Absent Septum Pellucidum

The septum pellucidum (translucent hedge) is a thin, triangular, vertical double membrane separating the anterior horns of the left and right lateral ventricles of the brain. The septum pellucidum extends from the anterior part of the body, the genu, and the rostrum of the corpus callosum to the superior surface of the fornix. Its anterior to posterior extent therefore is from the lamina terminalis to the splenium of the corpus callosum. It bridges the midline gap between the corpus callosum above and the fornix below. Its width varies from 1 .5 to 3.0 mm. It contains glial cells, some scattered neurons, fiber bundles, and veins that connect with the choroid plexus veins. It is lined with ependyma on the ventricular side. There is no midline septum pellucidum, as the bodies of the lateral ventricles diverge posterior to the interventricular foramina. The diverging medial wall of the body of each lateral ventricle is instead formed by the ependyma-lined alvei of hippocampi. These alvei form a linkage between the medial margins of the fornices and the inferior surface of the body of the corpus callosum.

The septum pellucidum serves as an important relay station; its most anatomically and functionally important fiber connections are with the hippocampus and the hypothalamus and not with the primary olfactory structures.

It is linked with the hippocampus by precommissural fornix fibers, with preoptic and hypothalamic nuclei by the phylogenetically regressive median forebrain bundle of Broca that also relates the limbic with the olfactory system, with the amygdaloid nuclei by way of the striae terminalis, with the habenula and colliculi by way of the striae medullaris, and with the midbrain by its connection with the anterior fornix. It has a connection with the infracallosal precommissural primordium hippocampi through the paraterminal body.



Neurons from the Septum Pellucidum. Nissl staining. Mag 10X.

Absence of the septum pellucidum occurs in septo-optic dysplasia, a rare developmental disorder also characterized by abnormal development of the optic disk and pituitary deficiencies. Symptoms of septo-optic dysplasia are highly variable and may include vision difficulties, low muscle tone, hormonal problems, seizures, intellectual problems, and jaundice at birth. Children born without any other cognitive issues, other than an absent septum pellucidum, usually progress through life normally, and usually have no learning or cognitive disabilities.

Case courtesy of Dr Bruno Di Muzio, Radiopaedia.org. From the case rID: 41832.

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