Presentation
Severe ventriculomegaly and increased size of third ventricle on fetal ultrasound.
Patient Data
Antenatal ultrasound selected images showing severe ventriculomegaly and increased head size (HC/BPD) for gestational age. The third ventricle is dilated.
Hand views show the thumb in an adducted state (see last stack images and cine loop). In addition, the repeated "snapping" movement of the thumb across the palm is abnormal and indicative of spasticity that is characteristic of this condition which involves abnormal underdevelopment of the corticospinal tracts and absence of the medullary pyramids, demonstrable on postnatal imaging and pathology but not on fetal US or MRI.
There is severe bilateral ventriculomegaly. The overlying parenchyma is severely thinned and stretched due to ventricular enlargement and, as a result, assessment of lamination of the hemispheres (an important landmark of normality at this GA) is impossible. The degree of opercularisation (Sylvian fissure deepening and closure) expected at this GA is not seen, again relating to gross ventricular dilatation. However, opercularisation failure is also characteristic of primary disorders of sulcation, such as lissencephalies and tubulinopathies, which may also be associated with ventriculomegaly.
None of the other primary hemispheric fissures (calcarine, parietooccipital, central) that should be visible in a normal fetus The expected high T2 signal is not visualized in the cerebral acqueduct. However, caution needs to be exercised with regard to interpreting this as being indicative of aqueduct stenosis in the fetus as the small size of the aqueduct and motion can obscure the linear T2 hyper intensity in the aqueduct seen more easily at later gestation.
Note that the ganglionic eminences are NOT enlarged - this is an important distinguishing feature of this condition compared with other causes of ventriculomegaly plus brainstem kink i.e. dystroglycanopathy and tubulinopathy.
There is also subtle cerebellar vermian dysplasia (the vermis is foliated but was <10th centile for GA in height and AP diameter and is not semicircular in outline as is normally the case). Transverse cerebellar diameter was <3rd centile.
There is a ventral kink at the pontomesencephalic junction.
Adduction of the thumb is noted. Being able to capture this on MR is probably an indication of the high proportion of time that the thumb spends in this position in the fetus!
Axial T2: Severe ventriculomegaly and adducted thumb (curved blue arrow)
Sag T2: Kinked brainstem (straight blue arrow) and mildly dysplastic vermis (curved purple arrow)
The pregnancy was terminated following genetic counseling. Sequence analysis of the L1CAM gene revealed a homozygous deletion c2215delC in the DNA obtained from a fetal blood sample.
Case Discussion
The differential diagnosis of severe ventriculomegaly with a kinked brainstem and vermian hypoplasia includes dystroglycanopathy, tubulinopathy and mutations in the L1CAM gene on the X-chromosome.
In this case, L1 CAM mutation was favored due to the following imaging features:
- presence of thumb adduction,
- absence of abnormal basal ganglia enlargement or cavitation, seen commonly in fetuses with various causes of lissencephaly including tubulinopathy and dystroglycanopathy, and
- absence of globe abnormalities (microphthalmia, buphthalmos, persistent hyperplastic primary vitreous and/or retinal detachment), which occur in some cases of dystroglycanopathy.
The patient also had a sister with an intellectually disabled son, suggesting a disorder with an X-linked mode of inheritance
Although there is some overlap in the imaging appearance of these conditions and genetic analysis is required, this case illustrates the utility and complementarity of US and MRI in guiding the diagnosis.