Purpose or Case Report: Illustrate the spectrum of imaging findings of prenatal and postnatal diencephalic-mesencephalic junction dysplasia (DMJD).
Methods & Materials: Retrospective review of five Fetal MRI studies performed during 2nd and 3rd trimesters due to hydrocephalus. Balanced Turbo Field Echo (BTFE) and T2-weighted sequences in axial, coronal and sagittal planes were evaluated by a pediatric neuroradiologist and a pediatric radiologist.
Results: DMJD is a rare congenital brainstem malformation characterized by partial or complete fusion of the rostral midbrain with the thalamus or hypothalamus. There are three imaging patterns characterized by MRI:

Type A DMJD is characterized by hypothalamic-mesencephalic fusion in the axial plane and a ventral cleft contiguous with the third ventricle (“butterfly sign”). We present a male fetus at 23 weeks who underwent MRI for hydrocephalus. Imaging shows severe hydrocephalus, stenosis of the aqueduct and persistent adducted thumbs. Sagittal view showed hypothalamus-midbrain fusion and a deepened midline sulcus communicating with the third ventricle. Dysgenesis of corpus callosum and borderline hypoplasia of the vermis were also visualized. The constellation of findings suggested L1CAM mutation.

Type B DMJD is characterized by incomplete cleavage of the midbrain and thalamus in the sagittal plane. A 23 week fetus with hydrocephalus and cerebellar hypoplasia on ultrasound showed partial thalamic - midbrain fusion, a deep ventral cleft, delayed sulcation pattern and cerebellar hypoplasia confirmed on postnatal MRI at 8 days old.

Type C DMJD is characterized by complete or near-complete thalamic-midbrain union in the sagittal plane. MRI of a 29 week male fetus shows severe hydrocephalus, aqueductal atresia, complete fusion of the hypothalamus-thalamus-midbrain, callosal dysgenesis, cerebellar hypoplasia and persistent adducted thumbs suggestive of L1 syndrome. Additional case at 21 weeks gestation showed similar fusion of the hypothalamus-thalamus and midbrain associated with hydrocephalus, cleft palate, giant omphalocele and scoliosis.
Conclusions: DMJD is characterized by abnormal cleavage of the diencephalon and mesencephalon. The spectrum of findings and associated abnormalities are variable. Three distinct types have been described based on cleavage pattern on MRI. While the developmental mechanisms are poorly understood, DMJDs are commonly associated with L1CAM mutations and pathogenic variants of PCDH-12, VRK-1 and GSX2.