The Cavum Septum Pellucidum (CSP) is conceptually considered a part of the longitudinal cerebral fissure that becomes walled off when the hemispheres that form the corpus callosum above and the fornix below unite. The Corpus callosum starts to develop by 12 weeks gestation from the lamina terminalis. This is associated with the development of the Septum Pellucida as two paired clear membranes. The space between the septa is one cavity but with two names- it is called CSP when it is anterior to the foramen of Monro and Cavum Vergae when it is posterior. The closure of the CSP begins at approximately 6 months gestational age and closure will have occurred in 97% of fetuses by term.
Several professional guidelines, reviews and societies including the American Institute of Ultrasound in Medicine, American College of Radiology and the American College of Obstetricians and Gynaecologists consider the assessment of the Cavum Septi Pellucidi (CSP) as an important part of the standard examination of fetal anatomy. An absent or CSP that is not visualized has important consequences for the fetus and pregnancy. These include several CNS conditions like Holoprosencephaly, Septo-optic dysplasia, Callosal dysgnesis and hypogenesis, chronic severe hydrocephalus, basilar encephalocoeles, schizencephaly, porencephaly/hydranencephaly and isolated septal deficiency.
Describing a Case
We describe a case of abnormality of ventricular system of the brain focused on the diagnosis, prognosis and follow up at 2 years. The parents of this subject presented in the second trimester for a second opinion. They were advised elsewhere that the fetus had a CNS abnormality and it may be better to terminate the pregnancy.
We decided to use a TIFFA scan of the fetus as the first step to identify any abnormalities. A unilateral venticulomegaly resulting from unilateral atresia of the Foramen of Munroe was noted on the ultrasound exam. We decided to explore this finding further with the use of a confirmatory fetal MRI exam. The fetal MRI exam done using SSFSE coronal sections of MRI confirmed the presence of unilateral atresia of Foramen of Munroe with a dilated lateral ventricle and normal contralateral ventricle. The cerebral Parenchyma, Corpus Callosum and Cerebellar Vermis were normal.
We discussed the findings with the parents and recommended an opinion from a neurosurgeon. The neurosurgeon reviewed the findings of the fetal MRI and confirmed the absence of associated CNS anomalies. The neurosurgeon recommended continuation of pregnancy and discussed surgical options with the parents. Post childbirth, the neurosurgeon did a septostomy in the Septum Pellucidum. There was no need for a ventriculoperitoneal shunt. The child has been doing well post surgery and is currently 2 years old.
Figure-1: Axial sections on ultrasound showing unilateral marked ventriculomegaly
FIGURE-2: MRI Coronal SSFSE (Single Shot Fast Spin Echo) images of Fetal Brain with unilateral Ventriculomegaly, Cavum Septum Pellucidum and normal contralateral ventricle and unilateral atresia of the Foramen of Munroe
There are several important points to consider
- It is important to look specifically for the CSP on ultrasound screening. Opt for a TIFFA scan in the second trimester.
- The CSP has a mean width of 3.4 mm (ranging from 2.08 to 4.72 mm based on 2 SD) at 19 to 20 gestation weeks.
- The CSP is routinely imaged on the 3 obligatory USG views of the fetal head
- In addition to the images of the ventricles and the posterior fossa, an axial view at the level of the paired thalami will yield both the BPD and the CSP
- The CSP can be visualized beyond approximately 18 to 20 gestation weeks
- Although CSP is a mandatory element of screening, one has to understand that it is not easy to visualize.
- Proper attention to technique is crucial to avoid calling the CSP as present when it is actually absent.
- The CSP has to extend from the medial wall of the frontal horn of one lateral ventricle to the medial wall of the frontal horn of the contralateral lateral ventricle. This is important. If you do not confirm this, you run the risk of missing the true absence of CSP and missing associated neurologic abnormalities.
- The presence of artefacts that mimic the presence of CSP in standard axial second trimester sonographic images has been described. If the CSP is not seen with certainty in standard axial images, obtain a series of coronal images and also attempt to use a mid-sagittal view to identify the Corpus callosum.
- Remember, it is easy to confuse the paired nerve columns of the fornix with the CSP. Paired hypoechoeic structures in the forebrain may lead to the fornix being misinterpreted as the CSP. The Normal CSP should appear as a dark box bounded by two white lines on its lateral margins. The fornix, however, will appear as a white line (lateral margin of one of the fornices), a dark area, a white line that is the interface between the paired fornices, a dark area and a white line that is the lateral margin of the other fornix.
