We report a rare case of partial rhombencephalosynapsis coexistent with Chiari II malformation in a 6-year-old girl and discuss the features of these entities on magnetic resonance imaging.published_or_final_versio

Ip, P

Khong, PL

Ooi, GC

Wan, SMY

Hong Kong Medical Journal

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Title Partial rhombencephalosynapsis and Chiari II malformationAuthor(s) Wan, SMY; Khong, PL; Ip, P; Ooi, GCCitation Hong Kong Medical Journal, 2005, v. 11 n. 4, p. 299-302Issued Date 2005URL http://hdl.handle.net/10722/44647Rights Creative Commons: Attribution 3.0 Hong Kong LicenseHong Kong Med J Vol 11 No 4 August 2005     299CASE REPORTKey words:Arnold-Chiari malformation;Cerebellum;Child;Magnetic resonance imaging;Rhombencephalon ! !"#$%&'()   !"#Hong Kong Med J 2005;11:299-302Partial rhombencephalosynapsis andChiari II malformation○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ !"#$%&'(ff !"#$%&'()Department of General Surgery, Norfolkand Norwich University Hospital, Norwich,United KingdomSMY Wan, BA, MBB ChirUniversity of Hong Kong, Queen MaryHospital, Pokfulam, Hong Kong:Department of Diagnostic RadiologyPL Khong, FRCRGC Ooi, FRCRDepartment of Paediatrics and AdolescentMedicineP Ip, MRCPCorrespondence to: Dr PL Khong(e-mail: plkhong@hkucc.hku.hk)SMY Wan  PL Khong  P Ip  GC Ooi  We report a rare case of partial rhombencephalosynapsis coexistentwith Chiari II malformation in a 6-year-old girl and discuss the fea-tures of these entities on magnetic resonance imaging. !"#S !"#$%&'()*+,-./0 ff !" !"#$%&'()*+,-./01234(5%678IntroductionRhombencephalosynapsis (RS) is a rare congenital malformation of theposterior cranial fossa characterised by vermal agenesis or hypogenesisand fusion of the cerebellar hemispheres. About 40 cases have been re-ported.1 Partial RS was reported for the first time recently whereby normaldevelopment of the anterior vermis and nodulus was noted but part ofthe posterior vermis was deficient.2 One case of RS associated with ChiariII malformation has also been reported.3 To the best of our knowledge,the coexistence of partial RS and Chiari II malformation and their featureson magnetic resonance imaging (MRI) have not been reported.Case reportA 6-year-old girl had spina bifida and hydrocephalus at birth. She was thesecond child of a non-consanguineous southern Chinese couple. Antenatalexamination by a private obstetrician including an ultrasound scan at 22weeks’ gestation was reported to be normal. There was no family historyof congenital malformations or any other remarkable medical problems.Her birth at full term was complicated by her large head and the deliverynecessitated a Caesarean section. A ruptured myelomeningocele over thelumbosacral region was also noted at birth and there was paucity of lowerlimb movement. Macrocephaly with features of increased intracranialpressure including dilated scalp veins and sunsetting of the eyes werefound. Computed tomographic scan confirmed hydrocephalus, withgrossly dilated lateral and third ventricles, and a myelomeningocele atthe L5-S1 level. The myelomeningocele was repaired surgically and aventriculoperitoneal shunt inserted on postnatal day 1 to relieve thehydrocephalus. Chromosomal analysis revealed normal karyotype of46,XX.She had recurrent urinary tract infections during infancy. Subsequentultrasound study of the urinary tract revealed a single horseshoe kidneywith bilateral hydronephrosis and a mildly irregular renal contour. Fur-ther dimercaptosuccinic acid study revealed scarring of the left side ofthe single kidney. Long-term low-dose prophylactic antibiotic therapywas prescribed.In the ensuing years, she was found to have global developmentalWan et al300      Hong Kong Med J Vol 11 No 4 August 2005delay, cerebellar ataxia, and mild dystonia. Sheentered a special child care centre at the age of 3 years,where she received intensive training. There