge, only a few published papers in the English-language literature (3, 4). The main point is whether the tela choroidea, by embryologic or anatomic definitions, includes the cisternal space of CVI at or near the term age of human brain development. According to Zellweger and van Epps (4), the tela choroidea of the third ventricle originates from the roof plate of the diencephalic region by a protrusion of a fold of pia mater into the primitive neural tube at about the third fetal month. With further development, the pia mater fold is pushed backward, forming the final tela choroidea of the third ventricle. It encloses a horizontal sac or fissure under the fornix, which opens behind and under the splenium of the corpus callosum where its pia mater is connected to the pia mater covering the median fissure of the cerebrum. The sac-like pia fold carries the name “transverse or choroidal fissure.” In the majority of cases, the choroidal fissure closes. When it persists, the choroidal fissure is called “CVI or cisterna interventricularis.” CVI is a true cisternal structure communicating with the quadrigeminal cistern, as has been shown by comparing pneumoencephalography and autopsy specimens (3). Some investigators suggest that it is a part of the anterior extension of the quadrigeminal cistern. There is no doubt that the tela choroidea forms the roof of the third ventricle; the tela choroidea itself is, by definition, the structure where the pia and ependyma approximate. It contains, however, a pair of internal cerebral veins, so it is debatable whether the CVI and its roof (the hippocampal commissure) still can be included as parts of tela choroidea or part of the third ventricle as it relates to radiologic anatomy. In the published literature, the anatomic location of this cavum has been described frequently as above or superior to the third ventricle. The internal cerebral veins course within the tela choroidea on the roof of the third ventricle. Thus, anatomically, they are inferior or lateral to the CVI CSF space. Although our study did not aim to provide the embryologic evidence of the origin of the CVI, we reported that the color-coded internal cerebral veins on sonographic studies are anatomic landmarks to CVI. They are inferior or inferolateral to the CVI, but do not enter it. This raises a similar question as to whether the mega cisterna magna is a part of the fourth ventricle because it is now widely accepted that the mega cisterna magna is formed by the evagination of the tela choroidea of the fourth ventricle.

In summary, we believe:

• The velum interpositum originates from a fold of pia mater protrusion, which forms the final tela choroidea of the third ventricle.

• The cavum veli interpositi is a true cistern situated above (but not communicating with) the third ventricle.

• The internal cerebral veins form parts of the inferolateral or lateral boundaries of the CVI but are not anatomically within it.

• Both terms “cavum veli interpositi” and “cavum velum interpositum” have been used interchangeably in the English-language literature, and the former is more appropriate.

• An enlarged suprapineal recess of the third ventricle is a frequent finding in patients with obstructive hydrocephalus, but is extremely rare if ever seen as a normal variant in neonates and infants.