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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