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\title{Beyond the obvious cranial abnormalities in fetal MMC. (Mini-commentary
on BJOG-20-1008.R1)}



\author[1]{Thierry Huisman}%
\affil[1]{Texas Children's Hospital, Department of Radiology}%


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  \date{June 12, 2024}


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Mini-commentary on BJOG-20-1008.R1: Cranial findings detected by second
trimester ultrasound in fetuses with myelomeningocele: a systematic
review

\textbf{Title: Beyond the obvious cranial abnormalities in fetal MMC}

\textbf{Author information:}Thierry A.G.M. Huisman, Department of
Radiology, Texas Children's Hospital Baylor College of Medicine 6701
Fannin Street, Suite 470 Houston, TX 77030 email:
huisman@texaschildrens.org

I write with pleasure a commentary to the excellent review by Dr.
Kunpalin and colleagues on the spectrum and frequency of cranial
findings that may be encountered by second trimester ultrasound in
fetuses diagnosed with non-skin covered spinal dysraphias (BJOG xxxx).
The early and adequate recognition of associated or presumed
non-associated/incidental imaging findings have a significant impact on
decision making of intrauterine corrective surgical procedures. The
association between a spinal dysraphia and the occurrence of a Chiari 2
malformation (C2M) are linked to the mechanical consequences of a
venting/leakage of cerebro-spinal fluid (CSF) from the developing caudal
embryonic rhombencepalic and metencephalic vesicles during early fetal
life. This unified theory does not explain the entire spectrum of
anatomical abnormalities that may be observed on imaging. Additional
complex molecular genetic and environmental factors also play a role.
The authors summarize 14 obvious sonographic findings that may be
detected during the second trimester. They reference that ``\emph{About
one-third are in the C2M spectrum and can be attributed to the
mechanical consequences of a CSF leakage. Some abnormalities may not be
obviously explained by the CSF leakage but their frequency indicate that
they are someway linked} ''. I completely agree with this conclusion.
Careful analysis of high-resolution anatomical and functional postnatal
imaging studies have shown that the brainstem, cerebellum and cerebrum
are highly abnormal beyond the obvious morphological distortion.
Diffusion tensor imaging (DTI) studies revealed an abnormal
micro-structural neuro-architecture and anomalous maturation of multiple
white matter tracts (Semin Ultrasound CT MR. 2016 Apr;37(20):129-142).
For example, qualitative DTI analysis and fiber tractography show a
reduction in size of the transverse pontine fibers as the most likely
explanation of pontine hypoplasia and anterior displacement of the
dentate nuclei. DTI also shows a decrease in the fiber density of the
middle cerebellar peduncles and cerebellar hypoplasia. Furthermore next
to the obvious corpus callosum (CC) hypo-dysplasia spectrum which is
likely secondary to a global white matter defect, in about 60\% of
children with C2M a dorsal callosal ridge is seen representing an
interhemispheric dysplastic small white matter bundle running over the
dorsal CC. Finally the increased incidence of true malformations of
cortical development (e.g. polymicrogyria), subependymal gray matter
heterotopias, malrotated and laterally displaced hippocampi, accessory
lobes, defined as gyri protruding into the posterior interhemispheric
cistern (20\% of patients), and an anomalous, possibly duplicated massa
intermedia confirm that the anomalous morphologically less obvious or
presumed non-associated malformations are secondary to the anomalous
CSF-hemodyamics and pressure dynamics interacting/disrupting the various
components of the complex programmed brain development (e.g. neuronal
migration, intracortical laminar organization, myelination/sulcation,
axonal guidance). Finally, the genetic association between the spinal
dysraphia and identified intracranial malformations is consequently much
more complex and linked than initially presumed. With ongoing
improvements in the diagnostic sensitivity and specificity of prenatal
imaging more detailed components of this complex malformation complex
will become ``detectable'' at an earlier phase of pregnancy and a
careful correlation of these findings with clinical outcome appears to
be a sine qua non for parental counselling and surgical decision making.

\textbf{No disclosures:} A completed disclosure of interest form is
available to view online as supporting information.

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