I' - f

of theexamination should benor'11181 postoperatively, barring the finding of an unsuspected myelomeningocele with thathave beenSuch a complication should be rare if magnified vision and nerve stimulation are used, The head circumference is followed closely to detect hydrocephalus, which will occur in 10 to 20 per cent of patients. The risk of hydroceph alus is less with lower-level meningo celes.?" The spine develops normally without scoliosis or kyphosis. assuming the vertebral bodies are normal. The incidence of late deterioration of neurological func tion is lower than in myelomeningocele, particularly if care is taken to relieve teth ering at the original operation. Genetic counseling should be arranged for theparents.THEARNOLD-CHIARI MALFORMATIONHistoricalBackground and DefinitionThe term "Amold-Chiari malformation' has been reinforced by 70 years of use, al though it does ignore the contribution of Cleland in 1883 and exaggerates the contri bution of Arnold in 1894.25,118 Cleland described an elongation of the vermis of the

and dilation of thespinal cord. In 1891 scribed

four types of hydrocephalusassociated with fida.lOl.114,llS.76I3n type 1, thespinal

canal with no involvement brainstem, In type II. a hypoplastk ] vermis

plus tonsils of thepocket-shapedelongation of the tricle and choroid plexus. and theoblongata are displaced into the upper vical

canal. A dorsal protuberance is ent at the cervicomedullary junction , cervical spinal cord is shortened. with I'OOtsclose together. and syringolia may be found in the distal spinal contCleland's 1883 description corresponds tothe Chiari type II deformity, Thefor type III have been modified intimes to include extension of the fourth'I,',,'11t1';;: le into an occ ipital encephala cele .1{:r ,:171 Type IV involves hypoplasia of rhe cerebellum and is not now considered a form of dvsraphisrn. In 1894, Arnold had described multiple anomalies in a newborninfant with spina bifida, but devoted most of the description to details of the visceral and lung abnormalities. The description and illustration of the cerebellar and hind-TABLE 35-8 PATHOLOGY OF TYPE I AND TYPE II ARNOLD-CHIARI MALFORMATlONFEATURE TYPE I TYPE IIAge groupCaudal displacement of cerebellar tonsilsCaudal displacement of inferior vermis and fourthventricleCaudal displacement of medulla oblongataDorsal kink of cervicomedullary junctionCourse of upper cervical nerve roots bifidaaperta

Adult Yes NoNo No NormalUsually absentMay be present; frequently nonprogressiveMay develop late

Infant Yes YesYesYesUsually cephalad..Always" present" Always" present:frequentlyMay develop early, P. W., and Markesbery, W. R.: Early descriptions of the Arnold-Ch" If t'on' "I!Iioj&J. Neurosurg., 37:543-547, 1972. ian ma arma' '"'"adherentneuralelementsduringdissection.

sils of theinferiorlobeof eachhemisphereextendintothe upper

definitionsof typesI andII are listed'Table 35-8, In typeIII, theentire cerebellum is displaced into the cervicalwith thefourth ventricle emptyinginto acervicalhydroencephalocele. Thec