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PubMed This is a summary of 12 peer-reviewed journal articles Updated
Cardiology

Symptoms, Triggers, and Avoiding Misdiagnosis

At a Glance

Long QT Syndrome (LQTS) causes sudden fainting that is often dangerously misdiagnosed as epilepsy. Unlike seizures, LQTS fainting happens without warning, and recovery is almost immediate. Triggers are specific to the genetic type and include exercise, loud noises, or sleep.

For many families, the road to a Long QT Syndrome (LQTS) diagnosis is confusing because the symptoms often look like something else entirely. Understanding how LQTS presents—and why it is so frequently mistaken for other conditions—is the first step in ensuring safety and getting the right treatment.

The Great Mimicker: LQTS vs. Epilepsy

One of the most common and dangerous pitfalls in diagnosing LQTS is mistaking it for epilepsy [1]. When the heart enters a dangerous rhythm like Torsades de Pointes, it momentarily stops pumping blood effectively to the brain [2]. This sudden drop in oxygen (cerebral hypoperfusion) can cause the body to stiffen or jerk [3].

To a bystander, these “convulsive movements” look exactly like a seizure [1].

  • The Danger of Misdiagnosis: If a patient is misdiagnosed with epilepsy, they may be prescribed anti-epileptic drugs. Some of these medications are on the “avoid” list for LQTS because they can further prolong the QT interval, potentially triggering the very cardiac events they were meant to “treat” [4][5].
  • The Key Difference: Most people who have a seizure experience a “post-ictal” state—a period of prolonged confusion and exhaustion afterward. In contrast, those who experience cardiac syncope (fainting due to a heart rhythm) often wake up and become alert almost immediately once blood flow returns to the brain [4][6].

Know Your Triggers

The “electricity” in an LQTS heart is often stable most of the time, only becoming unstable when pushed by specific triggers. These triggers are often linked to the specific genetic type of LQTS a person has [7].

LQTS Type Common Triggers Why it happens
LQT1 Exercise, especially swimming or diving [7][8] Adrenaline from physical exertion puts stress on the slow-to-reset potassium channels [7].
LQT2 Auditory stimuli (alarm clocks, phones, sirens) or sudden emotional stress [7][9] A sudden startle creates a “jolt” of adrenaline that the heart’s electrical system can’t process quickly [7].
LQT3 Sleep or rest [7][9] Events occur when the heart rate is slow, allowing “leaky” sodium channels to cause instability [10][7].

Symptoms and Red Flags

While some individuals with LQTS remain asymptomatic (showing no symptoms) until a major event occurs, others may experience warning signs [11]. It is vital to recognize these red flags:

  • Exercise-Induced Fainting: Fainting during physical activity (rather than after stopping) is a major warning sign of a cardiac event [4].
  • Fainting Due to Startle: Losing consciousness immediately after a loud noise [7].
  • Unexplained Drowning: A history of a “strong swimmer” suddenly struggling or drowning should always raise suspicion of LQT1 [9].
  • No Warning Signs: Unlike a typical “faint” where you might feel sweaty, nauseated, or see spots first, cardiac syncope often happens with zero warning—you are “out” before you hit the ground [6].

If you or your child has experienced any of these, the most critical next step is a 12-lead ECG to manually measure the QTc interval and rule out a heart rhythm issue [4][12].

Common questions in this guide

Can Long QT Syndrome be mistaken for epilepsy?
Yes, fainting caused by Long QT Syndrome is frequently misdiagnosed as epilepsy. When the heart rhythm becomes dangerously fast, the brain briefly loses oxygen, causing the body to stiffen or jerk in a way that looks exactly like an epileptic seizure.
What is the difference between an epileptic seizure and cardiac fainting?
After an epileptic seizure, a person usually experiences a prolonged period of confusion and exhaustion. In contrast, someone who faints from a cardiac event like Long QT Syndrome typically wakes up and becomes alert almost immediately once blood flow returns.
What are the most common triggers for Long QT Syndrome?
Triggers depend on your specific genetic type. LQT1 is often triggered by physical exercise or swimming, LQT2 by sudden loud noises or emotional stress, and LQT3 events typically happen during periods of sleep or deep rest.
What are the warning signs of a Long QT Syndrome event?
Cardiac events often happen with zero warning, meaning a person loses consciousness before hitting the ground. Fainting during physical activity, fainting after being startled, or an unexplained drowning incident are major red flags that require immediate medical evaluation.
What should I do if my child's fainting or seizures might be heart-related?
You should immediately ask your doctor for a 12-lead ECG to manually measure the QTc interval. This is a critical step to determine if the episodes are caused by a heart rhythm issue rather than a neurological one.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Could this 'seizure' actually be a cardiac syncopal event caused by LQTS?
  2. 2.Was a 12-lead ECG performed immediately after the event, and what was the manual QTc measurement?
  3. 3.Are any of the medications I/my child was prescribed (like certain anti-epileptics or antibiotics) on the list of drugs that prolong the QT interval?
  4. 4.Should we perform an exercise stress test or use a Holter monitor to see how the heart rhythm changes during specific triggers?
  5. 5.Given the triggers we've noticed, which genotype (LQT1, 2, or 3) is most likely, and how does that change our safety plan?

