Septum Thickness of 23 mm and Syncope: Implications for HCM Diagnosis and ICD Therapy
A septum thickness of 23 mm on echocardiography without LVOT obstruction meets diagnostic criteria for hypertrophic cardiomyopathy (HCM), and in the presence of syncope, an ICD is reasonable for primary prevention of sudden cardiac death.
Diagnosis of HCM
- A left ventricular wall thickness ≥15 mm that is not explained by abnormal loading conditions is diagnostic of HCM 1, 2
- The septum thickness of 23 mm in this case significantly exceeds this threshold, confirming the diagnosis of HCM, even without LVOT obstruction 1, 3
- HCM can present with various patterns of hypertrophy, and the absence of LVOT obstruction is seen in approximately 40-50% of HCM cases (non-obstructive HCM) 4
- While extreme hypertrophy is defined as wall thickness ≥30 mm, a thickness of 23 mm still represents significant hypertrophy and is associated with increased risk 1
Risk Stratification for Sudden Cardiac Death
According to the 2020 AHA/ACC guidelines, the following are established risk factors for sudden cardiac death in HCM 1:
- Family history of sudden death from HCM in first-degree relatives ≤50 years
- Massive LV hypertrophy (wall thickness ≥30 mm)
- Unexplained syncope, especially within 6 months of evaluation
- LV systolic dysfunction (EF <50%)
- LV apical aneurysm
- Extensive late gadolinium enhancement on CMR imaging (≥15% of LV mass)
- Nonsustained ventricular tachycardia on ambulatory monitoring
Significance of Syncope in HCM
- Syncope is a major risk factor for subsequent sudden cardiac death in HCM with a relative risk of approximately 5 1
- Syncope occurring within the previous 6 months carries the most prognostic importance 1
- The 2020 AHA/ACC guidelines specifically state that "≥1 unexplained episodes involving acute transient loss of consciousness, judged by history unlikely to be of neurocardiogenic (vasovagal) etiology, nor attributable to LVOTO" is a significant risk factor 1
ICD Recommendations Based on Risk Factors
- For patients with HCM and unexplained syncope within the preceding 6 months, an ICD is reasonable if meaningful survival greater than 1 year is expected (Class IIa recommendation) 1
- The 2017 AHA/ACC/HRS guidelines specifically state that in patients with HCM and "one or more episodes of unexplained syncope within the preceding 6 months," an ICD is reasonable 1
- The presence of syncope alone, without other risk factors, may be sufficient to justify consideration for prophylactic ICD implantation, particularly in younger patients 1
Role of Electrophysiology Study
- An invasive electrophysiological study with programmed ventricular stimulation should NOT be performed for risk stratification in HCM patients (Class III: No Benefit) 1
- The 2017 AHA/ACC/HRS guidelines explicitly state that "in patients with HCM, an invasive electrophysiological study with programmed ventricular stimulation should not be performed for risk stratification" 1
- Therefore, an EP study is not recommended for risk assessment in this patient with HCM and syncope 1
Clinical Decision Making
- The combination of significant septal hypertrophy (23 mm) and syncope places this patient at increased risk for sudden cardiac death 1
- While the 23 mm septal thickness doesn't meet the threshold for "massive hypertrophy" (≥30 mm), it still represents significant hypertrophy 1
- The presence of unexplained syncope is a well-established independent risk factor that warrants consideration for ICD therapy 1
- Additional risk assessment with cardiac MRI to evaluate for late gadolinium enhancement and 24-48 hour Holter monitoring to detect nonsustained VT would provide further risk stratification 1
Conclusion
In this patient with a septum thickness of 23 mm without LVOT obstruction and a history of syncope, the diagnosis of HCM is established, and an ICD is reasonable for primary prevention of sudden cardiac death based on current guidelines. An EP study is not recommended for risk stratification in HCM.