Andersons pediatric cardiology 1565
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fromthecerebrovascularcirculationtothepelvis.220
FIG.59.4 Magneticresonanceangiogramshowingdistinctivetortuosityof
theheadandneckvasculatureinapatientwithLoeys-Dietzsyndrome.
Homocystinuria
Homocystinuriaisanautosomalrecessivedisorderusuallyduetodeficiencyof
cystathioninesynthase,anenzymeneededforthemetabolismofmethionine.
AffectedpatientshaveabodyhabitussimilartothatofpatientswithMarfan
syndrome,albeittheirjointsusuallydemonstraterestrictedmobility.Cognitive
deficitsarealsocommon.Theprimarycardiaccomplicationisthatof
atheroscleroticdisease,whichoftenoccursbyadolescence.Endothelial
dysfunctioncontributestothearterialcomplicationsandproducesanincreased
riskformyocardialinfarction.221VitaminB6helpstoreducelevelsof
homocysteineinapproximatelyhalfofpatients,andthis,alongwith
supplementationofvitaminB12andfolicacid,formsthemainstayoftherapy.
NeuromuscularDiseases
MuscularDystrophies
DuchenneMuscularDystrophy
DuchennemusculardystrophyisanX-linkedrecessivedisease.Becauseofthis,
italmostalwaysafflictsmales.Itsincidenceinmalechildreniscalculatedat13
to33per100,000livebirths.SincefemaleswithTurnersyndromehaveonlyone
Xchromosome,theytoocaninheritthedisease,anditisseenrarelyinthis
setting.Thediseaseisduetoadefectinthegeneencodingdystrophin.
Dystrophinhelpstoformacomplexthatstabilizesthesarcolemmalmembrane
duringcontraction.Althoughtheexactmechanismformuscledegenerationhas
notbeenelucidated,abnormalnitricoxideregulation,abnormalsarcolemmal
fragility,andincreasedsusceptibilitytooxidativestressarefelttoplayimportant
roles.222
Theearliestsymptomsareclumsinessinwalking,atendencytofall,andthe
inabilitytorun.Thosewiththediseasealsohavedifficultyinclimbingstairsand
gettingupfromthefloor.Clinicalonsetisusuallymanifestbefore4yearsofage,
andthediagnosisisusuallymadearound6yearsofage.Deteriorationis
continuous.Mostareunabletowalkbytheageof10years.Lifeexpectancyis
increasingduetoimprovedmedicaltherapies,includingnocturnalventilation,
earlyandaggressivemanagementofcardiomyopathy,andperhapsalsotheuse
ofsteroids.223Muscularweaknessandatrophyinitiallyaffecttheproximal
musclegroupsoftheupperlimbs.Leginvolvementextendsfromthequadriceps
andglutealmusclestotheanteriortibialmuscles.Weaknesslateraffectsother
musclegroups.Morepowerisgenerallyretainedinthedistalmuscles.Slight
facialweaknessoccursinthelatestages.Scapular“winging”isalsoalater
phenomenon.Muscularhypertrophyofcalves,masticatorymuscles,anddeltoids
isfollowedbyapseudohypertrophicphaseoffattyreplacement.Tendonreflexes
arelostintheweakmuscles.Contracturesofthecalvesandhipflexormuscles
developaroundtheageof8years.Contracturesappearinthehips,knees,
elbows,andwrist,whenthepatientsareconfinedtowheelchairs.Severespinal
andthoracicdeformitiesareseenlateinthedisease.Kyphoscoliosisiscommon.
Theseresultfromdisuseandabnormalposture.Generalizeddecalcificationof
thebonesleadstofrequentpathologicfractures.
