Review of Cerebellar Embryology Key Imaging Findings and Associated Anomalies eEdE175 Abhijit Y Patil MD Vinay Kandula MD Michael Benstock MD Hemant A Parmar ID: 481058
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Rhombencephalosynapsis: Review of Cerebellar Embryology, Key Imaging Findings and Associated Anomalies eEdE-175
Abhijit Y. Patil, M.D.* Vinay Kandula, M.D. + Michael Benstock, M.D.#Hemant A. Parmar, M.D.*
*Department of Neuroradiology, University of Michigan Health System, Ann Arbor, MI
+
Nemours/Alfred I.
duPont
Hospital for Children, Wilmington, DE
#
Bryn Mawr Hospital, Bryn Mawr, PASlide2
Authors have no relevant financial disclosuresDisclosureSlide3
Rhombencephalosynapsis is a rare congenital malformation characterized by fusion of the cerebellar hemispheres and absence of the cerebellar vermis The exhibit will allow radiologists to:Identify classic features of rhombencephalosynapsis Recognize associated abnormalities such as aqueductal stenosis and other supratentorial anomalies
Be familiar with associated syndromes like Gómez-López-Hernández syndrome and VACTERLImprove communication with pediatric neurologistPurposeSlide4
After discussing normal cerebellar development and anatomy, five cases of rhombencephalosynapsis ranging from a newborn to an adult patient are presented to illustrate key imaging findings and associated anomaliesThe cases include isolated rhombencephalosynapsis as well as cases with associated supratentorial anomalies such as aqueductal stenosis and septo-optic dysplasia
Diffusion tensor imaging illustrating cerebellar fusion is shown for one patientApproachSlide5
Cerebellar Anatomy: LandmarksQuadrigeminal plateCerebral aqueduct Fourth ventricle
VermisCerebellar tonsilPonsTentorium cerebelliSuperior medullary velum Slide6
Cerebellar Anatomy: LandmarksVermisWhite matter of cerebellar hemisphere Slide7
Cerebellar Anatomy: FissuresPrimary Fissure (yellow arrow)Horizontal Fissure (
blue arrow)Suboccipital Fissure (green arrow)Slide8
Cerebellar Anatomy: LobesAnterior Lobe: Anterior to the primary fissure Posterior Lobe: Posterior to the primary fissure
Flocculonodular Lobe: Anterior to the posterolateral fissure * Posterolateral fissure Slide9
Cerebellar Anatomy: NucleiDentate Nucleus: Responsible for the planning, initiation and control of voluntary movements. Efferent fibers from this travel through red nucleus to contralateral ventrolateral thalamus
Interposed Nucleus*: Consists of globose and emboliform nuclei. Send efferent fibers to contralateral red nucleus. Origin for the rubrospinal tract that mainly influences limb flexor musclesFastigial Nucleus *: Afferents from vermis. Efferents via inferior cerebellar peduncle to vestibular nucleiSlide10
Cerebellar Anatomy: Peduncles Superior Cerebellar Peduncle *: Primary output of the cerebellum with mostly fibers carrying information to the midbrain. Most efferent fibers travel in this and arise from dentate nucleus (Tonsil
*)Middle Cerebellar Peduncle *: Carry input fibers from the contralateral cerebral cortex. Cortico-ponto-cerebellar form major fibers (Flocculus*)Inferior Cerebellar Peduncle: Connects the spinal cord and medulla oblongata with the cerebellum. Contain dorsal
spinocerebellar
tract,
olivocerebellar
and
vestibulocerebellar
tracts
Afferent fibers are far more numerous that efferent fibers by a ratio of 40:1Slide11
Cerebellar EmbryologyThe cerebellum represents a specific development of the alar plate from the rhombic lips of the metencephalon
At 50 days, the pontine flexure folds the metencephalon back against the myelencephalon. The rhombic lips of the pontine flexure will give rise to the cerebellumThe caudal region of the cerebellar primordia forms the more primitive flocculonodular
lobe, whereas the cranial region produces the much larger (and less primitive) vermis and cerebellar hemispheresSlide12
Rhombencephalosynapsis: DiscussionRhombencephalosynapsis (RES) is a rare midline brain malformation defined by fusion of the cerebellar hemispheres with partial or complete absence of the intervening vermisRES is a rare condition. Approximately 150 cases have been reported in the literature to date
