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1、ClinicalCorrelationsTheduramaybecomepartlycalcifiedorevenossifiedwithage.Insomecases,ofteninassociationwithlongstandinghydrocephalus,thefalxisfenestrated.Severaltypesofherniationofthebraincanoccur.Thetentoriumseparatesthesupratentorialandtheinfratentorialcompartments,andthetwospacescommunic
2、atebywayoftheincisurathatcontainsthemidbrain.Boththefalxandthetentoriumformincompleteseparations,andamassorexpandinglesionmaydisplaceaportionofthebrainaroundthesesepta,resultingineitherasubfalcialoratranstentorialherniation.Thelattertypemaybedownward(uncal,orcaudaltranstentorial,herniation)
3、orupward(rostraltranstentorialherniation).Theherniationofthecerebellartonsilsintotheforamenmagnumbyalesionisoftencalledconing.Transtentorialherniations,especiallythecaudaltype,arepotentiallylifethreateningbecausetheycandistortorcompressthebrainstemanddamageitsvitalregulatorycentersforrespir
4、ation,consciousness,bloodpressure,andotherfunctions(seeChapters18and20).ClinicalCorrelationsBlockingthecirculatorypathwayofcerebrospinalfluidusuallyleadstodilatationoftheventriclesupstream(hydrocephalus),becausetheproductionoffluidusuallycontinuesdespitetheobstruction(Figs11-10to11-14andTab
5、le11-2).Therearetwotypesofhydrocephalus:noncommunicatingandcommunicating.Innoncommunicating(obstructive)hydrocephalus,whichoccursmorefrequentlythantheothertype,thecerebrospinalfluidoftheventriclescannotreachthesubarachnoidspacebecausethereisobstructionofoneorbothinterventricularforamina,the
6、cerebralaqueduct(themostcommonsiteofobstruction,Figs11-9and11-10),ortheoutflowforaminaofthefourthventricle(medianandlateralapertures).Ablockatanyofthesesitesleadsrapidlytodilatationofoneormoreventricles.Theproductionofcerebrospinalfluidcontinues,andintheacuteobstructionphase,theremaybeatran
7、sependymalflowofcerebrospinalfluid.Thegyriareflattenedagainsttheinsideoftheskull.Iftheskullisstillpliable,asitisinmostchildrenunder2yearsofage,theheadmayenlarge(seeFig11-11).Incommunicatinghydrocephalus,theobstructionisinthesubarachnoidspaceandcanbethere