was asteady improvement and satisfactory progress in herdevelopment.Magnetic resonance imaging at 5 years of agerevealed absence of the inferior cerebellar vermis (Fig1a), hypoplasia of the superior cerebellar vermis, andpartial fusion of the two cerebellar hemispheres (Fig1b). The nodulus was present, although hypoplastic(Fig 1b). There was partial fusion of the thalami(Fig 2a), but the dentate nuclei, superior and inferiorcolliculi were not fused. The folia were transverselyoriented and extended across the whole of thecerebellum (Fig 1). There was a towering appearanceof the fused cerebellum at the region of the tentorialhiatus. In addition, the lateral aspects of the cerebellumwere displaced anterolaterally, ‘wrapping’ around thebrainstem, which was hypoplastic (Fig 1a). The fourthventricle was slightly dilated and had a keyhole shape.The tegmental plate appeared beak-shaped (Fig 2b).The posterior cranial fossa was small and the cerebellartonsils extended below the level of the foramen magnainto the spinal canal (Fig 2b). Supratentorially, thebrain was grossly abnormal. The ventricles weredecompressed such that the ventricular walls werenearly coapted. The corpus callosum was markedlyhypoplastic and distorted. The septum pellucidum wasnot apparent. There was also marked reduction ofcortical white matter. Gyral pattern in the parieto-occipital region was abnormal and the appearance wasin keeping with stenogyria. In addition, there wasinterdigitation of gyri across the interhemisphericfissure. The medial temporal lobe and the hippocampiappeared hypoplastic. These features were consistentwith the diagnosis of partial RS with a Chiari IImalformation.DiscussionRhombencephalosynapsis was first described byObersteiner in 19144 and the first case diagnosed byMRI was reported in 1991.5 It has been estimated thatRS occurs with a frequency of 0.13% in the paediatricpopulation6 but the true frequency could be higherthan the estimate, due to an increasing awareness ofthe malformation, as well as advancements in magneticresonance technology and its increasing availability.Both the new entity of partial RS and coexistent RSand Chiari II malformation were described for the firsttime within the last 2 years.2,3 Our findings of thepresence of the anterior vermis and nodulus, absenceof the posterior vermis, and partial fusion of the cer-ebellar hemispheres and thalami are similar to that ofthe case reported recently by Demaerel et al.2 InFig 1. (a) Axial T2-weighted magnetic resonance image showing absence of the inferior cerebellar vermis,transversely oriented cerebellar folia, and ‘wrapping’ of the cerebellum around the brainstem (arrows); (b) axialT2-weighted magnetic resonance image showing hypoplastic superior cerebellar vermis and nodulus (arrow)and fusion of the cerebellar hemispheres (arrowheads)(a) (b)Hong Kong Med J Vol 11 No 4 August 2005      301Partial rhombencephalosynapsiscontradistinction, the tonsils were present and her-niated through the foramen magnum in our patient andthere was no fusion of the colliculi. The recent view ofcerebellar development considers the cerebellarpremordium an unpaired structure with the develop-ment of the posterior vermis followed by the anteriorvermis.7,8 Rhombencephalosynapsis is thereforeattributed to a failure of vermian differentiationrather than an abnormal fusion of the two cerebellarhemispheres.8 The theory that the posterior vermisdevelops before the anterior vermis does not supportthe findings of absent posterior vermis in the presenceof an anterior vermis in both cases of partial RS.2 It ispossible that the defect seen in partial RS is due tosome intrinsic inability of the posterior vermis torespond to inductive signals for its differentiation.The theories for the pathogenesis of RS includegenetic defects; it has been suggested that RS probablyrepresents an underexpression of a dorsalisingorganiser