RES was first described by Obersteiner in 1914 from a routine autopsy of a 24-year-old male suicide victim RES is probably due to an exaggerated fusion of the two lateral cerebellar primordia. The cause of this exaggerated fusion, associated with a failure of vermian differentiation, is unknownThe anomaly is mostly sporadic. Although genetic basis has been suggested based on clinical observations, no single genetic cause has been ascertainedSlide13
Clinical Presentation of RESClinical presentation is variable due to associated supratentorial anomalies. This may account for the variable age difference at presentation in our cases
Wide spectrum of clinical presentation from symptoms such as generalized hypotonia, nystagmus, ataxia, mild to severe mental and motor developmental delays and early death, to a few reported cases diagnosed at a late age without significant associated clinical findings Slide14
Complete vs. Partial RESRhombencephalosynapsis can be considered an anomaly with variable degrees of severity as partial RES has been reported in the literature Complete RES: total absence of the vermis with complete fusion of cerebellar hemispheres
Partial RES: retained parts of the anterior and posterior vermis including the nodulus, with partly unfused cerebellar hemispheres One of the cases discussed in this review (Case #3) also demonstrated imaging findings suggestive of partial RES, similar to previously reported cases with fused posterior cerebellar hemispheres, unfused hemispheres anteriorly with intervening anterior vermis, presence of primary fissure as well as normal 4th ventricle fastigial recess
Partial RES can be explained by an alternative model of cerebellar development that considers RES as a failure of
vermian
differentiation with undivided development of the cerebellar hemispheres. This model considers cerebellar
primordium
as an unpaired structure where the
posterolateral
fissure is formed initially with subsequent development of the posterior vermis before the anterior vermisSlide15
Imaging Findings of RES: Sagittal Partial or complete absence of vermisAbnormal shape of 4th
ventricle *Absent flow void of aqueduct (yellow arrow)Absence of fissures Absent primary fissure (green arrow)Absent suboccipital
fissure (
red arrow
)
Hydrocephalus due to aqueductal stenosis
*Slide16
Imaging Findings of RES: AxialContinuity of transverse folia across midline without intervening vermis (red arrow)
Continuity or apposition of middle cerebellar peduncles Fusion or apposition of dentate nuclei (blue arrow)Keyhole or teardrop shape of 4th ventricle due to absence of vermisSlide17
Imaging Findings of RES: Coronal Fusion of inferior colliculli
Fusion or approximation of superior cerebellar pedunclesContinuity of normal appearing transverse folia across midline without intervening vermis (yellow arrow) Slide18
Neuropathological studiesTotal or partial absence of the cerebellar vermis, including its lobulesAbsent fastigial nuclei
Fusion of cerebellar hemispheres across midline without cleft Reduced transverse diameter of the cerebellumFusion or close approximation of dentate nuclei Nodulus preserved in subtotal rhombencephalosynapsis Present globose
and
emboliform
nuclei
Presence of
flocculi
Slide19
Neuropathological studies: contd.Absent incisura
cerebelli posterior Absent anterior medullary velum Normal appearing major sulciNormally oriented folia that are fused across midline
Absence or abnormality of
olivary
complexes
Midline facial anomalies Slide20
Associated Abnormalities Hydrocephalus : most common associated anomalyIn majority of cases due to aqueductal stenosis (green arrow
)In a study by Ishak et. al., aqueductal stenosis was present in 22 of 42 subjects and was strongly associated with the degree of cerebellar fusionOther less common reported causes of hydrocephalus include: thalamic fusion and obstruction of 4th ventricle by cerebellar tissue Prognosis and severity of clinical presentation depends on associated
supratentorial
anomalies
Imaging pearl
: In patients with aqueductal stenosis, look for presence of RESSlide21
Associated Abnormalities Absence of septum pellucidum (arrow)2nd
most common associated abnormality Septo-optic dysplasia has been reportedOther associated midlines defects such as fusion of fornices have been reported Slide22
Associated Abnormalities Dysgenesis of corpus callosum (yellow arrow)
Can be associated with septal aplasia Slide23
Other Associated Abnormalities HoloprosencephalyAbsent olfactory bulbs
Fusion of thalami Neural tube defectsNeurocortical dysplasia Slide24
