title: Rhombencephalosynapsis as a cause of aqueductal stenosis: an under-recognized association in hydrocephalic children.
abstract: BACKGROUND
  Rhombencephalosynapsis is a rare genetic aberration characterized by variable vermian hypoplasia/aplasia in conjunction with united cerebellar hemispheres. Genetic defects in the isthmic organizer at the mesencephalic-metencephalic junction are presumably responsible for the associated aqueductal stenosis.
  
  
  OBJECTIVE
  We performed a retrospective review of 20 children with rhombencephalosynapsis to evaluate for and emphasize the association of aqueductal stenosis and hydrocephalus.
  
  
  METHODS
  We retrospectively reviewed the MR and CT images of 20 children (0-11 years old) with rhombencephalosynapsis encountered at two academic children's hospitals. Rhombencephalosynapsis spectrum severity was graded based on pre-existing literature. We analyzed examinations for ventriculomegaly and degree of aqueductal stenosis. The collicular distances were measured from the collicular apices. Imaging studies were also analyzed for malformations of cortical and cerebellar development.
  
  
  RESULTS
  Thirteen of the 20 children (65%) with rhombencephalosynapsis presented with clinical or imaging evidence of hydrocephalus and aqueductal stenosis, principally involving the caudal cerebral aqueduct. All children with aqueductal stenosis had collicular fusion. All six children with complete rhombencephalosynapsis had aqueductal stenosis. The cerebral aqueduct varied from normal to stenotic in children with incomplete rhombencephalosynapsis. Corpus callosum dysgenesis was present in four children.
  
  
  CONCLUSIONS
  Aqueductal stenosis in the setting of rhombencephalosynapsis is an under-recognized cause of noncommunicating hydrocephalus. Our findings support the hypothesis that a defect involving the common gene(s) responsible for the differentiation and development of both the roof plate and midline cerebellar primordium at the mesencephalon/first rhombomere junction may be responsible for the association of aqueductal stenosis and rhombencephalosynapsis.
authors.affiliation: Department of Radiology, University of Tennessee Health Science Center, Memphis, TN, USA, matthewthomaswhitehead@gmail.com.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Whitehead
authors.forename: Asim F
authors.initials: AF
authors.lastname: Choudhri
authors.forename: John
authors.initials: J
authors.lastname: Grimm
authors.forename: Marvin D
authors.initials: MD
authors.lastname: Nelson
mesh_terms: D000293:Adolescent; D061085:Agenesis of Corpus Callosum / Q000473:pathology; D002529:Cerebellar Nuclei / Q000002:abnormalities / Q000473:pathology; D002531:Cerebellum / Q000002:abnormalities / Q000473:pathology; D002535:Cerebral Aqueduct / Q000473:pathology*; D002648:Child; D002675:Child, Preschool; D003251:Constriction, Pathologic / Q000209:etiology / Q000473:pathology; D005260:Female; D006801:Humans; D006849:Hydrocephalus / Q000150:complications*; D007223:Infant; D007231:Infant, Newborn; D008279:Magnetic Resonance Imaging; D008297:Male; D011247:Pregnancy; D012189:Retrospective Studies; D012249:Rhombencephalon / Q000002:abnormalities* / Q000473:pathology*; D012688:Septum Pellucidum / Q000002:abnormalities / Q000473:pathology; D014057:Tomography, X-Ray Computed; D055815:Young Adult