gene.9 Cerebellar midline and caudal midbrainabnormalities resembling that seen in RS have beenidentified in a mutant mouse model Dreher (dr) that hasa homozygous mutation in the Lmx1a gene.10 Furthercharacterisation and study of such dorsalising factorsmay shed light on the pathogenesis of RS.11Fig 2. (a) Axial T2-weighted magnetic resonance image showing partial fusion of the thalami; (b) sagittalT1-weighted magnetic resonance image showing features of the Chiari II malformation; a small posterior fossawith herniation of the cerebellar tonsils through the foramen magnum (arrow) and beaking of the tectum(arrowhead)(a) (b)It has been suggested that RS is sometimesmisinterpreted as Chiari II malformation by the unwarybecause of the rarity of RS and their overlappingimaging features: ventriculomegaly, hypoplasia of thecorpus callosum, and a relatively small-sized posteriorfossa leading to an impression of downward pointingof the cerebellar tonsils and a small fourth ventricle.6In our patient, as in the previously reported case ofcoexisting RS and Chiari II malformation,3 there was amyelomeningocele that is almost invariably present inChiari II malformations. Other associated features ofChiari II malformation, such as beaking of the tectum,gyral interdigitation through a fenestrated falx, anddisplacement of the cerebellum to ‘wrap’ round thebrainstem, were also seen in our patient. In addition, an18-fold increase in the incidence of fused kidneyswith meningomyelocele was reported12 and thiscoincides with the occurrence of a horseshoe kidneyin our patient. The reason for the association is notentirely clear. It has been speculated that the deformityof the spine in meningomyelocele may bring thedeveloping kidney into closer proximity than normalbecause they migrate caudally to assume their adultposition, encouraging fusion to occur.12This case report highlights the superiority of MRIWan et al302      Hong Kong Med J Vol 11 No 4 August 2005in diagnosing posterior fossa malformations, especiallyin the presence of complex pathology.References1. Toelle SP, Yalcinkaya C, Kocer N, et al.  Rhomb-encephalosynapsis: clinical findings and neuroimagingin 9 children. Neuropediatrics 2002;33:209-14.2. Demaerel P, Morel C, Lagae L, Wilms G. Partial rhomb-encephalosynapsis. AJNR Am J Neuroradiol 2004;25:29-31.3. Sener RN, Dzelzite S. Rhombencephalosynapsis and a ChiariII malformation. J Comput Assist Tomogr 2003;27:257-9.4. Obersteiner H. A cerebellum without vermis [in German]. ArbNeurol Inst Wien Univ 1914;21:124-36.5. Truwit CL, Barkovich AJ, Shanahan R, Maroldo TV. MR imagingof rhombencephalosynapsis: report of three cases and reviewof the literature. AJNR Am J Neuroradiol 1991;12:957-65.6. Sener RN. Unusual MRI findings in rhombencephalosynapsis.Comput Med Imaging Graph 2000;24:277-82.7. Sidman RL, Rakic P. Development of the human central ner-vous system. In: Haymaker W, Adams RD, editors. Histologyand histopathology of the nervous system. Springfield, Ill:Thomas; 1982:3-145.8. Utsunomiya H, Takano K, Ogasawara T, Hashimoto T,Fukushima T, Okazaki M. Rhombencephalosynapsis:cerebellar embryogenesis. AJNR Am J Neuroradiol 1998;19:547-9.9. Sarnat HB. Molecular genetic classification of cen-tral nervous system malformations. J Child Neurol2000;15:675-87.10. Millonig JH, Millen KJ, Hatten ME. The mouse Dreher geneLmx1a controls formation of the roof plate in the vertebrateCNS. Nature 2000;403:764-9 .11. Yachnis AT. Rhombencephalosynapsis with massivehydrocephalus: case report and pathogenetic considerations.Acta Neuropathol (Berl) 2002;103:301-4.12. Wilcock AR, Emery JL. Deformities of the renal tract inchildren with meningomyelocele and hydrocephalus, comparedwith those of children showing no such central nervous systemdeformities. Br J Urol 1970;42:152-7.

Partial rhombencephalosynapsis and Chiari II malformation

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