Echogenic cavum septi pellucidi is associated with mild callosal dysgenesis on postnatal MRI

Abstract

We present a previously unreported association between the sonographic finding of an echogenic cavum septi pellucidi (CSP), concurrent hypointense signal within the CSP on fetal MRI and postnatal MR evidence of dysgenesis of the rostrum and genu of the corpus callosum as well as abnormal shortening of the septal leaflets. An echogenic or non‐fluid filled CSP in the fetus at the midtrimester ultrasound is likely to represent a form of mild callosal dysgenesis. Perinatal assessment of vision and endocrine function and clinical follow‐up of neurodevelopment and growth are probably indicated in such cases.

Case report

The patient was a previously well 31‐year‐old primipara (G1P0). A thickened fetal nuchal translucency (5.6 mm) was noted on prenatal ultrasound (US) performed at 12 weeks. However, non‐invasive prenatal testing and microarray were normal, and nuchal thickening resolved on subsequent scans.

At the 20 week prenatal US (Figure 1a), it was noted that the cavum septi pellucidi (CSP) was not clearly seen. Whilst the leaflets were visible, the intervening cavum was uniformly echogenic, without visible fluid, and without any positive mass effect. The corpus callosum appeared present, and the examination was otherwise unremarkable.

Figure 1.

CSP (Straight Arrows) is Echogenic at the 21 Week Ultrasound (a) and Hypointense to CSF on Axial T2 Weighted MRI at 26 Weeks (b). There is No Abnormal High Signal in the Region of CSP on the T1 Weighted MRI (c). Note the Incomplete Visualisation of the Rostrum of the Corpus Callosum on Sagittal T2 Weighted MRI at 26 Weeks (d – Curved Arrow).

Fetal MRI scans performed at 26 weeks and then again at 28 weeks gestation (3T Excite, GE Healthcare, Milwaukee, WI, USA) confirmed the presence of the septal leaflets, but also the abnormal appearance of the cavum which was uniformly low in signal relative to hyperintense cerebrospinal fluid on T2 weighted images (Figure 1b). There was no evidence of a mass, haemorrhage or lipid content within this, based on T1 (Figure 1c) and susceptibility weighted imaging, and there was no change in this appearance between the two scans. The corpus callosum appeared present at 26 weeks, though the rostrum was poorly defined (Figure 1d). However, lack of clear visualisation of the rostrum on fetal MR is usual, due to the lower spatial resolution of fetal MR compared with postnatal MR and lack of silhouetting of the rostrum by fluid, unlike the splenium, making is more difficult to visualise. The corpus callosum was suboptimally seen at 28 weeks on the follow‐up MR, due to fetal motion. There was no other abnormality on either prenatal MR, and appropriate progression of sulcation occurred between the two scans.

The baby was a male born by normal vaginal delivery at term (39 weeks 6 days). He had no dysmorphic features and was assessment of the visual evoked potentials was normal.

The neonatal MRI shows a mild dysgenesis of the anterior corpus callosum with a hypoplastic genu and absent rostrum. This deficiency is best demonstrated by constructing the so – called MAC line (Figure 2a). This line, drawn through the Mammillary bodies and Anterior Commissure (MAC) on a sagittal MRI image, delineates the junction between body and genu of the corpus callosum1 and demonstrates the shortened genu and absent rostrum in this case compared with an age‐matched control subject (Figure 2b). The same image highlights the abnormally superiorly positioned fornices, resulting from the abnormally short, dysplastic leaflets of the CSP, compared with the normal control. There were no other abnormalities of the neonatal brain, and the pituitary stalk, optic nerves, olfactory bulbs and olfactory sulci were normal.

Figure 2.

Neonatal MRI in Our Subject (a) and in an Aged Matched Normal (b). The MAC Line (Dotted Line) Divides the Body of the Corpus Callosum from the Genu, Highlighting the Hypoplastic Genu and the Elevation of the Forniceal Columns Related to Dysgenesis of the Septal Leaflets.

Discussion

Identification of a normal midline cavum septi pellucidi is significant in antenatal imaging because its development links closely to the formation of the corpus callosum, forebrain and telencephalic commissural development in general.²

The failure to visualise the septal leaflets after 17 weeks gestation results from either destruction of the CSP due to severe hydrocephalus, or failure of formation associated with a disorder of commissural cerebral development such as callosal dysgenesis, holoprosencephaly spectrum disorders, or craniosynostosis. Absent septal leaflets may be associated in some cases with a postnatal diagnosis of septo‐optic dysplasia, but this diagnosis is clinical and cannot be made based on prenatal ultrasound findings alone.

The reason for abnormally increased echogenicity of the CSP as a precursor to later anterior callosal dysgenesis is not well understood. It has been previously described on ultrasound, ^3,4 but the fetal MRI correlate and ultimate postnatal MR appearance of this entity have not, to our knowledge, been previously described.

Malinger et al.3 describe 17 fetuses with an isolated finding of an abnormally echogenic cavum septum pellucidum who had a tertiary neurosonogram performed following referral for lack of visualisation of the CSP on a community ultrasound. Of these 17, six underwent fetal MRI. In four, callosal dysgenesis/abnormality was suspected and in the other two the ultrasound finding of obliteration of the CSP by solid tissue was confirmed. Neurodevelopment was obtained in 12 children with isolated abnormal echogenicity of the CSP (range, 4–36 month; median, 19.5 months) including the 4 with callosal abnormalities diagnosed antenatally with MRI, and was normal in all cases.

We hypothesise that this condition is due to abnormal underdevelopment of the median sulcus of the telencephalon and part of the spectrum of callosal dysgenesis and dysplasia of the septal leaflets. In our case, neonatal outcome including visual testing and endocrine function were normal but long term follow‐up of growth and neurodevelopment are being undertaken.

Conclusion

The ultrasound finding of an obliterated or echogenic CSP can be associated with postnatal mild anterior callosal dysgenesis. While published data, supported by the postnatal follow‐up in our case, suggests favourable neurodevelopmental outcome at least in the short term, fetal MRI was indicated here due to the uncertainty about why the septal leaflets were not appreciable on US. Specifically, the distinction between absence of the leaflets and obliteration of the CSP was not made with US and fetal MR helped to make this distinction as well as excluding anomalies known to be associated with absence of the septal leaflets. Data regarding long term neurodevelopmental outcomes for this entity are needed.

Ethical approval

The authors have no ethical conflicts to disclose.

Conflict of interest

The authors have no conflicts of interest to declare or ethical conflicts to disclose.

Author contribution

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