Download Hypertrophic cardiomyopathy in cats

Hypertrophiccardiomyopathyincats
Hypertrophiccardiomyopathy,orHCM,isadiseaseseenalmostexclusivelyincats.Itinvolvesa
thickeningofthenormalmuscleoftheheartwall,whichcanbegeneralisedormorepatchy.The
exactcauseisnotfullyunderstoodbutitisthoughtthatitrelatestoamutationinthepatient’s
geneticcode–thiscanbeinheritedinwhichcaseitispresentasapredispositionfrombirth,but
equallyitcanhappenatsomepointlaterintheanimal’slifeasanewmutation.
About12.5%ofcatshaveaheartmurmurassociatedwithdiseaseoftheheartandthevastmajority
ofthesewillhavesomedegreeofHCM.Formany,HCMisalmostavariationofnormalandtheywill
notexperiencesymptomsfromitandonlydiemanyyearslaterofanothercause.However,insome
cases,itwillleadtocongestiveheartfailure,arrhythmias,thromboembolismanddeath.
Unfortunately,atthemoment,wedonothaveamechanismtobeabletopredictwhichanimalswill
fallintothemoreseriousgroup.
WhenHCMcausesillness,theheartwallisthickened.Thisisseenasanoutwardsenlargement,but
alsoasanarrowingoftheinternaldimensionsoftheleftventricleasthethickeningmovesinwards.
Thismeansthatduringtherestingphase,whentheheartisnotcontractingandissupposedtobe
fillingwithbloodforthenextheartbeat,theamountofbloodthatcanentertheventricleisreduced
asthereislessroom.Hence,thereislessbloodtobeejectedintotheaortaandaroundthebody
whenthenextbeatcomes.Thisisexacerbatedbythefactthattheoutflowtractthroughwhich
bloodleavestheheartisoftenalsonarrowedandsobloodcannotflowoutoftheheartefficiently.
Therearetwoeffectsofthis:firstly,thebodyisdeprivedofitsfullsupplyofoxygenandnutrients
and,secondly,thebloodwhichcannotleavetheheartissqueezedbackintotheleftatriumwhich
becomesengorgedwithbloodandexpands.
Ultimately,asdiseaseworsens,theeffectsofoxygendeprivationonthebodyareseenasexercise
intolerance,inappetenceandweightloss,whilstbloodreturningfromthelungsisimpededsothat
thelungsbecomecongestedwithfluid(calledpulmonaryoedema)andthecatmayfinditdifficultto
breathe.
Theproblemknocksontotherightsideoftheheart-higherpressurespreventbloodreturningfrom
thebodyfromcomingin.Thisfluidaccumulatesinthespacesurroundthelungs(pleuraleffusion),
intheabdomen(ascites)andoccasionallyundertheskinofthelowerlegs.
Thebodyattemptstocorrecttheproblembyaskingthehearttobeatharderandfaster,whilst
contractingbloodvessels,whichallmakethebloodpressuregouptoforcecontinuedsupplyto
tissues.Varioushormonesareactivetohelpthisprocessaswellasstimulatetheretentionofwater
inthekidneystokeepcirculatingvolumeshigher.Unfortunately,whentheheartbeatsfaster,the
timeavailablebetweenbeatsforfillingwithbloodpriortothenextbeatisreducedandthisfurther
compromisestheamountofbloodwhichisabletoentertheventricletobepumpedroundthebody
onthenextbeat.Thesecountermechanismsworkupuntilapointandensurethetissuesremain
suppliedwithblood,butafterthispointtheybecomecounterproductiveandsimplymeanthatthe
hearthastoworkharderandharder.Fluidaccumulatesinthelungs,chestandabdomenwhenthe
heartcannotcopeanylonger.
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Examinationwillrevealaheartmurmur,withorwithoutcracklesonthelungswhichequatetofluid.
Thediseaseisthemostcommoncardiacproblemseenincats,whichmayleadtoapresumptive
diagnosis.
Investigationwillincludechestx-raysandheartultrasound(echocardiography)toassessthesizeand
functionoftheheartanditsvalves.Thethickenedmuscleisusuallyeasytoidentify.Inmostcases,
anECGwillbetakentoassesswhethertherhythmoftheheartisnormalandbloodworkwillbe
donetoseehowtherestofthebodyiscopingwithreducedoxygensupply.
Asmentionedabove,itisimpossibletoidentifywhichcatswithaheartmurmurandadiagnosisof
HCMwillgoontodevelopclinicaldisease.Itisalsonotclearwhich,ifany,treatmentswillhelpto
stopthosethataregoingtodevelopsymptomsfromdoingso.However,Idofeelthatintheyears
sincecatswithheartmurmurshavebeenroutinelyinvestigatedandtreatedwithlogicaldrugs,I
haveseenareductioninthenumberofcatsdevelopingcongestiveheartfailureor
thromboembolismandso,untildataisavailablewhichdisprovesthetheory,Ifeelitisprudentto
treatpriortotheonsetofheartfailure.Ingeneral,allcatswithHCMcanbemedicatedasa
precautionassideeffectsarefewandcostisminimalformostofthedrugsconcerned.Currently,it
seemspreferabletomedicatethemallthantowaittofindoutwhichreallyneedit,bywhichtimeit
isprobablytoolatetoeffectrealchangefortheseindividualpatients.Treatmentsusedwillvary
fromcasetocase,butmayincludeoneortwoofthefollowing:beta-blockers,calciumchannel
blockers,ACEinhibitorsand/oraspirin.
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