This fetus has absent cavum septum pellucidum, presumably as part of septooptic dysplasia.
A little anatomy before we go any further: The septum pellucidum consists of two thin membranes and separates the frontal horns of the lateral ventricles. This extends from the corpus callosum to the columns of the fornix, as shown below. This two layered septum is filled with fluid and may vary anatomically. If this fluid-filled area becomes large during organogenesis, there will be only one cavum, cavum septum pellucidum. If this space is narrow, two cavities will form. The anterior one would be cavum septum pellucidum and the posterior cavum vergae which communicate with each other. They usually fuse from back to front as the fetus approaches term or within the first few weeks after birth
In our case you can see that there is no distinct cavum septum pellucidum between the lateral ventricles. Rather, there is only one large midline fluid filled area which represents the fused frontal horns of the lateral ventricles. Those fused horns appear to have a square like roof and this appearance is characteristic of septooptic dysplasia (SOD). The corpus callosum is usually present in SOD; indeed in our case at least the front part of the corpus callosum appears to be present as proven by imaging of the pericallosal artery. Mild ventriculomegaly may also be a feature of SOD, as is the case with this fetus.
Sometimes the cavum septum pellucidum and/or cavum vergae may be quite prominent and create the false impression of SOD. In all theses cases you have to look carefully to see if you can identify the frontal horns of the lateral ventricles separately from the cavum. Coronal views may be especially helpful for that. The following images come from a different case, initially suspected to have SOD but which was normal as shown in the last two images:
The characteristic finding of absent cavum septum pellucidum with central fusion and squaring of the frontal horns in SOD is virtually identical to that encountered with lobar holoprocencephaly. Although the presence of a corpus callosum favors the diagnosis of SOD, corpus callosum may be present or simply hypoplastic in lobar holoprocencephaly as well. After birth a definite diagnosis may be made my MRI demonstrating optic tract hypoplasia or clinically with endocrine evaluation and visual assessment.
As a syndrome, SOD is characterized by anomalies of cerebral midline structures, such as absence of the septum pellucidum, congenital optic nerve dysplasia and panhypopituitarism, leading to multiple endocrine defects (diabetes insipidus, hypogonadotropic hypogonadism, hypothyroidism, adrenal insufficiency, diabetes insipidus etc). The clinical affects of SOD are variable. Visual impairment is usually present, but blindness is rare. Hypopituitarism is amenable to medical treatment. The developmental outcome is debated. Absence of the septum pellucidum and optic nerve hypoplasia are associated with an excess of cerebral palsy, mental retardation and seizures. However, abnormal development is usually limited to those cases with coexistent cerebral hemispheric anomalies.