Mid-Anterior Surface of the Callosal Splenium: Subependymal or Subpial?

We read with interest the recent article by Pekala et al in the AJNR (1). They described that focal high signal intensity on FLAIR images may be seen in the anterior “subependymal” region of the splenium after radiation therapy or with aging. We would like to point out the following anatomic information that is related to some of the statements made in the article.

The corpus callosum is the largest transverse commissure connecting the cerebral hemispheres. Posteriorly, the corpus callosum is attached with the fornix and hippocampal commissure. On each side, its inferior surface roofs the lateral ventricle, covered by the ependyma (2).

The velum interpositum is located in the roof of the third ventricle below the body of the fornix. The upper and lower walls of the velum interpositum are formed by two membranous layers of tela choroidea in the roof of the third ventricle. The layer that is attached to the lower surface of the fornix and hippocampal commissure forms the upper wall. The lower wall is attached to the superior surface of the pineal body and tectum posteriorly (3).

The double layers of pia mater, the tela choroidea, cover the ependymal roof of the third ventricle. The anterior aspect of the tela choroidea is closed at the foramen of Monro, where the pia mater folds on itself. The posterior ends remain open, and between the two ends is cistern of velum interpositum (also known as cistern of the transverse fissure or cistern of the roof of the third ventricle), which abuts on the midanterior surface of the splenium and communicates with the quadrigeminal cistern (4) (Fig 1).

In conclusion, the midanterior surface of the callosal splenium abuts on the subarachnoid cistern (cistern of velum interpositum), but not on the ventricles. The correct nomenclature is the anterior “subpial” region of the callosal splenium.

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Fig 1.

Sagittal (A), axial (B), and coronal (C) MDCT cisternograms showing filling of contrast material in the subarachnoid cisterns and fourth ventricle but not yet in the third and lateral ventricles. Cistern of velum interpositum (arrows) communicates with the quadrigeminal cistern around the internal cerebral veins (arrowhead). The midanterior surface of the callosal splenium abuts on the cistern of velum interpositum, not on the ventricle.

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