Questions For You

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References

References (12)
  1. 1

    Cerebral Seizures in an Adolescent with Jervell and Lange-Nielsen Syndrome: It May Not Be Epilepsy.

    Levaux J, Farhat N, Van Casteren L, et al.

    Clinics and practice 2022; (12(5)):677-685 doi:10.3390/clinpract12050070.

    PMID: 36136864
  2. 2

    A new paradigm for predicting risk of Torsades de Pointes during drug development: Commentary on: "Improved prediction of drug-induced Torsades de Pointes through simulations of dynamics and machine learning algorithms".

    McCauley MD, Darbar D

    Clinical pharmacology and therapeutics 2016; (100(4)):324-6 doi:10.1002/cpt.408.

    PMID: 27301674
  3. 3

    Overt long QT syndrome in children presenting with seizure disorders in Pakistan.

    Rashid U, Virk AO, Nawaz R, et al.

    Annals of pediatric cardiology 2021; (14(4)):485-489 doi:10.4103/apc.apc_10_21.

    PMID: 35527745
  4. 4

    CONGENITAL LONG QT SYNDROME: A SYSTEMATIC REVIEW.

    Galić E, Bešlić P, Kilić P, et al.

    Acta clinica Croatica 2021; (60(4)):739-748 doi:10.20471/acc.2021.60.04.22.

    PMID: 35734489
  5. 5

    Prolonged QT: A rare cause of cardiac arrest.

    Carlquist J

    JAAPA : official journal of the American Academy of Physician Assistants 2015; (28(8)) doi:10.1097/01.JAA.0000466588.56337.68.

    PMID: 26208008
  6. 6

    Prolonged QT Interval in Athletes: Distinguishing between Pathology and Physiology.

    Christou GA, Vlahos AP, Christou KA, et al.

    Cardiology 2022; (147(5-6)):578-586 doi:10.1159/000526385.

    PMID: 35947943
  7. 7

    Digenic heterozygous mutations of KCNH2 and SCN5A induced young and early-onset long QT syndrome and sinoatrial node dysfunction.

    Yang Z, Ma Y, Huang J, et al.

    Annals of noninvasive electrocardiology : the official journal of the International Society for Holter and Noninvasive Electrocardiology, Inc 2022; (27(1)):e12889 doi:10.1111/anec.12889.

    PMID: 34755423
  8. 8

    Inducible torsades de pointes during an acute face immersion test in an adolescent with type 2 long QT syndrome.

    Muneuchi J, Sugitani Y, Watanabe M

    Cardiology in the young 2020; (30(8)):1171-1172 doi:10.1017/S1047951120002176.

    PMID: 32690115
  9. 9

    Successful Management of a Young Athlete with Type 2 Long QT Syndrome by Genotype-specific Risk Stratification and Bridging Therapy with a Wearable Cardioverter Defibrillator.

    Kojima K, Kato K, Fujii Y, et al.

    Internal medicine (Tokyo, Japan) 2022; (61(8)):1179-1182 doi:10.2169/internalmedicine.8093-21.

    PMID: 35110475
  10. 10

    Long QT syndrome - Bench to bedside.

    Ponce-Balbuena D, Deschênes I

    Heart rhythm O2 2021; (2(1)):89-106 doi:10.1016/j.hroo.2021.01.006.

    PMID: 34113909
  11. 11

    Late-onset severe long QT syndrome.

    Asatryan B, Schaller A, Bartholdi D, Medeiros-Domingo A

    Annals of noninvasive electrocardiology : the official journal of the International Society for Holter and Noninvasive Electrocardiology, Inc 2018; (23(4)):e12517 doi:10.1111/anec.12517.

    PMID: 29194874
  12. 12

    The 2023 Canadian Cardiovascular Society Clinical Practice Update on Management of the Patient With a Prolonged QT Interval.

    Davies RA, Ladouceur VB, Green MS, et al.

    The Canadian journal of cardiology 2023; (39(10)):1285-1301 doi:10.1016/j.cjca.2023.06.011.

    PMID: 37827588

This page explains Long QT Syndrome symptoms and triggers for educational purposes only. It does not replace professional medical advice. Always consult a pediatric cardiologist or electrophysiologist for a proper diagnosis.

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