Fetal Diagnosis of Rhombencephalosynapsis

Prenatal Magnetic Resonance Imaging of Atypical Partial Rhombencephalosynapsis with Involvement of the Anterior Vermis: Two Case Reports

Rhombencephalosynapsis (RS) is a rare, sporadic cerebellar malformation that is characterized by the absence of the cerebellar vermis and continuity of the cerebellar hemispheres, dentate nuclei, and superior cerebellar peduncles leading to a characteristic keyhole-shaped appearance of the fourth ventricle.[1] The continuity of the cerebellar hemispheres is best seen as horizontal folial pattern on posterior coronal images. Although the genetic cause of RS is still unknown, it is most likely caused by a dorsoventral patterning defect in the rostral dorsal midline regions of rhombomere 1, just below the isthmus organizer and surrounding the upper fourth ventricle.[2] [3] The majority of patients are nonsyndromic and present with truncal and/or limb ataxia, abnormal eye movements, head stereotypies, and delayed motor development.[4] Long-term cognitive outcome varies from severe impairment to normal. In some patients RS is a key feature of Gómez-López-Hernández syndrome (parietal alopecia, trigeminal anesthesia, and craniofacial dysmorphic signs).[5]

RS was first described by Obersteiner in 1916 in a 28-year-old man, who was employed as a clerical assistant and who committed suicide.[6] With the advent and continuous progress of magnetic resonance imaging (MRI) and growing experience of pediatric neurologists and neuroradiologists, RS is being increasingly recognized pre- and postnatally.[7] [8] [9] The report of an increasing number of patients with RS revealed a spectrum of severity ranging from severe, complete RS with absence of the entire vermis including nodulus to mild, partial RS with fusion of the central part of the vermis, most involving the posterior vermis with some anterior vermis seen above the fusion.[1] In rare cases, there may be an atypical partial RS that is characterized by absence of the posterior vermis and nodulus and presence of some residual anterior vermis.[1]

In this issue of Neuropediatrics, Izzo et al report on the prenatal diagnosis of atypical partial RS in two fetuses by MRI at 21 and 23 weeks of gestation, respectively.[10] In both fetuses, bilateral ventriculomegaly as detected by fetal ultrasound was the indication to prenatal MRI. Fetal ventriculomegaly is a rather common prenatal neuroimaging finding. It may occur isolated or may be part of a more complex malformative syndrome and only the tip of the iceberg. A proper identification of the underlying etiology is essential for predicting outcome. RS is known to be associated with congenital ventriculomegaly in 45 to 65% of the cases.[1] [11] In RS, ventriculomegaly is mostly due to aqueductal stenosis. Obstruction may occur at different points along the aqueduct. In cases with thalamic fusion, obstruction may occur more cranial to the aqueduct, while in patients with crowding of the posterior fossa obstruction may be distal to the aqueduct.[1] In children with aqueductal stenosis, Ishak et al identified RS in 9%.[1] RS should be specifically evaluated by MRI in patients with aqueductal stenosis.

Based on the week of gestation, the prenatal diagnosis of morphological abnormalities of the vermis may be challenging. After 21 weeks of gestation, the posterior fossa structures are well defined: the tentorium has its definitive orientation and the sagittal shape of the brain stem as well as cerebellar vermis and hemispheres is easy to be recognized.[12] The primary fissure of the vermis may be identified at 21 weeks of gestation, the vermian folia become clearly visible after 24 weeks of gestation, and the convolutions of the cerebellar hemispheres are typically detectable a few weeks later.[12] Hence, some authors believe that the diagnosis of morphological abnormalities of the vermis should not be performed before 24 weeks of gestation.[13] In the fetuses reported in this article, the diagnosis of atypical partial RS was made at 21 and 23 weeks of gestation, respectively, and was confirmed by MR autopsy and neuropathology in one case (in the second fetus, MR autopsy was not performed and neuropathology was inconclusive as consequence of tissue lysis). This report shows that the diagnosis of prenatal morphological abnormalities of the cerebellar vermis may be made correctly before 24 weeks of gestation. High experience with the normal appearance and development of the fetal cerebellum for the different gestational ages and the wide spectrum of morphological anomalies involving the fetal vermis is crucial for a correct, early diagnosis of malformations of the cerebellar vermis. In cases of uncertainty, a follow-up fetal MRI study may be very helpful to confirm or exclude the presence of vermian abnormalities during cerebellar development.

This article is an editorial on “Prenatal Magnetic Resonance Imaging of Atypical Partial Rhombencephalosynapsis with Involvement of the Anterior Vermis: Two Case Reports” by Izzo et al (Neuropediatrics 2015;46(6):416–419).

• References

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