A 10-Year-Old Boy With Fainting Spells and Seizure Activity

Shatha M. Khatib, MD

Disclosures

September 25, 2019

Cardiac dysrhythmias can be initiated by an external trigger, such as emotional stress, exercise, or sudden loud noises (ie, an alarm clock or telephone); however, this is not always the case. Ventricular arrhythmias may also occur during sleep, which is commonly seen in patients with the LQT3 genotype. In fact, the kind of triggering event is often linked to the underlying mutation, with certain triggers more commonly associated with certain genotypes.

As many as 10% of patients are only diagnosed with LQTS at the time of sudden death.[6] Mortality can be as high as 70% in patients who remain untreated over a 10-year period.[3] This emphasizes the importance of presymptomatic diagnosis and treatment. Important clues indicating that a patient may have LQTS include abnormal ECG findings, a family history of unexplained death, or hearing loss (which is present in around 4% of patients with LQTS).[7]

In patients with suspected congenital LQTS, the initial evaluation should be directed at calculating the QTc interval on a resting ECG. The QTc interval is the QT interval corrected for heart rate because, under normal physiologic circumstances, the actual measured QT interval adjusts with the heart rate; in other words, it is longer at slower rates and shorter at faster rates. QTc is calculated by dividing the measured QT by the square root of the R-R interval (the Bazett formula), both of which are measured in seconds.

Prolongation of the QTc interval is defined on the basis of the following age-specific and sex-specific criteria (Table)[1]:

Table. Definitions of the QTc Interval

Population QTc Reference Range Borderline QTc Prolonged QTc
Males or females < 15 yr < 0.44 sec 0.44-0.46 sec > 0.46 sec
Males > 15 yr < 0.43 sec 0.43-0.45 sec > 0.45 sec
Females > 15 yr < 0.45 sec 0.45-0.46 sec > 0.46 sec

For practical purposes, any value > 0.45 second should be considered abnormal.

To increase accuracy, the QT interval can be manually measured on serial ECGs by using multiple leads, measuring several successive beats, and calculating their average for each ECG.[8] Nevertheless, approximately 10%-15% of patients with congenital LQTS (diagnosed by genetic tests) present with a normal QTc duration[1]; therefore, if suspicion is high despite a normal ECG, Holter monitoring or stress testing may be necessary to try to provoke or unmask the long QTc.

Other ECG features that help to establish the diagnosis of congenital LQTS include the following:

  • Abnormal T-wave morphology, including notched or biphasic T waves

  • The presence of T-wave alternans, which is defined as the regular alternation in T wave amplitude or polarity

  • Increased QT dispersion, which is defined as the variability in QT duration among different ECG leads

A scoring system for the diagnosis of congenital LQTS was established in 1985 by Schwartz and colleagues[9] and revised in 1993 but still serves as the best guide for clinicians today. It incorporates the ECG criteria (the measured resting QTc interval, history of torsade de pointes, presence of T-wave alternans or notched T wave on ECG, and low heart rate for age), the clinical criteria (syncope or congenital deafness), and family history (family members with definite LQTS or unexplained sudden death at < 30 years of age). Points ranging from 0.5 to 3 are assigned to each of the above criteria, and the points are added to calculate the LQTS score. Depending on the patient's score, the probability of having LQTS is rated as low (< 1 point), intermediate (2-3 points), or high (≥ 4 points). Additional testing, such as cold-water facial immersion or exercise testing, may be applied in patients in whom the diagnosis is still unclear.

Genetic testing for congenital LQTS is now available in specialized centers; however, the practical application of genetic testing is limited because of the complexity and heterogeneity of congenital LQTS. In addition, as many as 25% of patients have unknown mutations[10]; therefore, a negative test does not exclude the disease. Once an index case with congenital LQTS is identified, evaluation needs to extend to all first-degree relatives, and treatment must be established where indicated.

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