Associated Syndromes Gómez-López-Hernández syndrome (GLHS)
(aka Cerebello-trigeminal-dermal-dysplasia)Craniosynostosis causing tower-like skullTrigeminal anesthesia
Rhombencephalosynapsis
Bilateral parietal or temporal alopecia
Mental retardation, delayed speech
Imaging Pearls
:
Look for rhombencephalosynapsis in cases highly suggestive of GLHS
If rhombencephalosynapsis is diagnosed, the clinical signs of GLHS should be soughtSlide25
Associated Syndromes VACTERL associationFrequent occurrence of Vertebral anomalies, Anal atresia
, Cardiovascular anomalies, Trachea-oesophagEal fistula, Renal anomalies, Limb defects (VACTERL) as associated findings with RES has been reportedSlide26
Axial T2WI MR (a) shows transverse folia (red arrow) and fusion of dentate nuclei (blue arrow). Sagittal T2W MR (b) in the same child shows moderate to severe enlargement of the lateral ventricles (*), and to a lesser degree the third ventricle (*). There is stenosis at the cerebral aqueduct (green arrow) , with absence of the normal flow void. The fourth ventricle is normal in size. Absence of the primary fissure (
yellow arrow) in a child with Rhombencephalosynapsis (RES). RES is complete in this child. Axial CT (c) also clearly depicts fusion of cerebellar hemispheres (white arrow).Case 1: MRI brain at day 1 and CT at 8 monthsSlide27
Sagittal T2W MR (a) shows absence of the primary fissure (yellow arrow) in a child with isolated Rhombencephalosynapsis (RES). Foliation is that of the cerebellar hemisphere as no vermis is present. Note prominent corpus medullare (*).Coronal T2W MR (b) shows transverse folia (red arrow) and interfoliate sulci (green arrow) in the same child. RES is complete in this child. (c) DTI showing transverse fibers across the cerebellar hemispheres (white arrow).
Case 2: MRI brain at 2 years of ageSlide28
Case 3: MRI brain at 9 years of ageCoronal T2W MR (a) shows fusion of cerebellar hemispheres posteriorly with continuous transversely oriented folia and fissures extending across the midline (yellow arrow) in a child with partial Rhombencephalosynapsis (RES). Coronal T2W MR (b) shows unfused cerebellar hemispheres anteriorly with intervening anterior vermis (
blue arrow). Sagittal T1W MR (c) shows presence of normal primary fissure (green arrow), however, absence of prepyramidal fissure (red arrow). There is normal 4th ventricle fastigial recess (*). Atrophy of corpus callosum (*) as a result of old stroke superimposed on dysgenesis of corpus callosum with associated Wallerian degeneration (white arrow) in brainstem. Slide29
Case 4: MRI brain at 10 years of ageSagittal T2W MR (a) shows dysgenesis of corpus callosum (yellow arrow) in a child with complete Rhombencephalosynapsis (RES). Primary fissure (
green arrow) and prepyramidal fissure (red arrow) are absent. Axial T2W image (b) showing absence of septum pellucidum (white arrow). Coronal T1WI MR (c) shows associated mildly hypoplastic optic chiasm (blue arrow). Slide30
Case 5: MRI brain at 28 years of ageSagittal T1W MR (a) shows aqueductal stenosis (yellow arrow) causing hydrocephalus with enlarged lateral ventricles (*
) in an adult with complete Rhombencephalosynapsis (RES). Absence of the primary fissure (green arrow) and prepyramidal fissure (red arrow) is also seen. There is rounded 4th ventricle fastigial recess (*) secondary to hypoplasia of nodulus. Coronal T2WI MR (b) shows fusion of cerebellar hemispheres and continuous transversely oriented folia and fissures extending across the midline without intervening vermis (blue arrow). Axial T2W image (c) showing fused dentate nuclei without intervening vermis (white arrow).Slide31
Rhombencephalosynapsis (RES) although a rare congenital disorder, can be encountered in pediatric as well as occasionally in adult patientsRES is prone to be underreported due to radiologists’ relative lack of awareness of this condition Familiarity with classic imaging findings, associated supratentorial anomalies and related syndromes will allow correct identification and reporting of this rare condition
Conclusion Slide32
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ReferencesSlide33
Contact Information Abhijit Y. Patil, M.D.Fellow, Neuroradiology Division Dept. of Radiology, University of Michigan 1500 E. Medical Center Drive, UH B2-A209, Ann Arbor, MI 48109-5030
E-mail: abhijitp@med.umich.edu