Interventricular Septal Diameter Measurement in HCM Severity Assessment
The diastolic interventricular septal diameter (IVS(d)) determines the severity of hypertrophic cardiomyopathy, not the systolic measurement. This is the standard measurement used for diagnosis, risk stratification, and clinical decision-making in HCM.
Measurement Timing and Rationale
Maximum diastolic wall thickness is the established standard for assessing HCM severity because the septum is thickest and most clearly defined at end-diastole, providing the most accurate and reproducible measurement 1, 2. The American College of Cardiology/American Heart Association guidelines specifically recommend measuring "maximum diastolic wall thickness" using 2D short-axis views in all LV segments from base to apex, with measurements obtained at end-diastole 1.
Why Diastolic Measurement is Critical
- Diagnostic threshold: The conventional diagnostic criterion for HCM in adults is a maximal wall thickness ≥15 mm measured at end-diastole 1
- Risk stratification: Maximum diastolic wall thickness ≥30 mm is independently associated with sudden cardiac death and directly influences ICD decision-making 1
- Linear mortality correlation: There is a relatively linear association between maximal diastolic wall thickness and sudden death risk, with highest risk in patients with wall thickness ≥30 mm 3
- Prognostic significance: Severe septal hypertrophy (IVSd ≥25 mm) carries a 5-fold increased risk of all-cause mortality compared to IVSd <20 mm 4
Clinical Application
The diastolic measurement serves multiple critical functions:
- Surgical planning: Baseline IVSd determines the extent of myectomy required and predicts post-procedural outcomes 5, 4
- Intervention thresholds: Patients with extreme IVSd (≥30 mm) have higher incidence of adverse cardiovascular events and worse surgical outcomes, with 5-year survival significantly lower than those with IVSd <30 mm 5
- Family screening: IVSd measurement is used for periodic screening of first-degree relatives and genotype-positive, phenotype-negative individuals 6
Common Pitfall to Avoid
Do not confuse septal thickness measurement timing with LVOT gradient measurement timing. While maximal wall thickness must be measured during diastole, LVOT gradients are measured during systole using peak instantaneous velocity 1. The ACC/AHA guidelines emphasize avoiding this confusion, as measurements must be obtained at the correct cardiac phase: wall thickness during diastole and LVOT gradients during systole 1.
Measurement Protocol
- Use 2D short-axis views to assess all LV segments from base to apex 1, 6
- Measure at end-diastole when the septum is maximally thickened 1
- Document as "maximum diastolic wall thickness" or "IVSd" to avoid misinterpretation 1
- Consider contrast-enhanced echocardiography if images are suboptimal 6
Prognostic Implications by Thickness Category
Research demonstrates clear mortality stratification based on diastolic septal thickness:
- IVSd <20 mm: Baseline risk category 4
- IVSd 20-24 mm: Similar survival to baseline (HR 1.4) 4
- IVSd ≥25 mm: 5-fold increased mortality risk (HR 5.0) 4
- IVSd ≥30 mm: Highest risk with significantly reduced 5-year survival (85.7% vs 98.9% for IVSd <30 mm) 5
Baseline IVSd is inversely associated with survival, with each 1 mm increment carrying a hazard ratio of 1.13 for mortality 4.