- Demonstration of a fornix does not exclude abnormalities of the corpus callosum and other central forebrain malformations.
- A squared appearance to the frontal horns (on coronal views) indicate the need further exploration to confirm the absence of CSP.
- Once you detect/suspect an abnormality in the CNS of the fetus, always opt for a confirmatory fetal MRI.
- Fetal MRI adds useful information in fetuses with CNS abnormalities detected or suspected on USG; A fetal MRI is to be considered when USG is difficult- maternal obesity or oligohydramnios
- MRI is more sensitive especially in instances of CNS abnormalities of the fetus. The ability to evaluate the fetus in multiple planes within a larger field of view is an advantage of MRI.
- A review of the previous US scan is important so that a targeted MR imaging is done.
- Multiplanar sequences used may include T2-weighted Fast SE, half-Fourier single-shot fast SE, 0.5 signal-acquired single-shot fast SE, flow sensitive gradient echo (GRE) and echo planar imaging. Targeted images can be acquired after obtaining coronal, axial and sagittal images as localizers. Relocalization may be necessary if the fetus changes presentation or lie during the examination.
- T2 images best reveal ventricular morphology
- Imaging to Identify and Confirm the diagnosis/finding is an important step but is not the only step
- You have to consider the parents as well.
- Carefully consider your diagnosis, seek additional opinions and counsel the parents on further steps.
- Parents have a lot of physical and emotional investment in the growing fetus. Be considerate to their state and carefully discuss all options with them. Consider them as stakeholders in the care of their baby.
In this particular case, a confirmatory MRI and consultation with the neurosurgeon made a prognostic difference; the difference between a termination of pregnancy and surviving 2 year old is very obvious.
- American Institute of Ultrasound in Medicine. AIUM practice guideline for the performance of an antepartum obstetric ultrasound examination. J Ultrasound Med 2003; 22:1116–1125.
- American Institute of Ultrasound in Medicine. AIUM practice guideline for the performance of obstetric ultrasound examinations. J Ultrasound Med 2010; 29:157–166.
- American College of Obstetricians and Gynecologists. ACOG practice bulletin 58: ultrasonography in pregnancy. Obstet Gynecol 2004; 104:1449–1458.
- Angtuaco TL. Ultrasound imaging of fetal brain abnormal-ities: three essential anatomical levels. Ultrasound Q 2005; 21:287–294.
- International Society of Ultrasound in Obstetrics and Gynecology Education Committee. Sonographic examination of the fetal central nervous system: guidelines for performing the “basic examination” and the “fetal neurosonogram.” Ultrasound Obstet Gynecol 2007; 29:109– 116.
- Callen PW, Callen AL, Glenn OA, Toi A. Columns of the fornix, not to be mistaken for the cavum septi pellucidi on prenatal sonography. J Ultrasound Med 2008; 27:25–31
- Sherer DM, Sokolovski M, Dalloul M, Santoso P, Curcio J, Abulafia O. Prenatal diagnosis of dilated cavum septum pellucidum et vergae. Am J Perinatol 2004; 21:247–251.
- Serhatlioglu S, Kocakoc E, Kiris A, Sapmaz E, Boztosun Y, Bozgeyik Z. Sonographic measurement of the fetal cerebellum, cisterna magna, and cavum septum pellucidum in normal fetuses in the second and third trimesters of pregnancy. J Clin Ultrasound 2003; 31:194–200
- Lepinard C, Coutant R, Boussion F, et al. Prenatal diagnosis of absence of the septum pellucidum associated with septo-optic dysplasia. Ultrasound Obstet Gynecol 2005; 25:73–75
- Pilu G, Tani G, Carletti A, Malaigia S, Ghi T, Rizzo N. Difficult early sonographic diagnosis of absence of the fetal septum pellucidum. Ultrasound Obstet Gynecol 2005; 25:70–72.
- Barkovich AJ, Norman D. Absence of the septum pellu-cidum: a useful sign in the diagnosis of congenital brain malformations. AJR Am J Roentgenol 1989; 152:353–360.
- Jou HJ, Shyu MK, Wu SC, Chen SM, Su CH, Hsieh FJ. Ultrasound measurement of the fetal cavum septi pellucidi. Ultrasound Obstet Gynecol 1998; 12:419–421