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DESCRIPTIONCongenital long QT syndrome (LQTS) is an inherited disorder characterized by the lengthening of the repolarization phase of the ventricular action potential, increasing the risk of arrhythmic events, such as torsades de pointes, which may in turn result in syncope and sudden cardiac death. Management has focused on the use of beta blockers as first-line treatment, with pacemakers or implantation cardioverter defibrillators (ICD) as second-line therapy.
Congenital LQTS usually manifests itself before the age of 40 years, and may be suspected when there is a history of seizure, syncope, or sudden death in a child or young adult; this history may prompt additional testing in family members. It is estimated that more than one half of the 8,000 sudden unexpected deaths in children may be related to LQTS. The mortality of untreated patients with LQTS is estimated at 1%-2% per year, although this figure will vary with the genotype. Frequently, syncope or sudden death occurs during physical exertion or emotional excitement, and thus LQTS has received some publicity regarding evaluation of adolescents for participation in sports. In addition, LQTS may be considered when a long QT interval is incidentally observed on an EKG. Diagnostic criteria for LQTS have been established, which focus on EKG findings and clinical and family history (i.e., Schwartz criteria, see following section, "Clinical Diagnosis"). However, measurement of the QT interval is not well standardized, and in some cases, patients may be considered borderline cases.
In recent years, LQTS has been characterized as an “ion channel disease”, with abnormalities in the sodium and potassium channels that control excitability of the cardiac myoctyes. A genetic basis for LQTS has also emerged, with 7 different variants recognized, each corresponding to mutations in different genes. In addition, typical ST-T wave patterns are also suggestive of specific subtypes.
LQT1 is associated with mutations in the gene KNQ1 located on chromosome 11. LQT1 is responsible for about 50% of all LQTS, and arrhythmic events prompted by exercise may occur most commonly in this subtype. Therefore, patients with LQT1 may be advised to minimize exercise.
LQT2 is associated with mutations in the gene KCNH2 located on chromosome 7 and is seen in 45% of patients with LQTS. Arrhythmic events appear to be precipitated by auditory stimuli, and these patients may be advised to avoid clock alarms, etc.
LQT3 is associated with mutations in the gene SCN5A located on chromosome 3. This subtype is seen in 3%-4% of patients with LQTS. In this subtype, the majority of cardiac events occur during sleep. LQT3 variant is also known as the Brugada syndrome.
LQT 4-7 involve KCN genes located on chromosomes 21 and 17. These variants each account for less than 1% of LQTS.
The Schwartz criteria are commonly used as a diagnostic scoring system for LQTS. The most recent version of this scoring system is shown in the table below. A score of 4 or greater indicates a high probability that LQTS is present; a score of 2-3 an intermediate probability; and a score of 1 or less indicates a low probability of the disorder. Prior to the availability of genetic testing, it was not possible to test the sensitivity and specificity of this scoring system; therefore, the accuracy of this scoring system is ill-defined.
Diagnostic Scoring System for LQTS
The Familion® Test describes the analysis of the genes responsible for subtypes LQT 1-5. The test is offered in a variety of ways. For example, if a family member has been diagnosed with LQTS based on clinical characteristics, complete analysis of all five (5) genes can be performed to both identify the specific mutation and identify the subtype of LQTS. If a mutation is identified, then additional family members can undergo a focused genetic analysis for the identified mutation. If a specific type of LQTS is suspected based on the EKG abnormalities, genetic testing can focus on the individual gene. For example, subjects with suspected Brugada syndrome can undergo genetic analysis of the SCN5a gene.
All of the LQTS genes are large, and genetic testing has revealed multiple different mutations along their length. The pathophysiologic significance of each of the discrete mutations is an important part of the interpretation of genetic analysis. PGxHealth (New Haven, CT), the laboratory offering the Familion® test, compares the results to the PGxHealth Cardiac Ion Channel Variant Database, which includes data from over 750 individuals of diverse ethnic backgrounds. Therefore, the chance that a specific mutation is pathophysiologically significant is increased if it is the same mutation as that reported in several other cases of known LQTS. However, there may be many instances when the detected mutations are of unknown significance. Variants are placed into four classes, based on the probability that the variant identified represents an actual deleterious LQTS mutation:
The absence of a mutation does not imply the absence of LQTS; it is estimated that mutations are only identified in 60%-70% of patients with a clinical diagnosis of LQTS. For these reasons, the most informative result of testing would probably occur when a family member undergoes genetic testing for a specific genetic mutation that has been identified in symptomatic relatives known to have LQTS. Interpretation of the results will likely be improved as the database grows. Other laboratories have investigated different testing strategies. For example, Napolitano and colleagues propose a three tiered approach, first testing for a core group of 64 codons which have a high incidence of mutations, followed by additional testing for less mutations.
Another factor complicating interpretation of the genetic analysis is the penetrance of a given mutation or the presence of multiple phenotypic expressions. For example, approximately 50% of carriers of mutation never have any symptoms. There is variable penetrance for the LQTS, and penetrance may differ for the various subtypes. While linkage studies in the past indicated that penetrance was 90% or greater, more recent analysis by molecular genetics has challenged this number, and suggested that penetrance may be as low as 25% for some families.
POLICYGenetic testing in patients with known or suspected congenital long QT syndrome may be considered medically necessary for the following indications:
Individuals who do not meet the clinical criteria for LQTS, but who have:
* Determining the pretest probability of LQTS is not standardized. An example of a patient with a moderate to high pretest probability of LQTS is a patient with a Schwartz score of 2-3.
Genetic testing for LQTS to determine prognosis and/or direct therapy in patients with known LQTS is considered investigational.
POLICY GUIDELINESInvestigative service is defined as the use of any treatment procedure, facility, equipment, drug, device, or supply not yet recognized by certifying boards and/or approving or licensing agencies or published peer review criteria as standard, effective medical practice for the treatment of the condition being treated and as such therefore is not considered medically necessary.
The coverage guidelines outlined in the Medical Policy Manual should not be used in lieu of the Member's specific benefit plan language.
There is no specific CPT code for this test.
POLICY HISTORY7/27/2006: Approved by Medical Policy Advisory Committee (MPAC)
1/14/2008: Policy description updated. Genetic testing in patients with known or suspected LQTS changed from investigational to may be considered medically necessary for clinical criteria as outlined in POLICY section. Genetic testing for LQTS to determine prognosis and/or direct therapy in patients with known LQTS is investigational added to POLICY section.
3/27/2008: Reviewed and approved by MPAC
8/26/2008: Quarterly HCPCS code updates applied
1/05/2009: Policy reviewed. No changes.
08/11/2010: Policy reviewed; no changes.
08/11/2011: Policy reviewed; no changes.
09/25/2012: Policy reviewed. Policy statement unchanged. Added CPT codes 81280 - 81282 and ICD-9 codes 426.82 and 746.89 to the Code Reference section.
SOURCE(S)Blue Cross Blue Shield Association Policy # 2.04.43
CODE REFERENCEThis is not intended to be a comprehensive list of codes. Some covered procedure codes have multiple descriptions.
The code(s) listed below are ONLY covered if the procedure is performed according to the "Policy" section of this document.