michael s
(Michael S)
#1
83 How do I assess the patient with long QT?
Should I screen relatives, and how?
How do I treat them?
J Benhorin
Patients affected by the congenital long QT syndrome (LQT) are
often first assessed when syncope, documented ventricular
arrhythmia or aborted cardiac arrest affects them or a family
member. The diagnostic cut-offs (<100% sensitive) for a
congenitally-prolonged heart rate-corrected QT interval (QTc) on
standard 12-lead ECG (measured in lead II, or V5) are: >0.46 sec
(children <16 years), >0.45 sec (adult males), and >0.47 sec (adult
females), after drug induced QT prolongation has been excluded.
T wave morphology should also be carefully examined, in
particular for high takeoff, late onset, broad base, bifid
morphology with humps, and beat-by-beat alternating polarity
(T wave alternans). In several LQT variants, sinus bradycardia is
an additional common feature. Holter monitoring should be
performed to exclude repetitive ventricular arrhythmias of the
torsade de pointes type. Family screening by 12-lead ECG of all
first-degree relatives is mandatory in order to have a definite
diagnosis of hereditary LQT. In Romano-Ward syndrome
(1/20,000 births: autosomal dominant transmission with >90%
penetrance), 50% of offspring of one affected parent are predicted
to be similarly affected.
Six associated genetic loci (on chromosomes 3, 4, 7, 11, 21, 22)
have been identified, of which four relate to genes that encode
cardiac ion-channel proteins. Several mutations have been
described for each gene. Although only 50% of all LQT affected
families can be linked to one of these genes, genetic screening is
100% accurate amongst these, and can provide a definite
diagnosis in phenotypically borderline cases.
Medical therapy should be promptly started in symptomatic
LQT patients, and beta blockers are currently the first choice,
with the occasional need for pacemaker implantation. However,
recent evidence suggests that in symptomatic cases with aborted
cardiac arrest, automatic implantable cardiac defibrillator (ICD)
implantation, in addition to beta blocker therapy, is probably
indicated. In patients who do not respond to the above-
mentioned measures, high cervicothoracic sympathectomy might
