Aortic annulus
virtual basal ring · PLAX zoom, mid-systole, inner-edge to inner-edge, hinge-point to hinge-point
Normal range & thresholds
Men 23 ± 2 mm · Women 21 ± 2 mm by 2D TTE; indexed ≈13 mm/m² in both sexes.[1] MDCT area-derived diameter runs 1–1.5 mm larger than TTE because the ring is elliptical, and MDCT (area 338–683 mm² depending on device) — not TTE — is the sizing standard for TAVI.[36] Small annulus is conventionally <20 mm / area <400 mm².
Pathophysiology
The annulus is not a true anatomical ring but the virtual plane through the three leaflet nadirs, structurally continuous with the LVOT and the aortic root. It enlarges only when the whole ventriculo-aortic junction enlarges — medial elastin loss (syndromic and bicuspid aortopathy) or chronic diastolic volume overload from aortic regurgitation stretching the root outward. It cannot dilate in isolation, so an enlarged annulus with normal sinuses should first raise suspicion of a measurement error.[18]
Raised by
- Annuloaortic ectasia — Marfan, Loeys-Dietz, vascular Ehlers-Danlos, familial TAAD (ACTA2, MYH11, TGFBR1/2, SMAD3)
- Bicuspid aortic valve, root phenotype
- Chronic severe aortic regurgitation (root stretch by volume load)
- Aortitis — Takayasu, giant cell, syphilitic, IgG4-related
- Long-standing hypertension plus age (modest, usually <2 mm)
Lowered by
- Constitutionally small annulus — short stature, women (a size, not a disease)
- Heavy annular and leaflet calcification encroaching on the ring
- Rheumatic and radiation valve disease with commissural fusion
- Unicuspid valve, congenital subaortic/valvular complex hypoplasia
Technique & pitfalls
- Zoom the PLAX; measure in mid-systole, when the ring is largest and the hinge points are crispest.
- Inner-edge to inner-edge, from the hinge point of the right coronary cusp to that of the non-coronary cusp, perpendicular to the aortic long axis.
- Never measure at the leaflet tips or at the sinus level — both are systematic overestimates.
- The same measurement is the LVOT diameter used in the continuity equation: a 1 mm error propagates to ≈10% error in aortic valve area, because the diameter is squared.[5]
- If a valve intervention is planned, the number that matters is the CT annular area and perimeter, not this one.[36]
Pseudo-change & artefact
- Oblique or foreshortened PLAX cuts the ring obliquely and overestimates.
- Diastolic measurement underestimates by 1–2 mm.
- Calcium blooming widens the apparent lumen; drop gain and use the shoulder of the calcium.
- Leading-edge convention (used for the root) adds ≈1 mm if wrongly applied here.
- TTE reports a minor-axis diameter of an ellipse; a ‘growth’ seen only on CT is usually the geometry, not the patient.
Treatment thresholds
- No intervention is triggered by annulus size itself. It is a sizing and feasibility variable, not a disease severity variable.
- Feeds LVOT area → stroke volume → AVA and stroke volume index (<35 mL/m² defines low-flow aortic stenosis).[5]
- Drives TAVI prosthesis selection and the risk of annular rupture (oversizing) or paravalvular leak (undersizing).[36]
- Predicts prosthesis–patient mismatch: projected indexed EOA ≤0.85 cm²/m² (moderate), ≤0.65 (severe) — the trigger for root enlargement or a supra-annular/transcatheter valve.[93,107]
Next step
- Next: repeat in mid-systole on a zoomed, non-foreshortened PLAX before accepting a discordant AVA.
- Then: ECG-gated contrast CTA for annular area, perimeter, coronary heights and access — mandatory before TAVI.[36]
- Then: 3D TOE with multiplanar reconstruction if CT contrast is contraindicated.
- If dilated: measure sinuses, STJ and ascending aorta; if the whole root is large, go to card 02.
Drugs
- Nothing shrinks an annulus.
- In syndromic aortopathy, beta-blockers and losartan slow root growth — that is prevention of further dilatation, not regression.[9]
- Fluoroquinolones are best avoided in known aortopathy.[9]
Reversibility
≈0% spontaneous reversibility. The virtual basal ring does not remodel back. It is fixed by surgery (valve-sparing root replacement, annular enlargement) or by a transcatheter frame. After AVR for chronic severe AR the root proximal to the graft may appear 1–2 mm smaller purely because the volume load has gone.[9,18]