A midline sagittal T1-WI (Fig. A) demonstrates absence of the normal cerebellar vermis. Cerebellar tissue is present in the midline, but note that it does not have the normal vermian lobular configuration.
A coronal T2-WI (Fig. B)shows midline fused cerebellar lobes with a small fourth ventricle. Cerebellar folia cross the midline, with no intervening vermis.
An axial T2-WI at the level of the medulla (Fig. C) confirms fusion of the cerebellar hemispheres across the midline and lack of formation of the vermis.
An axial T2-WI (Fig. D) at the level of the superior cerebellar peduncles shows fusion of the cerebellar hemispheres with no definable midline cerebellar vermis. The fourth ventricle is “heart” shape.
An axial CT section (Fig. E) of posterior fossa shows a mass of cerebellar tissue with poorly defined vermis and hemispheres.
An axial CT section (Fig. F) at a slightly higher level shows the “heart” shaped posteriorly pointing fourth ventricle

Rhombencephalosynapsis

Iyappan Ponnuswamy


History: A 2 Yrs old Male child presented with history of aggressive behavior and delayed developmental milestones. Clinical examination revealed cerebellar ataxia


Findings:

MRI 1: A midline sagittal T1-WI (Fig. A) demonstrates absence of the normal cerebellar vermis. Cerebellar tissue is present in the midline, but note that it does not have the normal vermian lobular configuration. :: MRI 1 ::
A midline sagittal T1-WI (Fig. A) demonstrates absence of the normal cerebellar vermis. Cerebellar tissue is present in the midline, but note that it does not have the normal vermian lobular configuration.







MRI 2: A coronal T2-WI (Fig. B)shows midline fused cerebellar lobes with a small fourth ventricle.  Cerebellar folia cross the midline, with no intervening vermis. :: MRI 2 ::
A coronal T2-WI (Fig. B)shows midline fused cerebellar lobes with a small fourth ventricle. Cerebellar folia cross the midline, with no intervening vermis.







MRI 3: An axial T2-WI at the level of the medulla (Fig. C) confirms fusion of the cerebellar hemispheres across the midline and lack of formation of the vermis. :: MRI 3 ::
An axial T2-WI at the level of the medulla (Fig. C) confirms fusion of the cerebellar hemispheres across the midline and lack of formation of the vermis.







MRI 4: An axial T2-WI (Fig. D) at the level of the superior cerebellar peduncles shows fusion of the cerebellar hemispheres with no definable midline cerebellar vermis. The fourth ventricle is “heart” shape. :: MRI 4 ::
An axial T2-WI (Fig. D) at the level of the superior cerebellar peduncles shows fusion of the cerebellar hemispheres with no definable midline cerebellar vermis. The fourth ventricle is “heart” shape.







CT 1: An axial CT section (Fig. E) of posterior fossa shows a mass of cerebellar tissue with poorly defined vermis and hemispheres. :: CT 1 ::
An axial CT section (Fig. E) of posterior fossa shows a mass of cerebellar tissue with poorly defined vermis and hemispheres.







CT 2: An axial CT section (Fig. F) at a slightly higher level shows the “heart” shaped posteriorly pointing fourth ventricle :: CT 2 ::
An axial CT section (Fig. F) at a slightly higher level shows the “heart” shaped posteriorly pointing fourth ventricle









Discussion:
Rhombencephalosynapsis is an extremely rare cerebellar malformation that involves vermian agenesis or severe hypogenesis, fusion of the cerebellar hemispheres, and apposition or fusion of the dentate nuclei.
The first case diagnosed by MRI was reported in 1991. There are less than 50 cases described in the literature so far.

Etiology

Cause is unknown. No teratogenic factor can be identified so far. RS seems to be sporadic in occurrence with no familial recurrence. No particular chromosomal abnormality could be attributed. The pathogenesis is related to a disturbed development of the cerebellum between 28 and 41 days of gestation.
Absence or hypoplasia of the normal midline vermis in combination with absence of distention of the rhombencephalic vesicle allows the cerebellar hemispheric primordia (rhombic lips) to fuse across the midline.

Clinical Features

Clinical presentation can be variable. Patients with supratentorial anomalies and rudimentary cerebellum seem to be the most symptomatic ones. Manifestations include hypotonia, motor and cerebellar dysfunction, strabismus and developmental delays. Many patients die in infancy or early childhood, and most reports are of pediatric patients, although there are scattered cases of adult subjects in the literature. No correlation between neuroimaging findings and clinical manifestations could be made so far.

Pathology
Gross
• Single-lobed cerebellum which is typically hypoplastic
• Folia and fissures are oriented horizontally
• The vermis is absent or hypoplastic, the flocculi are rudimentary, and the tonsils are usually fused
Microscopic
• The inferior olivary nuclei are typically absent

Imaging Findings

CT

.Till date, no case was diagnosed based upon CT findings solely.
• A single mass of cerebellar tissue with poorly defined vermis and hemispheres is present
.Sometimes, the fourth ventricle may adopt the configuration of a keyhole or heart shape.

MR
• There is continuity of gray and white matter of the folia across the midline of the single-lobed cerebellum
• Absence or severe hypoplasia of the vermis
. The nodulus may be formed if the fastigium is present in a sagittal section.
• Apposition or fusion of the middle cerebellar peduncles and dentate nuclei
• A small, narrow fourth ventricle with a ‘‘keyhole’’or “diamond” shape fourth ventricle, instead of the normal crescent shape, is seen on axial sections
• Small brainstem, posterior fossa, and cisterna magna
. Deficiency or absence of the septum pellucidum, dysgenesis of the corpus callosum and the anterior commissure, fused fornices, and fused thalami may also be present.

Treatment
None

Prognosis
Intellectual impairment and life expectancy are variable, although many affected individuals die in infancy and childhood





Differential diagnosis: None: this is a pathognomonic appearance

Diagnosis confirmation: Imaging including endoscopy



Category: Neuro

Region / Organ: Head-Brain and brain nerves

Etiology: congenital

References:
1. Hidetsuna Utsunomiya, Koichi Takano, Tetsuzo Ogasawara, Takeo Hashimoto, Takeo Fukushima, and Masatoshi Okazaki. Rhombencephalosynapsis: Cerebellar Embryogenesis.AJNR Am J Neuroradiology 1998;19:547–549

2. FCY Lam, PD Barnes. A Case Report of Rhombencephalosynapsis. HK J Paediatr (new series) 2006;11:157-159

3. J.L.F. Mendonça, M.R.C. Natal, S.L. Viana, P.P.A. Coimbra, M.A.C.B. Viana, M. Matsumine. Rhombencephalosynapsis: CT and MRI findings. Neurology India 2004;52:118-120

4. Lisa l. Guyot, Chris D. Kazmierczak and Daniel b. Michael. Adult Rhombencephalosynapsis. J Neurosurg 2000;93:323–325

5. Philippe Demaerel, Catherine Morel, Lieven Lagae, and Guido Wilms. Partial Rhombencephalosynapsis. AJNR Am J Neuroradiology 2004;25:29–31




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