Presentation
Tertiary obstetric referral after increased NT at 12 weeks (5.3mm). Low risk NIPT. CVS showed normal molecular karyotype. US 15 and 20 week US showed prominent renal pelvices, normal growth.
Patient Data
Selected ultrasound images are shown. There was normal fetal biometry.
The corpus callosum was identified. The CSP however could not be clearly seen.
There was bilateral renal pyelectasis (not shown), but otherwise normal anatomy.
The cavum septum pellucidum is absent and there is an abnormal bridge of tissue across the midline at the level of the CSP.
Intermediate to high T2 signal structure centered within the CSP, measuring 4 x 6 x 7mm.
There is no associated diffusion restriction, susceptibility artefact or T1 high signal.
Ultrasound Day 3
Fetal MRI shows hypogenesis of the rostrum, which was difficult to appreciate on fetal imaging.
The MAC line is a reference line, drawn on a sagittal midline image through mammillary body and anterior commisure, intersecting the corpus callosum. It indicates the anatomic transition from genu to body of corpus callosum, and helps to distinguish the normal from the abnormal genu.
In this case, note the foreshortened genu (cf an aged matched normal) and absent rostrum, indicating a subtle callosal dysgenesis that was not easily appreciated on prenatal imaging. Also note the abnormally close positioning of the fornix to corpus callosum, indicating that the CSP between them was also abnormally formed.
Case Discussion
This is an unusual abnormality of the CSP, associated with a subtle callosal dysgenesis.
In general, possible reasons non visualization of a normal CSP antenatally include:
1. Septal leaflets not formed (most common scenario, but clearly not the case in this patient)
2. Leaflets formed, but cavity subsequently filled in, eg by cyst, neoplasm, hematoma, hamartoma.
3. Failure of formation of the cavity.
The latter option is thought to be the cause for the appearance in this case. The normal CSP is formed from the 8th week of embryonal development from a structure called the median sulcus (sulcus medianus telencephali medii), in a process that aids normal cerebral commisurization.
Developmental etiology is supported by the finding of subtle callosal dysgenesis, which was best depicted on the postnatal MRI (as demonstrated on the annotated image).
Ongoing monitoring of endocrine function and growth will be required, as these abnormalities may manifest later in life.
Acknowledgement: Prof Stacy Goergen.
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