Wall thickness ≥1.3 cm with family history of HCM, or ≥1.5 cm without, meets diagnostic threshold. Measure at end-diastole, perpendicular to septum.

Asymmetric septal hypertrophy (IVS:LVPW ratio > 1.3) is the classic HOCM pattern.

Any SAM with septal contact is significant. Duration of contact correlates with gradient severity.

Measure 1 cm below AV at end-systole, inner edge to inner edge. Required for VTI-based stroke volume.

Place M-mode cursor through the aortic valve leaflets in PLAX. Normal: leaflets open fully in early systole and remain open (box shape) until end-systole. HOCM: as the LVOT gradient peaks in mid-systole, the Venturi effect and SAM-septal contact cause partial or complete AV leaflet re-closure — producing a notch or 'W' shape on M-mode. Severity of notching correlates with gradient: mild notch = moderate obstruction; complete mid-systolic closure = severe obstruction (gradient often >50 mmHg). Provocation (Valsalva, amyl nitrite) worsens the notching. Key distinction from AS: AS shows globally reduced leaflet separation throughout systole (no notch, no normal early opening); HOCM shows full early opening followed by mid-systolic notch.